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SPI emuslion-离子浓度和加热对性质和氧化稳定性的影响

SPI emuslion-离子浓度和加热对性质和氧化稳定性的影响
SPI emuslion-离子浓度和加热对性质和氧化稳定性的影响

Characteristics and oxidative stability of soy protein-stabilized oil-in-water emulsions:In ?uence of ionic strength and heat pretreatment

Yun Shao a ,Chuan-He Tang a ,b ,*

a

Department of Food Science and Technology,KLGPNPS,South China University of Technology,Guangzhou 510640,Guangdong Province,People ’s Republic of China b

State Key Laboratory of Pulp and Paper Engineering,South China University of Technology,Guangzhou 510640,People ’s Republic of China

a r t i c l e i n f o

Article history:

Received 2August 2013Accepted 31October 2013Keywords:

Protein-stabilized emulsions Soy protein isolate Emulsifying property Emulsion stability Oxidative stability

a b s t r a c t

Some physicochemical characteristics,microstructure and stability of native and preheated (95 C,15min)soy protein isolate (SPI)-stabilized emulsions,formed at varying protein concentrations (c ;0.5e 4.0%,w/v)in the absence

or presence of 300mM NaCl,were characterized.The emulsifying ability,?occulated state of droplets,microstructure,interfacial protein concentration (G )of the fresh emulsions,as well as stability of these emulsions against coalescence,?occulation,creaming and even lipid oxidation upon storage up to 2weeks were evaluated.In general,increasing c was favorable for the emulsi ?cation ef ?ciency,but the ?occulated state of oil droplets or size of the ?ocs in the fresh emulsions was more affected by the presence of salt,and/or the heat pretreatment.Increasing ionic strength or application of a heat pretreatment resulted in remarkable increases in extents of droplet ?occulation in the fresh emulsions,as well as amount and concentrat,ion of adsorbed proteins at the interface.All the emulsions exhibited an extraordinary stability against coalescence and/or ?occulation.Increasing c led to a progressive increase in stability against creaming,especially for the preheated SPI emulsions with 300mM NaCl.All the emulsions at c ?1%or above exhibited a similarly high oxidative stability upon storage up to 9days.Even at c ?0.5%,the oxidative stability of the formed emulsions could be greatly improved by increasing ionic strength,and/or application of a heat pretreatment.The ?ndings have important implications for the development of an important kind of protein-stabilized emulsions with industrial relevance.

ó2013Elsevier Ltd.All rights reserved.

1.Introduction

Soy proteins have been widely applied in a wide range of food formulations,due to its good nutritional,and functional properties,and even health effects (FDA,1999).Besides the excellent heat-induced gelling properties,these proteins also exhibit good emul-sifying properties (Akoi,Taneyama,&Inami,1980;Hettiarachchy &Kalapathy,1998).To date,the emulsifying properties of soy proteins are mainly utilized as processing aids in concentrated emulsions,such as meat emulsions in the comminuted meat ?eld,and their use as an emulsifying agent in dilute emulsion products is very limited.One of the main reasons causing this situation is that soy proteins are a mixture of multi-components (e.g.,7S and 11S globulins)with complex oligomeric structure,and their properties

not only highly dependent on the type,composition and nature of the proteins,e.g.,extent of thermal denaturation and/or aggrega-tion,but also affected by a variety of processing and environmental parameters,e.g.,pH,temperature,salts and the presence of other components.For emulsions to be evaluated,the choice of emulsi-?cation or homogenization technique is also important for the emulsion properties.Thus,it is not surprising that much less in-formation is available addressing the emulsion properties of soy protein products as compared to milk proteins,e.g.,whey protein and caseins (Damodaran,2005;Dickinson,1994;McClements,2004).

Using a shear homogenization as the emulsi ?cation technique (e.g.,20,000rpm,30s),Wagner and coworkers mainly investigated the in ?uence of protein denaturation and protein concentration (c )on the coalescence stability of soy proteins (whey and isolate)-stabilized emulsions,formed at oil fraction (?)?0.25e 0.33and c ?0.1e 1.0%(w/v),and found that 1)irrespective of the applied c ,native soy protein emulsions exhibited better stability against coalescence than those stabilized by denatured proteins;2)the

*Corresponding author.Department of Food Science and Technology,KLGPNPS,South China University of Technology,Guangzhou 510640,Guangdong Province,People ’s Republic of China.Tel.:t862087114262;fax:t862087114263.

E-mail address:chtang@https://www.sodocs.net/doc/2911801359.html, (C.-H.Tang).Contents lists available at ScienceDirect

Food Hydrocolloids

journal homepa ge:https://www.sodocs.net/doc/2911801359.html,/locate/foodhyd

0268-005X/$e see front matter ó2013Elsevier Ltd.All rights reserved.https://www.sodocs.net/doc/2911801359.html,/10.1016/j.foodhyd.2013.10.030

Food Hydrocolloids 37(2014)149e 158

presence of NaCl (e.g.,0.15M)improved the coalescence stability of these emulsions,especially in the case at higher c values (e.g.,>0.5%)(Mitidieri &Wagner,2002;Palazolo,Mitidieri,&Wagner,2003).In these works,the creaming stability was not addressed,though the emulsions formed under the conditions investigated might be very unstable against creaming.In a following work addressing this issue,they observed a seemly contrasting phe-nomenon that the cream phase of denatured soy protein emulsions was more stable against coalescence and ?occulation than that of native protein counterparts (Palazolo,Sorgentini,&Wagner,2005).

The inconsistency might be due to the differences in interfacial nature and ?occulated state of droplets in the system.They attributed the higher stability in the cream phase (of denatured proteins-stabilized emulsions)to formation of hydrated ?ocs with a gel-like network structure (Palazolo et al.,2005).

On the other hand,several studies have been available addressing the properties and microstructure of soy protein-stabilized emulsions,produced by a series of emulsi ?cation tech-niques with high energy levels (Floury,Desrumaux,&Legrand,2002;Puppo,Sorgentini,&A?ón,2003;Roesch &Corredig,2002,2003).In these works,the applied c was relatively high ranging from 1to 10%,with ?varying within the range 0.1e 0.6.The test soy proteins included soy protein concentrate (SPC),soy protein isolate (SPI)and soy 11S globulin.Interestingly,it was found that the creaming stability of the emulsions could be greatly improved by increasing the c or ?,and even a preheating treatment of the pro-teins prior to the emulsi ?cation.The improvement of creaming stability was largely attributed to gel-like structure of the emul-sions (Roesch &Corredig,2002,2003).In a recent work of ours,we characterized the rheological properties and microstructure of these gel-like emulsions stabilized by unheated and preheated SPI,at a relatively high c value of 6%and varying ?values of 0.2e 0.6,and found that increasing the ?,or increasing the ionic strength pro-gressively increased the stiffness of these gel-like emulsions (Tang &Liu,2013).Thus,the gel-like network formation seems to be highly associated with bridging ?occulation of oil droplets,through inter-droplet hydrophobic interactions of the proteins adsorbed at the interface (Puppo et al.,2003;Tang &Liu,2013).

Based on the mentioned-above works,it can be generally recognized that the stability of soy protein emulsions,e.g.against coalescence and creaming,is not only highly related to the dena-tured state of proteins,the applied c and ionic strength in the aqueous phase and ?,but also depends on the energy level of emulsi ?cation technique.However,we can see that the properties and stability of soy protein emulsions at relatively low c and ?values,produced by a high energy level of emulsi ?cation,are little characterized.The development of these dilute soy protein emul-sions has important implications for many food formulations,e.g.soy protein beverages with emulsi ?ed ingredients.

Therefore,the present work was to systematically investigate the in ?uence of c in the aqueous phase (0.5e 4.0%,w/v),the pres-ence of 300mM NaCl and/or a heat pretreatment (at 95 C for 15min)on the stability against coalescence,?occulation and creaming of SPI-stabilized emulsions.The emulsions were formed by micro ?uidization at a speci ?c ?value (0.2).The emulsifying ability (or ef ?ciency),?occulated state of oil droplets in fresh emulsions,and coalescence,?occulation and creaming stability were evaluated.To help understand the mechanism for the emul-sion stability,some physicochemical parameters including zeta-potential and amount (and concentration)of adsorbed interfacial proteins of droplets,as well as microstructure of the fresh emul-sions were also evaluated.On the other hand,oxidative stability of oils in the SPI emulsions upon storage was also evaluated,due to the consideration that lipid oxidation can occur rapidly in oil-in-water emulsions due to their large surface area that facilitates

lipid oxidation and subsequent rancidity of the emulsions (Waraho,McClements,&Decker,2010).2.Materials and methods 2.1.Materials

SPI was prepared from defatted soy ?our,provided by Shandong Yuwang Industrial and Commercial Co.Ltd.(China),according to the same process described in the previous work (Tang,Chen,&Foegeding,2011).The protein content was 91.5%on the wet basis,as determined by the Kjeldahl method (N ?6.25).Nile Blue A and Nile Red was obtained from Sigma e Aldrich (Sigma Chemical Co.,St.Louis,MO,USA).Sodium dodecyl sulfate (SDS),isooctane,iso-propanol,butanol,methanol,trichloroacetic acid,thiobarbituric acid,ammonium thiocyanate,cumene hydroperoxide,and 1,1,3,3-tetraethoxypropane were purchased from Aladdin Reagent Cor-poration (Shanghai,China).Bovine serum albumin (BSA)was ob-tained from Fitzgerald Industries International Inc.(Concord,MA,USA).Soy oil was purchased from a local supermarket in Guangz-hou (China).All other chemicals,used with no further puri ?cation,were of analytical grade.

2.2.Preparation of oil-in-water emulsions

All the emulsions,stabilized by native or preheated SPI,were prepared at an oil volume fraction of 0.2,with varying c values of 0.5e 4.0%(w/v)in the continuous phase.Two sets of the unheated SPI dispersions at a speci ?c c value were prepared by dissolving the SPI powder in de-ionized water and stirring using a magnetic stirrer for 2h at room temperature.The dispersions were then stored overnight at 4 C to allow complete hydration.Sodium azide (0.02%,w/v)was used as an antimicrobial agent.The preheated SPI dispersions were prepared by heating one set of the above un-heated SPI dispersions,in a water bath at 95 C for 15min,and then immediately cooling in ice bath to room temperature.After that,each set of the SPI dispersions (unheated and preheated)were further divided into two subsets,with one subset of SPI dispersions added with 300mM NaCl.The salt was added by mixing the NaCl powder with the dispersions under stirred conditions,little by lit-tle.Finally,the pH of all the SPI dispersions was adjusted to 7.0using 1M NaOH or 1M HCl,if necessary.

For the emulsion formation,any SPI dispersion with a constant volume (about 50mL)was mixed with soy oil at ??0.2.The mixtures were pre-homogenized using a high-speed dispersing and emulsifying unit (model IKA-ULTRA-TURRAX T25basic,IKA Works,Inc.,Wilmington,NC)at 10,000rpm for 2min,to produce coarse emulsions.Then,the obtained coarse emulsions were further homogenized though a Micro ?uidizer (M110EH model,Micro ?uidics International Corporation,Newton,MA)for one pass at a pressure level of 40MPa.To help identi ?cation,the obtained emulsions from the four kinds of SPI dispersions were denoted as the emulsions I e IV as follows:I,native SPI without 300mM NaCl;II,native SPI with 300mM NaCl;III,preheated SPI without 300mM NaCl;IV,preheated SPI with 300mM NaCl.All the obtained emulsions were directly subject to analysis.

2.3.Determination of volume-average droplet size (d 4,3)

The volume-average droplet size (d 4,3)of freshly prepared or stored (24h)SPI emulsions were determined using a Malvern MasterSizer 2000(Malvern Instruments Ltd,Malvern,Worcester-Y.Shao,C.-H.Tang /Food Hydrocolloids 37(2014)149e 158

150

(1.456)to that of the continuous phase (1.33).Droplet size mea-surements are reported as the volume-average diameter,

d 4,3?P n i d i 4/P n i d i 3,and surface-averag

e diameter,d 3,2?P n i d i 3

/P n i d i 2

,where n i is the number of droplets with diameter d i .All determinations were conducted at least in duplicate.2.4.Confocal laser scanning microscopy (CLSM)

CLSM observations of the fresh emulsions were performed on a Leica TCS SP5Confocal Laser Scanning head mounted on a Leica DMRE-7(SDK)upright microscope (Leica Microsystems Inc.,Hei-delberg,Germany)equipped with a 20?HC PL APO/0.70NA oil immersion objective lens.The various emulsion samples were stained with an appropriate amount of 1.0%(w/v)Nile Blue A (?uorescent dye)in distilled water,or the mixture of Nile Blue A (1%,w/v)and Nile Red (0.1%,w/v)in 1,2-propanediol (containing distilled water,20m L/g),according to the process described by Auty,Twomey,Guinee,and Mulvihill (2001).The stained emulsions were placed on concave confocal microscope slides (Sail;Sailing Medical-Lab Industries Co.Ltd.,Suzhou,China),covered with glycerol-coated cover slips,and examined with a 100magni ?cation lens and an argon/krypton laser having an excitation line of 514nm and a Helium Neon laser (HeNe)with excitation at 633nm for the dye Nile Blue A alone,and 488and 633nm for combined dyes,respectively.

2.5.Determination of percentage of adsorbed proteins (AP%)and interfacial protein concentration (G )

The AP%and G of various freshly prepared emulsions were determined using the method as described by Ye (2008)and Puppo et al.(2011),with a few modi ?cations.In brief,1mL of fresh emulsion was centrifuged at 10,000g for 30min at room temperature.After the centrifugation,two phases were observed:the creamed oil droplets at the top of the tube and the aqueous phase of the emulsion at the bottom.The cream layer was care-fully removed using a syringe,and the subnatant was ?ltered through a 0.22m m ?lter (Millipore Corp.).The protein concen-tration of the ?ltrate (C f )was determined with the Lowry method using BSA as the standard.The initial protein solution was also centrifuged at the same conditions to allow determination of protein concentration (C s )in the supernatant.The AP%and G were calculated as follows:

AP%?

C s àC f

?100=C o (1)G

mg =m 2

?

C s àC f

?d 3;2=6?

(2)

where C o (mg/mL)is the initial protein concentration applied for the emulsion preparation,and d 3,2and ?are the surface-average diameter (see Section 2.3),and oil fraction (0.2in the present work),respectively.

2.6.Determination of percentage of ?occulation index (FI%)

The percentage of ?occulation index (FI%)of oil droplets in the fresh or stored emulsions was calculated as the following equation:

FI%?

?àd 4;3-water á àd 4;3-1%SDS áà1:0??100

(3)

where d 4,3-water and d 4,3-1%SDS of any emulsion after 0(fresh)or 24h

of storage are the d 4,3values as determined using water and 1%SDS as the dispersants,respectively.

2.7.Determination of percentage of creaming index (CI%)

The various emulsions formed under various conditions were applied for CI determination.Percentage of creaming index (CI%)was determined as described by Firebaugh and Daubert (2005),with some modi ?cations.Ten millimeters (10mL)of each emulsion was ?lled into a glass test tube (1.5-cm internal diameter ?12-cm height)and then stored at ambient temperature (placed in a perpendicular state).The height of the serum (H s )and the total height of emulsions (H t )were recorded after storage up to 7days at ambient temperature for different period times.The mean and standard deviations of three replicates were reported.The CI%was reported as (H s /H t )?100.

2.8.Measurements of lipid oxidation

2.8.1.Lipid hydroperoxides

Emulsions (6mL)were placed in test tubes and allowed to oxidize at 50 C for 9days.Formation of lipid peroxides was eval-uated according to the method of Shantha and Decker (1994),with a few modi ?cations.In brief,aliquots (0.3mL)of the emulsions,after storage of a speci ?c incubation period,were mixed with 1.5mL of isooctane/2-propanol (3:1,v/v)by vortexing 3times with each of approximately 10s.The organic solvent phase of the mix-tures was collected by centrifugation at 1000g for 2min.The organic solvent phase (200m L)was added to 2.8mL of a meth-anol:1-butanol mixture (2:1,v/v),followed by addition of 15m L of 3.94M ammonium thiocyanate and 15m L of ferrous iron solution (prepared by mixing 0.132M BaCl 2and 0.144M FeSO 4).The absorbance of the resultant solutions were measured at 510nm 20min after addition of the iron.Lipid hydroperoxide concentra-tions (mmol/L of emulsion)were determined using a standard curve made from cumene hydroperoxide.

2.8.2.Thiobarbituric acid-reactive substances (TBARS)

TBARS content of the emulsions upon storage was determined using a process as described by McDonald and Hultin (1987),with a few modi ?cations.In brief,a series of an emulsion with varying volume of 0.1e 1.0mL were added with distilled water to a constant volume (1.0mL),and these diluted emulsions were mixed with 2.0mL of TBA reagent [15%(w/v)trichloroacetic acid and 0.375%(w/v)thiobarbituric acid in 0.25M HCl]in test tubes.The resultant mixtures were heated in a boiling water bath for 15min,and then cooled in the air to room temperature for approximately 10min.Instead of the conventional centrifugation at 1000g for 15min,the heated mixtures were ?ltered with a 1.5m m microporous mem-brane,in the present work.The absorbance of the ?ltrates was recorded at 532nm.The TBARS concentration (m mol/L of emulsion)was determined according to a standard curve with 1,1,3,3-tetraethoxypropane.2.9.Statistical analysis

An analysis of variance (ANOVA)of the data was performed,and a least signi ?cant difference (LSD)with a con ?dence interval of 95%was used to compare the means.3.Results and discussion

3.1.Fresh emulsion characteristics:in ?uence of protein concentration

3.1.1.Droplet size (in the absence or presence of SDS)

Table 1shows the d 4,3values of droplets in different sets (I e IV)of fresh SPI-stabilized emulsions,at varying c values (0.5e 4.0%),

Y.Shao,C.-H.Tang /Food Hydrocolloids 37(2014)149e 158151

determined using water or 1%SDS as the dispersing solvent.The d 4,3of de ?occulated droplets in the emulsions,e.g.,in the presence of SDS,can re ?ect ability of the proteins to help dispersion of oil phase into an aqueous medium.In general,we can see that for any type of emulsions,increasing c from 0.5to 4.0%resulted in a pro-gressive reduction in d 4,3(in 1%SDS)(Table 1),suggesting that at higher c values,more proteins participated in the stabilization of interfacial ?lms of oil droplets.

The d 4,3value (in 1%SDS)of the type-I emulsions at any c value was basically the same as those of the type-II emulsion at a com-parable c value (Table 1).In contrast,a heat pretreatment (95 C,15min)resulted in distinct decreases in d 4,3(in 1%SDS)of the emulsions at any test c value and a given NaCl concentration (0or 300mM;Table 1).The observations suggested that the emulsifying ability of SPI was unaffected by the ionic strength,but remarkably improved by the heat pretreatment.The improvement of the emulsifying ability agrees with a well-recognized fact that partial denaturation and subsequent structural unfolding of globular proteins upon heating are favorable for their emulsifying properties (Damodaran,1996).Interestingly,we observe that the improve-ment by the preheating was more distinct in the presence of 300mM NaCl than that without NaCl;for example,the protein amount approximately at a c of 1%seems to be enough to fully cover the interface of oil droplets,in the preheated case with the presence of 300mM NaCl,while it would need a c of at least 2%for the type-I or II emulsions (Table 1).The better improvement in the presence of 300mM NaCl might be partially because the electrostatic screening facilitates the adsorption of the proteins at the interface and sub-sequent protein e protein lateral interactions of adsorbed proteins.

Besides the emulsifying ability,the ?occulation state of droplets in the fresh emulsions,using ?occulation index at 0h (FI 0)as the index,is also an important parameter affecting their stability upon storage.For the type-I emulsions,the FI 0values were basically very low (5e 18.0)(Table 1),indicating that most of droplets in fresh emulsions were present in the un ?occulated form.This signi ?es that in this case,the inter-droplet electrostatic repulsion played a vital role in the stabilization of the emulsions.For the type-II emulsions,the FI 0values progressively increased from about 260to 1700with the c increasing from 0.5to 4.0%(Table 1),re ?ecting a

severe ?occulation of droplets,in a protein concentration-dependent way.This observation con ?rmed that the electrostatic repulsion was of vital importance for the ?occulated state of droplets in the emulsions.

On the other hand,we can see that at any test c value,the preheating of the proteins prior to the emulsi ?cation led to remarkable increases in extent of droplet ?occulation in the fresh emulsions,irrespective of the presence of NaCl or not,as evidenced by considerably increased FI 0(Table 1).The observation is reason-ably expected,since the preheating (95 C,15min)might result in complete denaturation and/or aggregation of the proteins in SPI (Luo,Liu,&Tang,2013),thus greatly in favor of the inter-droplet attractive interactions in the emulsions.However,it can be still observed that the changing patterns of the size of the ?ocs or FI 0upon the c increasing were dependent on whether the surface charge of the proteins was screened or not.For the type-III emul-sions,although the FI 0progressively increased with increasing the c (from 0.5to 4.0%),the size of the ?ocs (or d 4,3in water)on the contrary decreased (Table 1).Whereas for the type-IV emulsions,the d 4,3data (in water)were almost independent of the c ,while the FI 0progressively increased from about 2200to 8800upon the c increasing (Table 1).At any tested c value,the d 4,3data (in water)and the FI 0of the type-IV emulsions were considerably higher than those of the type-III counterparts (Table 1),con ?rming that the electrostatic screening further increased the inter-droplet attrac-tive interactions,and subsequently,extent of droplet ?occulation in the fresh emulsions.

3.1.2.Microstructure of the fresh emulsions

The ?occulated state of oil droplets in four types (I e IV)of the fresh emulsions was also evaluated using CLSM technique,as dis-played in Fig.1.As expected,the droplet microstructure of these emulsions considerably varied with the type of the emulsions (stabilized by native or preheated SPI,with or without 300mM NaCl)and the c .For the type-I emulsions (native SPI;without 300mM NaCl),it can be observed that most of the droplets were present in the separated and un ?occulated form,and the droplet size progressively decreased with increasing the c (Fig.1,row I).As expected,the electrostatic screening by the presence of 300mM

Table 1

Emulsion characteristics,including mean droplet size (d 4,3),?occulation index (FI),and adsorbed protein percentage (AP%)and interfacial protein concentration (G )of SPI-stabilized emulsions at different c levels (0.5e 4%),freshly formed (0h)or after storage of 24h.All values are the means of at least duplicate measurements.Emulsion types

C (%)

d 4,3(m m)FI Interfacial protein 0h 24h 0h

24h

AP%

G (mg/m 2)

Water

1%SDS Water 1%SDS I (native)

0.5% 3.79i 3.46a 3.93j 3.45a 9.7m 13.9k 19.7e 2.06i 1% 2.41j 2.04c 2.38k 2.05c 17.8l 16.1j 19.9e 3.04h 2% 1.19k 1.13f 1.16l 1.12e 5.0n 3.6m 27.2d 3.84g 4%0.76l 0.69g 0.75m 0.69g 10.3m 9.3l 19.2e 3.21h II (NaCl)

0.5%10.8f 2.98b 11.3fg 2.92b 262k 288i 47.7c 4.85c 1%12.1e 1.60d 12.2f 1.60d 659i 664h 42.0cd 4.94c 2%11.0f 0.91f 11.2fg 0.92f 1112g 1126f 47.0c 5.63b 4%11.2f 0.62gh 10.8g 0.621704e 1651e 43.1cd 6.52a III

0.5%16.2d 3.00b 19.9d 3.03b 439j 557h 31.3d 3.07h 1%15.0d 1.46e 14.7e 1.43de 924h 932g 42.5cd 3.77g 2%9.59g 0.73g 9.14h 0.72g 1212fg 1174fg 49.7c 4.40d 4% 6.00h 0.42h 6.05i 0.45h 1339f 1244f 45.1cd 3.77g IV

0.5%33.5b 1.45e 34.4c 1.58d 2206c 2076d 84.8a 3.16h 1%32.7b 0.86f 34.6c 0.85f 3703c 3980c 69.4b 3.602%31.9c 0.49h 39.5b 0.46h 6452b 8513b 81.4ab 4.18f 4%

36.9a

0.41h

62.8a

0.41h

8817a

15337a

80.4ab

6.54a

Emulsion types:I,the emulsions stabilized by native SPI,in the absence of NaCl;II,the emulsions stabilized by native SPI,in the presence of 300mM NaCl;III,the emulsions stabilized by preheated SPI,in the absence of NaCl;IV,the emulsions stabilized by preheated SPI,in the presence of 300mM NaCl.The preheating prior to emulsi ?cation was performed by heating the SPI solutions in a bath at 95 C for 15min,and then immediately cooling in ice bath to room temperature.Different superscripts indicate signi ?cant difference at p <0.05level among the same column.

Y.Shao,C.-H.Tang /Food Hydrocolloids 37(2014)149e 158

152

NaCl greatly increased the extent of droplet ?occulation (Fig.1,row II).In this case,we can see that the contour of the ?occulated oil

droplets (or ?ocs)gradually became irregular,and at higher c values (e.g.,4%),most of the droplets were present in the clustered form.

The microstructure of the type-III emulsions at any tested c value was basically similar to that of the type-II counterparts,except that at c ? 4.0%,no distinct droplet ?occulation was observed (Fig.1,rows II and III).In this type of emulsions (III),we can observe that upon the c increasing,the droplet morphology gradually became vague,and even not recognizable at c ?4.0%.The microstructure of the type-III emulsion at c ?4%is almost the same as that reported for the preheated SPI emulsion gel (at c ?6.0%)induced by transglutaminase (Yang,Liu,&Tang,2013),where the droplets were homogenously entrapped within the gel network.The additional electrostatic screening by the presence of 300mM NaCl further increased the droplet ?occulation,as evi-denced by formation of more clustered and irregular ?ocs (Fig.1,rows III and IV),con ?rming the above argument that the elec-trostatic screening accelerated the instability of the emulsions against ?occulation.In this case,we can interestingly observe that the ?ocs or droplet clusters became more interconnected upon increasing the c ,and at higher c values,e.g.4%,a homogenous gel-like network could be distinctly observed (Fig.1,row IV).The gel-like network formation for the emulsions stabilized by preheated SPI in the presence of 300mM NaCl has been well con ?rmed in our previous work (Tang &Liu,2013).These CLSM observations are basically consistent with the droplet size and/or FI 0data (Table 1),con ?rming that the ?occulated state of droplets in the fresh emulsions was dependent on the c and the denatured na-ture of the proteins.

Fig.1.Typical CLSM images of four types (I e IV)of SPI-stabilized emulsions,formed at c values of 0.5(a),1.0(b),2.0(c)and 4.0%(d),respectively.The emulsion types I e IV are the same as in the legend of Table 1.

Y.Shao,C.-H.Tang /Food Hydrocolloids 37(2014)149e 158153

3.1.3.AP%and interfacial protein concentration(G)

The AP%and G data of various fresh emulsions at varying c values(0.5e4.0%)are included in Table1.It can be seen that the AP%

and G data changed with type of the emulsions and the applied c,in a very complex way.In general,for any type of emulsion at any

tested c value,both the presence of300mM NaCl and the pre-

heating resulted in accumulating increases in AP%(Table1).For

example,the AP%was in the range19.2e27.2%,42.0e47.7%,31.3e

49.7%and69.4e84.8%for the type I,II,III and IV emulsions,

respectively.The data indicated that both the electrostatic

screening and the protein denaturation and/or aggregation were

favorable for the protein adsorption at the interface.Interestingly,

in the presence of300mM NaCl,the AP%for the type II or IV

emulsions was approximately kept constant,even when the c

increased from0.5to4.0%(except for the type-IV emulsions at

c?1.0%)(Table1),signifying that under the charge-screening

conditions,the protein adsorption at the interface seemed to be

mainly related to the total interface area of oil droplets.

The G is an important parameter affecting stability of oil drop-lets against coalescence.In usual,the higher the G is,the greater the emulsion stability against coalescence.For the type-I emulsions,

the G progressively increased from2.06to3.84mg/m2(highest), upon the c increasing from0.5to2.0%,and a further increase in c

(up to4%),on the contrary,led to a slight decline in G(Table1).The G at c?0.5%(2.06mg/m2)is slightly lower than that(2.2mg/m2) reported by Mitidieri and Wagner(2002)for native SPI emulsion at

c?0.1%.The G at c?1.0%(3.04mg/m2)is almost the same as that (3.03mg/m2)previously reported for untreated SPI emulsion at the same c,though in the latter case,the pH was8.0(vs7.0in the present work)(Puppo et al.,2005).In other previous works with a much higher c(7%),Puppo and others reported that the emulsions stabilized by native SPI,7S or11S globulins at pH8.0exhibited a G value of8.6e9.6mg/m2(Puppo et al.,2005,2011),considerably higher than that at c?2.0or4.0%(3.84or3.04mg/m2),in the present work.The extraordinary higher G might be largely attrib-uted to the difference in nature of applied SPI,e.g.extent of protein denaturation and/or aggregation.Mitidieri and Wagner(2002)had indicated that the emulsion stabilized by denatured SPI exhibited a much higher G than that by native SPI.

The progressive increase in G is consistent with the fact that at higher c values,more proteins could be adsorbed at the interface (per a speci?c interfacial area).The decreased G at c?4.0%(relative to2.0%)might be partially related to the size effect of oil droplets, since the size of droplets,produced during the emulsi?cation at this high c,was remarkably lower.As compared with the type-II emulsions at any tested c value,the G of the type-II emulsions was distinctly increased(Table1).In this case,it can be observed that the G progressively increased from4.85to6.52mg/m2,as the c increased from0.5to4.0%.The improvement of G by the presence of300mM NaCl was clearly attributed to enhanced attractive in-teractions(especially hydrophobic interactions)between adsorbed proteins and those in the continuous phase,as a result of consid-erably decreased electrostatic repulsion(data not shown).

The improvement of the G,due to enhanced hydrophobic in-teractions,can be observed when a heat pretreatment of the pro-teins was applied prior to the emulsi?cation,as evidenced by the comparison of the G between the type-I and III emulsions(Table1). Similar to the type-I case,highest G value(4.40mg/m2)was observed at c?2.0%for the type-III emulsions.On the other hand, although the type-IV emulsions exhibited a similar changing pattern of the G upon increasing the c to that for the type-II emulsions,their G values at c?0.5e2.0%was distinctly lower (Table1).The decreased G values are in fact consistent with their decreased d4,3values(in1%SDS;Table1),con?rming the formation of emulsions with larger interfacial area.3.2.Emulsion stability

3.2.1.Coalescence and?occulation stability

The coalescence stability of oil droplets in an emulsion can be evaluated by the changes in its d4,3value(in1%SDS)before and after storage for a speci?c incubation period.In the present work,we see that all the emulsions at any test c value(0.5e4.0%)exhibited a high stability against coalescence upon storage of24h incubation,as evidenced by insigni?cant changes in d4,3values(in1%SDS)be-tween0and24h(Table1).Even upon storage up to1week,all the test emulsions did not show any signi?cant increase in d4,3values (in1%SDS)(data not shown),indicating extraordinary coalescence stability.The high coalescence stability of these emulsions might be largely attributed to their relatively high G values(2.06e6.54mg/ m2),and formation of viscoelastic interfacial?lms.

The?occulation stability of these emulsions upon storage of 24h incubation was also evaluated.Since all the emulsions dis-played high coalescence stability upon storage,the?occulation stability can be thus re?ected by the relative changes in FI before and after the storage.Interestingly,we can observe that only for the type-IV emulsions at c values of2.0%or above,the storage(of24h incubation)resulted in a remarkable increase in FI, e.g.with increasing extents of about32and74%for c values of2.0and4.0%, respectively(Table1).The observation indicated that the native SPI emulsions,at varying c values of0.5e4.0%,exhibited high?occu-lation stability(upon storage of24h),despite the presence of 300mM NaCl,while for the preheated SPI emulsions,only those without300mM NaCl showed high?occulation stability.The c-dependent instability against?occulation for the type-IV emulsions might be related to the inter-droplet hydrophobic interactions be-tween adsorbed and aggregated proteins at the interface.

3.2.2.Creaming stability

Fig.2shows the changes in percentage of creaming index(CI%) for all the test emulsions,upon storage up to2weeks.For the type-I emulsions,distinctly visual creaming occurred only after storage of 6days at c?0.5%and of12days at c?1.0%,respectively(Fig.2A). Although there was no distinct creaming for the emulsions at c above1.0%upon storage of2weeks,it can be still observed that the bottom of the emulsions became less visually turbid(Fig.3).For the type-II emulsions(with300mM NaCl),we can generally observe that as compared with the type-I emulsions,the initiation of distinctly visual creaming was greatly accelerated(for example, distinct creaming could be observed within a storage period of 12h);at any test c value,the CI%progressively increased over storage period;increasing the c resulted in progressive decreases in the creaming rate(during the initial period,e.g.less than2days) and the CI%at the end of the storage(Fig.2B).After storage of2 weeks,the CI%(about48%)of the type-II emulsions at c?0.5%was considerably lower than that(w70%)for the type-I emulsions at the same c.In this case,the serum layers were much less turbid than in the type-I emulsions(Fig.3).The observations indicated that the creaming behavior of the emulsions was not only related to the ?occulated state of droplets in the fresh emulsions,but also dependent on the inter-droplet interactions of separated or?oc-culated droplets.

For the type-II emulsions,the progressive decrease in CI%(e.g., at the end of storage)upon the c increasing seems to be contrasting from the observations that the FI(at0or24h)of the fresh emul-sions progressively increased with increasing the c(Table1),since it is well recognized that for a conventional emulsion(e.g.stabi-lized by means of electrostatic repulsion)the droplet?occulation usually facilitates the creaming.Herein,it should be to note that the G of the fresh emulsions(type-II)progressively increased with increasing the c(Table1).A similar inconsistency between the

Y.Shao,C.-H.Tang/Food Hydrocolloids37(2014)149e158 154

creaming behavior and the droplet ?occulation (and even G )has been observed for the preheated soy 11S globulin stabilized emulsions,where a gel-like microstructure formed in the emul-sions has been con ?rmed to account for the improved creaming stability at higher extent of droplet ?occulation (Luo et al.,2013).

Similar to the type-I emulsions,the type-III emulsions at c values above 1%did not exhibit any distinct creaming upon storage up to 2weeks (Fig.2A,C).However,we can still observe that although the heat pretreatment greatly accelerated the initiation of the creaming at c values of 0.5e 1.0%,the CI%at the end of storage was remarkably decreased,as compared to the type-I emulsions at comparable c values (Fig.2A,C).Considering the observation that

the FI of the fresh type-III emulsions progressively increased upon the c increasing (Table 1),it thus can be reasonably hypothesized that the formation of the gel-like microstructure played a vital role in the creaming behavior of these emulsions,though there was high electrostatic repulsion between the droplets or ?ocs.On the other hand,we see that the electrostatic screening by the presence of 300mM NaCl further inhibited the creaming of the preheated SPI emulsions at these c values (0.5e 1.0%;Fig.2C,D).The observation is well in accordance with the FI data of the fresh emulsions (Table 1),further

supporting that the formation of gel-like network composed of the ?ocs was favorable for the creaming stability of the emulsions.

2

4

6

8

10

12

14

C I (%)

Storage Time (days)

A

2

4

6

8

10

12

14

B

C I (%)

Storage Time (days)

2

4

6

8

10

12

14

20

40

60

80

C

C I (%)

Storage Time (days)

2

4

6

8

10

12

14

D

C I (%)

Storage Time (days)

Fig.2.Evolution of creaming index percentage (CI%)of the native or preheated SPI-stabilized emulsions at varying c values (0.5e 4.0%),upon storage up to 2weeks.Panels A e D represent the emulsion types I e IV,respectively (as described in the legend of Table 1).Each data are the means and standard deviations of triplicate measurements.

Fig.3.Typical visual images of the four types of the emulsions after storage of 2weeks.The types (I e IV)for the emulsions are the same as described in the legend of Table 1.

Y.Shao,C.-H.Tang /Food Hydrocolloids 37(2014)149e 158155

3.3.Oxidative stability

Lipid oxidation is a major cause of quality deterioration in food emulsions (Coupland &McClements,1996).For protein-stabilized emulsions,the proteins can provide a protection against lipid oxidation,through formation of relatively thick viscoelastic interfacial ?lms,scavenging of free radicals,as well as chelation of metal ions (Coupland &McClements,1996;Hu,McClements,&

L i p i d H y d r o p e r o x i d e

(m m o l /L e m u l s i o n )

Time (days)A

L i p i d H y d r o p e r o x i d e

(m m o l /L e m u l s i o n )

Time (days)

B

L i p i d H y d r o p e r o x i d e

(m m o l /L e m u l s i o n )

Time (days)C

L i p i d H y d r o p e r o x i d e

(m m o l /L e m u l s i o n )

Time (days)

D

Fig.4.Evolution of lipid hydroperoxides in the four types (I e IV)of SPI-stabilized emulsions upon accelerated storage up to 9days.Data are expressed as the means and standard deviations of triplicate measurements.The types (I e IV)of the emulsions are the same as in the legend of Table 1.

A B

C D

Fig.5.Evolution of thiobarbituric acid-reactive substances (TBARS)in the four types of SPI-stabilized emulsions upon accelerated storage up to 9days.Data are expressed as the means and standard deviations of triplicate measurements.The types (I e IV)of the emulsions are the same as in the legend of Table 1.

Y.Shao,C.-H.Tang /Food Hydrocolloids 37(2014)149e 158

156

Decker,2003).The oxidative stability of the different SPI-stabilized emulsions at varying c values of0.5e4.0%was deter-mined by monitoring both lipid hydroperoxide and thiobarbituric acid-reactive substances(TBARS)formation during the storage up to9days,as displayed in Figs.4and5.For the hydroperoxide formation of the type-I emulsions,we can see that at c?0.5%,the lipid hydroperoxide progressively increased up to about14mmol/ L emulsion with storage time up to9days,but the hydroperoxide formation was considerably inhibited by increasing the c to1.0% or above(Fig.4A).A similar phenomenon has been reported for a commercial SPI-stabilized emulsion at the same c and at a lower oil fraction(0.05),with corn oil as the core(Hu et al.,2003).The inhibition of lipid oxidation at c?1.0e4.0%relative to that at c?0.5%is well in agreement with the higher G values at c values of1.0e4.0%relative to that at c?0.5%(Table1),indicating that the oxidation inhibition was mainly due to the formation of thicker viscoelastic interfacial membrane at higher c values.As indicated in Table1,the G value of the emulsions(even at c?0.5%)was increased by the electrostatic screening(with 300mM NaCl)and/or a heat pretreatment.As a consequence,the lipid oxidation for the three types(II e IV)of the emulsions was expected to be greatly inhibited(as compared to that for the type-I emulsion at c?0.5%)(Fig.4).

On the other hand,the increase in TBARS for the different emulsions at varying c values was basically similar to that of the hydroperoxide(Figs.4and5),indicating the importance of increasing the G for the bene?cial effects in inhibiting the increase of secondary oxidation products.However,some differences in the increases of TBARS and the hydroperoxide existed for the type-III emulsions(Figs.4and5).In this case,it can be observed that the increase of TBARS at c?0.5%was distinctly higher than that at c values of1.0e4.0%.The underlying reason for this inconsistency is still unknown.

4.Conclusions

The present work clearly indicated that the physicochemical and microstructural characteristics,storage stability and oxidative sta-bility of SPI-stabilized emulsions were closely associated with the applied initial c and ionic strength in the continuous phase,as well as the nature(native or denatured)of proteins.In general, increasing ionic strength or application of a heat pretreatment of the proteins(prior to the emulsi?cation)resulted in a remarkable increase in inter-droplet attractive interactions,and subsequently, an increased extent of droplet?occulation.However,the enhanced inter-droplet interactions were favorable for the creaming stability, especially when combined treatments of increasing ionic strength and a preheating were applied.All the emulsions,stabilized by native or heat-treated SPI at varying c values of0.5e4.0%,in the presence or absence of300mM NaCl,exhibited high stability against coalescence.The underlying mechanisms for the enhanced stability at higher c values varied with the emulsions stabilized by native or denatured SPI.In the native SPI cases,the emulsion sta-bility(e.g.,?occulation and creaming)was largely determined by the inter-droplet electrostatic repulsion,while in the denatured SPI cases,the creaming stability was more contributed by formation of gel-like network composed of the droplet?ocs than by the inter-droplet repulsion.All the emulsions at c values of1%or above exhibited a similarly high oxidative stability upon storage up to9 days.Even at c?0.5%,the oxidative stability of the formed emul-sions could be greatly improved by increasing ionic strength,or application of a heat pretreatment.These results are of great importance for the understanding the physicochemical character-istics and oxidative stability of soy protein-stabilized emulsions that have displayed a great potential to be applied in the food formulations.

Acknowledgment

This work is supported by the National Natural Science Foun-dation of China(serial number:31171632),and Program for New Century Excellent Talents in University(NCET-10-0398). References

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碳正离子重排规律

有机化学中重排反应 有机化学中重排反应很早就被人们发现,研究并加以利用。第一次被Wohler发现的,由无 机化合物合成有机化合物,从而掀开有机化学神秘面纱的反应一加热氰酸铵而得到尿素,今 天也被化学家归入重排反应的范畴。一般地,在进攻试剂作用或者介质的影响下,有机分 子发生原子或原子团的转移和电子云密度重新分布,或者重键位置改变,环的扩大或缩小,碳架发生了改变,等等,这样的反应称为是重排反应。 按照反应的机理,重排反应通常可分为亲核反应、亲电反应、自由基反应和周环反应四大类。 也有按照不同的标准,分成分子内重排和分子间重排,光学活性改变和不改变的重排反应,一、亲核重排 重排反应中以亲核重排为最多,而亲核重排中又以1,2重排为最常见。 (一)亲核1, 2重排的一般规律 1?亲核1,2重排的三个步骤:离去基团离去,1, 2基团迁移,亲核试剂进攻2?发生亲核1,2重排的条件 (1 )转变成更稳定的正离子(在非环系统中,有时也从较稳定的离子重排成较不稳定的离子) (2)转变成稳定的中性化合物 (3 )减小基团间的拥挤程度,减小环的张力等立体因素。 (4)进行重排的立体化学条件:带正电荷碳的空 p轨道和相邻的C—Z键以及a碳和B碳应共平面或接近共平面 (5)重排产物在产物中所占的比例不仅和正电荷的结果有关,而且和反应介质中存在的亲核试剂的亲核能力有关 3?迁移基团的迁移能力 (1)多由试验方法来确定基团的固有迁移能力 (2 )与迁移后正离子的稳定性有关 (3)邻位协助作用 (4 )立体因素 4?亲核1, 2重排的立体化学: (1 )迁移基:构象基本保持,没有发现过构型反转,有时有部分消旋 (2)迁移终点:取决于离去及离去和迁移基进行迁移的相对时机 5?记忆效应:后一次重排好像和第一次重排有关,中间体似乎记住了前一次重排过程 (二)亲核重排主要包括基团向碳正离子迁移,基团向羰基碳原子迁移,基团向碳烯碳原子 迁移,基团向缺电子氮原子转移,基团向缺电氧原子的迁移,芳香族亲核重排,下面就这六种迁移作简要介绍: 1.基团向碳正离子迁移: (1)Wagner-Meerwein重排:烃基或氢的1, 2移位,于是醇重排成烯 (2)片那醇重排:邻二醇在酸催化下会重排成醛和酮

碳正离子

第一节碳正离子 含有带有正电荷的三价碳原子的基团,是有机化学反应中常见的活性中间体。很多离子型的反应是通过生成碳正离子活性中间体进行的,同时碳正离子也是研究得最早、最深入的活性中间体,很多研究反应历程的基本概念和方法都起始于碳正离子的研究,因此,有人认为碳正离子的研究是理论有机化学的基础。 一.碳正离子的生成 碳正离子可以通过不同方法产生,主要有下面三种: 1.中性化合物异裂,直接离子化 化合物在离解过程中,与碳原子连接的基团带着一对电子离去,发生共价键的异裂,而产生碳正离子,这是生成碳正离子的通常途径。 明显的实例如: 在这样的过程中,极性溶剂的溶剂化作用是生成碳正碳离子的重要条件。反应生成难溶解的沉淀也可影响平衡,使反应向右进行,而有利于碳正离子的生成,例如Ag+可以起到催化碳正离子生成的作用。 R-Br + Ag → AgBr↓ + R+ SbF5作为Lewis酸,又可生成稳定的SbF6-,也有利于碳正离子的生成。 R-F + SbF5 → R+ + SbF6- 在酸或Lewis酸的催化下,醇、醚、酰卤也可以离解为碳正离子,例如: 利用酸性特强的超酸甚至可以从非极性化合物如烷烃中,夺取负氢离子,而生成碳正离子。 由于碳正离子在超酸溶液中特殊的稳定性,很多碳正离子结构和性质的研究是在超酸中进行的,利用超酸可以制备许多不同碳正离子的稳定溶液。 2.正离子对中性分子加成,间接离子化

质子或带电荷的基团在不饱和键上的加成也可生成碳正离子。 如烯键与卤化氢的加成,第一步生成碳正离子。 羰基酸催化的亲核加成,首先质子化形成碳正离子,更有利于亲核试剂进攻。 芳环上的亲电取代反应,如硝化是由+NO2正离子进攻,形成σ络合物,这是离域化的碳正离子。 3.由其他正离子生成 碳正离子可以由其他正离子转变得到,例如重氮基正离子就很容易脱氮而生成芳基正离子。 也可以通过一些较易获得的正离子而制备更稳定但难于获得的碳正离子,例如用三苯甲 基正离子可以夺取环庚三烯的负氢离子而获得离子。 二.碳正离子的结构 碳正离子带有正电荷,中心碳原子为三价,价电子层仅有六个电子,其构型有两种 可能:一种是中心碳原子处于杂化状态所形成的角锥形构型,一种是的杂化状 态所形成的平面构型。不论还是,中心碳原子都是以三个杂化轨道,与三个成键原子或基相连构成三个σ键,都余下一个空轨道。不同的是前者的空轨道是杂化轨道,而后者空着的是未杂化的轨道。

1碳正离子的稳定性顺序为

项目七 练习题 一、填空 1.碳正离子的稳定性顺序为 > > > 。 2.活性自由基的稳定性顺序为 > > > 。 3.马尔科夫尼科夫规则是指当卤化氢与不对称烯烃加成时,卤化氢中的氢原原子加到 ,卤原子加到 。 4.用碘作碘化试剂与芳烃作用时,由于生成的碘化氢具有 ,必须将其除去,除去的方法有 、 、 。 5.醇的碘取代反应一般用 或 作碘化试剂。 二、判断题 1.氯、溴与烯烃的加成不但易于发生,而且在很多情况下是定量进行的。 ( ) 2.碘的活性较低,通常它是不能与烯烃发生加成反应的。 ( ) 3.卤化氢与烯烃的加成反应是离子型机理还是自由基机理,只要根据反应条件来判断就可以了。 ( ) 4.卤化氢与烯烃的离子型加成机理产物是反马氏规则的。 ( ) 5.和烯烃相比,炔烃与卤素的加成是较容易的。 ( ) 6.苯胺的卤代若用卤素作卤化试剂,则主要得到三卤化苯胺。 ( ) 7.卤化亚砜(SOCl2)特别适用于伯醇的卤取代反应。 ( ) 8.氟代芳烃也可以用直接的方法来制备。 ( ) 9.在光和过氧化物存在下,不对称炔烃与溴化氢的加成也是自由基加成反应,得到的是反马氏规则的产物。 ( ) 10.在侧链的取代卤化反应中,工业上采用衬玻璃、衬搪瓷或衬铅的反应设备。( ) 三、完成下列反应式 1. 2. 3. 4. 5. CH 3O COCH 3Br 2/AcOH CH 3O COCH 3Fe ,Br 2CH 2CHCOOCH 3+Br H CH 3O CH 2(CH 2)2CH 2COC 6H 5NBS ,光照CCl 42,光照

参考答案: 一、填空题 1.叔碳正离子、仲碳正离子、伯碳正离子、甲基正离子 2.叔碳自由基、仲碳自由基、伯碳自由基、甲基自由基 3.含氢较多的不饱和碳上,含氢较少的不饱和碳上 4. 还原性、加入氧化剂、加碱、加入能与碘化氢形成难溶于水的碘化物的金属氧化物 5.碘化钾加磷酸、碘加红磷 二、判断题 1.√ 2.√ 3.√ 4.× 5.× 6.√ 7.√ 8.× 9.√ 10.√ 三、完成下列反应式 CH3O COCH2Br 1. 2.CH O COCH3 3 Br 3.CH3CHBrCOOCH3 4.CH3O CH(CH2)2CH2COC6H5 Cl

1碳正离子的稳定性顺序为.

项目七练习题 一、 填空 1. 碳正离子的稳定性顺序为 2. 活性自由基的稳定性顺序为 ______________ > __________ > _____________ > 3. 马尔科夫尼科夫规则是指当卤化氢与不对称烯烃加成时,卤化氢中的氢原原子加 到 ________________________ ,卤原子加到 ________________ 4?用碘作碘化试剂与芳烃作用时,由于生成的碘化氢具有 除去的方法有 、 5 ?醇的碘取代反应一般用 二、 判断题 1 .氯、溴与烯烃的加成不但易于发生, 而且在很多情况下是定量进行的。 2.碘的活性较低,通常它是不能与烯烃发生加成反应的。 3 ?卤化氢与烯烃的加成反应是离子型机理还是自由基机理,只要根据反应条件来判断 就可以了。 4. 卤化氢与烯烃的离子型加成机理产物是反马氏规则的。 5. 和烯烃相比,炔烃与卤素的加成是较容易的。 6. 苯胺的卤代若用卤素作卤化试剂, 则主要得到三卤化苯胺。 7?卤化亚砜(SOC12)特别适用于伯醇的卤取代反应。 &氟代芳烃也可以用直接的方法来制备。 o 作碘化试 剂。 ,必须将其除去, ( ( ( ( ( ( ) ) ) ) ) ) 9?在光和过氧化物存在下,不对称炔烃与溴化氢的加成也是自由基加成反应,得到的 是反马氏规则的产物。 ( 10.在侧链的取代卤化反应中, 工业上采用衬玻璃、 衬搪瓷或衬铅的反应设备。( 三、 完成下列反应式 1. 3. 4. 5. CH 3O COCH 3 CH 3O COCH 3 Br 2 /AcOH Fe Br 2 CH 2=CHCOOCH 3+ HBr CH 2(CH 2)2CH 2COC 6H 5 NBS ,光照 = CCl 4 [XI Cl 2,光照

碳正离子机理

第五讲 与酸和亲电试剂有关的反应 一、 碳正离子 酸性介质中的反应可能涉及到碳正离子。碳正离子的稳定性为: 3o > 2o > 1o > +CH3 1. 碳正离子的形成 (1) 离解 a. 醇发生质子化后,碳氧单键发生异裂,得到碳正离子。 例如: C CH 3 3 H 3C OH + H + C CH 33 H 3C OH 2 +C CH 33 H 3C + 一般仅限制于生成稳定的碳正离子(SN1或E1反应的中间体为碳正离子)。 b. 极性介质中,反应物分子中又存在好的离去基团,不需要酸的催化,也会发生键的异裂,生成碳正离子。 MeO CH 2O S O OCF 3H O CF 3SO 3 -+MeO CH 2+ (2) 亲电试剂对双键的加成 3H + - OSO 3H (3) 羰基的质子化 3H + - OSO 3H (4) 羰基化合物与Lewis 酸的反应 + C O Cl CH 3 O 3 例如,Friedel-Crafts 酰基化反应 该反应的机理是:

C CH 3 CH 3O 3Cl CH 33-C Cl CH 3 AlCl 3O + -AlCl 333 +- H + CH 3 O 2.碳正离子的重排 碳正离子重排的驱动力是:生成更稳定的碳正离子。重 排通常涉及到碳正离子中心原子的α-C 上的烷基、苯基或 – H 的迁移。 (1)– H 的迁移 Br OH + H 2SO 4 2 (2)烷基的迁移 下列反应涉及到烷基的迁移,为它提出一个合理的机理。 O H 3O + H 机理 O H 3O + O H 实例1:二烯酮-酚的重排反应 反应: O H 3O + 机理:

碳正离子

有机活性中间体——碳正离子的研究 一、碳正离子的生成 在有机化学反应中碳正离子可以通过不同的方法产生,主要有以下三种。 1、直接离子化[1] 在化合物的离解过程中,以共价键的异裂方式产生碳正离子。最常见的为卤代烃的异裂,在离解过程中,与碳原子相连的卤原子带着一对电子离去,产生碳正离子。 R —X →R + +X - 在这个反应中,极性溶剂的溶剂化作用是生成碳正离子的重要条件。反应是可逆的,反应生成难溶物或用SbF 5作为Lewis 酸生成稳定SbF 6一, 会使反应向右进行,有利于碳正离子的生成。R —Br+ Ag +→R ++ AgBr ; R —F+SbF5→R ++SbF 6-。但是醇、醚、酰卤在酸或Lewis 酸的催化下也可以离解为碳正离子。 R 一0H → R +-OH 2→R ++H 20 ; CH 3COF+BF 3-→CH 3CO ++BF 4- 利用超强酸可以从非极性化合物如烷烃中,夺取负氢离子而生成碳正离子。 (CH 3)3CH + SbF 5·FSO 3H →(CH 3)3C ++ SbF 5·FSO 3-+H 2 2、间接离子化[2] 主要由其它正离子对中性分子加成而产生的碳正离子,最常见的为烯烃的亲电加成反应和芳环上的亲电取代反应。 C C H ; + NO2 2 3、其它生成的途径 由其它较容易获得的碳正离子转换成较稳定的难以获得的碳正离子。常见的有重氮基正离子脱N 2而生成碳正离子。 RN R +N2 ; N 2+N2 二、碳正离子的结构 碳正离子带有正电荷,其结构是由其本身所决定的,碳正离子的中心碳原子为三价,价电子层仅有六个电子,根据杂化轨道理论,其构型有两种可能:一种是中心碳原子处于sp 3 杂化状态下的角锥构型,另一种是中心碳原子处于sp 2杂化状态下的平面构型(见下图)。

论述碳正离子重排1

一、亲核重排 重排反应中以亲核重排为最多,而亲核重排中又以1,2重排为最常见。 (一)亲核1,2重排的一般规律 1.亲核1,2重排的三个步骤:离去基团离去,1,2基团迁移,亲核试剂进攻 2.发生亲核1,2重排的条件 (1)转变成更稳定的正离子(在非环系统中,有时也从较稳定的离子重排成较不稳定的离子) (2)转变成稳定的中性化合物 (3)减小基团间的拥挤程度,减小环的张力等立体因素。 (4)进行重排的立体化学条件:带正电荷碳的空p轨道和相邻的C-Z键以及α碳和β碳应共平面或接近共平面 (5)重排产物在产物中所占的比例不仅和正电荷的结果有关,而且和反应介质中存在的亲核试剂的亲核能力有关 3.迁移基团的迁移能力 (1)多由试验方法来确定基团的固有迁移能力 (2)与迁移后正离子的稳定性有关 (3)邻位协助作用 (4)立体因素 4.亲核1,2重排的立体化学: (1)迁移基:构象基本保持,没有发现过构型反转,有时有部分消旋 (2)迁移终点:取决于离去及离去和迁移基进行迁移的相对时机 5.记忆效应:后一次重排好像和第一次重排有关,中间体似乎记住了前一次重排过程 (二) 亲核重排主要包括基团向碳正离子迁移,基团向羰基碳原子迁移,基团向碳烯碳原子迁移,基团向缺电子氮原子转移,基团向缺电氧原子的迁移,芳香族亲核重排,下面就这六种迁移作简要介绍: 1.基团向碳正离子迁移: (1)Wagner-Meerwein重排:烃基或氢的1,2移位,于是醇重排成烯 (2)片那醇重排:邻二醇在酸催化下会重排成醛和酮 (3)Demyanov重排,Tiffeneau-Demyanov扩环以及有关反应 (4)二烯酮-酚重排:4,4-二取代环己二烯酮经酸处理重排成3,4-二取代酚的反应 (5)醛酮同系物的合成:醛或酮和重氮甲烷作用生成高一级的同系物 (6)烯丙基重排:烯丙基系统中双键发生位移的反应 2.基团向羰基碳原子迁移: (1) Benzil-Benzilic Acid重排:α-二酮经强碱处理会发生重排,生成α-羟基乙酸盐 (2) 酸催化下醛酮的重排:在烃基的交换后,醛重排成酮,酮则重排成另一种酮

碳正离子机理

第五讲与酸和亲电试剂有关的反应 一、碳正离子 酸性介质中的反应可能涉及到碳正离子。碳正离子的稳定性为: 3o > 2o > 1o > +CH3 1.碳正离子的形成 (1)离解 a.醇发生质子化后,碳氧单键发生异裂,得到碳正离子。 例如: 一般仅限制于生成稳定的碳正离子(SN1或E1反应的中间体为碳正离子)。 b.极性介质中,反应物分子中又存在好的离去基团,不需要酸的催化,也会发生键的异裂,生成碳正离子。 (2)亲电试剂对双键的加成 (3)羰基的质子化 (4)羰基化合物与Lewis酸的反应 例如,Friedel-Crafts 酰基化反应 该反应的机理是: 2.碳正离子的重排 碳正离子重排的驱动力是:生成更稳定的碳正离子。重排通常涉及到碳正离子中心原子的α-C上的烷基、苯基或–H 的迁移。 (1)–H 的迁移 (2)烷基的迁移 下列反应涉及到烷基的迁移,为它提出一个合理的机理。 机理O H3O+ H

实例1:二烯酮-酚的重排反应 反应:O H 3O + 机理: 实例2: 片呐醇重排 反应 机理: 3. 涉及缺电子氮的正离子重排 下列反应为Beckmann 重排: 机理: Ph Ph N O P Cl Cl Cl Cl PCl 5的作用是增大底物分子中氮氧键的极性,帮助其异裂。迁移基团处于离去基团的对面。最后一步可以看成是酮式与烯醇式的互变异构。 二、 亲电加成 亲电加成是脂肪族π键的典型反应,这类加成涉及到两步:亲电试剂对亲核双键的加成得到碳正离子中间体;碳正离子与亲核试剂结合。 典型的亲电试剂有:Br 2 、Cl 2、 H + (HCl 、HBr 、HI 、 H 2SO 4、H 3PO 4)、Lewis 酸和碳正离子。 第二步的亲核试剂常常是与亲电试剂相连的阴离子,例如Cl —、B r —、I — 等,或者是象水和乙酸这样的亲核性溶剂。在第一步可能产生稳定性不一样的碳正离子,因而有着区域 选择性。 例如: + HI I 机理: I + H + + I - 三、 酸催化的羰基化合物的反应

碳正离子的构型与稳定性

碳正离子的构型与稳定性 学院: 食品工程与生物技术学院 学号: 14144107 姓名: 田永明 碳正离子的构型与稳定性 食品工程与生物技术学院 14144107 田永明摘要:在自然界中存在着许多有机化合物,它们每时每刻都在发生着变化,而许多的反应都是多步完成的。在这些过程中存在着一个或多个活性中间体。论证这些活性中间体存在及它们的结构是研究有机反应机理的重要环节。由于有机化合物发生的反应不同,化学键断裂方式也不同,它们的活性中间体也不同,其中最多的活性中间体有碳正离子、碳负离子、自由基、碳烯、苯炔等。碳正离子是有机化学反应过程中产生的活性中间体,它的稳定性及立体构型甚至直接决定着化学反应产物的产生速度和产率。然而活性中间体碳正离子的寿命是极其短暂的,一个反应机制的主要内容就是说明一个中间体的形成和消灭的过程,活性中间体的形成和消灭的过程也就是一个反应的反应机制,因而得到这些活性中间体的证据及碳正离子稳定性理论,成为研究有机反应机制的一个重要重要成果。对于碳正离子的存在和结构确定碳正离子的生成、碳正离子的稳定性及对反应的影响很有意义。可以更加深化碳正离子存在的反应的机理,就有利于碳正离子的稳定性的原理的应用及指导有机合成路线的选择和设计。 关键词:碳正离子;稳定性;碳正离子的反应类型; 一、碳正离子 1、碳正离子是带有正电荷的含碳离子,是一类重要的活性中间体,可用R3C+表示(R为烷基)。碳正离子及其反应于20世纪20年代由C(K(英戈尔德等提出的。碳正离子可以认为是通过共价C-C单键中一对电子的异裂反应而产生。

2、碳正离子的结构 碳正离子带有正电荷,其结构是由其本身所决定的,碳正离子的中心碳原子为三价,价电子层仅有六个电子,根据杂化轨道理论,其构型有两种可能:一种是中心碳原子处于sp3杂化状态下的角锥构型,另一种是中心碳原子处于sp2杂化状态下的平面构型(见下图) 在这两种构型中,以平面构型比较稳定,这一方面是由于平面构型中与碳原子相连的三个基团相距最远,空间位阻最小;另一方面是sp-杂化的s成份较多,电子更靠近于原子核,也更为稳定;再一方面空的p轨道伸展于平面两侧,便于溶剂化。因此,一般碳正离子是sp2杂化的平面构型,正电荷集中在未参加杂化的且垂直于该平面的P轨道上。这也通过红外光谱,Raman光谱和核磁共振谱得到证实。但也有一些例外,如三苯甲基碳正离子,由于三个苯基的空间作用,不处在同一平面,苯环之间彼此互为54?角,呈螺旋桨形结构;苯正离子和炔正离子的正电荷不可能处在p轨道上,而是分别处在sp2和sp杂化轨道上。二、碳正离子的类型碳正离子作为一种有机化学反应活性中间体有很多,根据不同的分类标准,可以得到不同的归类。 1(按碳正离子所连接的基团及所处的位置分类 (1)链状碳正离子(链状碳正离子又分为以下两种) A、非共轭型碳正离子:其正电荷集中于中心碳原子上,最常见的为烷基碳正离子。如:-CH3+ B、共轭型碳正离子:由于中心碳原子的P轨道与不饱和键上的π键发生共轭,使正电荷不再集中于中心碳原子上,而是离域到整个共轭体系,如: (2)环状碳正离子,如: 2(据中心碳原子所连基团的多少分为伯、仲、叔碳正离子。 3(除上述经典的碳正离子外,还有一种非经典碳正离子。这种碳正离子一般由经典碳正离子转化而成。

碳正离子和缺电子重排

姓名:李广申学号:250967 碳正离子和缺电子重排 碳正离子通常被分为具有定域电荷的经典碳正离子及具有离域电荷的非经典碳正离子。经典碳正离子可以被一个“Lewis结构”代表,仅包括两电子二中心键。其中带正电荷的碳原子的价电子层有六个电子,形成三个共价键,这就是通常所指的碳正离子。如CH3+,CH2=CHCH2+ 等。非经典碳正离子不能被一个“Lewis结构”所代表,带正电荷的碳原子外面有八个电子,其中一对电子为三中心键。如降冰片正离子。 1 23 4 5 6 7 这里主要对经典碳正离子的形成及其性质进行一下讨论。 一、碳正离子的形成 1、中性分子的异裂 使中性分子发生异裂是生成碳正离子最常用的方法。和碳原子直接相连的原子或原子团带着一对成键电子离去。R—X R+ +X-,如: (CH3)3C—Cl(CH3)3C+ +Cl- 一般叔碳正离子或其他较稳定的碳正子(苯甲型、烯丙型,二苯甲基碳正离子、三苯甲基碳正离子),较容易通过直接离解形成,而且介质的极性愈大,离解时所需的能量愈小。例如:氯代叔丁烷再空气中离解成碳正离子,所需能量为628.5KJ/mol,而在水溶液中形成碳正离子,离解所需能量仅为83.74KJ/mol. 离去基团愈容易离去,也愈有利于碳正离子的形成。有时离去基团较难离去时,可加路易士酸予与帮助。 R—Br +AlBr3R+ +AlBr4-(芳烃的傅—克烷基化反应) R—X + Ag+R+ +AgX (卤代烃与AGNO3的醇溶液反应)CH3COF +BF3 CH3—C+=O +BF4-

(CH 3)CF +SbF 5 (CH3)C + +SbF 6- 利用超酸溶剂可以制备碳正离子的稳定溶液。例:用100% H 2SO 4制备三苯甲基碳正离子。 (C 6H 5)3COH + 2H 2SO 4 (C 6H 5)3+ +H 3O + + 2H 2SO 4 2. 质子或其他带电荷的原子团与不饱和体系加成 最常见的正离子是H +离子。烯烃酸化水合生成醇就包括着H+与C=C 双键的 加成。例如: C H 3C CH 3+ C CH 3C CH 3 C CH 3C CH 3 + OH 2 C CH 3 3C CH 3 OH 这个反应是可逆的。逆反应是醇酸催化脱水生成烯烃。 质子也可以发生在 C=O 双键的氧原子上。如: R 2C=O +H 2SO 4 R 2C=OH +HSO 4- 应用正离子转移试剂也可以生成碳正离子。例如CH 3Cl 和ALCl 3可以把CH 3+转移到六甲苯上,生成七甲基环己二烯基正离子 CH 3 CH 3CH 3 H 3C H 3C CH 3 CH 3 CH 3CH 3 H 3C H 3C CH 3 +CH 3Cl +AlCl 3 +AlCl 4 七甲基环己二烯基正离子是个共振(或共轭)碳正离子: H 3C CH 3 CH 3CH 3 CH 3 H 3C H 3C H 3C CH 3CH 3CH 3 CH 3 H 3C H 3C H 3C CH 3 CH 3CH 3 CH 3 3C 3C H 3C CH 3 CH 3CH 3 CH 3 H 3C H 3C 3、从其他正离子生成 碳正离子也可以从其他正离子的离解得到。翁离子(onium ion )的分解就是 生成碳正离子的一类重要反应。例如:

碳正离子

碳正离子 碳正离子是一种带正电的极不稳定的碳氢化合物。分析这种物质对发现能廉价制造几十种当代必需的化工产品是至关重要的。欧拉教授发现了利用超强酸使碳正离子保持稳定的方法,能够配制高浓度的碳正离子和仔细研究它。他的发现已用于提高炼油的效率、生产无铅汽油和研制新药物。碳正离子与自由基一样,是一个活泼的中间体。碳正离子有一个正电荷,最外层有6个电子。带正电荷的碳原子以SP2杂化轨道与3个原子(或原子团)结合,形成3个σ键,与碳原子处于同一个平面。碳原子剩余的P轨道与这个平面垂直。碳正离子是平面结构。1963年有报道,直接观察到简单的碳正离子,证明了它的平面结构,为它的存在及其结构提供了实验依据。根据带正电荷的碳原子的位置,可分为一级碳正离子,二级碳正离子和三级碳正离子。碳正离子的结构与稳定性直接受到与之相连接的基团的影响。它们稳定性的一般规律如下:(1)苄基型或烯丙型一般较稳定;(2)其它碳正离子是:3°>2°>1°;碳正离子越稳定,能量越低,形成越容易,加成速度也越快,可见碳正离子的稳定性决定烯烃加成的取向。碳正离子根据结构特点不同可分为:经典碳正离子和非经典碳正离子 碳负离子 (Carbanion)指的是含有一个连有三个基团,并且带有一对孤对电子的碳的活性中间体。碳负离子带有一个单位负电荷,通常是四面体构型,其中孤对电子占一个sp3 杂化轨道。通过比较相应酸的酸性大小,可以大致判断碳负离子的稳定性大小。一般地,具有能稳定负电荷的基团的碳负离子具有较高的稳定性。这些基团可以是苯基、电负性较强的杂原子(如O,N,基团如-NO2、-C(=O)-、-CO2R、-SO2-、-CN和-CONR2等)或末端炔烃(也可看作电负性的缘故),例如,三苯甲烷、三氰基甲烷、硝基甲烷和1,3-二羰基化合物具有较强的酸性。除此之外,不同于缩酮,缩硫酮的α氢也具有较强的酸性。这可以用硫的3d轨道与C-S键σ*轨道的超共轭效应来解释。硫代硝基苯基甲烷的去质子化表明,硫的可极化性起主要作用。有机金属化合物,如Grignard试剂和有机锂试剂也可看作是碳负离子源。叶立德,如磷叶立德和硫叶立德等,都含有具有碳负离子结构的共振杂化体。碳负离子可进行SN2反应。 实验事实表明碳正离子和碳自由基具有平面结构,而碳负离子则呈角锥状,因此杂化轨道理论指出在碳正离子和碳自由基中,碳原子都采用sp2杂化方式,并使用3个sp2杂化轨道形成3个σ键,形成一个平面的分子。不同的是,在碳正离子中,2p轨道上没有电子,而在碳自由基中,2p轨道上有一个单电子。

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