Esterification is a kind of organic chemical reaction, mainly involving alcohol andcarboxylic acidOr inorganic oxyacid generationesterReaction with water.It can be divided into three types: reaction of carboxylic acid with alcohol, reaction of inorganic oxyacid with alcohol and reaction of inorganic strong acid with alcohol.carboxylic acidThe esterification reaction with alcohol is reversible and generally very slow, so concentrated sulfuric acid is often used as a catalyst.Polycarboxylic acids react with alcohols to form various esters.The reaction rate of inorganic strong acid with alcohol is generally fast.Typical esterification reaction includes the reaction of ethanol and acetic acid, which produces aromaticethyl acetateIt is the raw material for manufacturing dyes and medicines.Esterification reaction is widely used inOrganic synthesisAnd other fields.
Chinese name
esterification
Foreign name
esterification
Definition
Reaction of alcohol with carboxylic acid or oxygen-containing inorganic acid to form ester and water
Formation of two compoundsester(Typical reaction is the reaction of acid and alcohol to form ester), which is called esterification reaction.
There are two cases:carboxylic acidReact with alcohols and inorganic oxyacids react with alcohols.
The reaction process of carboxylic acid and alcohol is generally: the hydroxyl in the carboxylic acid molecule combines with the hydrogen atom of the hydroxyl in the alcohol molecule to form water, and the rest combines with each other to form esters.
Formula: dehydrogenation of acid dehydroxylated alcohol (dehydrogenation of acid dehydroxylated alcohol).Esterification of carboxylic acid with alcohol is reversible and generally very slow, so it is commonly usedconcentrated sulfuric aciddocatalyzer。Polycarboxylic acids react with alcohols to form various esters.
Examples are as follows:
（1）acetic acidandethanolIt is formed by reaction under the condition of concentrated sulfuric acid heatingethyl acetateAnd water:
(2) Acetic acid and methanol can produce methyl hydrogen oxalate orDimethyl acetate
(3) Alcohol reacts with oxygen-containing inorganic acid to lose a molecule of water and generate inorganic ester[3]
The reaction rate of inorganic strong acid with alcohol is generally fast.Polyols react with inorganic oxygen containing strong acids to form esters.
Generally speaking, in addition to the direct esterification of acids and alcohols to produce esters, there are three types of substances that can react (but not necessarily esterification) to produce esters:Acyl halideandalcohol、phenol、Sodium alkoxideReaction;Acid anhydrideReact with alcohol, phenol and sodium alkoxide;KeteneReact with alcohol, phenol and sodium alkoxide;
Reaction characteristics
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belong toReversible reactionGenerally, the reaction is not complete. According to the reaction balance principle, to increase the yield of ester, it is necessary to separate one component from the product or make one component of the reactant excessive to make the reaction proceed in the positive direction.Esterification is a one-way two-way reaction.
There are a series of reversible equilibrium reaction steps in esterification reaction.Step ② is the control step of esterification reaction, while step ④ is the control step of ester hydrolysis.This reaction is SNtwoReaction, through addition elimination process.
The oxygen of ester comes from hydroxyl group, and the oxygen of water comes from carboxyl group
SN2 principle reaction formula
Isotopic labeling of alcohols confirmed that the water generated in the esterification reaction was from the hydroxyl group of carboxylic acid and the hydrogen of alcohols.However, the esterification of carboxylic acid with tertiary alcohol is that the alkoxy bond of alcohol is broken, and carbonium ions are generated in the middle.
Another criterion for judging that the oxygen atom in water generated in esterification reaction comes from carboxyl group is that the ester formed by the photoactive alcohol still has photoactivity, because if the oxygen comes from hydroxyl group, the oxygen of carboxyl group will cause racemization when attacking the asymmetric carbon of alcohol, that is, the generated ester will lose photoactivity due to racemization.[1]
In esterification reaction, alcohol acts asNucleophileThe nucleophilic attack is carried out on the carbonyl group of the carboxyl group. In the presence of the proton acid, the carbonyl carbon is more electron deficient and conducive to the nucleophilic addition of alcohol with it.Without the presence of acid, the esterification of acid and alcohol is difficult.
Methanol>primary alcohol>secondary alcohol>tertiary alcohol>phenol (the above mechanism is applicable to secondary alcohol, that is, 2 ℃ alcohol).
Reaction type
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Fischer esterification
Esterification reaction is generallyReversible reaction。The traditional esterification technology uses acid and alcohol in acid (usuallyconcentrated sulfuric acid）Catalytic heating reflux reaction.This reaction is also called Fischer esterification.Concentrated sulfuric acid acts as a catalyst and water absorbent, which can hybridize the carbonyl matrix of carboxylic acid and enhance theElectrophilicity, makereaction rateAcceleration;It can also remove the by-product water of the reaction and improve the yield of ester.
If the raw materials are low-grade carboxylic acids and alcohols, which are soluble in water, water can be added to the reaction solution after reaction (saturated if necessarysodium carbonateSolution), and put the reaction solution in the separating funnel for separating treatment, collect the upper ester layer that is difficult to dissolve in water, so as to purify the ester generated by the reaction.Sodium carbonate reacts with carboxylic acid to formCarboxylate, increase the solubility of carboxylic acid and reduce the solubility of ester.If the boiling point of the product ester is low, the ester can also be continuously evaporated during the reaction to shift the reaction balance to the right, and the volatile ester can be collected by condensation.
However, there are also a few esterification reactions in which the hydroxyl groups of acids or alcohols are protonated, water leaves and generatesacylPositive ion orCarbocationThe intermediate reacts with alcohol or acid to form ester.These reactions do not follow the rule of "acid out of hydroxyl alcohol out of hydrogen".
carboxylic acidafterAcyl chlorideIt reacts with alcohol to form ester.The reactivity of acyl chloride is stronger than that of carboxylic acid, so this method is a common method for preparing esters, and the yield is generally higher than that of direct esterification.For less reactiveAcyl halideAnd alcohol, a small amount ofalkali, such assodium hydroxideorpyridine。 HthreeC-COCl + HO-CHtwo-CHthree→ HthreeC-COO-CHtwo-CHthree+H-Cl carboxylic acid passes throughAcid anhydrideIt reacts with alcohol to form ester.Carboxylic acid throughCarboxylateRegroupHalohydrocarbonThe reaction produces an ester.The reaction mechanism is that the carboxylate anion hasNucleophilic substitution reaction。
Esterification under the action of sulfoxide chloride
The basic method is to dissolve the acid in excess lower alcohol (usually methanol or ethanol), and then drop it at low temperatureSulfoxide chlorideThe method is characterized by mild conditions, convenient operation, short reaction time and high yield, which is particularly suitable for esterification of amino acids, and generally does not affect the alcohol hydroxyl group in the acid due to the excessive amount of lower alcohols in the reaction.
In addition, the Vilsmeier Haack type complex composed of sulfoxide chloride and DMF can be used for esterification of sterically hindered alcohols.Kaul et al. used the reagent to activate the carboxyl group to esterify various primary alcohols, including sterically hindered alcohols and polyols, with a near quantitative yield.[2]
Steglich esterification reaction
Carboxylic acid and alcohol inDCCAnd a small amountDMAPEsterification in the presence of.This method is especially suitable for esterification of tertiary alcohols.DCC is a water loss agent in the reaction, and DMAP is a commonly used esterification catalyst.
Yamaguchi esterification
2,4,6-trichlorobenzoyl chlorideIt reacts with carboxylic acid substrate to generate mixed anhydride to activate carboxylic acid, and then smoothly reacts with alcohol to form ester.DMAP is the catalyst for esterification.
Typical applications
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Esterification reaction to produce ethyl acetate
Esterification of ethanol and acetic acid (commonly known as acetic acid) to produce aromaticethyl acetateIt is the raw material for manufacturing dyes and medicines.In the cooking process of some dishes, if vinegar and wine are added at the same time, partial esterification will also be carried out to generate esters, which will make the dishes taste more delicious.If the reaction is to meet the industrial requirements, sulfuric acid should be used as a catalyst, and sulfuric acid should also absorb the water generated in the reaction process to make the esterification reaction more thorough.The reaction equation is shown in the figure:
Alcohols andInorganic acidIt can also undergo esterification reaction, such as methanol and sulfuric acidDimethyl sulfate, is a methylation reagent, which can becarbohydrateIntroduce methyl.
