Esterification

It is a chemical reaction that takes place during the formation of the ester. Esterification is the chemical process that combines alcohol (ROH) and an organic acid (RCOOH) to form an ester (RCOOR) and water. This chemical reaction results in forming at least one product of ester through an esterification reaction between a carboxylic acid and an alcohol.

Below is the reaction for esterification.

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What is Meant by Esterification Reaction?

When carboxylic acids are heated with alcohol with the help of an acid catalyst, esters are produced. The catalyst in use is usually concentrated sulphuric acid. Alternatively, dry hydrogen chloride gas is useful in some cases which incline towards aromatic sweet-smelling esters, the ones with the benzene ring. The chemical reaction which occurs during the formation of the ester is known as the esterification reaction.

CH3COOH + CH3CH2COOH → CH3COOCH2CH3

The Esterification Mechanism

The process of esterification involves five steps known as the esterification mechanism. The steps are as follows:

Step 1: Formation of Cation

The first step is where the ethanoic acid takes a proton (a hydrogen ion) from the concentrated sulphuric acid. This proton attaches to one of the single pairs on the oxygen which is double-bonded to the carbon.

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Step 2: Carbonation Delocalized

Methanol acts as a nucleophile to a carbocation remembering that there is an excess of methanol molecules in the solution as always in this reaction. Here the Carboxyl oxygen gets protonated giving a delocalized carbocation in turn making the carbocation one better electrophile.

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Step 3: Proton Transfer

The protonated ether leaves as methanol but does not accomplish anything. A proton transfers to one of the hydroxyl groups making it a good leaving group.

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Step 4: Pi Bond Formation

The oxygen alcohol atom from the hydroxy group donates a pair of electrons to the atom of carbon making a π bond while eliminating water. This eliminated water is not a viable nucleophile that reverses the reaction because its concentration is low as against the concentration of the methanol.

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Step 5: Formation of Ester

The water is extremely low for a concentration to reverse the whole reaction.

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Methods of Esterification

The process of esterification takes place in three ways as given below:

1. From alcohol and acid chloride.

2. From alcohol and acid anhydride.

3. From alcohol and carboxylic acid.

1. From Alcohol and Acid Chloride

This method works for alcohols and phenols. In this case of phenols, the reaction improves by the conversion of the phenol firstly into a form which is more reactive.

When adding an acyl chloride or acid chloride to alcohol, one gets a vigorous or even a violent reaction even at room temperature thereby producing an ester and clouds full of steamy acidic fumes consisting of hydrogen chloride. Here given is one of the ester examples - if one adds liquid ethanoyl chloride to ethanol, then they produce a burst of hydrogen chloride along with hey liquid ester ethyl ethanoate.

CH3OCl+CH3CH2OH ⟶ CH3COOCH2CH3+HCl (1)

The substance usually called "phenol" is the simplest phenol in the family. Phenol consists of an -OH group attaching itself to a benzene ring - and nothing more. The reaction happening between ethanoyl chloride and phenol is similar enough to the ethanol reaction although not so vigorous. Phenyl ethanoate forms along with hydrogen chloride gas.

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The formula for Benzoyl chloride is C₆H₅COCl. The -COCl group attaches itself directly to a benzene ring. Benzoyl chloride is much less reactive than acyl chlorides such as ethanoyl chloride. The phenol first converts into the ionic compound sodium phenoxide (sodium phenate) through dissolving it in sodium hydroxide solution.

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The phenoxide ion thus reacts more rapidly with benzoyl chloride as compared to the reaction of the original phenol. However, one has to shake it along with benzoyl chloride for about 15 minutes to get solid phenyl benzoate. 

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2. From Alcohol and Acid Anhydride

This is a reaction that is useful to make esters from phenols and alcohols again. Though the reactions are slower as against the corresponding reactions with an acid chloride, they usually warm the mixture as per their requirement. In the case of Amphenol, it reacts with the sodium hydrochloride solution initially to produce the more reactive phenoxide ion.

One can take ethanol to react with ethanoic anhydride which is the typical reaction involving alcohol, it is a slow reaction which is at room temperature. However, there is no visible change to be seen in these colorless liquids, but a mixture of ethanoic acid and ethyl ethanoate is formed.

(CH₃CO)₂O + CH₃CH₂OHCH₃ ⇒ COOCH₂CH₃ + CH₃COOH (2)

The reaction with phenol is similar but slower. Phenyl ethanoate is formed together with ethanoic acid.

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This reaction is unimportant itself, but a very similar esterification reaction example is the manufacture of aspirin. If the phenol is initially converted into sodium phenoxide by mixing with sodium hydroxide solution, then the reaction is much faster. Phenyl ethanoate is formed again, but this time the other product is sodium ethanoate and not ethanoic acid.

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3. From Alcohol and Carboxylic Acid

This method is useful for converting alcohol into esters. However, this method does not work with phenols or compounds where the -OH group is attached to the benzene ring. Since phenols react very slowly with carboxylic acids this reaction is not usable for the purpose of preparation.

When carboxylic acids are heated with alcohols in the presence of a catalyst which is acid at such time esters are produced. The catalyst is usually concentrated sulfuric acid. Alternatively, dry hydrogen chloride gas is useful in some cases which incline towards aromatic sweet-smelling esters, the ones with the benzene ring.

The esterification reaction is very slow and reversible. The equation for the reaction between alcohol R'OH and an acid RCOOH is as follows:

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(where R and R' can be either the same or different)

So, for example, the equation of ethyl ethanoate made from ethanoic acid and ethanol would be:

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Properties of Esters

• An ester gets its name from the carboxylic acid that is used in the esterification reaction.

• Esters have a pleasant smell.

• They are highly useful in the perfume and food industries.

• They are organic compounds usually found in fats and oils.

Uses of Esters

Some common uses of esters are as follows.

• Esters are fragrant and so are highly useful in perfumes cosmetics and food flavorings.

• They are useful as an organic solvent.

• An ester Nitroglycerin is known and famous for its explosive properties.

• They are useful in the manufacturing of surfactants like soaps and detergents.

• They have present in pheromones if they occur naturally.

• Phosphoesters from the backbone of DNA molecules.

• Esters called polyesters are used to produce plastic.