AS/A-Level Chemistry - Esters

In A-Level Chemistry - Organic chemisrtry, we look at different functional groups in hydrocarbons. Let's look at the group of esters and their properties here.  

 Esters are derived from carboxylic acids. A carboxylic acid contains the -COOH group, and in an ester the hydrogen in this group is replaced by a hydrocarbon group of some kind. This could be an alkyl group like methyl or ethyl, or one containing a benzene ring like phenyl. Esters have the structural formula: 

Esterification

Carboxylic acids react with alcohols in the presence of concentrated sulphuric acid (as a catalyst) in a reversible reaction to form esters:

Acid +alcohol <----> ester + water

Naming

Esters are named after the alcohol residue, R’, followed by the name of the carboxylate group, RCOO: 

· HCOOCH₃ is methyl methanoate 

· CH₃COOCH₂CH₃ is called 1-propyl ethanoate --> the prefix 1 indicates theposition of the ester linkage in the propyl chain 

Hydrolysis

Esters are hydrolysed when heated under reflux with either aqueous acid or aqueous alkali: 

CH₃COOC₂H₅ + H₂O <---H⁺(aq)---> CH₃COOH + C₂H₅OH

Here, the acid is a catalyst and the reaction is reversible, therefore the yield of acid and alcohol is low.  

CH₃COOC₂H₅+ NaOH ---> CH₃COONa + C₂H₅OH 

This reaction is not reversible, so there is a good yield ofthe salt of the carboxylic acid and the alcohol. If the organic acid is required, the solution is cooled and excess dilute strong acid, such as HCl is added.

The alkaline hydrolysis of the big esters found in animal and vegetable fats and oils can be used to make soap. If the large esters present in fats and oils are heated with concentrated sodium hydroxide solution exactly the same reaction happens as with the simple esters. A salt of a carboxylic acid is formed, which is theimportant ingredient of soap. An alcohol is also produced. The alkaline hydrolysis of esters is sometimes known as saponification.  

 Physical properties of esters

· Boiling Points: 

The small esters have boiling points, which are similar to those of aldehydes and ketones with the same number of carbon atoms. They are polar molecules and so have dipole-dipole interactions as well as London forces. Most esters are liquids at room temperature.  

· Solubility in water: 

Despite being polar molecules, all esters are insoluble in water because they cannot form hydrogen bonds with water molecules because do not have any δ+ hydrogen atoms and the δ- oxygen atoms are sterically hindered, preventing close approach by water molecules.  

 Transesterification 

In transesterification with another organic acid, the acid part of the ester is replaced by the acid reactant. The simplest example is the reaction between ethyl ethanoate and methanoic acid in the presence of an acid catalyst. The products are ethyl methanoate and ethanoic acid:  

CH₃COOC₂H₅ + HCOOH <---> HCOOC₂H₅ + CH₃COOH

This type of reaction is used in the manufacture of low-fat margarine, where the incoming acid is saturated and the product acid is unsaturated.  

 Biodiesel

Biodiesel is made from natural vegetable oils. One such is rapeseed oil, but another is the seeds from the Jatropha tree, which grows on poor soil so will not use land that could be used for food production. Biodiesels are obtained by transesterification, where the natural oil ester is mixed with methanol and a catalyst. Transesterification takes place forming the methyl esters of the fatty acids present in the vegetable oils.  

 Esters are used as:  

· Perfumes and flavourings:Perfumes are complex mistures of esters and ketones.  

· Solvents: Nail varnish contains some ethyl ethanoate, as the solvent of the varnish 

· Anaesthetics: Benzocaine and procaine re esters and are used as local anaesthetics, the latter in dentistry 

· Biofuels: Methyl esters of long-chain acids are used as biofuels 

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