20.4 - Condensation reactions
20.4.1 :Describe, using equations, the reactions of alcohols with carboxylic acids to form esters and state the uses of esters. Mechanisms will not be assessed
Alcohols contain the -OH group which can react with the -COOH group of carboxylic acids to form an ester linkage with the elimination of water. This sort of reaction is described as a condensation reaction.
Condensation means the joining together of two molecules with the elimination of a small molecule. The small molecule may be water, a hydrogen halide or ammonia.
In the reaction of ethanol with ethanoic acid:
This reaction is an equilibrium and the conditions are adjusted to ensure that the equilibrium is pulled to the right hand side. This is achieved by heating with concentrated sulphuric acid, which acts as a catalyst to attain equilibrium rapidly and also a dehydrating agent to absorb the water formed on the right hand side.
In the ester product the part of the molecule that originates from the acid is shown first, while the name actually starts with the alkyl chain that originates from the alcohol!
The group of atoms that join the two alkyl chains together is called the ester linkage, -COO-.
20.4.2: Describe, using equations, the reactions of amines with carboxylic acids. Mechanisms will not be assessed
The reaction of amines with carboxylic acids is also condensation.
In this case the product is an N-substituted amide.
The importance of this reaction lies in its role in biochemical processes. Proteins are made of aminoacids linked together in chains. Each aminoacid is linked to another by amide linkages (called peptide in biochemistry), formed by the reaction of amine groups and carboxylic acid groups on different aminoacid molecules.
20.4.3: Deduce the structures of the polymers formed in the reactions of alcohols with carboxylic acids. Emphasise the need for two functional groups on each monomer. Include the polyester formed from ethane-1,2-diol and benzene-1,4-dicarboxylic acid. Include the identification of the repeating unit.
Molecules that have two alcohol groups (diols) can condense with molecules that have two carboxylic acid groups (-dioic acids) to form polymeric chains, where each molecule is attached to the next by an ester group. The resulting molecule is called a polyester.
20.4.4: Deduce the structures of the polymers formed in the reactions of amines with carboxylic acids. Emphasise the need for two functional groups on each monomer. Include the polyamide formed from 1,6-diaminohexane and hexanedioic acid. Include the identification of the repeating unit.
In the same way that polyesters can be formed by condensation of molecules with two functional groups, so can polyamides be formed when a molecule with two amine groups condenses with a molecule with two carboxylic acid groups. In this case they are joined together by amide (peptide) linkages.
Hexa-1,6-diamine reacts with hexanedicarboxylic acid forming the polymer 'Nylon'.
The name 'Nylon' is derived from the two cities New York and London, where the two research groups working for the DuPont chemical company developed the manufacturing process.
20.4.5: Outline the economic importance of condensation reactions.
The development of condensation polymers gave another push to the blossoming plastics and artificial fibres industry. Condensation polymers were used for artificial silks (Nylon) and cotton (terylene).
The properties of the polymers can be modified by changing the structure of the monomers, for example chain length and attachments, while keeping the two functional groups on each monomer.
Nylon is a very flexible material (no pun intended) that can be used for packaging, insulation and construction.