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Carboxylic acids (Topic 17C)
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When a carboxylic acid is deprotonated, a carboxylate anion is formed.
The pi bond electrons delocalise to include both oxygen atoms. This more delocalised system stabilises the charge making the loss of a proton in solution more thermodynamically feasible than in an alcohol’s .
The carboxylic acid group is trigonal planar about the carbonyl carbon, with bond angles approximately 120°.
The shorter carboxylic acids, up to and including butanoic acid, are completely soluble in water due to the formation of hydrogen bonds between the carboxylic acid group and water molecules.
As the hydrocarbon chain increases and exerts a more nonpolar influence, the carboxylic acids become less soluble.
Carboxylic acids can be prepared by the oxidation of aldehydes and the oxidation of primary alcohols. The reaction must be completed under reflux with a suitable oxidising mixture such as .
Carboxylic acids can also be formed by the acidic hydrolysis of nitriles. The group reacts readily with warm dilute acid such as forming a carboxylic acid and ammonia.
Carboxylic acids can be reduced back to the primary alcohol, by the powerful reducing agent, lithium aluminium hydride (). The reaction is conducted in a solvent of dry ethoxyethane as the reacts violently with water.
The reaction takes place in two stages with the reduction first to the aldehyde, followed by further reduction to the primary alcohol.
A 1:4 ratio of carboxylic acid to reducing agent, , is required.
The alternative milder reducing agent does not reduce the carboxylic acid group.
Acyl chlorides have a similar structure to a carboxylic acid but with a chlorine atom instead of the group.
This highly electronegative forms a stable anion and is an excellent leaving group. This makes the acyl chloride very reactive, especially towards nucleophilic attack.
Carboxylic acids react with the white solid phosphorus (V) chloride to form acyl chlorides.
Another liquid product, phosphorus trichloride oxide,, is formed alongside the acyl chloride, as well as steamy fumes of .
Ethanoic acid forming ethanoyl chloride is the most widely used reaction of this type.
The acyl chloride may be separated by distillation.
Acyl chlorides are much more reactive than carboxylic acids and so the conversion is important in chemical synthesis.
Esterification with acyl chlorides is not a reversible reaction and requires less energy to heat the reaction mixture than the carboxylic acid equivalent.
Esters, RCOOR’, can be formed by a carboxylic acid and an alcohol.
The structures of both reactant molecules are recognisable in the ester structure.
Carboxylic acids react slowly and reversibly with alcohols in the presence of a strong acid catalyst to form esters.
A water molecule is eliminated. It forms from the group of the carboxylic acid and the of the alcohol’s group.
This is a condensation reaction.
When naming the ester product, the prefix comes from the alcohol, and the suffix comes from the carboxylic acid.
Acyl chlorides react readily with water to form a carboxylic acid and gaseous hydrogen chloride as steamy fumes.
Ethanoyl chloride reacts with water to form ethanoic acid and hydrochloric acid.
The products are formed in a nucleophilic addition–elimination reaction.
Acyl chlorides react with alcohols to form esters and water.
Acyl chlorides are much more reactive than carboxylic acids, so esters are readily formed at room temperature in a non-reversible reaction that does not require a catalyst.
This is a nucleophilic addition–elimination reaction.
Acyl chlorides are very reactive towards nucleophiles such as ammonia and the amines.
Ethanoyl chloride reacts with ammonia to form the primary amide, ethanamide, and hydrochloric acid in a nucleophilic addition–elimination reaction.
Acyl chlorides react very vigorously with primary amines, forming a secondary amide and hydrochloric acid.
The products are N-ethyl ethanamide (a secondary amide) and hydrochloric acid.
Esters hydrolyse in aqueous solution to reform alcohols and carboxylic acids.
The hydrolysis reaction is very slow in neutral conditions, so we use acidic or alkaline conditions to accelerate the hydrolysis.
In hot aqueous acid, ester hydrolysis forms a carboxylic acid and an alcohol.
This is the reverse of the ester formation reaction. This means the products of acid hydrolysis can, in turn, regenerate the ester.
The acid-catalysed hydrolysis of methyl butanoate is shown below:
In hot aqueous alkali, hydrolysis of esters yields a carboxylate salt, and an alcohol.
Alkaline hydrolysis is not reversible, as the products do not react together to reform the ester. The carboxylate salt does not contain an leaving group and so cannot be attacked by the alcohol.
The alkaline-catalysed hydrolysis of methyl butanoate is shown below:
Alkaline hydrolysis is also called saponification; it is a reaction used in soap manufacture.
Dicarboxylic acid monomers and diol monomers undergo condensation polymerisation to form polyesters, and water as a by-product.
This is an example of two monomers synthesising a polymer product. The polyester formed from this reaction features an ester bond within the repeat unit.
Two molecules of water are produced for every repeat unit in the chain; there is an ester link formed within the repeat unit, as well as one connecting to the main chain.
Diacyl chloride monomers and diol monomers undergo condensation polymerisation to form polyesters, and hydrochloric acid as a byproduct.
The polyester formed from this reaction features an ester bond within the repeat unit.
The use of a diacyl chloride will produce the same polyester as with a dioic acid, but the reaction will be faster.
This comes with the disadvantage of giving off toxic hydrochloric acid gas, (g), rather than water as a by-product.
Polyesters can be formed from a single monomer with both the carboxylic acid and alcohol group on the same carbon skeleton.
These monomers are known as hydroxycarboxylic acids.
It is important to note that in this reaction the repeat unit does not contain an ester group.
If a polyester is made from two different monomers, a dicarboxylic acid (or diacyl chloride) and a diol, the order of the ester connectivity will alternate across the polymer and both monomers must appear in the repeating unit.
To draw the repeat unit:
- Draw the ester group in the middle of the structure.
- Add the dicarboxylic acid (or diacyl chloride) carbon skeleton to the left of the group, and terminate the left side with the last group, a trailing bond, and a repeat unit bracket.
- Draw the diol carbon skeleton on the right of the central from the ester and terminate on the right with the group, a trailing bond, and a repeat unit bracket.