Carboxylic acids and esters — OCR A-Level Chemistry

Carboxylic acids contain the carboxyl group, COOH, in which a carbonyl group is joined to a hydroxyl group on the same carbon. The O–H hydrogen is acidic: the acid donates a proton to water or to a base, forming a carboxylate ion and, for example, a salt with a metal carbonate or hydroxide. The carboxylate ion is stabilised because the negative charge is delocalised over the two oxygen atoms, so carboxylic acids are weak acids that partially dissociate in water. They react with carbonates to release carbon dioxide, with reactive metals to release hydrogen, and with alcohols in esterification. The carbon chain affects solubility: small acids mix with water, while longer chains are less soluble.

Carboxylic acids dissolve in water because their molecules can form hydrogen bonds with water molecules. The carboxyl group contains a polar O–H bond, so the hydrogen atom carries a partial positive charge (δ⁺) and can be attracted to a lone pair on the oxygen of a water molecule. The C=O oxygen carries a partial negative charge (δ⁻) and can accept a hydrogen bond from a water molecule's O–H. These solute–solvent hydrogen bonds release energy that offsets the energy needed to separate the acid molecules and the water molecules, so small carboxylic acids such as methanoic and ethanoic acid mix freely with water. As the hydrocarbon chain lengthens, the non-polar part cannot hydrogen bond, so solubility falls; butanoic acid is far less soluble than ethanoic acid.

In aqueous solution a carboxylic acid donates a proton to a base, acting as a Brønsted–Lowry acid and forming a carboxylate salt plus water or hydrogen. With a reactive metal such as magnesium, the acid gives a carboxylate salt and hydrogen gas, for example 2CH₃COOH + Mg → (CH₃COO)₂Mg + H₂. With a metal oxide such as copper(II) oxide, the acid gives a salt and water: 2CH₃COOH + CuO → (CH₃COO)₂Cu + H₂O. With a carbonate such as sodium carbonate, the acid gives a salt, water and carbon dioxide: 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂. With an alkali such as sodium hydroxide, the acid is neutralised to give a salt and water: CH₃COOH + NaOH → CH₃COONa + H₂O. The salt is named from the metal cation and the carboxylate anion.

Esters are compounds formed when a carboxylic acid reacts with an alcohol, releasing water. The functional group is the ester link, written as –COO–, in which a carbonyl carbon is bonded to an oxygen that is itself bonded to a carbon chain. Naming follows the pattern alkyl alkanoate: the alcohol supplies the first part of the name and the acid supplies the second, so ethanol with ethanoic acid gives ethyl ethanoate. Esters are typically volatile, sweet-smelling liquids with boiling points lower than the parent acids because they cannot hydrogen-bond to each other through an –OH group. They are used as flavourings, perfumes and solvents, and they hydrolyse back to acid and alcohol with water.

Esterification is the reaction in which a carboxylic acid and an alcohol combine to form an ester and water. The reaction is slow at room temperature, so it is heated under reflux with a small amount of concentrated sulfuric acid catalyst. The acid protonates the carbonyl group, making it more susceptible to attack by the alcohol, and water is eliminated. The equation for ethanoic acid with ethanol is CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O. The reaction is reversible, so yield can be improved by removing water or using an excess of one reactant. The process is also called condensation because a small molecule, water, is lost.

A carboxylic acid reacts with an alcohol to form an ester and water; this is esterification, a reversible condensation. A few drops of concentrated sulfuric acid are added as a catalyst, and the mixture is heated under reflux so the reaction can reach equilibrium without losing volatile components. The acid catalyst protonates the carboxylic acid, making the carbonyl carbon more electrophilic so the alcohol can attack; the catalyst is regenerated at the end. Because the reaction is reversible, a catalyst speeds up the approach to equilibrium but does not change the position of equilibrium or the equilibrium yield. Yield is improved instead by using an excess of one reactant (often the alcohol) or by removing water or the ester as it forms, which shifts the equilibrium to the right by Le Chatelier's principle. The ester is often recognised by its characteristic sweet smell.

This row covers the reaction of an acid anhydride with an alcohol to form an ester and a carboxylic acid. An acid anhydride can be viewed as two acyl groups joined through an oxygen, so ethanoic anhydride reacting with ethanol gives ethyl ethanoate and ethanoic acid. Compared with the carboxylic acid route, the anhydride reacts faster and does not need a strong acid catalyst, and it avoids the reversible equilibrium, which makes it useful for making esters such as aspirin. Learners should write the word equation, identify both products, name the ester, and explain why an anhydride is often preferred over a carboxylic acid for esterification.

Ester hydrolysis is the cleavage of the ester linkage by water, and OCR H432 requires you to know it happens under two sets of conditions: hot aqueous acid and hot aqueous alkali. This row covers the general process. In acid conditions the reaction is reversible and gives a carboxylic acid plus an alcohol; in alkali conditions the carboxylic acid is deprotonated as it forms, so the organic product is a carboxylate salt and the alcohol, and the reaction goes essentially to completion. Recognise that water is the reagent that attacks the ester, that the C–O single bond of the ester is broken, and that the products differ between the two routes. Practise writing a balanced equation for a named ester such as ethyl ethanoate and predicting which functional groups appear in each product.

Under hot aqueous acid conditions an ester reacts with water to give a carboxylic acid and an alcohol. The acid acts as a catalyst: it protonates the carbonyl oxygen, making the carbonyl carbon more electrophilic so water can attack, and a proton is lost to regenerate the catalyst. The reaction is an equilibrium, so it is reversible and does not go to completion unless a product is removed. For example, ethyl ethanoate with hot dilute hydrochloric acid gives ethanoic acid and ethanol. Learn to write the balanced equation, to identify the two products by functional group, and to explain why the acid is described as a catalyst rather than a reactant consumed in the reaction.

This clause describes alkaline hydrolysis of esters: heating an ester with hot aqueous alkali such as aqueous sodium hydroxide breaks the ester link and gives a carboxylate salt plus an alcohol. For example, heating ethyl ethanoate with hot aqueous NaOH produces sodium ethanoate and ethanol. The alkali supplies the hydroxide that attacks the carbonyl carbon, and the resulting carboxylate is isolated as its salt rather than the free acid, because the alkaline conditions deprotonate the acid formed. Contrast this with acid hydrolysis, which is reversible and yields the carboxylic acid itself. In an MCQ, identify the two products correctly and recognise that the salt, not the acid, is formed under alkaline conditions.

Acyl chlorides are reactive derivatives of carboxylic acids with the general formula RCOCl, where the –OH of the acid is replaced by –Cl. The carbonyl carbon is strongly electrophilic, so acyl chlorides react vigorously with water to give a carboxylic acid and hydrogen chloride, with alcohols to give an ester and HCl, and with ammonia or amines to give an amide and HCl. The HCl produced fumes in moist air and is often described as steamy. Because they react readily, acyl chlorides are useful for making esters and amides under mild conditions. In an MCQ, match the nucleophile to the correct product and remember that HCl is released each time.

Carboxylic acids react with thionyl chloride, SOCl₂, to give an acyl chloride. The acid's –COOH group loses –OH and the acyl chloride gains –Cl, so ethanoic acid becomes ethanoyl chloride. SOCl₂ supplies the chlorine and removes the –OH as SO₂ and HCl, both gases that leave the mixture, which helps drive the reaction and gives a purer product than some alternatives. The equation is RCOOH + SOCl₂ → RCOCl + SO₂ + HCl. The reaction is fast at room temperature and needs no separate catalyst. In an MCQ, track which atoms move: the carbon–chlorine bond forms where the carbon–oxygen single bond of the acid was, and the acid's C=O is retained.

Acyl chlorides, RCOCl, are reactive acid derivatives because the carbonyl carbon carries a partial positive charge and the chloride is a good leaving group, so nucleophiles attack readily. Water hydrolyses an acyl chloride to the carboxylic acid, RCOOH, releasing HCl; an alcohol gives an ester, RCOOR′, plus HCl; ammonia gives a primary amide, RCONH₂; and a primary amine, R′NH₂, gives a secondary amide, RCONHR′. Each reaction is a nucleophilic addition–elimination, is fast at room temperature and often needs no catalyst, which is why acyl chlorides are useful in synthesis. The HCl by-product is acidic, so a base such as pyridine is often added to remove it.