AS Level Chemistry · Carboxylic acids and derivatives. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.
Carboxylic acids contain the –COOH group. They are weak acids: in water only a small fraction of the molecules ionise, RCOOH ⇌ RCOO− + H+. They still show all the usual acid reactions and form salts called carboxylates, such as sodium ethanoate.
They are much stronger acids than alcohols. A carboxylic acid gives carbon dioxide with a carbonate, and an alcohol does not. This is the simple test for –COOH.
Carboxylic acids are made by oxidising a primary alcohol or an aldehyde under reflux, or by hydrolysing a nitrile or an ester. They react with alcohols to form esters, and LiAlH4 reduces them to primary alcohols.
After hydrolysis with dilute alkali, students write the carboxylic acid as the product. In alkali the product is the carboxylate salt, for example sodium ethanoate; the acid forms only after the mixture is acidified.
Answer: 0.360 dm3 of hydrogen.
Try each one, then open the answer.
Answer: C. Alkaline hydrolysis gives sodium propanoate and methanol; adding acid converts the propanoate ion into propanoic acid. The -oate part of the ester name identifies the acid.
Answer: A. Acid hydrolysis converts the –CN group to –COOH with no change in carbon number: CH3CH2CN + 2H2O + H+ → CH3CH2COOH + NH4+. Three carbons in, three carbons out.
Answer: B. Refluxing with excess oxidising agent keeps the ethanal intermediate in the flask until it is fully oxidised to ethanoic acid. Distilling immediately collects ethanal instead.
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