Free Carboxylic Acids MCQs with Answers

1. Carboxylic acids are more acidic than alcohols because

• A. they contain more hydrogen atoms
• B. the carboxylate ion formed is stabilised by delocalisation of the negative charge over two oxygen atoms
• C. they are liquids at room temperature
• D. their molecules are larger

Explanation: Losing the acidic hydrogen gives an ion whose charge is shared equally between the two oxygens, so the two carbon to oxygen bonds become identical in length and the ion is far more stable than an alkoxide. That stability makes the loss of the proton much more favourable. This is why an acid liberates carbon dioxide from a carbonate while an alcohol does not.

2. Ethanoic acid reacts with sodium carbonate to give

• A. sodium ethanoate, water and carbon dioxide
• B. sodium ethanoate and hydrogen
• C. ethanol and carbon dioxide
• D. no reaction

Explanation: Being a stronger acid than carbonic acid, ethanoic acid displaces carbon dioxide from a carbonate, and the effervescence produced is the standard test that distinguishes a carboxylic acid from both alcohols and phenols. With sodium metal the acid would give hydrogen instead. Neutralisation with an alkali gives only the salt and water.

3. The presence of chlorine atoms on the carbon next to the carboxyl group, as in chloroethanoic acid, makes the acid stronger because chlorine

• A. donates electrons to the carboxyl group
• B. increases the molar mass
• C. withdraws electron density, spreading and stabilising the negative charge of the anion
• D. makes the molecule non polar

Explanation: The electron withdrawing inductive effect of chlorine pulls charge away from the carboxylate oxygens, so the ion is more stable and the acid ionises more readily, and trichloroethanoic acid is stronger still because three chlorines act together. The effect weakens rapidly with distance along the chain. Electron releasing alkyl groups have the opposite effect, which is why ethanoic acid is weaker than methanoic acid.

4. Carboxylic acids have unusually high boiling points because in the liquid state their molecules

• A. are ionised
• B. form dimers held together by two hydrogen bonds
• C. form covalent bonds with one another
• D. are non polar

Explanation: Two molecules pair up so that the hydroxyl hydrogen of each bonds to the carbonyl oxygen of the other, giving a cyclic dimer with double the effective molar mass, so ethanoic acid boils at 118 degrees Celsius, well above ethanol despite similar size. The same dimerisation occurs in non polar solvents and even in the vapour just above the boiling point. Only in water do the dimers break up, replaced by hydrogen bonds to the solvent.

5. Ethanoic acid can be prepared by the oxidation of

• A. ethanol under reflux with acidified potassium dichromate
• B. ethene with bromine water
• C. propanone with Tollens reagent
• D. ethane with oxygen at room temperature

Explanation: Refluxing keeps the intermediate ethanal in the flask so that oxidation continues to the acid, whereas distilling the product off as it forms would stop the reaction at the aldehyde. The dichromate turns from orange to green as it is reduced. Vinegar is made industrially by the bacterial oxidation of ethanol instead.

6. Hydrolysis of a nitrile such as ethanenitrile with dilute acid gives

• A. an amine
• B. an aldehyde
• C. an alcohol
• D. a carboxylic acid with the same number of carbon atoms as the nitrile

Explanation: The carbon of the nitrile group becomes the carbon of the carboxyl group, so hydrolysing ethanenitrile gives ethanoic acid and ammonia is released as its salt. Since the nitrile itself was made by adding cyanide to an alkyl halide with one fewer carbon, the two steps together lengthen the chain by one. Reduction rather than hydrolysis of a nitrile gives a primary amine.

7. The reaction between a carboxylic acid and an alcohol in the presence of concentrated sulphuric acid is called

• A. saponification
• B. esterification
• C. hydrolysis
• D. decarboxylation

Explanation: Esterification joins the two molecules with the elimination of water and is reversible, so the sulphuric acid serves both as catalyst and as a dehydrating agent that pulls the equilibrium towards the ester. Saponification is the reverse process carried out with alkali, which produces a soap from a fat. Decarboxylation removes carbon dioxide and shortens the chain.

8. The hydrolysis of an ester with hot sodium hydroxide solution is called saponification and gives

• A. the carboxylic acid and an alkene
• B. an aldehyde and water
• C. the sodium salt of the acid and an alcohol
• D. an amide and hydrogen

Explanation: Alkaline hydrolysis produces the carboxylate salt rather than the free acid, and because that salt cannot react back with the alcohol the reaction goes to completion, unlike acidic hydrolysis which is reversible. When the ester is a natural fat the sodium salt produced is soap, which is the origin of the name. Glycerol is the alcohol released in that case.

9. The most reactive of the carboxylic acid derivatives towards nucleophilic attack is

• A. the amide
• B. the ester
• C. the carboxylate salt
• D. the acyl chloride

Explanation: Chlorine is a good leaving group and withdraws electron density strongly, so the carbonyl carbon of an acyl chloride is highly electron deficient and it reacts violently even with cold water. Reactivity falls in the order acyl chloride, anhydride, ester, amide, since each successive group donates electron density more effectively. This is why acyl chlorides are the reagents of choice for making esters of unreactive alcohols and phenols.

10. Ethanoyl chloride reacts with ethanol to give

• A. ethyl ethanoate and hydrogen chloride
• B. ethanoic acid and chlorine
• C. ethane and water
• D. ethanal and hydrogen chloride

Explanation: The alcohol attacks the carbonyl carbon and chloride is expelled, giving the ester rapidly and irreversibly along with fumes of hydrogen chloride. This route is preferred over direct esterification when a high yield is needed, since no equilibrium limits it. The vigorous reaction with water is the reason acyl chlorides must be kept dry.