Free Carboxylic Acids MCQs with Answers

11. Reduction of a carboxylic acid with lithium aluminium hydride gives

• A. an aldehyde only
• B. a primary alcohol
• C. a ketone
• D. an alkane

Explanation: Lithium aluminium hydride is powerful enough to reduce the carboxyl group all the way to a primary alcohol, passing through the aldehyde without stopping there. Milder reducing agents such as sodium borohydride will not touch a carboxylic acid at all, which is why the choice of reagent matters. The reaction must be carried out in dry ether, since the reagent reacts violently with water.

12. Which acid is the strongest?

• A. Ethanoic acid
• B. Propanoic acid
• C. Trichloroethanoic acid
• D. Butanoic acid

Explanation: Three electronegative chlorine atoms withdraw electron density powerfully and stabilise the carboxylate ion, so trichloroethanoic acid is a strong acid, comparable in strength to some mineral acids. Among the unsubstituted acids, strength falls slightly as the alkyl chain lengthens because alkyl groups release electron density. This is why methanoic acid is the strongest of the simple aliphatic acids.

13. Soap is manufactured by the alkaline hydrolysis of

• A. ethanol
• B. benzene
• C. starch
• D. natural fats and oils, which are esters of glycerol

Explanation: Boiling a fat with sodium hydroxide splits the three ester linkages and gives sodium salts of long chain fatty acids, the soap, together with glycerol as a valuable by product. Soap cleans because the hydrocarbon tail dissolves in grease while the ionic head remains in the water. Using potassium hydroxide instead gives a softer, more soluble soap.

14. The functional group of an amide is

• A. CONH2, in which nitrogen is bonded directly to the carbonyl carbon
• B. COOH
• C. COOR
• D. COCl

Explanation: In an amide the nitrogen lone pair is delocalised into the carbonyl group, which is why amides are almost neutral rather than basic like amines, and why they are the least reactive of the acid derivatives. The peptide link joining amino acids in a protein is exactly this group. COOR is the ester group and COCl the acyl chloride.

15. Methanoic acid differs from other carboxylic acids in that it also

• A. fails to react with alkalis
• B. acts as a reducing agent, giving a silver mirror with Tollens reagent
• C. contains no carboxyl group
• D. is insoluble in water

Explanation: Its carboxyl carbon also carries a hydrogen, so the molecule contains an aldehyde group as well and can be oxidised further to carbon dioxide, which no other carboxylic acid can do. This is why methanoic acid reduces Tollens and Fehling's reagents. It is otherwise a typical, and in fact the strongest, of the simple aliphatic acids.

16. Carboxylic acids up to four carbon atoms are completely miscible with water because

• A. they ionise completely
• B. they are non polar
• C. the carboxyl group forms hydrogen bonds with water molecules
• D. they react with water chemically

Explanation: Both the carbonyl oxygen and the hydroxyl hydrogen can take part in hydrogen bonding with water, so the smaller acids dissolve freely, and solubility then falls as the hydrocarbon chain grows and dominates the molecule. Complete ionisation is not the reason, since these are weak acids only slightly dissociated in solution. Dissolving is a physical process, not a chemical reaction.

17. Ethanoic acid reacts with phosphorus pentachloride to give

• A. ethanoyl chloride, hydrogen chloride and phosphorus oxychloride
• B. chloroethane and water
• C. ethanol and chlorine
• D. ethanal only

Explanation: The hydroxyl group of the acid is replaced by chlorine, giving the far more reactive acyl chloride, and the misty fumes of hydrogen chloride released make this a useful test for a hydroxyl or carboxyl group. Thionyl chloride achieves the same conversion with gaseous by products that are easier to remove. The acyl chloride is then used to prepare esters and amides.

18. Which reagent would distinguish ethanoic acid from phenol?

• A. Sodium hydroxide solution
• B. Sodium metal
• C. Sodium hydrogen carbonate solution
• D. Litmus paper alone

Explanation: Ethanoic acid gives brisk effervescence of carbon dioxide with hydrogen carbonate while phenol, being too weak an acid, gives no reaction at all. Sodium hydroxide is no use, since both compounds are neutralised by it, and both also react with sodium metal because both contain a hydroxyl hydrogen. This is the standard sequence for telling apart an alcohol, a phenol and a carboxylic acid.

19. The IUPAC name of CH3CH2COOH is

• A. ethanoic acid
• B. butanoic acid
• C. propanoic acid
• D. propan-1-ol

Explanation: The molecule has three carbon atoms in total, and the carboxyl carbon is always counted as part of the chain and numbered one, so the name is propanoic acid. Forgetting to count the carboxyl carbon gives ethanoic acid, which is the standard error. The parent acid name is formed by replacing the final e of the alkane with the ending oic acid.

20. An ester is named ethyl ethanoate. This tells us that it was formed from

• A. ethanoic acid and ethanol
• B. ethanol and methanoic acid
• C. ethanoic acid and methanol
• D. ethanal and ethanol

Explanation: The first part of an ester name comes from the alcohol and the second from the acid, so ethyl ethanoate is made from ethanol and ethanoic acid. Methyl ethanoate would come from methanol and the same acid, which is the closest distractor. Reading ester names in this order is essential when working backwards to identify the reactants.