Free Functional Groups MCQs with Answers

1. The functional group present in an aldehyde is

• A. a hydroxyl group
• B. a carbonyl group at the end of the chain, written CHO
• C. a carboxyl group
• D. a carbonyl group between two carbon atoms

Explanation: In an aldehyde the carbonyl carbon carries at least one hydrogen and so must be terminal, whereas in a ketone the same carbonyl group lies between two carbon atoms. This structural difference is why aldehydes are readily oxidised to carboxylic acids and give a positive test with Tollens reagent while ketones do not. The carboxyl group of an acid combines a carbonyl and a hydroxyl on the same carbon.

2. The general formula CnH2n could represent

• A. an alkane
• B. an alkyne
• C. an alcohol
• D. an alkene or a cycloalkane

Explanation: Both an alkene, with one double bond, and a cycloalkane, with one ring, have two hydrogens fewer than the corresponding alkane, so they share this general formula and are functional isomers of each other. Alkanes are CnH2n+2 and alkynes CnH2n-2. This is why a general formula alone can never identify a compound with certainty.

3. The functional group of a carboxylic acid is

• A. COOH
• B. CHO
• C. OH
• D. CO

Explanation: The carboxyl group combines a carbonyl and a hydroxyl on the same carbon atom, and this arrangement makes the hydrogen far more acidic than in an alcohol, because the resulting carboxylate ion is stabilised by delocalisation. CHO is the aldehyde group and CO between two carbons is a ketone. Acids therefore liberate carbon dioxide from carbonates, which alcohols do not.

4. The functional group present in an ester is

• A. COOR, formed by replacing the acidic hydrogen of a carboxylic acid
• B. OH attached to a benzene ring
• C. NH2 attached to a carbon chain
• D. a carbon to carbon triple bond

Explanation: An ester is formed when a carboxylic acid reacts with an alcohol, water being eliminated, and the resulting group is responsible for the pleasant fruity smells used in flavourings and perfumes. A hydroxyl on a benzene ring makes a phenol and an NH2 group makes an amine. Esterification is reversible and is catalysed by concentrated sulphuric acid.