Free Alkynes MCQs with Answers

1. Ethyne differs from ethene in that ethyne contains

• A. one sigma and two pi bonds between the carbon atoms, with sp hybridisation
• B. two sigma bonds between the carbon atoms
• C. only sigma bonds
• D. three pi bonds between the carbon atoms

Explanation: A triple bond is always one sigma bond plus two pi bonds, formed from two unhybridised p orbitals on each sp hybridised carbon, and the molecule is linear as a result. Ethyne also shows weak acidity, unlike ethene, because the higher s character of the sp orbital holds the bonding electrons closer to carbon. Any multiple bond, however high the order, contains exactly one sigma bond.

2. Ethyne can be prepared in the laboratory by the action of water on

• A. calcium carbonate
• B. calcium oxide
• C. calcium hydroxide
• D. calcium carbide

Explanation: Calcium carbide reacts vigorously with water to give ethyne and calcium hydroxide, which was once used in carbide lamps and is still the classic laboratory preparation. Calcium carbonate with acid would give carbon dioxide instead, which is the tempting confusion between the carbide and the carbonate. The ethyne produced this way smells unpleasant because of phosphine impurities.

3. Terminal alkynes such as ethyne react with ammoniacal silver nitrate to give a precipitate because

• A. the triple bond adds silver
• B. the hydrogen attached to the triply bonded carbon is weakly acidic and is replaced by silver
• C. silver catalyses polymerisation
• D. alkynes are strong bases

Explanation: The sp hybridised carbon holds its bonding electrons closer, so the attached hydrogen is slightly acidic and can be replaced by a metal to form an acetylide precipitate, which is the standard test distinguishing a terminal alkyne from an internal one. Alkenes and alkanes give no such reaction. The dry precipitates are explosive and must be destroyed with acid.

4. Terminal alkynes show acidic character due to:

• A. Terminal carbon atoms are sp hybridized
• B. Terminal carbon atoms are sp2 hybridized
• C. Terminal carbon atoms are sp3 hybridized
• D. Terminal carbon atoms show hydrogen bonding

Explanation: An sp hybrid orbital has 50 per cent s character, so it holds the bonding electrons closer to the carbon nucleus and the attached hydrogen is released more easily as a proton. This is why ethyne gives a precipitate with ammoniacal silver nitrate while ethene and ethane do not. Acidity therefore rises with s character in the order sp3 to sp2 to sp.