Chemistry 2001 JAMB Past Questions - Question 43: A characteristic reaction of the compounds with the general formula CnH2n is

Correct Answer

B

Explanation

Compounds with the general formula CnH2n  where n is a positive integer, are hydrocarbons belonging to the alkynes class. Alkynes are unsaturated hydrocarbons that contain at least one carbon-carbon triple bond (C≡C). The characteristic reaction of alkynes is their ability to undergo addition reactions across the triple bond.

The triple bond consists of one sigma (σ) bond and two pi (π) bonds. The pi bonds are relatively weak compared to sigma bonds, making the triple bond reactive. Alkynes can undergo addition reactions with various reagents, leading to the breaking of the pi bonds and the formation of new sigma bonds.

Common reactions of alkynes include:

1. **Hydrogenation:** Alkynes can be hydrogenated to form alkanes. The triple bond is reduced to a single bond by adding hydrogen (Hâ‚‚) in the presence of a catalyst such as palladium or platinum.

  \[C_nH_2 + nH_2 \rightarrow C_nH_{2n+2}\]

2. **Halogenation:** Alkynes react with halogens (e.g., bromine or chlorine) to form dihalides. The halogen atoms add to the carbon atoms of the triple bond.

  \[C_nH_2 + Br_2 \rightarrow C_nH_{2n}Br_2\]

3. **Hydration:** Alkynes can undergo hydration, where water is added across the triple bond to form a carbonyl group (aldehyde or ketone).

  \[C_nH_2 + H_2O \rightarrow C_nH_{2n}O\]

4. **Acidic Hydrolysis:** Alkynes can be hydrolyzed in the presence of an acid to form carboxylic acids.

  \[C_nH_2 + 2H_2O \rightarrow C_nH_{2n}COOH\]

These are just a few examples, and alkynes can participate in various other reactions based on the reagents and conditions used. The reactivity of alkynes arises from the presence of the triple bond, which contains high-energy pi bonds that can be readily broken to form new bonds with other atoms or groups.