Question

(a) What is the special feature of the structure of:
i) $C_2{H}_4$
ii) $C_2{H}_2$

(b) What type of reaction is common to both these compounds? Why does methane not undergo this type of reaction?

[4 MARKS]

Answer 1

Each part: 2 Marks

(a) i) Structure of $C_2{H}_4$ (ethene) : Each carbon atom in ethene is attached with two hydrogen atoms by single covalent bonds and to another carbon atom by a double bond. There are four C - H single covalent bonds and one C - C double bond. It is a planar (flat) molecule. All bond angles (H - C- H and H - C - C) are ${120}^{\circ }$ as shown below:


ii) Structure of $C_2{H}_2$ (acetylene) : Each carbon atom in acetylene is attached with one hydrogen atom by single covalent bond.
The molecular, electronic and structural formula are given below.

The shape of ethyne molecule is linear. The H - C - C bond angle is ${180}^{\circ }$

(b) Both compound (i) as well as (ii) give addition reactions.
Methane doesn't undergo this type of reaction as it is a saturated hydrocarbon. Saturated hydrocarbons undergo only substitution reactions. Addition reactions are not possible in case of saturated hydrocarbons. Only unsaturated hydrocarbons like ethene and ethyne give addition reactions since carbon - carbon double and triple bonds break up easily.