# Question 00280

Sep 27, 2016

Let the chlorination reaction of Ethane $\left({C}_{2} {H}_{6}\right)$ with chlorine replaces n atoms from its molecule producing chloro ethane of formula ${C}_{2} {H}_{6 - n} C {l}_{n}$

Considering atomic masses as

$C = 12 \frac{g}{\text{mol}}$

$H = 1 \frac{g}{\text{mol}}$

$C l = 35.5 \frac{g}{\text{mol}}$

We get the molar mass of chloroethane as

$2 \times 12 + \left(6 - n\right) \times 1 + n \times 35.5$
$= 30 + 34.5 n$

And % Cl in the compound as calculated from its molecular formula ${C}_{2} {H}_{6 - n} C {l}_{n}$ will be

$= \frac{35.5 \times n \times 100}{30 + 34.5 n}$

Equating this with the given percentage we can write

$\frac{35.5 \times n \times 100}{30 + 34.5 n} = 71.7$

$\implies \frac{35.5 \times n \times 100}{71.7} = \left(30 + 34.5 n\right)$

$\implies 49.5 n = \left(30 + 34.5 n\right)$

$\implies 49.5 n - 34.5 n = 30$

$\implies n = \frac{30}{15} = 2$

(A) Hence the molecular formula of chloroethane is ${C}_{2} {H}_{4} C {l}_{2}$

(B) The condensed structural formula and name of the isomers

$C {H}_{3} C H C {l}_{2} \to 1 , 1 - \text{dichloroethane}$

$C {H}_{2} C l C {H}_{2} C l \to 1 , 2 - \text{dichloroethane}$

(C) Between the two isomers the major product will be
$C {H}_{3} C H C {l}_{2} \to \text{1,1-dichloroethane}$

The reaction follows free radical mechanism.

(1) $C l \cdot$ free radical is first produced by homolytic fission of $C l - C l \text{ } \sigma b o n d$ under the influence of light or heat energy.

(2)This $C l \cdot$ free radical extracts $H$ atom from $C {H}_{3} C {H}_{3}$ molecule forming $C {H}_{3} C {H}_{2} \cdot$ free radical

(3) $C {H}_{3} C {H}_{2} \cdot$ free radical then combines with $C l \cdot$ free radical to form $C {H}_{3} C {H}_{2} C l$ ,monochloroethane.

(4) Now the extraction of $H \text{-atom}$ from $C {H}_{3} C {H}_{2} C l$ by $C l \cdot$ again occurs with two possibilities forming $C {H}_{3} C H C l \cdot$ or $\cdot C {H}_{2} C {H}_{2} C l$

(5) The first one $\textcolor{b l u e}{C {H}_{3} C H C l \cdot}$ is more stable free radical as it has got more alpha"H# in respect of free radical providing higher hyper conjugative effect. So the first one which is formed in higher concentration in the reaction mixture,will combine with $C l \cdot$ free radical more and thus the major product will be

$\textcolor{red}{C {H}_{3} C H C {l}_{2} \to \text{1,1-dichloroethane}}$ 