Question

An aqueous mixture of sodium ethanoate and sodium propanoate is electrolyzed. The product of the reaction is __________.
A. $C{H_3}C{H_2}C{H_3}$
B. $C{H_3}C{H_2}C{H_2}C{H_3}$
C. $C{H_3}C{H_3}$
D. all of these

Answer 1

Hint:When an aqueous mixture of sodium salts of carboxylic acids is electrolyzed, alkanes are obtained as the product. The type of alkanes obtained depends upon the number of carbon atoms present in the carboxylic acid salts.

Complete step by step solution:
In aqueous solution, both sodium ethanoate ($C{H_3}COONa$) and propanoate ($C{H_3}C{H_2}COONa$) will split up into their constituent ions as follows:
$C{H_3}COONa \rightleftharpoons C{H_3}CO{O^ - } + N{a^ + }$
$C{H_3}C{H_2}COONa \rightleftharpoons C{H_3}C{H_2}CO{O^ - } + N{a^ + }$
At the anode, oxidation will take place and the carboxylate ions will get oxidized into alkyl free radicals. One electron from each molecule is released into the solution. Also, carbon dioxide gas is liberated. The equations at the anode are as follows:
$C{H_3}CO{O^ - } \to \mathop C\limits^ \bullet {H_3} + {e^ - } + C{O_2}$
$C{H_3}C{H_2}CO{O^ - } \to C{H_3}\mathop C\limits^ \bullet {H_2} + {e^ - } + C{O_2}$
These alkyl free radicals are highly unstable compounds, and combine together rapidly to form alkanes.
Two methyl free radicals can combine together to form ethane:
$\mathop C\limits^ \bullet {H_3} + \mathop C\limits^ \bullet {H_3} \to C{H_3}C{H_3}$
Two ethyl free radicals can combine to form butane:
$C{H_3}\mathop C\limits^ \bullet {H_2} + C{H_3}\mathop C\limits^ \bullet {H_2} \to C{H_3}C{H_2}C{H_2}C{H_3}$
And finally, one methyl free radical and one ethyl free radical can combine to form propane:
$\mathop C\limits^ \bullet {H_3} + C{H_3}\mathop C\limits^ \bullet {H_2} \to C{H_3}C{H_2}C{H_3}$
Hence, we can see that all three alkanes are being formed.
Therefore, the correct option to be marked is option D.

Note:
This form of electrolysis of sodium salts of carboxylic acids is a common method of producing alkanes, and is known as Kolbe’s electrolysis. When only one carboxylic acid salt is taken, the product is always an alkane with an even number of carbon atoms. Note that oxidation always happens at the anode and reduction always happens at the cathode. The process by which two free radicals combine together to form a bigger, more stable molecule is known as dimerization.