Question

Alkanes having odd carbons cannot be prepared as a major product in:
(a) Wurtz reaction
(b) Frankland reaction
(c) Kolbe’s electrolysis.
(d) Sabatier-sendersen reaction

Answer 1

Hint: First you should know about the chemical reactions of the alkanes properly and then, you can easily identify the reactions which give alkanes having even number of carbon atoms and alkanes with odd number of carbon atoms. Now solve it.

Complete answer:
We will discuss the given options one by one:
(a) Wurtz’s reaction:- In this reaction, two molecules of alkyl halide react with the sodium metal in the presence of dry ether and results in the formation of the alkanes.
The general reaction is supposed to occur as;
$R-X+2Na+X-R\to R-R+2NaX$
The alkanes obtained by this method contain an even number of carbons and alkanes with an odd number of carbon atoms cannot be obtained by this method.

(b) Frankland reaction:- This reaction is similar just like the wurtz’s reaction but in place of sodium metal , zinc metal is used.
The general reaction is supposed to occur as;
$R-X+2Zn+X-R\to R-R+2ZnX$
The alkanes obtained by this method contain an even number of carbons and alkanes with an odd number of carbon atoms cannot be obtained by this method.

(c) Kolbe’s electrolysis:- In this reaction, sodium or potassium salt of dicarboxylic acids on electrolysis gives alkanes.
The general reaction is supposed to occur as;
$2RCOOK\underset{-2C{O}_2}{\overset{-2e^{-}}{\to }}R-R$
The alkanes obtained by this method also contain an even number of carbons and alkanes with an odd number of carbon atoms cannot be obtained by this method.

(d) Sabatier-senderens reaction:- In this reaction, alkene or the alkyne undergoes reduction in the presence of nickel to form the alkanes.
The general reaction is supposed to occur as;
$R-C=C-R+H_2\stackrel{Ni}{\to }R-C-C-R$
The alkanes obtained by this method contain an odd number of carbon atoms.
So, thus from the above it is clear that alkanes having odd carbons cannot be prepared as a major product in Wurtz reaction, Frankland reaction and Kolbe’s electrolysis.

Hence, options (a), (b) and (c) are correct.

Note:
Alkanes are the simplest hydrocarbons among all i.e. the alkenes and the alkynes which consist of the carbon-carbon single bond in their compounds and hence are saturated hydrocarbons.
These have the general chemical formula as $C_n{H}_{2n+2}$ .