Question

Hydrocarbon (A) reacts with bromine by substitution to form an alkyl bromide which by Wurtz reaction is converted to gaseous hydrocarbon containing less than four carbon atoms. (A) is _____________.
A.$C{H_3} - C{H_3}$
B.$CH \equiv CH$
C.$C{H_4}$
D.$C{H_2} = C{H_2}$

Answer 1

Hint: To solve this question, one needs to have knowledge on the Wurtz reaction. The formation of alkanes from alkyl halides in the presence of dry ether and sodium metal, in the complete absence of moisture, is the Wurtz reaction. The smallest alkane that can be synthesized from this reaction is ethane, methane cannot form from this reaction.

Complete step by step answer:
At first it should be mentioned here that only alkanes are formed from Wurtz reaction, alkenes, and alkynes are not formed.
Hence, options (B), ethyne and (D), ethene are ruled out at first.
Now, in the question, it is given that the hydrocarbon formed after the Wurtz reaction has less than four carbons. Thus, option (A) is also incorrect. As Wurtz reaction is a coupling reaction, the reaction of ethyl halide will result in a hydrocarbon with four carbon atoms.
The reaction will go as follows:
$C{H_4} + {X_2} \to C{H_3}X + HX$
This methyl halide will react in the presence of sodium metal as:
$2C{H_3}X + 2Na \to C{H_3} - C{H_3} + 2NaX$

Hence option C is correct.

Note:
1. Wurtz reaction should be carried in a medium where there is no moisture at all, also oxygen should not be allowed in the reaction medium, this is because the reaction is carried out in the presence of sodium which is highly reactive to both oxygen and moisture.
2. Alkanes with even number of carbon atoms can be synthesized through this method hence, methane, propane, pentane, etc. cannot be prepared though this method.