Why is Wurtz reaction not preferred for the preparation of alkanes containing odd number of carbon atoms? Illustrate your answer by taking one example.
The Wurtz reaction is not preferred for the preparation of alkanes containing an odd number of carbon atoms because it requires taking two different alkyl halides. When two different alkyl halides are used, a mixture of three different alkanes is produced instead of a single desired product, making separation very difficult.
For example, to prepare propane (an alkane with 3 carbon atoms) using the Wurtz reaction, we would need to react bromomethane () and bromoethane () with sodium in dry ether. This reaction produces a mixture of ethane (), butane (), and the desired propane ().
Since these alkanes have closely related physical properties, they cannot be easily separated from the mixture. Hence, this method is unsuitable for preparing alkanes with an odd number of carbon atoms.
Explanation
The textbook introduces the Wurtz reaction using the same alkyl halides to produce alkanes with an even number of carbon atoms. It then prompts the student to consider what happens if two different alkyl halides are taken. To get an odd-numbered alkane, two different halides must be used, which leads to cross-coupling and self-coupling, yielding a mixture of three products that are hard to separate.
Solution Steps
Step 1: State that preparing an odd-carbon alkane requires two different alkyl halides.
Step 2: Explain that using two different alkyl halides results in a mixture of three alkanes.
Step 3: Provide the example of reacting and to form ethane, butane, and propane.
Step 4: Conclude that the mixture is difficult to separate, making the method unsuitable.