Problem 102 - Advanced Problems in Organic Chemistry for Competitive Examinations

Question

Which of the following alkane can not be synthesised by Wurtz reaction in good yield? (a) \(\left(\mathrm{CH}_{3}\right)_{2}-\mathrm{CH}-\mathrm{CH}_{2}-\mathrm{CH}-\left(\mathrm{CH}_{3}\right)_{2}\) (b) \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CH}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{CH}-\left(\mathrm{CH}_{3}\right)_{2}\) (c) \(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{C}\left(\mathrm{CH}_{3}\right)_{2}-\mathrm{CH}_{2}-\mathrm{CH}_{3}\) (d) \(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{CH}_{3}\)

Step-by-Step Solution

Answer

Option (a) cannot be synthesized in good yield by the Wurtz reaction.

Step 1: Understanding the Wurtz Reaction

The Wurtz reaction involves treating alkyl halides with sodium metal to form higher alkanes by coupling two alkyl radicals. It works well for symmetric linear or simple alkanes but tends to give lower yields with bulky groups due to side reactions or because the intermediate radicals are hard to form.

Step 2: Analyze Option (a)

Option (a) represents a highly branched alkane. The Wurtz reaction is typically less effective for producing highly branched alkanes due to steric hindrance and the difficulty in forming and coupling the necessary radicals.

Step 3: Analyze Option (b)

Option (b) is also a highly branched alkane that would face the same issues as option (a) in the Wurtz reaction. The reaction would struggle to produce this compound in good yield due to steric hindrance.

Step 4: Analyze Option (c)

Option (c) is moderately branched but not excessively, suggesting that it might fare somewhat better in the Wurtz reaction. However, due to its structure, side reactions are still likely, which may reduce the yield.

Step 5: Analyze Option (d)

Option (d) is a linear alkane (butane). The Wurtz reaction is most successful with simple, linear alkanes like this one as there are no bulky groups impeding the reaction or causing side reactions.

Step 6: Conclude the Analysis

Since the Wurtz reaction does not favor the formation of highly branched alkanes in good yield, the best candidates would be linear alkanes like option (d). Option (a) and (b), being highly branched, are less likely to be synthesized effectively by the Wurtz reaction.

Key Concepts

Alkanes

Alkanes are the simplest type of hydrocarbons, consisting of carbon and hydrogen atoms connected by single bonds. This means they have a saturated chemical structure, so they can't bond with more hydrogen atoms. Generally, they are non-polar and less reactive, with a hydrophobic nature, meaning they don't mix well with water. Alkanes are named using the suffix "-ane," and their molecular formula follows the general \\[ C_{n}H_{2n+2} \] pattern, where \( n \) represents the number of carbon atoms. They can be found in different forms:

• Linear Alkanes: Carbon atoms are arranged in a single chain. For example, butane is a simple linear alkane.
• Branched Alkanes: Alkanes that have side chains or branches of carbon atoms along the main carbon chain. Due to their structure, they can cause steric hindrance, affecting reactions like the Wurtz reaction.

Alkyl Halides

Alkyl halides, also known as haloalkanes, are compounds where one or more hydrogen atoms in an alkane are replaced by halogen atoms (such as chlorine, bromine, or iodine). Their general formula is \\[ RX \] , where \( R \) is the alkyl group and \( X \) is the halogen. These molecules possess polar bonds due to the electronegativity difference between the carbon and the halogen, making them more reactive compared to alkanes.
The Wurtz reaction involves these alkyl halides. They are the starting materials, which, when reacted with sodium metal, transform into alkanes. The reactivity of the alkyl halide can vary depending on the type of halogen and the structure of the molecule, impacting the yield of the desired product in the reaction.

Steric Hindrance

Steric hindrance refers to the impediment to chemical reactions due to the physical size and arrangement of atoms within a molecule. When large bulky groups are present, they create crowding around reactive sites, making it difficult for reacting species to come close and engage in effective interaction.
In the context of the Wurtz reaction, steric hindrance can severely impact the formation and coupling of radicals, especially in highly branched alkanes. The branched structure restricts access to the reaction sites, leading to side reactions or preventing the desired reaction altogether. This is why highly branched alkanes, like those in options (a) and (b), are less likely to be synthesized with high yield through this reaction.

Radicals Coupling

Radicals are atoms, molecules, or ions with unpaired electrons, which makes them extremely reactive. In the Wurtz reaction, radicals are formed as intermediate species. When two alkyl radicals collide, they couple to form a new carbon-carbon bond, resulting in the synthesis of alkanes.
However, the success of radicals coupling depends heavily on several factors:

• Stability of Radicals: Stable radicals have better chances of coupling to form desired products.
• Reaction Environment: Clean and controlled conditions improve coupling efficiency.
• Structure of Alkyl Halides: Smaller, less hindered radicals couple more efficiently than bulkier ones.

In branched systems, like options (a) and (b), radicals often face difficulties aligning properly due to steric hindrance, which leads to poor yields in the Wurtz reaction.