Question

Why is Wurtz reaction not preferred for the preparation of alkanes containing odd number of carbon atoms? Illustrate your answer by taking one example.

Short answer

Wurtz reaction forms a mixture of alkanes, leading to low yield of desired odd-carbon products.

Step-by-step solution

Understanding Wurtz Reaction

The Wurtz reaction involves reacting two alkyl halides with sodium metal to form a higher alkane. During the reaction, two molecules of an alkyl halide couple to form a longer chain alkane.

Identifying the Limitations for Odd Carbon Alkanes

When identical alkyl halides are used, only even-numbered alkanes are produced because two identical fragments couple. If different alkyl halides are utilized to target an odd-numbered alkane, a mix of products is formed with no control over the chain lengths.

Example of Mixed Alkyl Halides

Consider using bromomethane (CH₃Br) and ethyl bromide (C₂H₅Br) in Wurtz reaction. The reaction leads to a mixture including ethane (C₂H₆), propane (C₃H₈), and butane (C₄H₁₀), among others.

Analyzing the Product Mixture

In this example, a desire for propane (C₃H₈), an odd-numbered alkane, also results in formation of ethane and butane due to uncontrolled coupling. The products result from each alkyl fragment randomly coupling with others, leading to a statistical mix of even and odd alkanes.

Conclusion

The Wurtz reaction yields a variety of coupling products including alkanes with even and odd carbon numbers when applied to mixed alkyl halides, which complicates the isolation of pure odd-carbon alkanes. This lack of selectivity renders this method inefficient for preparing odd-numbered alkanes.

Key concepts

Alkyl Halides

Alkyl halides are organic compounds that contain one or more halogen atoms substituted for hydrogen atoms in an alkane. These compounds have the general formula R-X, where "R" represents the alkyl group and "X" stands for the halogen atom such as chlorine, bromine, or iodine. They serve as fundamental starting materials in many organic reactions due to their high reactivity.
In the context of the Wurtz reaction, alkyl halides play a critical role. When two molecules of alkyl halides react in the presence of sodium metal, they undergo coupling to form a higher alkane. The reactivity of alkyl halides is primarily attributed to the presence of the carbon-halogen bond, which is polar. This makes the carbon atom electrophilic and open to nucleophilic attack, facilitating the process of nucleophilic substitution essential for the Wurtz reaction.
Alkyl halides can vary in structure, including primary, secondary, and tertiary forms, influencing the reaction outcome and the types of alkanes that can be synthesized.

Sodium Metal

Sodium metal is a crucial component in the Wurtz reaction, serving as a reducing agent. It is a soft, silvery-white, highly reactive metal, typically handled with care due to its reactivity with water and air. In the Wurtz reaction, sodium provides electrons that facilitate the coupling of alkyl halides.
The mechanism involves sodium donating electrons to the alkyl halide, leading to the formation of radical intermediates. These radicals then couple, forming a new carbon-carbon bond, which ultimately yields the desired alkane. The use of sodium metal is vital in breaking the carbon-halogen bond and catalyzing the coupling process.
Due to its reactivity, sodium metal must be used under controlled conditions, usually in an inert atmosphere to preclude unwanted reactions that might affect the reaction's efficiency.

Alkane Synthesis

Alkane synthesis via the Wurtz reaction is a well-known method for constructing larger carbon chains. This synthesis involves the coupling of two alkyl halide molecules in the presence of sodium metal, resulting in the formation of a higher alkane with simultaneous elimination of halogen atoms.
The Wurtz reaction is straightforward and can yield different products based on the starting alkyl halides used. When identical alkyl halides are used, the reaction results in the formation of an even-numbered alkane. The elegant simplicity and utility of this reaction make it popular for synthesizing symmetrical alkanes.
However, when the target product is an odd-numbered alkane, challenges arise due to the random coupling that occurs when different alkyl halides are used. This lack of selectivity can lead to a mix of alkanes, complicating the isolation of the desired odd-number product.

Odd-Numbered Alkanes

Odd-numbered alkanes, as implied by the name, contain an odd number of carbon atoms. These can be more challenging to synthesize using certain methods like the Wurtz reaction due to the inherent difficulties in controlling the chain length during product formation.
The primary challenge is the statistical nature of the coupling when mixed alkyl halides are used. Attempts to synthesize a specific odd-numbered alkane often result in mixtures of products. For example, using bromomethane and ethyl bromide in a Wurtz reaction might lead to the creation of not only propane, but also ethane and butane, making it hard to purify the desired odd-numbered alkane.
Due to this unpredictable outcome and reduced selectivity, alternative synthetic routes might be preferred when targeting odd-numbered alkanes, unless specific conditions or strategies are employed to improve selectivity.