Problem 35 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following alkane can be prepared in large amounts by Wurtz reaction? (a) \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CHCH}_{2} \mathrm{CH}\left(\mathrm{CH}_{3}\right)_{2}\) (b) CCC (c) \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{CCH}_{2} \mathrm{CH}_{3}\) (d) \(\mathrm{CH}_{3} \mathrm{CH}\left(\mathrm{CH}_{3}\right) \mathrm{CH}_{2} \mathrm{CH}_{3}\)

Step-by-Step Solution

Answer

Option (d) 2-methylbutane is best prepared by Wurtz reaction.

Step 1: Understanding the Wurtz Reaction

The Wurtz reaction involves the coupling of two alkyl halides using sodium in dry ether to form a higher alkane. This method is best suited for forming symmetrical alkanes, typically when the two alkyl groups are identical.

Step 2: Evaluating Option (a)

Option (a) \[\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CHCH}_{2} \mathrm{CH}\left(\mathrm{CH}_{3}\right)_{2}\], represents 2,3-dimethylpentane. This structure suggests an unsymmetrical compound, indicating that it is less likely to be efficiently produced by the Wurtz reaction, as it often leads to a mixture of products when unsymmetrical starting materials are used.

Step 3: Evaluating Option (b)

Option (b), represented by the SMILES notation "CCC", is essentially propane. Producing propane via Wurtz reaction requires coupling identical ethyl groups, which is viable. However, forming smaller alkanes like propane can sometimes be inefficient due to competing side reactions.

Step 4: Evaluating Option (c)

Option (c) \[\left(\mathrm{CH}_{3}\right)_{3} \mathrm{CCH}_{2} \mathrm{CH}_{3}\] stands for 2,2-dimethylbutane. Preparing this compound using two different alkyl halides might result in multiple products, showing unsymmetrical alkane formation that complicates Wurtz reaction efficiency.

Step 5: Evaluating Option (d)

Option (d) \[\mathrm{CH}_{3} \mathrm{CH}\left(\mathrm{CH}_{3}\right) \mathrm{CH}_{2} \mathrm{CH}_{3}\] represents 2-methylbutane. This molecule can potentially be synthesized by coupling two propyl radicals, forming a potentially efficient route to a higher symmetrical alkane via the Wurtz reaction.

Step 6: Conclusion and Answer Selection

After analyzing each option, we find that option (d), 2-methylbutane, can be constructed more feasibly as it would form primarily from symmetric coupling reactions. The Wurtz reaction is less efficient when unsymmetrical alkanes are involved, as in options (a) and (c).

Key Concepts

Alkane Synthesis

The synthesis of alkanes is a fundamental topic in organic chemistry, with various reactions designed to efficiently build these simple hydrocarbons. One particularly important method is the Wurtz reaction, which allows the creation of larger alkanes from smaller precursors. This is achieved by coupling two alkyl halides in the presence of sodium and dry ether.

• The primary goal in alkane synthesis is to form a desired chain length without introducing complexity or branching that can hinder the target product.
• Reactions like the Wurtz reaction are favored for their ability to couple simpler molecules into larger, more useful alkanes that can be further manipulated or utilized in syntheses.

Understanding this reaction's mechanisms and limitations is crucial for effectively using it to produce alkanes.

Symmetrical Alkanes

Symmetrical alkanes are a unique class of hydrocarbons where the alkane's structure is mirrored around a central point. This characteristic makes them particularly suitable for synthesis through certain chemical processes, such as the Wurtz reaction.

• Symmetrical structures often arise from the coupling of two identical alkyl groups. This greatly simplifies the outcome of reactions, as the potential for side product formation is reduced.
• Using symmetrical alkanes in the Wurtz reaction leads to more predictable and efficient outcomes because it minimizes the array of possible structural variations.

This is especially beneficial in industrial chemistry, where the purity and yield of a product can significantly impact production costs and efficiencies.

Alkyl Halides Coupling

The coupling of alkyl halides is a key mechanism in organic synthesis, particularly in the creation of larger hydrocarbons. The Wurtz reaction utilizes this concept by promoting the formation of carbon-carbon bonds between two alkyl halides using sodium metal.

• Alkyl halides are organic compounds containing carbon, hydrogen, and halogen atoms. They are relatively reactive, making them excellent candidates for coupling reactions.
• The sodium metal acts as a reducing agent in the reaction, causing the elimination of halogen atoms and encouraging the bonding of terminal carbon atoms.

Effectively managing these transformations requires a nuanced understanding of the substrates involved, as the presence of identical or different alkyl groups will dictate the final product distribution.

Organic Chemistry Reactions

Reactions in organic chemistry are central to the synthesis, transformation, and modification of organic compounds. They serve as the backbone for creating everything from simple hydrocarbons to complex pharmaceuticals. The Wurtz reaction plays a critical role among these reactions, as it enables the construction of larger alkanes from simpler substances.

• Organic reactions often involve changes in functional groups, the formation of new bonds, or both. They are catalyzed by various factors, including heat, light, catalysts, or reagents like sodium.
• The choice of reaction conditions, such as solvent and temperature, is crucial for steering the reaction toward a specific outcome.

Mastery of these reactions enables chemists to design pathways that efficiently produce desired molecules with minimal waste and energy expenditure.