Problem 34 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following alkane can be prepared 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 CCCC(C)C (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

The suitable alkane for Wurtz reaction is (d) \(\mathrm{CH}_{3} \mathrm{CH}(\mathrm{CH}_{3}) \mathrm{CH}_{2}\ \mathrm{CH}_{3}\).

Step 1: Understand the Wurtz Reaction

The Wurtz reaction is a coupling reaction between two alkyl halides in the presence of sodium metal to form a higher alkane. It generally works well for symmetrical alkanes, resulting from identical alkyl halides.

Step 2: Analyze Each Option

Evaluate each alkane for its feasibility to be formed by Wurtz reaction: (a) \((\mathrm{CH}_{3})_{2} \mathrm{CHCH}_{2}\ \mathrm{CH}(\mathrm{CH}_{3})_{2}\) is a complex branched alkane.(b) Both options under (b) correspond to hexane (\(\mathrm{C}_{6}\)) and cannot be differentiated based on given information.(c) \( (\mathrm{CH}_{3})_{3} \mathrm{CCH}_{2} \mathrm{CH}_{3}\) is a branched alkane with a tertiary carbon, which does not prefer this reaction.(d) \(\mathrm{CH}_{3} \mathrm{CH}(\mathrm{CH}_{3}) \mathrm{CH}_{2}\ \mathrm{CH}_{3}\) is straight chain alkane, which can form directly from a single type of alkyl halide.

Step 3: Consider Reaction Suitability

For Wurtz reaction to occur successfully in large amounts, both alkyl groups should ideally be symmetrical to avoid side reactions. Hence, simpler alkanes like straight chains that can be formed from the combination of identical alkyl halides are most favorable.

Step 4: Determine the Most Suitable Alkane

Among the given options, \(\mathrm{CH}_{3} \mathrm{CH}(\mathrm{CH}_{3}) \mathrm{CH}_{2}\ \mathrm{CH}_{3}\) (option d) can be prepared efficiently since it can be synthesized from two equivalents of \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Br}\) or similar halides, which are symmetrical and form a straight-chain product.

Key Concepts

Alkanes

Alkanes are a type of hydrocarbon, which simply means they are composed entirely of hydrogen and carbon atoms. They are saturated compounds, meaning they have single bonds between their carbon atoms. This makes them relatively stable and less reactive compared to other hydrocarbons such as alkenes or alkynes.

• Alkanes have the general formula: \(C_nH_{2n+2}\), where \(n\) is the number of carbon atoms.
• They are found in natural gases and gasoline and are part of everyday fuels.
• Alkanes can be either straight-chain or branched, affecting their physical properties like melting and boiling points.

One of the important reactions involving alkanes is the Wurtz reaction, which synthesizes larger alkanes from smaller fragments. In this reaction, the simple structure and single bonds of alkanes make them excellent targets because they remain stable throughout the reaction process.

Alkyl halides

Alkyl halides, also known as haloalkanes, are compounds that result when an alkane's hydrogen atom is replaced by a halogen (such as chlorine, bromine, or iodine). This substitution results in a class of organic compounds that are highly useful in various chemical syntheses.

• They are often identified by their carbon skeleton followed by the halide, e.g., ethyl chloride (\(CH_3CH_2Cl\)).
• The presence of the halogen atom in these molecules makes them more reactive than the parent alkanes.
• Due to their reactivity, they are frequently used as intermediates in the preparation of other compounds.

In the context of the Wurtz reaction, alkyl halides are essential as they undergo coupling in the presence of sodium metal to form larger alkanes. This reaction is particularly effective when the halides are symmetrical, which helps to shift the reaction towards the desired alkane product rather than a mixture of possible products.

Chemical synthesis

Chemical synthesis is the process of using chemical reactions to construct compounds from simpler constituents. It is a cornerstone of organic chemistry, allowing chemists to create a variety of complex molecules required for applications in medicine, industry, and research.

• Methods in chemical synthesis include separation of products, combining reactants and designed pathways to desired compounds.
• Each synthetic process can involve multiple steps and the strategic use of reagents and catalysts.
• Reactions like the Wurtz reaction are utilized to extend carbon chains and construct larger molecules.

In exercises like choosing an appropriate alkane for a Wurtz reaction, understanding synthesis helps to discern which starting materials will most efficiently lead to the desired product. It is this knowledge of synthesis pathways and reaction conditions that allows chemists to predict the outcome and yields of their reactions, an essential skill in both theoretical and applied chemistry.