Question

Which one of the following reactions will not lead to the desired ether formation in major proportion?
(iso-Bu = isobutyl, sec-Bu = sec-butyl, nPr = n-propyl, tBu = tert-butyl, Et = ethyl)

• A.
• B.
• C.
• D.

Hint:

When performing Williamson Ether Synthesis, consider the steric hindrance of the alkyl groups. The reaction is more favorable when the nucleophile is less hindered. Secondary and tertiary alkyl groups show less reactivity in comparison to primary alkyl groups.

Answer 1

The Correct Option is C

To determine which reaction will not lead to the desired ether formation in major proportions, we must consider the mechanism by which ethers are typically formed in chemical reactions, specifically via the Williamson Ether Synthesis. This synthesis involves the reaction of an alkoxide ion with a primary alkyl halide to form ethers.

The Williamson Ether Synthesis generally works best with primary alkyl halides because they undergo S_N2 reactions readily. Secondary and tertiary alkyl halides are less ideal because they tend to undergo elimination or other side reactions rather than substitution.

In this problem, we assess each option to determine which might not lead to efficient ether formation:

1. In the reaction option provided as the correct answer (), t-butyl bromide (a tertiary alkyl halide) is present. Tertiary halides are poor substrates for S_N2 reactions due to steric hindrance, making them more likely to undergo elimination (E2 reaction) rather than ether formation.
2. Other options involve primary or secondary alkyl halides such as isobutyl, sec-butyl, or n-propyl, which are more conducive to S_N2 mechanisms and likely to produce ethers in good yield.

Given this understanding, the reaction involving t-butyl bromide will not efficiently form ethers due to steric hindrance leading to an elimination pathway instead. Therefore, the option with t-butyl bromide () is not suitable for obtaining the desired ether in substantial proportion.

Answer 2

The question involves determining which reaction will not primarily lead to the desired ether. This is a classic example of understanding the Williamson Ether Synthesis and considerations of steric hindrance in reactions involving bulky groups.

Step 1: Understand the Williamson Ether Synthesis

The Williamson Ether Synthesis is a method used to prepare ethers from an alkyl halide and a deprotonated alcohol (alkoxide). The general reaction is:

\(R-O^{-} + R'-X \rightarrow R-O-R' + X^{-}\)

Here, \(R-O^{-}\) is the alkoxide ion and \(R'-X\) is the alkyl halide.

Step 2: Factors to Consider

The reaction is nucleophilic substitution, typically proceeding via an \(\text{S}_\text{N}2\) mechanism. Key considerations include:

• Steric Hindrance: A bulky alkyl halide hinders the reaction as it blocks the approach of the nucleophile.
• Reactivity of Alkyl Halide: Primary alkyl halides are more favorable for this reaction than secondary or tertiary.

Step 3: Analyze Each Option

• In theoption, the alkyl halide is not sterically hindered, allowing smooth ether formation.
• features a moderately hindered (secondary) alkyl halide, still feasible for ether synthesis.
• The optiondoes not present significant hindrance; thus, ether formation is probable.
• The critical option is. The combination of a bulky tert-butyl group with a nucleophile can lead to \text{E}_\text{2} elimination rather than ether formation, leading to an alkene instead.

Conclusion: The reaction in

will not give the desired ether as the major product due to steric hindrance and tendency towards elimination. Therefore, it is the correct choice.