Problem 105 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following alkyl halides would be the most reactive in an SN reaction? (a) \(\mathrm{C}_{6} \mathrm{H}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{Br}\) (b) CC(CBr)CBr (c) CC(C)(Br)c1ccccc1 (d) CC(Br)Cc1ccccc1

Step-by-Step Solution

Answer

Option (d) is the most reactive in an SN reaction due to its benzylic position.

Step 1: Identify Structure and Type of Each Alkyl Halide

First, we need to consider the structure of each of the given options in order to identify whether they are primary, secondary, or tertiary alkyl halides as this will influence their reactivity in an SN reaction. Option (a) is a primary alkyl halide with a phenyl group attached to the fourth carbon. Option (b) is a dibromo alkane with one of the bromines on a secondary carbon and the other bromine on a tertiary carbon. Option (c) is a compound with a tertiary carbon attached to both a bromine and an aromatic ring, thus making it a tertiary alkyl halide. Option (d) has a primary bromine connected to a benzylic carbon, making it a benzylic primary alkyl halide.

Step 2: Determine the Reactivity Based on Structure

We need to apply the concept that SN1 reactions are favored by tertiary carbons, while SN2 reactions are favored by primary and secondary carbons. Benzylic positions, like in option (d), enhance reactivity due to resonance stabilization of the carbocation intermediate formed in SN1 reactions.

Step 3: Compare Reactivity

Comparing all types: - Primary alkyl halides (option a) are generally more reactive in SN2 but less so than benzylic positions in SN1. - Option (b) contains a tertiary carbon that is hindered by steric effects. - Tertiary alkyl halide (option c) is less reactive in SN2 but more in SN1. - Option (d) is a primary benzylic halide, highly reactive due to resonance stabilization of the intermediate, enhancing both SN1 and SN2 pathways.

Step 4: Identify the Most Reactive Compound for SN Reactions

Combining considerations, option (d) is expected to be the most reactive due to the benzylic position of the bromine, allowing for resonance stabilization through the adjacent aromatic ring. This makes it highly favorable for SN1 reactions, while still maintaining high reactivity in SN2 as well.

Key Concepts

Primary Alkyl Halide

A primary alkyl halide is an organic compound in which the halogen atom is attached to a primary carbon atom. This means the carbon is connected to only one other carbon and the rest are hydrogen atoms. Primary alkyl halides such as option (a) in our exercise can participate in SN2 reactions, which proceed with a direct attack by the nucleophile on the carbon bearing the halogen.

Some key points about primary alkyl halides in SN reactions include:

• They are usually more reactive in SN2 mechanisms due to less steric hindrance.
• They make poor candidates for SN1 reactions because the primary carbons do not form stable carbocations.

In option (a), which is a primary alkyl halide, the reactivity will be mainly influenced by the more straightforward SN2 reaction path.

Tertiary Alkyl Halide

A tertiary alkyl halide has a halogen atom bonded to a tertiary carbon, which is a carbon atom connected to three other carbons. These compounds, such as shown in option (c), are traditionally more complex in terms of reactivity because of their ability to hinder nucleophilic attacks due to steric effects.

Here are some characteristics of tertiary alkyl halides in SN reactions:

• They are less favorable for SN2 reactions because of the steric bulk around the reactive site.
• They prefer to undergo SN1 reactions since the formation of a stable carbocation is more feasible.

In option (c), the presence of a bulky tertiary carbon means sterics will dominate, leading to a reduced rate for SN2 reactions and an increased likelihood of SN1 pathway occurring.

Benzylic Position

The benzylic position refers to a carbon atom that is directly bonded to an aromatic ring, particularly a benzene. This position is unique because it can stabilize carbocations using resonance with the aromatic system.

Some important attributes of benzylic positions in SN reactions include:

• Carbocations at the benzylic position are highly stabilized by resonance with the aromatic ring.
• This enhances reactivity in SN1 reactions due to the readily formed, stable intermediate.
• Benzylic positions can also be favorable for SN2 due to the reduced energy barrier for bond breakage.

In option (d), the bromine atom is at a benzylic position, which makes it highly reactive and suggests significant resonance stabilization if the SN1 pathway is taken.

Resonance Stabilization

Resonance stabilization occurs when the distribution of electrons across multiple atoms in a molecule leads to increased stability for certain intermediates, like carbocations. This is a central concept for understanding the reactivity patterns in SN reactions, especially SN1.

Some benefits of resonance stabilization include:

• Increases the lifetime of intermediates in a chemical reaction by distributing the positive charge over multiple atoms.
• Allows a molecule such as an alkyl halide to react through an SN1 mechanism by stabilizing the intermediate ion.
• Is often seen in compounds with aromatic rings, influencing their reactivity significantly.

In option (d), resonance stabilization is a key factor that makes the compound very reactive in SN1 reactions, contributing to its overall increased reactivity in both SN1 and SN2 pathways.