Problem 60 - A Complete Resource Book in Chemistry for JEE Main

Question

Isopropyl chloride undergoes hydrolysis by (a) \(\mathrm{S}_{\mathrm{N}} 1\) and \(\mathrm{S}_{\mathrm{N}}^{2}\) mechanisms (b) neither \(\mathrIsopropyl chloride undergoes hydrolysis by (a) \)\mathrm{S}_{\mathrm{N}} 1\( and \)\mathrm{S}_{\mathrm{N}}^{2}\( mechanisms (b) neither \)\mathrm{S}_{\mathrm{N}}_{1}\( nor \)\mathrm{SN}_{2}\( mechanisms (c) \)\mathrm{S}_{\mathrm{N}}_{1}\( mechanism only (d) \)\mathrm{S}_{\mathrm{N}^{2}}\( mechanism onlym{S}_{\mathrm{N}}_{1}\) nor \(\mathrm{SN}_{2}\) mechanisms (c) \(\mathrm{S}_{\mathrm{N}}_{1}\) mechanism only (d) \(\mathrm{S}_{\mathrm{N}^{2}}\) mechanism only

Step-by-Step Solution

Answer

Isopropyl chloride undergoes hydrolysis primarily by the \( S_N1 \) mechanism.

Step 1: Understand the Reaction Type

Isopropyl chloride is a secondary alkyl halide. Hydrolysis refers to the reaction where the halogen is replaced by hydroxide ion (OH-), often proceeding through nucleophilic substitution mechanisms: \( S_N1 \) and \( S_N2 \).

Step 2: Evaluate \( SN1 \) Mechanism

The \( SN1 \) mechanism involves the formation of a carbocation intermediate. Secondary alkyl chlorides, such as isopropyl chloride, can form relatively stable carbocations, making the \( SN1 \) pathway feasible under conditions that favor carbocation formation (e.g., polar protic solvents).

Step 3: Evaluate \( SN2 \) Mechanism

The \( SN2 \) mechanism involves a one-step bimolecular nucleophilic attack. For \( SN2 \) reactions, steric hindrance significantly affects the reaction rate. While secondary alkyl halides can undergo \( SN2 \) reactions, they are less favorable compared to primary alkyl halides due to increased steric hindrance around the carbon atom.

Step 4: Consider Solvent and Reaction Conditions

If we consider aqueous or polar protic solvents, \( SN1 \) becomes more favorable due to carbocation stabilization. \( SN2 \) reactions are favored in polar aprotic solvents, which wouldn't support isopropyl chloride as effectively due to its secondary structure.

Key Concepts

SN1 mechanism

In organic chemistry, reactions that proceed through an SN1 mechanism are characterized by a two-step process. To begin with, you have the formation of a carbocation. This is a positively charged species that results when the leaving group, such as a chloride in isopropyl chloride, departs. This first step is crucial as the formation of the carbocation is the rate-determining step. It means that the speed of the whole reaction depends on how quickly the carbocation can form.

SN1 reactions are typically found in environments that can stabilize the carbocation, such as polar protic solvents like water and alcohols. These solvents help stabilize the intermediate by solvating the carbocation. This makes SN1 mechanisms quite common with secondary alkyl halides, where the resulting carbocation can be more stable due to the electron-donating effects of nearby carbon groups.

• Two-step reaction involving a carbocation intermediate
• Favored by polar protic solvents
• More common with secondary and tertiary alkyl halides due to carbocation stability

SN2 mechanism

The SN2 mechanism is a one-step process that involves a direct displacement of a leaving group by a nucleophile. A key feature of SN2 reactions is that they are bimolecular, involving simultaneous interactions between the substrate and the nucleophile. This kind of reaction is also characterized by an inversion of configuration, often referred to as a "backside attack."

For SN2 reactions, steric hindrance is a critical factor. This means that less crowded environments, such as with primary alkyl halides, favor SN2 reactions since the nucleophile can easily access the carbon atom. In the case of secondary alkyl halides like isopropyl chloride, while SN2 is still possible, it occurs less efficiently due to increased steric hindrance.

Ideal conditions for SN2 include polar aprotic solvents like DMSO and acetone. These solvents do not solvate the nucleophile strongly, allowing it to act more freely and react with the substrate. Secondary alkyl halides present a challenge here because the transition state is significantly hindered compared to primary halides.

• One-step bimolecular reaction
• Involves inversion of configuration
• Affected by steric hindrance; less favorable for secondary alkyl halides

Secondary Alkyl Halide

When discussing secondary alkyl halides, it's essential to focus on the position of the halide group. In a secondary alkyl halide like isopropyl chloride, the halogen replaces a hydrogen on a carbon atom that is connected to two other carbon atoms. This positioning imparts unique properties.

Secondary alkyl halides are more reactive than primary ones but less reactive than tertiary ones when it comes to substitution reactions. This is because the carbon atom bonded to the halogen in secondary alkyl halides has a moderate amount of steric hindrance and carbocation stability. As a result, secondary alkyl halides can undergo both SN1 and SN2 mechanisms, although the reaction mechanism selected often depends on other conditions like the solvent and temperature.

• Carbon bonded to two other carbons
• Moderate reactivity in substitution reactions
• Can participate in both SN1 and SN2 mechanisms

Carbocation Stability

One of the pivotal aspects of understanding SN1 reactions is grasping the concept of carbocation stability. Carbocations are positively charged species with electron deficiency, making them highly reactive. The stability of a carbocation determines the feasibility of SN1 reactions.

Stability is influenced by several factors:

• The number of alkyl groups attached: More alkyl groups mean more electron donation through hyperconjugation, thus providing stability.
• Resonance effects: If the carbocation is allylic or benzylic, resonance can offer additional stabilization.
• The nature of the solvent: Polar protic solvents help stabilize carbocations by solvation, making SN1 reactions more favorable.

In the case of isopropyl chloride, being a secondary alkyl halide implies that the resulting carbocation is stabilized by alkyl groups donating electron density. This stabilization is sufficient for the SN1 mechanism to proceed effectively, especially in a suitable solvent like water.