Problem 30 - Advanced Problems in Organic Chemistry for Competitive Examinations

Question

Which of the following is most reactive towards aqueous HBr? (a) 1-Phenyl-2-propanol (b) 1-Phenyl-1-propanol (c) 3-Phenyl-1-propanol (d) 2-Phenyl-1-propanol

Step-by-Step Solution

Answer

(b) 1-Phenyl-1-propanol is most reactive due to the formation of a stable tertiary carbocation.

Step 1: Understanding Reactivity with HBr

When alcohols react with aqueous HBr, they undergo a substitution reaction in which the hydroxyl group is replaced by a bromide ion. The reactivity of alcohols in such reactions is influenced by the stability of the carbocation intermediate formed during the reaction. Hence, tertiary carbocations are more stable and reactive compared to secondary, which are more reactive than primary.

Step 2: Analyzing Each Compound

- (a) **1-Phenyl-2-propanol**: The hydroxyl group is on a secondary carbon, but with no direct strong stabilization from a phenyl group proximity for carbocation stability. - (b) **1-Phenyl-1-propanol**: The hydroxyl is on a tertiary carbon, offering more stability due to resonance with the phenyl group, leading to a stable tertiary carbocation. - (c) **3-Phenyl-1-propanol**: The hydroxyl is on a primary carbon which generally forms the least stable carbocation. - (d) **2-Phenyl-1-propanol**: The hydroxyl is on a secondary carbon, with potential resonance stabilization but less than a tertiary carbocation.

Step 3: Identifying the Most Reactive

1-Phenyl-1-propanol forms a tertiary carbocation when the -OH group is protonated and leaves, allowing for the most stable intermediate due to resonance stabilization with the phenyl group. This stabilization makes it the most reactive towards aqueous HBr.

Key Concepts

Carbocation Stability

Carbocation stability is a central concept when discussing reactivity in organic chemistry, particularly in substitution reactions involving alcohols. When an alcohol undergoes a substitution reaction with HBr, the hydroxyl group is replaced by a bromide ion, forming a carbocation as an intermediate. The stability of this carbocation dramatically influences how fast and efficiently the reaction proceeds.
A tertiary carbocation, where the positively charged carbon is attached to three other carbon atoms, is much more stable than a secondary carbocation, and significantly more stable than a primary one. This stability can be attributed to the ability of surrounding carbon atoms to donate electron density and help disperse the positive charge, a phenomenon known as hyperconjugation. Moreover, the presence of nearby groups that can participate in resonance also enhances stability by allowing the charge to be spread over a larger area.

Substitution Reactions

Substitution reactions are a type of chemical reaction where an atom or group of atoms in a molecule is replaced by another atom or group of atoms. In the context of alcohols reacting with HBr, the hydroxyl (-OH) group is replaced by a bromide ion (Br^-).
The reaction typically involves three main steps:

• The protonation of the alcohol, where the hydroxyl group becomes a good leaving group as water.
• The formation of a carbocation intermediate after the water molecule departs.
• The subsequent attack by the bromide ion (a nucleophile) on the carbocation, forming the alkyl bromide product.

Tertiary alcohols generally react faster than secondary or primary alcohols due to the greater stability of the tertiary carbocation formed during the reaction.

Phenyl Group Influence

The phenyl group, a ring of carbon atoms, plays a crucial role in carbocation stabilization through resonance. Resonance allows electrons to be delocalized over a larger area, effectively stabilizing adjacent positive charges such as those found in carbocations.
When a phenyl group is directly attached to the carbon of a carbocation, it can engage in resonance stabilization. This makes compounds like 1-Phenyl-1-propanol particularly reactive. The tertiary carbocation formed here not only benefits from typical tertiary stability but also gains additional stabilization from the resonance with the phenyl group. This dual stabilization effect significantly boosts the overall reactivity of such compounds toward substitution reactions.

Tertiary vs. Primary Alcohols

The classification of alcohols into tertiary, secondary, and primary types is based on the number of alkyl groups attached to the carbon bearing the hydroxyl group.

• **Tertiary alcohols**: These have three alkyl groups attached to the hydroxyl-bearing carbon, making the resultant carbocation after protonation very stable. This high stability often results in faster and more successful substitution reactions, as seen with 1-Phenyl-1-propanol.
• **Primary alcohols**: These have only one alkyl group attached, leading to the formation of less stable carbocations. As a result, primary alcohols typically react much more slowly than their tertiary counterparts when exposed to HBr.

This difference in stability leads to markedly different reactivity patterns, with tertiary alcohols generally exhibiting higher reactivity than primary alcohols when subjected to the same chemical conditions.