Problem 13 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

HBr reacts fastest with : (a) 2-methylpropan-2-ol (b) propan-1-ol (c) propan-2-ol (d) 2-methylpropan-1-ol

Step-by-Step Solution

Answer

(a) 2-methylpropan-2-ol reacts fastest with HBr due to forming a stable tertiary carbocation.

Step 1: Identify the Reaction Type

The reaction between HBr and an alcohol is generally a substitution reaction where the hydroxyl group (-OH) is replaced by a bromine atom (Br). This type of reaction proceeds faster with more stable carbocations.

Step 2: Analyze Carbocation Stability

The stability of the carbocation intermediate is key to the speed of the reaction. Tertiary carbocations are more stable than secondary, which are more stable than primary due to hyperconjugation and inductive effects from surrounding alkyl groups.

Step 3: Evaluate Each Option

Let's evaluate the carbocation formed from each alcohol during the reaction: - (a) 2-methylpropan-2-ol forms a tertiary carbocation. - (b) propan-1-ol forms a primary carbocation. - (c) propan-2-ol forms a secondary carbocation. - (d) 2-methylpropan-1-ol forms a primary carbocation.

Step 4: Determine the Fastest Reacting Alcohol

Based on the evaluated carbocation stability, the alcohol that forms the most stable carbocation will react fastest. Therefore, 2-methylpropan-2-ol, which forms a tertiary carbocation, will react fastest with HBr.

Key Concepts

Carbocation Stability

In organic chemistry, the stability of a carbocation is a crucial factor in determining the rate of many reactions, including substitution reactions. A carbocation is a positively charged carbon atom, and its stability is influenced by the number of alkyl groups attached to it.
Each alkyl group provides an electron-donating effect called hyperconjugation, which disperses the positive charge.
This effect stabilizes the carbocation. In comparison, tertiary carbocations, which have three alkyl groups, are more stable than secondary carbocations with two, and primary carbocations with only one.

• Tertiary Carbocation: The most stable, benefiting from maximal hyperconjugation and inductive effects.
• Secondary Carbocation: Less stable, with moderate hyperconjugation.
• Primary Carbocation: The least stable, with minimal hyperconjugation.

The stability of the carbocation plays a critical role in reaction kinetics. A more stable carbocation forms more readily, leading to faster reactions. So, when considering the speed of reactions, such as the substitution of alcohols by HBr, carbocation stability provides valuable insight.

Substitution Reaction

Substitution reactions are a fundamental type of chemical reaction where one atom or group of atoms in a molecule is replaced by another atom or group of atoms. In the case of alcohols reacting with HBr, we're dealing with a nucleophilic substitution reaction.
This specific process involves an alcohol group (-OH) being replaced by a bromine atom (Br). Such reactions follow a path that often includes the formation of a carbocation intermediate, particularly when tertiary or secondary alcohols are involved.

• SN1 Mechanism: This is a two-step mechanism predominantly seen in tertiary or secondary alcohols. It starts with the formation of a carbocation intermediate after the hydroxyl group leaves, followed by the nucleophilic attack by bromide.
• SN2 Mechanism: A one-step mechanism where a nucleophile attacks the alcohol and the departure of the leaving group occurs simultaneously. It is more common with primary alcohols due to the lack of carbocation intermediate formation.

Substitution reactions vary greatly in rates, influenced by factors like the structure of the alcohol, the nature of the leaving group, and the strength of the nucleophile.
Understanding the mechanism can help predict how swiftly or slowly a substitution reaction will proceed.

Alcohol Reactivity

Alcohols display a range of reactivities in chemical reactions, primarily affected by molecular structure and the type of substituents present. Their reactivity in substitution reactions, such as those with hydrogen bromide (HBr), is generally determined by the ability to form stable carbocations.
Different alcohols behave differently due to their structure
, which in turn affects how readily they can participate in substitution reactions.

• Tertiary Alcohols: Characterized by three alkyl groups attached to the carbon with the hydroxyl, making them the most reactive in substitution reactions due to the formation of stable tertiary carbocations.
• Secondary Alcohols: Have two alkyl groups attached, offering moderate reactivity as they form intermediate stability carbocations.
• Primary Alcohols: Less reactive in typical substitution processes due to the formation of less stable primary carbocations.

By understanding these variations in alcohol reactivity, one can predict the outcome of their reactions with acids like HBr. Tertiary alcohols tend to react fastest, forming stable carbocations, while primary alcohols are less reactive due to their unstable carbocations.
Such insights are crucial in synthetic organic chemistry for planning efficient synthesis routes.