Problem 27 - A Complete Resource Book in Chemistry for JEE Main

Question

On the addition of \(\mathrm{HBr}\) to propene in the absence of peroxides, the first step involves the addition of (a) \(\mathrm{H}^{+}\) (b) \(\mathrm{Br}^{-}\) (c) \(\dot{H}\) (d) \(\mathrm{Br}\)

Step-by-Step Solution

Answer

(a) \\(H^+\\) adds first to propene in the reaction.

Step 1: Understanding the Reaction

When \(HBr\) adds to propene without peroxides, it typically follows the Markovnikov's rule. This rule states that in an electrophilic addition reaction, the positive part of the adding reagent (here \(H^+\)) bonds to the carbon with more hydrogen atoms, while the negative part bonds to the other carbon.

Step 2: Identifying the Electrophile

In the addition of \(HBr\) to an alkene like propene, \(H^+\) acts as the electrophile because it seeks electrons. Since hydrogen is more positive in an \(HBr\) molecule, it tends to add first, leading to the formation of a carbocation on the alkene.

Step 3: Select the Correct Option

Based on the mechanism described, the first step of this reaction involves the addition of the electrophile \(H^+\), making option (a) the correct choice.

Key Concepts

Electrophilic Addition

The concept of electrophilic addition is an essential mechanism found in organic chemistry, particularly when dealing with alkenes. In this type of reaction, an electrophile, which is an atom or molecule that is electron-deficient, interacts with a compound possessing a double bond, like an alkene. This process involves two main steps:

• The formation of a carbocation intermediate by the initial addition of the electrophile to the alkene.
• The subsequent combination of the carbocation with a nucleophile, which in this context is an electron-rich species.

The reaction begins when the electrophile (\(H^+\)in our example) approaches the electron-rich double bond of the alkene. This interaction often leads to the electrons from the double bond being donated to the electrophile, thus forming the carbocation intermediate. This is a crucial part of Markovnikov's Rule, which guides the regioselectivity of electrophilic additions.

Alkenes

Alkenes are hydrocarbons featuring at least one carbon–carbon double bond. This double bond is characterized by significant electron density compared to single bonds, making alkenes reactive sites for many reactions. Because of this electron-rich zone, they act as nucleophiles, seeking out and reacting with electrophilic species.
Alkenes behave uniquely in reactions because the double bond provides both stability and reactivity. This dual nature is essential when considering reactions like electrophilic addition. As the double bond reacts with the electrophile, one of its pairs of electrons forms a new single bond, altering the compound's structure significantly. Understanding this process is crucial when predicting how alkenes will behave in various chemical reactions.

Carbocation Formation

Carbocation formation is a pivotal moment in many organic reactions, such as when an electrophile adds to an alkene. A carbocation is a positively charged carbon atom with only three bonds, making it highly reactive. In the case of the addition of \(HBr\) to propene, the initial phase involves the electrophile \(H^+\) bonding to the double bond, resulting in the creation of a carbocation.
Carbocations can be categorized based on the number of alkyl groups attached to the positively charged carbon:

• Primary carbocations have one alkyl group.
• Secondary have two.
• Tertiary have three.

Typically, more alkyl groups imply greater carbocation stability because these groups help disperse the positive charge. In propene's reaction with \(HBr\), the most stable carbocation forms in line with Markovnikov's rule, showing how these principles work together to drive organic reactions.