Problem 93 - A Complete Resource Book in Chemistry for JEE Main

Question

The reaction of propene with HOCl proceeds via the addition of (a) \(\mathrm{H}^{+}\)in the first step (b) \(\mathrm{Cl}^{+}\)in the first step (c) \(\mathrm{OH}\) - in the first step (d) \(\mathrm{Cl}^{+}\)and \(\mathrm{OH}^{-}\)in a single step

Step-by-Step Solution

Answer

The first step involves the addition of Cl+ (option b).

Step 1: Identify the Reagents

Identify the reagents involved in the reaction. In this case, propene (CH2=CH-CH3) is reacting with HOCl, which is hypochlorous acid.

Step 2: Understand the Reaction Mechanism

Understand how the reaction typically proceeds. The addition of HOCl to an alkene such as propene follows a Markovnikov addition, where the OH group attaches to the more substituted carbon atom, and the Cl attaches to the less substituted one.

Step 3: Determine the First Intermediate

In general reactions of HOCl, the molecule can split into H+ and OCl-. However, the Cl+ ion is an active electrophile in reactions with alkenes. This cation seeks out areas of high electron density to react.

Step 4: Identify the Correct First Step

In the reaction with propene, Cl+ can initially add to the double bond as it targets the electron-rich pi bond, causing the formation of a chloronium ion intermediate.

Step 5: Conclusion on Initial Step for HOCl

From the analysis, the correct first step in the reaction of propene with HOCl is the addition of Cl+. This aligns with (b) \(\mathrm{Cl}^{+}\) in the first step.

Key Concepts

Markovnikov addition

When discussing reactions like the addition of HOCl to an alkene such as propene, one key principle is the Markovnikov addition. This rule helps predict the site of attachment for different groups in reactions involving asymmetric alkenes.
In the context of this reaction, Markovnikov addition dictates that the hydroxyl group (-OH) will bind to the carbon atom with more substituents, while the chlorine atom will bind to the less substituted one.
This preference occurs because more substituted carbon forms a more stable carbocation (though here a different intermediate is involved), which further stabilizes the reaction through hyperconjugation and the inductive effect.

• This rule applies because the more substituted carbocation formed has greater stabilization due to the proximity of electron-releasing alkyl groups.
• Even though a carbocation is not formed in this specific reaction, understanding Markovnikov's rule is essential for grasping the general mechanism of electrophilic additions.

This behavior ensures that the reaction's product is stable and formed under typical conditions, following predictable patterns of reactivity.

Electrophilic addition

The addition of HOCl to an alkene is an example of electrophilic addition, a fundamental type of reaction in organic chemistry where alkenes react with electrophiles.
An electrophile is a species that seeks out electron-rich areas in molecules. In our case, the chlorine ion ( +Cl ) acts as the electrophile.
Initially, the chlorine ion approaches the alkene's double bond, a region rich in electrons owing to pi bonding, and forms a novel bond with the carbon atom of the alkene, resulting in an intermediate.

• This step fixes the path of the reaction, with the electrophile leading the charge in breaking the double bond.
• The process proceeds as the nucleophile (often the hydroxide ion from the dissociation of HOCl) subsequently bonds with the positively charged carbon.

Understanding this mechanism is crucial because it outlines how intermediates are formed and how the functional groups that finally appear in the product are arranged and bonded.

Chloronium ion intermediate

The formation of a chloronium ion intermediate is a unique twist in the addition of HOCl to alkenes.
Once the electrophilic chlorine ion ( +Cl ) interacts with the electron-rich double bond of propene, a cyclic chloronium ion is created. This intermediate plays a critical role in determining the reaction's pathway and outcome.
This intermediate configuration is a three-membered ring where chlorine forms two covalent bonds with adjacent carbon atoms.

• The chloronium ion is positively charged, and its formation relieves the tension induced by the electron repulsion within the pi bonds.
• The subsequent step involves the opening of this cyclic ion by a nucleophile, usually the hydroxide ion, which attacks the more substituted carbon atom, consistent with Markovnikov addition.

Grasping this intermediate step is essential because it depicts an important divergence from typical carbocation pathways, showing how different ionic and molecular forces shape organic reactions.