Problem 106 - A Complete Resource Book in Chemistry for JEE Main

Question

Propyl benzene reacts with bromine in presence of light or heat to give (a) CC(Br)Cc1ccccc1 (b) CCC(Br)c1ccccc1 (c) CCCc1ccccc1Br (d) BrCCCc1ccccc1

Step-by-Step Solution

Answer

The major product is (b) CCC(Br)c1ccccc1.

Step 1: Understand the Reaction Type

The reaction involves propyl benzene and bromine in the presence of light or heat. This is a free radical substitution reaction, where a bromine atom replaces a hydrogen atom after the formation of free radicals.

Step 2: Identify the Most Reactive Site

In free radical halogenation, tertiary hydrogens are more reactive than primary or secondary hydrogens. Propyl benzene has a side chain with a tertiary carbon atom, which will preferentially form a radical.

Step 3: Predict the Major Product

The bromine radical will react with the hydrogen atoms bonded to the tertiary carbon in the propyl group (the middle carbon), creating a more stable benzylic radical. This radical will then react with another bromine molecule to form the final product with a bromine atom at the tertiary carbon.

Step 4: Match with Given Options

The major product of this reaction is the third option, (b) CCC(Br)c1ccccc1, where bromine is attached to the middle carbon in the propane chain of propyl benzene.

Key Concepts

Propyl Benzene

Propyl benzene is a type of aromatic compound, which features a benzene ring connected to a propyl group. The propyl group is a chain of three carbon atoms, represented as \( \text{C}_3\text{H}_7 \). Think of it as a little tail attached to the benzene ring. In the structure of propyl benzene, this propyl group can attach to different positions on the benzene ring, but it's important in these reactions to focus on the positioning of the carbon atoms in the side chain.

This side chain is crucial for understanding how reactions like bromination occur, as it can influence which hydrogen atoms are more likely to be replaced by other atoms or groups in reactions. The propyl chain in propyl benzene can form a primary, secondary, or even a tertiary position depending on which carbon atom is bonded to the benzene ring. In the context of halogenation reactions, identifying whether the carbons in the propyl group are primary, secondary, or tertiary helps predict reaction outcomes.

Bromination Reaction

Bromination involved in the free radical halogenation process requires a source of energy, such as heat or light, to initiate the reaction. This type of reaction pertains to substitution, where a bromine atom takes the spot of a hydrogen atom in the organic molecule. During this reaction, the bromine molecules dissociate into bromine radicals under the presence of light or heat. This initial step is known as the initiation step.

Once the radicals are formed, these highly reactive species attack hydrogen atoms in the propyl benzene. The choice of the hydrogen atom for attack largely depends on the stability of the resulting radical. This attack part of the process is called the propagation step. After substituting a hydrogen atom and forming a more stable radical, the reaction continues to progress and eventually reaches completion with the formation of the desired alkyl bromide product.

Radical Stability

Radical stability is a key factor in determining the major product of a free radical halogenation reaction. When a radical forms during halogenation, the stability of that radical can determine how the reaction progresses. In general, radical stability follows the order: tertiary > secondary > primary. This means that radicals formation on tertiary carbons is favored as they are more stable than those on secondary or primary carbons.

In the case of propyl benzene during bromination, the tertiary carbon in the propyl side chain is the most favored site for radical formation. This is because tertiary carbon radicals benefit from hyperconjugation and the inductive effect, both of which provide additional electron density to the radical, making it more stable. In the exercise, this stable radical formation leads to the bromine atom being placed at the most reactive site in the propyl chain, which results in the major product of the reaction.