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

Short answer

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

Step-by-step solution

Understanding Propylbenzene's Reactivity

Propylbenzene is an aromatic hydrocarbon with a benzene ring and a three-carbon propyl side chain. When it reacts with bromine in the presence of light or heat, a free radical substitution reaction occurs on the alkyl chain, typically at the carbon that forms the most stable radical.

Identify Site of Bromination

For free radical bromination, the bromine prefers to add to the most stable radical, which is generally the carbon that results in the most stable intermediate. In a propyl group, the secondary carbon in the middle of the chain forms a more stable radical intermediate than the primary carbons at the ends.

Predict the Product from the Reaction

Given the stability of radicals, the secondary carbon in the propyl chain will preferentially be brominated. This means that the bromine atom will attach to the second carbon of the propyl chain connected to the benzene, resulting in the structure (b) CCC(Br)c1ccccc1.

Confirm the Correct Structure

Verify that the structure predicted aligns with the SMILES notation. The correct product for this reaction is (b), which is described by the SMILES notation as CCC(Br)c1ccccc1, indicating the bromine is on the second carbon of the propyl group adjacent to the benzene ring.

Key concepts

Stability of Radicals

When considering free radical reactions, the stability of the free radicals formed is a key factor in determining the outcome of the reaction. Radicals are highly reactive species with an unpaired electron, and they tend to stabilize themselves by delocalizing this unpaired electron. The more stable a radical is, the more likely it is to form during a reaction.
Secondary radicals, like the one formed at the middle carbon of a propyl group, are generally more stable than primary radicals because the unpaired electron can be spread out across neighboring carbon atoms, reducing the energy of the radical.
Tertiary radicals are even more stable, but in a propylbenzene system, the secondary radical is the most stabilized option available. Therefore, during bromination, it is the most likely site for radical formation and subsequent bromine attachment.

Alkyl side chain substitution

In free radical bromination reactions, the alkyl side chain of an aromatic hydrocarbon undergoes substitution rather than the aromatic ring itself.
This is because the aromatic ring of propylbenzene is highly stable and less reactive compared to the more vulnerable alkyl chain.
The alkyl chain, in this case, propyl, provides a location for the radicals to form, which allows for the substitution to occur. Free radical bromination on the alkyl chain usually targets the most stable radical form, leading to specific substitution at the secondary carbon of the propyl group since this radical has the greatest stability among available carbons.

Aromatic Hydrocarbons

Aromatic hydrocarbons like propylbenzene contain benzene rings, which are stable structures consisting of six carbon atoms arranged in a hexagon with alternating double bonds.
This configuration gives benzene its characteristic stability due to the delocalization of electrons across the ring, making it much less reactive under normal conditions.
While the benzene ring is stable, it does not partake in the free radical bromination because the reaction conditions and the nature of radical intermediates favor substitution at the alkyl side chain instead. This propensity ensures that aromatic hydrocarbons can have reactive alkyl side chains without disrupting the integrity of the aromatic system.

Reactivity of Benzene Derivatives

Benzene derivatives, such as propylbenzene, exhibit a specific type of reactivity due to the presence of additional functional groups or side chains attached to the benzene ring.
The reactivity is largely determined by the nature of these substituents.
In the case of propylbenzene, the propyl side chain alters the substitution pattern during bromination. The benzene ring remains unaffected due to its inherent stability, therefore most reactions occur at the weaker, more reactive alkyl chains. This reactivity pattern is vital in synthetic organic chemistry, allowing chemists to selectively modify molecules by targeting the more reactive side chains of benzene derivatives, thereby maintaining the aromatic system intact while introducing new functional groups.