Question

Give structures and names of the principal organic products expected from the reaction of n-propylbenzene with each of the following: (1) \(\mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}, \mathrm{H}_{2} \mathrm{SO}_{4}\), heat (3) \(\mathrm{Cl}_{2}, \mathrm{Fe}\) (2) \(\mathrm{HNO}_{3}, \mathrm{H}_{2} \mathrm{SO}_{4}\) (4) \(\mathrm{Br}_{2}\), heat, light (5) cyclohexene, \(\mathrm{HF}\)

Short answer

(1) Propylbenzoic acid (3) 1-chloro-4-n-propylbenzene (2) 4-nitro-n-propylbenzene (4) 3-bromo-n-propylbenzene (5) n-propylbenzene connected to cyclohexene ring

Step-by-step solution

(1) Reaction with \(K_2Cr_2O_7\), \(H_2SO_4\), and heat

When n-propylbenzene reacts with \(K_2Cr_2O_7\), \(H_2SO_4\), and heat, the principal reaction is oxidation. The chromic acid formed from \(K_2Cr_2O_7\) and \(H_2SO_4\) oxidizes the n-propyl side chain to form a carboxylic acid attached to the benzene ring. The major product for this reaction is propylbenzoic acid.

(3) Reaction with \(Cl_2\), \(Fe\)

When n-propylbenzene reacts with \(Cl_2\) in the presence of a catalyst \(Fe\), the principal reaction is electrophilic aromatic substitution. Chlorine, acting as an electrophile, will attach to the benzene ring at its most reactive position. Since the propyl group is an ortho-para director, the major product for this reaction is 1-chloro-4-n-propylbenzene.

(2) Reaction with \(HNO_3\), \(H_2SO_4\)

When n-propylbenzene reacts with \(HNO_3\) and \(H_2SO_4\), the principal reaction is nitration, another example of electrophilic aromatic substitution. The nitronium ion, \(\mathrm{NO}_{2}^{+}\), generated from \(HNO_3\) and \(H_2SO_4\), will attack the benzene ring at its most reactive position. As the propyl group is an ortho-para director, the major product for this reaction is 4-nitro-n-propylbenzene.

(4) Reaction with \(Br_2\), heat, light

When n-propylbenzene reacts with \(Br_2\) under heat and light, the principal reaction is free radical bromination. The light initiates the formation of bromine radicals, which abstract a hydrogen atom from the n-propyl side chain, forming a new radical at the alkyl group. Subsequent radical recombination results in bromine addition. Since the reaction occurs at the allylic position, the major product for this reaction is 3-bromo-n-propylbenzene.

(5) Reaction with cyclohexene, \(HF\)

When n-propylbenzene reacts with cyclohexene in the presence of \(HF\), the principal reaction is an acid-catalyzed alkylation of the aromatic ring by the alkene using Friedel-Crafts mechanism. The major product of this reaction is an isopropylbenzene connected to a cyclohexyl group, or n-propylbenzene connected to the cyclohexene ring.

Key concepts

Oxidation of Alkylbenzenes

When an alkylbenzene such as n-propylbenzene undergoes oxidation, the side chain adjacent to the benzene ring is typically transformed into a carboxylic acid group, if at least one hydrogen atom is present on the alpha-carbon (the carbon directly attached to the benzene ring). During the oxidation, strong oxidizing agents like potassium dichromate (K_2Cr_2O_7) in the presence of sulfuric acid (H_2SO_4) and heat are often used to facilitate the reaction.

The oxidation process transforms the n-propyl group into propylbenzoic acid. This reaction is particularly important in organic synthesis as it provides a pathway to introduce functional groups onto the benzene ring, thereby expanding the possibilities for further chemical modifications.

• Potassium dichromate (K_2Cr_2O_7) and sulfuric acid (H_2SO_4) are commonly used oxidizing agents in this reaction.
• Heat acts as a catalyst to accelerate the reaction rate.
• The oxidation typically targets the alpha-carbon of the alkyl side chain.

Electrophilic Aromatic Substitution

Electrophilic aromatic substitution is a fundamental reaction mechanism in organic chemistry where an electrophile replaces a hydrogen atom on a benzene ring. This type of reaction is central to the chemistry of aromatic compounds, including n-propylbenzene.

In one scenario, chlorine (Cl_2) can be introduced to the benzene ring in the presence of iron (Fe) to form chlorinated products, where chlorine acts as the electrophile. Similarly, nitration, which involves introducing a nitro group (NO_2), follows the same pattern of reactivity using nitric (HNO_3) and sulfuric (H_2SO_4) acids to generate the nitronium ion (NO_2^+), which then acts as the electrophile.

• The position of substitution is usually directed by the substituent already present on the benzene ring.
• n-Propyl group directs further substitution to the ortho and para positions.

Free Radical Bromination

Free radical bromination is a type of chain reaction that is initiated by heat or light, which induces homolytic cleavage in molecular bromine (Br_2), generating highly reactive bromine atoms. These bromine atoms can abstract a hydrogen atom from aliphatic side chains of aromatic compounds such as n-propylbenzene, resulting in the formation of a carbon-centered radical.

This radical can then react with another bromine molecule to give a brominated alkylbenzene. However, the reaction tends to proceed at the most stable radical site, often that is the allylic position (the carbon atoms adjacent to a double bond), leading to products like 3-bromo-n-propylbenzene.

• Light or heat is required to generate bromine radicals.
• The reaction typically occurs at the allylic position of the side chain.

Friedel-Crafts Alkylation

The Friedel-Crafts alkylation reaction involves adding alkyl groups to an aromatic ring, such as in the reaction between n-propylbenzene and cyclohexene. Hydrogen fluoride (HF) is used as a catalyst, and the mechanism involves the formation of a carbocation intermediate from the alkene, which then interacts with the benzene ring.

This reaction is useful for introducing alkyl groups onto benzene rings, which may be otherwise difficult to attach due to the stability of the benzene molecule. The Friedel-Crafts alkylation can involve rearrangements if more stable carbocations can form, which can lead to a variety of structural possibilities in the final product.

• Hydrogen fluoride (HF) acts as a catalyst in this reaction.
• Rearrangement of the carbocation intermediate to a more stable carbocation is possible.