Question

Chlorination of toluene in the presence of light and heat followed by treatment with aqueous \(\mathrm{NaOH}\) gives (a) p-cresol (b) o-cresol (c) benzoic acid (d) 2,4 -dihydroxytoluene

Short answer

(c) benzoic acid

Step-by-step solution

Identify the Reaction Type

The reaction of toluene with chlorine in the presence of light involves free-radical halogenation. This usually leads to the substitution of hydrogen atoms in the methyl group of toluene to form benzyl chloride.

Analyze the Conditions

The presence of light and heat indicates a radical reaction. Chlorine radicals will substitute hydrogen atoms from the side chain of toluene rather than the aromatic ring.

Understand the Product Formation

The free-radical chlorination of toluene forms benzyl chloride when hydroxylated with aqueous NaOH. The hydroxide ions substitute the chlorine atoms, converting benzyl chloride to benzyl alcohol.

Examine Further Hydroxylation

Continued reaction with aqueous NaOH can further convert benzyl alcohol to benzoic acid through oxidation, depending on conditions. This means that benzoic acid can form as an end product.

Conclude with Final Product Analysis

Given the pathway determined by the reaction conditions and sequence, benzoic acid is the most likely product from further hydroxylation and oxidation processes of the initial benzyl chloride compound.

Key concepts

Reaction Mechanism

In free-radical halogenation, a reaction mechanism not only describes what happens in a chemical reaction but also how it happens. It details every step from the reactants to the final products. This particular mechanism involves a series of steps initiated by the presence of light and heat, which generate free radicals capable of participating in various reactions.

• Initiation: Light provides the energy required to dissociate chlorine molecules into two chlorine radicals. This step requires enough energy to break the Cl-Cl bond, resulting in two neutral chlorines, each with an unpaired electron.
• Propagation: These radicals are highly reactive. A chlorine radical will react with the hydrogen atom of toluene's methyl group, forming hydrogen chloride and a benzyl radical. This new radical then reacts with a chlorine molecule to form benzyl chloride and another chlorine radical, propagating the chain reaction.
• Termination: Eventually, the radicals react to form stable molecules, thus halting the chain process. Termination can occur when two radicals combine, destroying the reactive intermediates.

Understanding each step of the reaction mechanism allows chemists to control and predict the products safely and efficiently.

Organic Chemistry

Organic chemistry is the branch of chemistry that deals with the structure, properties, and reactions of organic compounds, which are primarily compounds containing carbon atoms. In this field, free-radical halogenation is a common reaction type. It involves substituting a hydrogen atom in an organic compound with a halogen atom, such as chlorine or bromine. This substitution plays a critical role in organic synthesis.

• Hydrocarbons: Organic compounds primarily made of hydrogen and carbon, like toluene, serve as the base for many reactions in organic chemistry.
• Functional Group Transformation: The reactions often involve converting one functional group to another, such as from alkanes to alcohols, which is observed when benzyl chloride transforms into benzyl alcohol upon treatment with NaOH.
• Synthetic Applications: Understanding these reactions helps in synthesizing complex molecules in pharmaceuticals and materials chemistry.

Thus, mastering these reactions can enable the creation of new compounds with desirable chemical and physical properties.

Chlorination of Hydrocarbons

Chlorination of hydrocarbons, like toluene, is a critical process in both industrial and academic research settings. This process, especially within the realm of free-radical mechanisms, targets the alkyl side chains attached to aromatic rings rather than the stable aromatic system itself.

• Selective Chlorination: The reaction prefers substituting hydrogen atoms in the side chains over those in the aromatic ring, resulting in the formation of compounds like benzyl chloride instead of chlorobenzene.
• Role of Light and Heat: This reaction typically requires external energy, such as light and heat, to generate chlorine radicals needed for the initiation step.
• Importance of Conditions: The careful control of reaction conditions, including temperature and chlorine concentration, affects the selectivity and extent of chlorination, guiding the reaction toward the desired product.

The ability to selectively chlorinate hydrocarbons allows chemists to add functionality to simpler organic molecules, facilitating the production of valuable end products like benzoic acid.