Case: Read the passage and answer the following questions

Alcohol can be prepared in many ways. Alcohols are prepared from alkenes by acid catalysed hydration and by hydroboration–oxidation. Alcohols are even prepared from ketones and aldehydes by reduction. The reduction of carboxylic acid and ester leads to the formation of alcohol. Alcohols are produced by the reaction of Grignard reagents with aldehydes and ketones. Phenol, also known as carbolic acid, was first isolated in the early nineteenth century from coal tar. Nowadays, phenol is commercially produced synthetically. In the laboratory, phenols are prepared from benzene derivatives in many ways.

The process of converting alkyl halides into alcohols involves_____________.

Substtution reaction

The correct answer is option 2. Substtution reaction.

Let us go through the process of converting alkyl halides into alcohols in detail, focusing on why it is a substitution reaction.

Alkyl Halide \((R-X)\) A compound where an alkyl group \((R)\) is bonded to a halogen atom (\(X\), such as chlorine, bromine, or iodine).

Mechanism of Substitution Reaction:

In this context, substitution refers to replacing the halogen atom \((X)\) in the alkyl halide with a hydroxyl group \((-OH)\) to form an alcohol \((R-OH)\).

Types of Substitution Reactions:

There are two main types of substitution reactions that can occur with alkyl halides:

1. \(S_N1\) (Unimolecular Nucleophilic Substitution): Mechanism:

The alkyl halide undergoes a two-step process.

The halogen leaves first, forming a carbocation intermediate.

The hydroxide ion (OH⁻) then attacks the carbocation, forming the alcohol.

Example: Tertiary alkyl halides (3°) commonly undergo \(S_N1\) reactions due to the stability of the carbocation intermediate.

2. \(S_N2\) (Bimolecular Nucleophilic Substitution): Mechanism:

The hydroxide ion (OH⁻) directly attacks the carbon atom bonded to the halogen from the opposite side, displacing the halogen in a single step. This reaction proceeds through a backside attack, leading to the inversion of configuration if the carbon is chiral.

Example: Primary (1°) and secondary (2°) alkyl halides commonly undergo \(S_N2\) reactions due to the lesser steric hindrance around the carbon.

General Reaction:

\(\text{R-X} + \text{OH}^- \rightarrow \text{R-OH} + \text{X}^-\)

Reactants: Alkyl halide \((R-X)\) and hydroxide ion \((OH^-)\).

Products: Alcohol \((R-OH)\) and halide ion \((X⁻)\).

Why It Is a Substitution Reaction:

Substitution: The halogen atom \((X)\) in the alkyl halide is replaced (substituted) by the hydroxyl group \((OH^-)\).

Nucleophilic Attack: The hydroxide ion \((OH^-)\) acts as a nucleophile, attacking the carbon atom bonded to the halogen.

Leaving Group: The halogen atom \((X)\) leaves as a halide ion \((X^-)\).

Example: Conversion of Bromoethane to Ethanol:

Bromoethane (CH₃CH₂Br) reacts with hydroxide ion (OH⁻):

\(\text{CH}_3\text{CH}_2\text{Br} + \text{OH}^- \rightarrow \text{CH}_3\text{CH}_2\text{OH} + \text{Br}^- \)

Product: Ethanol (CH₃CH₂OH) and bromide ion (Br⁻).

Other Reaction Types (For Comparison):

Addition Reaction: Involves adding elements to a molecule, usually without removing any part of it. Common in reactions involving alkenes and alkynes.

Hydrogenation Reaction: Involves adding hydrogen (H₂) to a molecule, typically used to convert alkenes or alkynes to alkanes.

Dehydrogenation Reaction: Involves removing hydrogen from a molecule, typically used to convert alkanes to alkenes or alcohols to aldehydes/ketones.

Conclusion:

The process of converting alkyl halides into alcohols is a substitution reaction because it involves the replacement of the halogen atom (X) in the alkyl halide with a hydroxyl group (OH⁻).