Problem 181 - A Complete Resource Book in Chemistry for JEE Main

Question

Match the following Column-I (a) Neohexyl alcohol (b) Sec -butyl alcohol (c) n- hexyl alcohol (d) t- butyl alcohol\begin{aligned} &\text { Column-II } \\ &\hline \text { (p) fastest reaction with } \\ &\text { Luca's reagent } \\ &\text { (q) highest boiling point } \\ &\text { (r) positive iodoform } \\ &\text { test } \\ &\text { (s) highest water } \\ &\text { solubility } \\ &\text { (t) susceptible to } \\ &\text { chromate ( }\left(\mathrm{CrO}_{3} / \mathrm{H}^{+}\right) \\ &\text {oxidation } \\ &\hline \end{aligned}

Step-by-Step Solution

Answer

(a) Neohexyl alcohol: (s) (b) Sec-butyl alcohol: (r) & (t) (c) n-Hexyl alcohol: (q) (d) t-Butyl alcohol: (p)

Step 1: Analyze Neohexyl Alcohol

Neohexyl alcohol has a branched structure. It is a tertiary alcohol.

Step 2: Analyze Sec-butyl Alcohol

Sec-butyl alcohol is a secondary alcohol, meaning it has two alkyl groups attached to the carbon bearing the -OH group. This structure allows it to be oxidized by chromates and gives a positive iodoform test.

Step 3: Analyze n-Hexyl Alcohol

n-Hexyl alcohol is a straight-chain primary alcohol, which generally has the highest boiling point due to strong intermolecular forces among the options given.

Step 4: Analyze t-Butyl Alcohol

t-Butyl alcohol is a tertiary alcohol and thus reacts instantly with Lucas reagent to form a cloudiness, indicating the fastest reaction.

Key Concepts

Lucas Reagent Reaction

The Lucas reagent is commonly used to classify alcohols based on their reactivity. This reagent is a mixture of concentrated hydrochloric acid and zinc chloride.
The reaction with Lucas reagent tests the alcohol's ability to form alkyl halides.

• The key here is that tertiary alcohols react the fastest with Lucas reagent, forming an insoluble alkyl chloride that causes the solution to turn cloudy.
• Secondary alcohols react more slowly, while primary alcohols show little to no reaction at room temperature.

For instance, t-butyl alcohol is a tertiary alcohol and reacts swiftly with Lucas reagent, causing the solution to become cloudy almost instantly.

Iodoform Test

The iodoform test is a qualitative chemical test that is used to detect the presence of certain types of alcohols. Specifically, it checks for alcohols that can be oxidized to methyl ketones—most notably secondary alcohols with the formula \(CH_3CH(OH)R\).

• Secondary alcohols containing at least one methyl group adjacent to the hydroxyl group will give a positive iodoform test.
• This results in the formation of yellow, crystalline iodoform (CH\(_3\)I\(_3\)).

Sec-butyl alcohol, being a secondary alcohol with the correct structure, gives a positive iodoform test. This characteristic helps in differentiating it from other types of alcohols.

Alcohol Oxidation

Alcohol oxidation refers to the chemical process where an alcohol loses electrons, and usually gains oxygen or loses hydrogen.
The degree of oxidation depends heavily on the type of alcohol:

• Primary alcohols are oxidized to aldehydes and can further oxidize to carboxylic acids.
• Secondary alcohols are oxidized to ketones.
• Tertiary alcohols generally do not undergo oxidation because they lack the necessary hydrogen atom on the carbon containing the hydroxyl group.

For instance, sec-butyl alcohol can be oxidized by chromates to form a ketone. This property aids in identifying secondary alcohols.

Boiling Point of Alcohols

The boiling point of alcohols is influenced by the ability of these compounds to form hydrogen bonds.

• Primary alcohols generally have higher boiling points than their secondary and tertiary counterparts because they can form more extensive hydrogen bonding networks.
• This leads to a higher amount of energy required to separate the molecules.

N-hexyl alcohol, being a primary alcohol with a straight chain, has the highest boiling point among the given options due to these strong intermolecular forces and an unbranched structure, which permits extensive hydrogen bonding.

Alcohol Solubility

Alcohol solubility in water is primarily affected by two factors: the length of the carbon chain and the presence of hydrogen bonding.

• Shorter carbon chains typically mean higher solubility in water, mainly because the hydroxyl group can form hydrogen bonds with water molecules.
• As the carbon chain increases in length, the solubility generally decreases since the hydrophobic alkyl part outweighs the hydrophilic nature of the hydroxyl group.

T-butyl alcohol, due to its tertiary structure with a relatively smaller carbon chain compared to other alcohols, is quite soluble in water. The bulky tert-butyl group ensures that despite the tertiary structure, solubility remains high.