Problem 180 - A Complete Resource Book in Chemistry for JEE Main

Question

$$ \begin{array}{ll} \hline \text { Column-I } & \text { Column-II } \\ \hline \text { (a) Neohexyl alcohol } & \text { (p) highest boiling point } \\\ \text { (b) Sec-butyl alcohol } & \text { (q) positive iodoform } \\ \text { test } \\ \text { (c) } \mathrm{n} \text { - hexyl alcohol } & \text { (r) highest water } \\ \text { solubility } \\ \text { (d) t- butyl alcohol } & \text { (s) susceptible to } \\ \text { chromate }\left(\mathrm{CrO}_{3} / \mathrm{H}^{*}\right) \\ \text { oxidation } \\ \text { (t) Shows instant reac- } \\ \text { tion with } \mathrm{ZnCl}_{2}+ \\ \text { conc. HCl } \\ \hline \end{array} $$

Step-by-Step Solution

Answer

(a) - (r), (b) - (q), (c) - (p), (d) - (t)

Step 1: Identifying Neohexyl Alcohol Characteristics

Neohexyl alcohol is a primary alcohol with a neopentane structure. Primary alcohols generally have increased solubility compared to secondary and tertiary alcohols due to their ability to form multiple hydrogen bonds with water.

Step 2: Examining Sec-butyl Alcohol

Sec-butyl alcohol, a secondary alcohol, typically gives a positive iodoform test due to the presence of a methyl group adjacent to the hydroxyl-bearing carbon. This signifies it should be matched with property (q).

Step 3: Analyzing n-Hexyl Alcohol

n-Hexyl alcohol has a long and straight carbon chain making it less soluble in water compared to the branched or tertiary alcohols. However, linear alcohols often exhibit higher boiling points due to stronger van der Waals forces. Thus, it would likely have the highest boiling point (p).

Step 4: Considering t-Butyl Alcohol

t-Butyl alcohol is a tertiary alcohol. Tertiary alcohols react instantly with ZnCl2 and concentrated HCl (Lucas test) due to the stability of the resulting carbocation. Therefore, it should match with property (t).

Key Concepts

Neohexyl Alcohol

Neohexyl alcohol is a primary alcohol, derived from a neopentane structure. This unique structure gives it certain characteristics. Primary alcohols, like neohexyl alcohol, have a specific ability to form multiple hydrogen bonds with water molecules.

This results in their increased solubility in water compared to secondary and tertiary alcohols. Neohexyl alcohol has branching in its structure, which affects its oral and dermal bioavailability, but its major feature in the context of solubility is its primary alcohol properties.

Sec-butyl Alcohol

Sec-butyl alcohol is classified as a secondary alcohol because of its hydroxyl group attached to a secondary carbon atom. A distinctive property of sec-butyl alcohol is its ability to give a positive iodoform test.

The iodoform test detects the presence of a methyl group adjacent to the hydroxyl-bearing carbon. This test is an indicator of certain types of alcohols having a specific skeletal structure, making sec-butyl alcohol easily identifiable.

n-Hexyl Alcohol

Given its straight-chain structure, n-hexyl alcohol has a lengthy carbon backbone. This structure results in substantial van der Waals forces, impacting its physical properties like boiling point.

Higher boiling points are usually observed in alcohols with longer chains due to these forces. Despite its increased boiling point, n-hexyl alcohol's linear form makes it less soluble in water compared to branched or tertiary alcohols.

t-Butyl Alcohol

Tertiary alcohols, such as t-butyl alcohol, have distinct chemical behaviors. An important characteristic is demonstrated through the Lucas test. This test involves reactivity with zinc chloride ( ZnCl_2 ) and concentrated hydrochloric acid (HCl).

t-Butyl alcohol's structure provides stability to the resulting carbocation, allowing for an instant reaction in the Lucas test. This rapid reaction distinguishes tertiary alcohols from primary and secondary ones.

Iodoform Test

The iodoform test is a qualitative chemical test for the detection of methyl ketones and secondary alcohols with a methyl group adjacent to the carbon carrying the hydroxyl group. This test involves iodine and a base, usually used to identify the presence of compounds like sec-butyl alcohol.

A positive result in the iodoform test is marked by the formation of a yellow precipitate, which indicates the presence of the characteristic molecular structure for a positive test.

Boiling Point

Boiling point in alcohols can vary significantly depending on their molecular structure. Generally, alcohols with longer and straighter carbon chains, such as n-hexyl alcohol, have higher boiling points.

This phenomenon occurs due to increased van der Waals forces. In contrast, alcohols with more branching or smaller chains might exhibit lower boiling points, affecting their volatility and physical state at room temperature.

Solubility

Solubility of alcohols in water is heavily influenced by their hydroxyl groups, which form hydrogen bonds with water. Primary alcohols generally display better solubility.

The number and type of carbon branches can affect solubility. Alcohols with more branches might display reduced solubility, while straight-chain alcohols such as n-hexyl alcohol might display lesser solubility compared to highly branched ones, despite having primary alcoholic characteristics.

Lucas Test

The Lucas test is an effective qualitative method to distinguish between primary, secondary, and tertiary alcohols. It involves treating the alcohol with zinc chloride in concentrated hydrochloric acid.

A rapid formation of a cloudy solution or separate layer signifies a positive test. Tertiary alcohols like t-butyl alcohol provide positive results almost instantaneously due to the stability of the intermediate carbocation. Secondary alcohols show delayed results, and primary alcohols are largely unreactive under these conditions.