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

Question

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

Step-by-Step Solution

Answer

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

Step 1: Understanding the Options

Before matching, identify the key characteristics of each alcohol. - Neohexyl alcohol: a primary alcohol with a branched chain. - Sec-butyl alcohol: a secondary alcohol. - n-hexyl alcohol: a primary alcohol with a straight chain. - t-butyl alcohol: a tertiary alcohol.

Step 2: Matching Highest Boiling Point

The boiling point depends on intermolecular forces, such as hydrogen bonding and surface area. - (a) Neohexyl alcohol and (c) n-hexyl alcohol are primary alcohols, but (c) has a longer chain, thus higher boiling point due to increased van der Waals forces. Therefore, (c) n-hexyl alcohol matches with (p) highest boiling point.

Step 3: Matching Positive Iodoform Test

The iodoform test is positive for alcohols with the structure R-CH(OH)-CH3, typically secondary alcohols. - (b) Sec-butyl alcohol has this structure, so it matches with (q) positive iodoform test.

Step 4: Matching Highest Water Solubility

Water solubility often increases with branching and decreases with longer carbon chains. - (d) t-butyl alcohol is highly branched, making it more soluble in water. Thus, it matches with (r) highest water solubility.

Step 5: Matching Susceptibility to Chromate Oxidation

Chromate oxidizes primary and secondary alcohols effectively. - (a) Neohexyl alcohol is a primary alcohol, so it matches with (s) susceptible to chromate oxidation.

Step 6: Matching Reaction with ZnCl2 + conc. HCl (Lucas Test)

The Lucas test gives instant reactivity with tertiary alcohols due to rapid formation of carbocations. - (d) t-butyl alcohol is tertiary, matching with (t) shows instant reaction with ZnCl2 + conc. HCl.

Key Concepts

Boiling point

The boiling point of alcohols varies significantly depending on their molecular structure. Primarily, it is influenced by:

• Hydrogen bonding capabilities
• Van der Waals forces
• Molecular size and branching

Alcohols, like n-hexyl alcohol, exhibit a higher boiling point due to the uninterrupted chain that allows for greater surface interaction and van der Waals forces. Comparatively, primary alcohols like neohexyl alcohol with branched chains have lower boiling points because branching reduces surface area and, consequently, intermolecular forces. Thus, longer, unbranched molecules like n-hexyl alcohol tend to have the highest boiling points due to these enhanced forces.

Iodoform test

The iodoform test identifies methyl ketones and secondary alcohols containing the R-CH(OH)-CH3 structure. When treated with iodine (9) and a base, such as sodium hydroxide (NaOH), alcohols yield a yellow precipitate of iodoform (CHI3) if positive. Alcohols like sec-butyl alcohol fit this structural requirement. They undergo free-radical halogenation and subsequent oxidation by iodine, resulting in the production of iodoform. Thus, a positive iodoform test is a clear indicator of a specific structural group within alcohols, particularly effective for identifying secondary alcohols like sec-butyl alcohol.

Water solubility

Water solubility in alcohols is determined by the balance between hydrophilic and hydrophobic parts within the molecule. Key factors include:

• Branching of the molecule
• Length of the carbon chain

Alcohols with short, highly branched chains—like t-butyl alcohol—possess higher solubility due to their compact structure, which allows easier accommodation within water's polar matrix. As the chain length increases, the hydrophobic character diminishes solubility. Branching tends to enhance solubility as it reduces the effective size of the hydrophobic segment.

Chromate oxidation

Chromate oxidation, using agents like chromic acid (9), is a method to oxidize primary and secondary alcohols to aldehydes or ketones. Primary alcohols (such as neohexyl alcohol) are susceptible due to their hydroxyl group being on the terminal carbon. These reactions employ chromate ions (CrO3) in an acidic medium (H⁺). The alcohol supplies electrons, converting the Cr(VI) to Cr(III), while the alcohol itself is oxidized. A change in color of the chromate solution, usually from orange to green, indicates the oxidation process, making it easy to confirm.

Lucas test

The Lucas test distinguishes alcohols based on their reactivity with Lucas reagent, a mixture of zinc chloride (ZnCl2) in concentrated hydrochloric acid (HCl). This test is particularly rapid for tertiary alcohols, such as t-butyl alcohol, which readily form carbocations. Upon reacting with the Lucas reagent, they exhibit instant turbidity due to the formation of an insoluble organic chloride.
Secondary alcohols react more slowly, while primary ones show little to no reaction. The speed of turbidity formation provides insight into the alcohol's structure, with immediate reaction indicating a tertiary alcohol like t-butyl alcohol.