Alcohols are the compounds containing hydroxyl group attached to the alkyl group, whereas phenols are hydroxyl derivatives of aromatic hydrocarbons in which hydroxyl group is directly attached to carbon atom of aromatic ring. Alcohols are versatile compounds and can behave both as nucleophiles as well as electrophiles.
Phenols, too are reactive compounds and are known to undergo reactions on : phenolic group, benzene ring as well as various other name reactions like Reimer-Tiemann reaction, Kolbe's reaction etc.

In which of the following case, only ether is obtained, when reaction is carried out at \(413 K\) and with concentrated \(H_2SO_4\)?

Dehydration of ethanol

The correct answer is option 4. Dehydration of ethanol.

Dehydration of alcohols with concentrated sulphuric acid (\(H_2SO_4\)) can lead to either the formation of ether or alkene, depending on the reaction conditions (temperature, type of alcohol).

At lower temperatures (~413 K or 140°C), the dehydration of alcohols generally favors the formation of ether (via intermolecular dehydration).

At higher temperatures (~443 K or 170°C), the dehydration reaction tends to produce an alkene (via intramolecular dehydration).

Let us evaluate each case:

1. Dehydration of isopropyl alcohol:

Isopropyl alcohol is a secondary alcohol. At 413 K, secondary alcohols typically favor alkene formation (intramolecular dehydration). Therefore, ether formation is not favored.

2. Dehydration of tertiary alcohol:

Tertiary alcohols are very prone to forming alkenes through dehydration due to the stability of the tertiary carbocation. Ether formation is not favored with tertiary alcohols, even at lower temperatures like 413 K.

3. Dehydration of secondary alcohol:

Similar to isopropyl alcohol, secondary alcohols tend to form alkenes at 413 K.

4. Dehydration of ethanol:

Ethanol is a primary alcohol. At 413 K, the dehydration of ethanol with concentrated \(H_2SO_4\) produces diethyl ether through intermolecular dehydration.

The reaction proceeds as:

Mechanism:

There are three key steps.

First of all, one equivalent of alcohol is protonated to its conjugate acid – which has the good leaving group, \(OH_2\) (water, a weak base).

Next, another equivalent of the alcohol can now perform nucleophilic attack at carbon (\(S_N2)\), leading to displacement of \(OH_2\) (water) and formation of a new \(C-O\) bond.  This is an \(S_N2\) reaction.

The final step is deprotonation of the product by another equivalent of solvent (or other weak base), resulting in our ether product.

Ether formation is favored at this temperature for ethanol.

Conclusion:

At 413 K and in the presence of concentrated sulfuric acid, ethanol (a primary alcohol) undergoes intermolecular dehydration to form diethyl ether. Therefore, the correct answer is 4. Dehydration of ethanol.