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 \(H_2SO_4\)?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Dehydration of ethanol
Alcohols are dehydrated by protic acids such as \( \ce{H2SO4} \), and the product, an alkene or an ether, depends on the conditions. For a primary alcohol at a lower temperature (413 K), one protonated alcohol molecule is attacked by a second alcohol molecule (SN2) and water is lost, giving the ether; at 443 K elimination wins and the alkene forms:
\( \displaystyle \ce{2C2H5OH} \xrightarrow{\text{conc. }\ce{H2SO4},\ \text{413 K}} \ce{C2H5OC2H5 + H2O} \)
\( \displaystyle \ce{C2H5OH} \xrightarrow{\text{conc. }\ce{H2SO4},\ \text{443 K}} \ce{CH2=CH2 + H2O} \)
So ethanol gives ethoxyethane at 413 K.
Option (1) Dehydration of isopropyl alcohol — Incorrect. Isopropyl alcohol is secondary; its carbocation forms easily and loses a proton, so propene is the main product, not an ether.
Option (2) Dehydration of tertiary alcohol — Incorrect. Tertiary alcohols dehydrate most easily and give alkenes; a bulky tertiary carbon also resists the SN2 attack needed for an ether.
Option (3) Dehydration of secondary alcohol — Incorrect. A secondary alcohol reacts by the SN1 path through a secondary carbocation, so elimination competes over substitution and the alkene forms instead of the ether.
NCERT (2026-27 edition):
By Dehydration of Alcohols – “Alcohols undergo dehydration in the presence of protic acids (H2SO4, H3PO4). The formation of the reaction product, alkene or ether depends on the reaction conditions. For example, ethanol is dehydrated to ethene in the presence of sulphuric acid at 443 K. At 413 K, ethoxyethane is the main product. … The method is suitable for the preparation of ethers having primary alkyl groups only. The alkyl group should be unhindered and the temperature be kept low. Otherwise the reaction favours the formation of alkene. The reaction follows SN1 pathway when the alcohol is secondary or tertiary about which you will learn in higher classes. However, the dehydration of secondary and tertiary alcohols to give corresponding ethers is unsuccessful as elimination competes over substitution and as a consequence, alkenes are easily formed.”