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CBSE 12th Standard Chemistry Subject Alcohols , Phenols and Ethers HOT Questions 2 Mark Questions 2021

Published on: 28/05/2021

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Std: 12th Standard CBSE - Chemistry

chapter: - HOT Questions

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Questions:

Generic 2

3 x 2 = 6

Ethers are cleaved by acids not by bases. Why?

Arreange the following in order of increasing boiling. State reason
CH₃CH₂CH₂OH, CH₃CH₂CH₂CH₃, CH₃CH₂OCH₂CH₃, CH₃CH₂CH2CHO

Explain the following
(a) Cyclohezanol is more soluble in water than 1-Hezanol
(b) Propane 1,3-diol is more soluble in the water than propan-1-ol

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Answers Key:

Generic 2

3 x 2 = 6

The C — O — C bond in ethers like the C — OH bond in alcohols is quite strong. In order to weaken it, the oxygen atom must be protonated. A subsequent nucleophile attack by a strong nucleophile such as Br– as I– ion on the less hindered carbon atom of the protonated:
\(C-\overset { + }{ \underset { \overset { | }{ H } }{ O } - } C\)
bond brings about the cleavage of ethers to form an alcohol and an alkyl halide. The acids only can provide the H+ ion required for protonation of O atom of ether and therefore only acids can bring about the cleavage of ethers and not bases.

CH₃CH₂CH₂CH₃ < C₂H₅OC₂H₅ < CH₃CH₂CH₂CHO < CH₃ (CH₂ )₂OH
\((hydrocarbon)\overset { (ether) }{ \underset { increase\quad in\quad bond\quad polarity }{ \longrightarrow } } (aldehyde)(alcohol)\).

(a) The alkyl group in cyclohexanol is more compact than the alkyl group in Hexanol-1. The -OH group of cyclohexanol is more exposed and is more available for Hydrogen bonding with water. Hence, Cyclohexanol is more soluble in water than hexanol-1.
(b) In Propane1, 3-diol, there are two - OH groups present which increase the no. of H bonds between water and the diol. Hence Propane 1, 3-diol is more soluble in water than propan-1-ol. .