Chemistry Class 12 NCERT Solutions: Chapter 11 Alcohols Phenols and Ethers Part 9

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Q: 26. How is 1-propoxypropane synthesised from propan-1-ol? Write mechanism of this reaction.

Answer:

1-propoxypropane can be synthesized from propan-1-ol by dehydration.

Propan-1-ol undergoes dehydration in the presence of protic acids (such as , ) to give 1-propoxypropane.

The mechanism of this reaction involves the following three steps:

Step 1: Protonation

Q 26 1 Protonation

Q 26 1 Protonation

Q 26 1 Protonation

Step 2: Nucleophilic attack

Q 26 2 Nucleophilic attack

Q 26 2 Nucleophilic Attack

Q 26 2 Nucleophilic attack

Step 3: Deprotonation

Q 26 3 Deprotonation

Q 26 3 Deprotonation

Q 26 3 Deprotonation

Q: 27. Preparation of ethers by acid dehydration of secondary or tertiary alcohols is not suitable method. Give reason.

Answer:

The formation of ethers by dehydration of alcohol is a bimolecular reaction ( involving the attack of an alcohol molecule on a protonated alcohol molecule. In the method, the alkyl group should be unhindered. In case of secondary or tertiary alcohols, the alkyl group is hindered. As a result, elimination dominates substitution. Hence, I place of ethers, alkenes are formed.

Q: 28. Write the equation of the reaction of hydrogen iodide with:

(i) 1-propoxypropane

(ii) Methoxybenzene and

(iii) Benzyl ethyl ether

Answer:

(i)

(ii)

Q 28 ii Methoxybenzene and Phenol

Q 28 Ii Methoxybenzene and Phenol

Q 28 ii Methoxybenzene and Phenol

(iii)

Q 28 iii Benzylethyl Ether

Q 28 Iii Benzylethyl Ether

Q 28 iii Benzylethyl Ether

Q: 29. Explain the fact that in aryl alkyl ethers

(i) The alkoxy group activates the benzene ring towards electrophilic substitution and

(ii) It directs the incoming substituents to ortho and para positions in benzene ring.

Answer:

(i)

Q 29 i The fact that in aryl alkyl ethers

Q 29 I the Fact That in Aryl Alkyl Ethers

Q 29 i The fact that in aryl alkyl ethers

In aryl alkyl ethers, due to the effect of the alkoxy group, the electron density in the benzene ring increases as shown in the following resonance structure.

Q 29 i 1 The fact that in aryl alkyl ethers

Q 29 I 1 the Fact That in Aryl Alkyl Ethers

Q 29 i 1 The fact that in aryl alkyl ethers

Thus, benzene is activated towards electrophilic substitution by the alkoxy group.

(ii) It can also be observed from the resonance structures that the electron density increases more at the ortho and para positions than at the meta position. As a result, the incoming substituents are directed to the ortho and para positions in the benzene ring.