Question

The acidic hydrolysis of ether (X) shown above is fastest when:

A. one phenyl group is replaced by a methyl group
B. one phenyl group is replaced by a para – methoxyphenyl group
C. two phenyl groups is replaced by two para - methoxyphenyl group
D. no structural change is made to X

Answer 1

Hint: In hydrolysis the rate of reaction is directly proportional to the stability of carbo-cation formed and stability of carbocation is dependent on + I effect. So phenyl group(s) is replaced to increase the carbocation stability.

Complete answer: 

- As we know ethers are a class of organic compounds with an oxygen atom connected to two alkyl or aryl groups.

- Note that Ethers can be symmetrical and asymmetrical.

Simple or symmetrical ethers → R = R’

Mixed or Asymmetrical ethers→ R≠R’

- We name ethers as Alkoxy Alkane, where Alkoxy group is the smaller group of the two groups.

- Now let’s see the nature of C-O-C bonds in ethers. In ethers the C-O bonds are polar, therefore ethers are weakly polar and have weak dipole – dipole interaction. Ether has a bent structure like that of water.

- We know Ethers have lower boiling points than corresponding alcohols because alcohols have inter – molecular H – bonding while others do not have.

- Also, solubility of ethers in water is similar to alcohol because of their tendency to make H – bonds with water. Solubility decreases with increase in the size of the alkyl group.

- Now for acidic hydrolysis of ethers we generally study reaction with acid halide, the reaction depends upon the feasibility of Nu attack (SN2), therefore the smaller group converts into alkyl halide.

- The stability of the intermediate carbo – cation depends on +I effect and +R effect.

- So, to increase the rate of reaction of the given reaction we have to replace the phenyl groups by groups that have greater +I and +M effect, which is para - methoxyphenyl group.

- To increase +I and +M effect we replace 2 phenyl groups by 2 para - methoxyphenyl groups which increases carbo-cation stability.

Hence the correct option is C. two phenyl groups are replaced by two para - methoxyphenyl groups.

Note: 

-Methyl group shows +I effect but para - methoxyphenyl group shows high +M effect so for higher +M effect 2 para - methoxyphenyl group are being used.

-For electron withdrawing substituent the mesomeric effect is negative and positive for electron donating substituent.

-The electronic effect due the polarisation of σ bonds within a molecule or ion is called inductive effect.