Q. 195.0( 1 Vote )

Give reasons for

Answer :

(i) Phenol is more acidic than methanol because the conjugate base of phenol formed after release of proton is more stable than the conjugate base of methanol.

Conjugate base of phenol i.e. phenoxide ion is more stable due to delocalization of negative charge in benzene ring and thus, phenol is more acidic. On the other hand methoxide ion (conjugate base of methanol) is destabilized due to +I effect of CH3 group making it less acidic.

(ii) The C-O-H bond angle in alcohols is slightly less than the tetrahedral angle (109°28’) because of the repulsion between the two lone pairs of electrons on oxygen atoms. Due to this lp- lp repulsion C-O-H bond angle is less than tetrahedral angle.

(iii) (CH3)3C – O – CH3 on reaction with HI gives (CH3)3C - I and CH3–OH as the main products and not (CH3)3C – OH and CH3- I because reaction of (CH3)3C – O – CH3 with HI follows SN1 mechanism. In SN1 mechanism, product is formed on the basis of stability of carbocation formed.

When (CH3)3C – O – CH3 reacts with HI there is possibility of formation of (CH3)3C+ (3°) or CH3+ carbocation. Since, tertiary carbocation is more stable than –CH3 carbocation therefore, nucleophile i.e. I will attack on (CH3)3C+ and thus, (CH3)3C - I and CH3–OH are formed as the main products.

(CH3)3C – O – CH3 + HI (CH3)3C+ + CH3OH (CH3)3C-I + CH3-OH

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