Question

The best combination of reactants for preparing CH3CH2CH2CH2I is:

a. CH3CH2CH2CH2OH + NaI (acetone)

b. CH3CH2CH2CH2Br + NaI (acetone)

c. CH3CH2CH2CH3 + NaI (acetone)

d. CH3CH2CH2CH3 + HI

Answer 1

Ans : b - CH₃CH₂CH₂CH₂Br + NaI (acetone)

CH₃CH₂CH₂CH₂Br + NaI (acetone) ---------> CH₃CH₂CH₂CH₂I

Reason :

I^- is a strong nucleophile which will attack the primary carbon containing bromine in SN² fashion.

In SN² reaction, nucleophile attacks from backside and the reactivity of alkyl halides in SN² reaction is

CH₃ > primary> secondary > tertiary.

Br is a good leaving group because it can hold the negative charge on it's own.

So CH₃CH₂CH₂CH₂Br + NaI (acetone) is the best combination to prepare CH₃CH₂CH₂CH₂I.

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c and d do not have a leaving group so not possible to prepare CH₃CH₂CH₂CH₂I.

a – OH is a good nucleophile as NaI so that reaction would be useless.