Question

Arrange the following alkyl bromides in decreasing
order of rates of SN2 displacement reaction.

(a) II > III > I > IV
(C) III > IV > I > II
(b) I> II > IV > III
(d) III > IV > II >I​

Answer 1

(c) is the right answer.

As order of reactivity towards SN2 reaction decreases from primary to tertiary alkylbromides,

III will be the least reactive and IV will be more reactive than III but lesser than the I and II primary alkylbromides. Thus. ( I,II ) > IV > III

For comparison between I and II. Reactivity towards SN2 reaction increases for primary alkylbromides if respective carbocation is more stable, which here, is more stable in case of I than II. I > II

Hence I > II > IV > III

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Answer 2

The SN2 rate is I> II > IV > III.

• SN2 reactions are anti reactions. Here the nucleophile enters at the opposite to leaving group.
• The reaction rate of substrate follow the below order-
• 1°>2°>3°
• (I) is a 1° alkyl halide so, it will show the highest SN2 reaction rate.
• (II) is a 1° alkyl halide but it is has a little steric hindrance so, it will show a high SN2 reaction rate.
• (III) is a 3° alkyl halide so, it will show the lowest SN2 reaction rate.
• (IV) is a 2° alkyl halide so, it will show the normal SN2 reaction rate.
• The reaction rate order is-
• I> II > IV > III