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SN2 Reaction Mechanism

The decreasin...

Question

The decreasing order of rate of $S_{N} 2$ is :
(I) $B r - C H_{2} - C H_{2} - B r$ (II) $N = C - C H_{2} - B r$
(III) $C H_{3} - C | B r H - C H_{3}$ (IV) $C H_{3} - C H_{2} - B r$

I I > I > I V > I I I

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I I > I I I > I > I V

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I I > I > I I I > I V

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I V > I > I I > I I I

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Solution

The correct option is D $I I > I > I V > I I I$
The decreasing order of rate of $S_{N} 2$ is $I I > I > I V > I I I .$
The rate is maximum for II as two electron withdrawing groups $C N$ and $B r$ are present.
They decrease the electron density on methylene carbon atom and make it more electrophilic.
The rate is minimum for III as it contains only one electron withdrawing $B r$ atom and is a secondary alkyl halide.

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S_{N} 1

C_{6} H_{5} - C | H C H_{3} - B r

C_{6} H_{5} - C H_{2} - B r

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C_{6} H_{5} - C H_{2} - I

S_{N} 2

C H_{3} - C l

C H_{3} - C | | O - C H_{2} - C l

C H_{3} - C H | C H_{3} - C H_{3} - C l

C H_{3} - C H_{2} - C l

β

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C H_{3} - H | C | C H_{3} - C H_{2} B r

C H_{3} - C H_{2} - B r

C H_{3} - C H_{2} - C H_{2} - B r

S_{N} 2

K I

C H_{3} - C H_{2} - C H_{2} - C H_{2} - C l a n d C H_{3} - C H ∣ C H_{3} - C H_{2} - C l

(I) (I I)

C H_{3} - C H_{2} - C H_{2} - C l a n d C H_{3} - C H_{2} - C H_{2} - B r

(I) (I I)

C H_{3} - C H ∣ C H_{3} - C H_{2} - C H_{2} - C l a n d C H_{3} - C H_{3} ∣ C ∣ C H_{3} - C H_{2} C l

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C H_{3} - C H_{2} - C H_{2} - B r ∣ C H - C H_{3} a n d C H_{3} - C H_{3} ∣ C H - C H_{2} - B r ∣ C H - C H_{3}

(I) (I I)

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