Arrange the following compounds in increasing order of their reactivity towards $S_N2$ reaction:

(A) $CH_3CH_2CH_2CH_2Br$
(B) $(CH_3)_2CHCH_2Br$
(C) $CH_3CH_2CH(Br)CH_3$
(D) $(CH_3)_3CBr$

Choose the correct answer from the options given below:

(D), (C), (B), (A)

The correct answer is Option (1) → (D), (C), (B), (A)

The increasing order of reactivity towards SN2 reaction is:
tert-butyl bromide < secondary alkyl bromide < branched primary bromide < straight-chain primary bromide

So, (D) < (C) < (B) < (A)

Key concept of SN2 reaction

• SN2 is a one-step reaction

• Nucleophile attacks from the backside

• Reaction is highly affected by steric hindrance

 Less crowding = faster reaction

(D) (CH₃)₃CBr

• Tertiary alkyl halide

• Very bulky

• Backside attack is almost impossible

→ Least reactive

(C) CH₃CH₂CH(Br)CH₃

• Secondary alkyl halide

• Moderate steric hindrance

→ Reactivity increases compared to tertiary

(B) (CH₃)₂CHCH₂Br

• Primary alkyl halide but branched

• Some steric hindrance due to branching

→ More reactive than secondary but less than straight chain

(A) CH₃CH₂CH₂CH₂Br

• Primary alkyl halide

• Least steric hindrance

→ Most reactive in SN2