Arrange the following in the order of increasing reactivity towards SN _ 2 reactions. array ll ( CH ₃ )₃ CCH ₂ Br ( P ) ; & CH ₃ ( CH ₂ )₃ Br ( Q ) ; & CH ₃ CH ₂- CH ( CH ₃ )- CH ₂ Br ( R ) ; & ( CH ₃ )₂ CHCH ₂ CH ₂ Br ( S ) array

Step-by-step solution

S_N2 reactivity decreases with increasing steric hindrance around the carbon bearing the leaving group. P: (CH₃)₃CCH₂Br (neopentyl bromide) — branching at -carbon with three bulky methyl groups most hindered, least reactive. R: CH₃CH₂-CH(CH₃)-CH₂Br — branching at -carbon (one methyl) significant hindrance. S: (CH₃)₂CHCH₂CH₂Br — branching at -carbon, further from reaction centre less hindrance than R. Q: CH₃(CH₂)₃Br (n-butyl bromide) — straight chain, no branching least hindered, most reactive. Increasing reactivity