The correct order of increasing reactivity of the following alkyl halides, CH$_3$CH$_2$CH(Br)CH$_3$ (I), CH$_3$CH$_2$CH$_2$Br (II), (CH$_3$)$_2$CClCH$_2$CH$_3$ (III) and CH$_3$CH$_2$CH$_2$Cl (IV) towards S$_N$2 displacement is
Show Hint
S$_N$2 rule:
Primary > Secondary >> Tertiary
Br > Cl > F (leaving group ability)
Concept:
S$_N$2 reaction depends on:
• Steric hindrance (less hindrance → faster)
• Nature of leaving group (Br better than Cl)
• Structure: Primary > Secondary >> Tertiary
Analysis of compounds: • (III) (CH$_3$)$_2$CClCH$_2$CH$_3$ → highly hindered (tertiary-like) → slowest
• (I) Secondary bromide → moderate
• (IV) Primary chloride → better than secondary but Cl is poor LG
• (II) Primary bromide → least hindered + good LG → fastest
Final order:
\[
III < I < IV < II
\]