Home › UP Board Class 12 › chemistry › Haloalkanes and Haloarenes › Which haloalkane is MOST reactive toward $\mathr…
Which haloalkane is MOST reactive toward $\mathrm{S_N 2}$ substitution (with a typical nucleophile like $\mathrm{OH^-}$)?
A$\mathrm{CH_3Cl}$ (methyl chloride)
B$\mathrm{(CH_3)_3CCl}$ (tert-butyl chloride)
C$\mathrm{(CH_3)_2CHCl}$ (isopropyl chloride)
D$\mathrm{CH_3CH_2Cl}$ (ethyl chloride)
Answer & Solution
Correct answer: A. $\mathrm{CH_3Cl}$ (methyl chloride)
1. SN2 mechanism: ONE-STEP backside attack of the nucleophile on the carbon, with simultaneous expulsion of the leaving group. Transition state has 5 substituents around the central C.
2. STERIC FACTORS dominate: smaller alkyl groups around the central C → less crowded transition state → faster SN2.
3. Reactivity order for SN2: methyl > 1° > 2° > 3° (steric crowding by bulky groups around the central C SLOWS SN2).
4. Of the given choices, $\mathrm{CH_3Cl}$ has only HYDROGENS around the central C — least crowded, fastest SN2.
5. Tertiary $\mathrm{(CH_3)_3CCl}$ is the SLOWEST for SN2 (option B), but it's the FASTEST for SN1.
6. The remaining options fall between these extremes.
_Source: NCERT Class 12 Chemistry Part 2, Ch 6, §6.5.1 (SN2 mechanism — steric effects on rate), p. 11–12._
Related questions
$\mathrm{CHCl_3}$ (chloroform) on prolonged storage in the air may be partially oxidised tChlorobenzene ($\mathrm{C_6H_5Cl}$) is MUCH LESS reactive toward nucleophilic substitutionSAYTZEFF'S RULE for elimination reactions states thatWhen 1-bromopropane is heated with concentrated alcoholic KOH, the major product isA GRIGNARD REAGENT is formed by reacting an alkyl halide withThe IUPAC name of $\mathrm{(CH_3)_2CH{-}CH_2Br}$ isA SN2 reaction on a chiral carbon (with a single stereocentre) results inThe reactivity of haloalkanes toward nucleophilic substitution generally follows the order