Question
(JEE Main 2024 style) Which of the following will react fastest in conditions? (a) (1°) (b) (2°) (c) (3°) (d) (allyl)
Solution — Step by Step
proceeds through a carbocation intermediate. The rate-determining step is C-X bond breaking to form the carbocation.
So the rate depends on carbocation stability — more stable cation = faster .
- (a) : primary carbocation (least stable)
- (b) : secondary
- (c) : tertiary (very stable)
- (d) : allyl cation (resonance-stabilised, comparable to or better than 2°)
General order: for typical haloalkanes.
But specifically, tertiary carbocations are more stable than allylic in most contexts due to the cumulative hyperconjugation (9 alpha-hydrogens) and the absence of the requirement for resonance stabilisation.
So the fastest is (c) .
reacts fastest in conditions because it forms the most stable (tertiary) carbocation.
Why This Works
is unimolecular — only one substrate molecule is involved in the slow step. The rate equation is rate , independent of nucleophile concentration.
The slow step is C-X bond ionisation to form a carbocation. Once the cation forms, the nucleophile attacks rapidly. So the rate is set by how easy it is to form the cation — i.e., cation stability.
Quick rule:
- favours: tertiary > allyl/benzyl > secondary > primary > methyl
- favours: methyl > primary > secondary > tertiary (steric crowding)
These two trends are OPPOSITE because the mechanisms are opposite.
Alternative Method — Compare Polar Protic Solvent Effects
is favoured in polar protic solvents (water, alcohols) which stabilise the carbocation intermediate via solvation. If the question gives the solvent, that hints at vs .
Common Mistake
Students often pick the primary or methyl haloalkane as fastest in — confusing with . The orders are completely reversed.
Another classic: thinking allyl chloride is always faster than tert-butyl chloride in . Tertiary carbocations actually beat allyl due to the combination of inductive donation and hyperconjugation. Allyl wins only against primary cations.
JEE Main 2023 had two haloalkane reactivity problems across shifts. NEET 2024 included an vs comparison. Both exams test the reactivity orders annually — pure scoring topic if memorised.
The reverse-order trick is one of the highest-yield concepts in organic chemistry. likes stable cations; likes accessible carbon centres.