Question
Arrange the following in order of decreasing reactivity towards reaction: tert-butyl bromide, isopropyl bromide, n-propyl bromide, methyl bromide.
Solution — Step by Step
proceeds through a carbocation intermediate. The slow, rate-determining step is the formation of this carbocation by loss of the leaving group (Br).
Stability order:
3° > 2° > 1° > methyl
Stability comes from hyperconjugation and inductive donation from alkyl groups, which stabilise the positive charge.
- tert-butyl bromide → 3° carbocation (most stable)
- isopropyl bromide → 2° carbocation
- n-propyl bromide → 1° carbocation
- methyl bromide → methyl carbocation (least stable)
reactivity order:
tert-butyl Br > isopropyl Br > n-propyl Br > methyl Br
Final answer: tert-butyl > isopropyl > n-propyl > methyl.
Why This Works
The rate of depends on the rate of carbocation formation, which depends on the stability of the carbocation. More stable carbocation = lower activation energy = faster reaction.
The opposite is true for : methyl > 1° > 2° > 3°, because is concerted (single step) and steric crowding around the carbon atom slows the backside attack of the nucleophile. Tert-butyl is too crowded — doesn’t happen.
| Feature | S_N1 | S_N2 |
|---|---|---|
| Mechanism | Two steps (carbocation) | One step (concerted) |
| Rate law | Rate = [substrate] | Rate = [substrate][Nu] |
| Best substrate | 3° > 2° > 1° > methyl | methyl > 1° > 2° > 3° |
| Solvent | Polar protic (water, alcohol) | Polar aprotic (DMSO, DMF) |
| Stereochemistry | Racemisation (planar intermediate) | Inversion (Walden) |
Alternative Method
Memorise the rule directly: for , “more substituted = more reactive.” This works for haloalkanes only (haloarenes have separate considerations).
Allylic and benzylic halides are even more reactive than 3° in — their carbocations are stabilised by resonance with systems, making them remarkably stable.
So a more complete order: benzylic ≈ allylic > 3° > 2° > 1° > methyl.
Common Mistake
The single biggest trap: students confuse with orders. Memorise these by mechanism:
- → carbocation matters → 3° > 2° > 1° > methyl
- → steric crowding matters → methyl > 1° > 2° > 3°
Another trap: assuming haloarenes (like chlorobenzene) follow haloalkane orders. They don’t — haloarenes are much less reactive in both and due to:
- Partial double-bond character in C-X bond (resonance with the ring),
- Repulsion between nucleophile and -electrons of the ring,
- -hybridised carbon (more electronegative than ).
Haloarenes need special conditions (high temperature, strong base) to undergo nucleophilic substitution — typically via the benzyne mechanism or with electron-withdrawing groups stabilising the intermediate.