- 2069 Bhadra · 3+2 marks
SN1 vs SN2 in haloalkane; regulating factors
Short answer
SN1 is two-step, carbocation intermediate, first order (k[RX]), favours 3° substrates, polar protic solvents. SN2 is one-step backside attack, second order (k[RX][Nu−]), favours CH3/1° substrates, polar aprotic solvents. Factors: substrate structure, nucleophile strength, leaving group, solent polarity.
How do SN1 and SN2 reactions differ in haloalkane? Mention the factors which regulates the reaction.
Difference between SN1 and SN2
| Basis | SN1 | SN2 |
|---|---|---|
| Steps | Two steps; carbocation intermediate | One step; transition state |
| Rate law | Rate = k[RX] (first order) | Rate = k[RX][Nu⁻] (second order) |
| Substrate order | 3° > 2° > 1° | CH₃ > 1° > 2° > 3° |
| Nucleophile | Strength unimportant | Strong nucleophile needed |
| Solvent | Polar protic (water, alcohol) | Polar aprotic (acetone, DMF, DMSO) |
| Stereochemistry | Racemisation (inversion + retention) | Complete inversion (Walden) |
| Rearrangement | Possible | Not possible |
| Example | (CH₃)₃CBr + aq. NaOH | CH₃Br + aq. NaOH |
Factors regulating the reaction
- Structure of the substrate: 3° halides favour SN1 (stable carbocation); CH₃ and 1° halides favour SN2 (less steric hindrance).
- Nucleophile: a strong, high-concentration nucleophile favours SN2; a weak one favours SN1.
- Leaving group: a better leaving group (I⁻ > Br⁻ > Cl⁻ > F⁻) speeds up both.
- Solvent: polar protic solvents favour SN1; polar aprotic solvents favour SN2.
✓ Answer checked by Claude Opus 5.5 (high effort)
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