SN1 vs SN2 + Stereochemistry
Haloalkanes & Haloarenes
42
JEE Qs
12%
Hard
100
min
Master the interplay of substrate structure, nucleophile strength, leaving group ability, and solvent effects to correctly predict the mechanism (SN1/SN2) and stereochemistry for any given reaction.
🧮 Key Formulas
✅ Key Points for JEE
- 1SN1 reactions are two-step, involve a carbocation intermediate, and their rate depends only on the substrate concentration. They are favored by 3°, allylic, or benzylic substrates, weak nucleophiles, and polar protic solvents. The stereochemical outcome is racemization (often with slight excess of inversion) if the chiral center is involved.
- 2SN2 reactions are one-step, concerted processes involving a transition state, with backside attack of the nucleophile. Their rate depends on both substrate and nucleophile concentrations. They are favored by methyl, 1°, or small 2° substrates, strong nucleophiles, and polar aprotic solvents. The stereochemical outcome is complete inversion of configuration (Walden inversion).
- 3Identify the nature of the substrate (primary, secondary, tertiary, allylic, benzylic) as the primary determinant for SN1 vs. SN2 pathway. Steric hindrance around the leaving group strongly disfavors SN2.
- 4Always evaluate the leaving group ability (good leaving groups are weak bases, e.g., I⁻ > Br⁻ > Cl⁻ > F⁻) as it is crucial for both SN1 and SN2 reactions. Also, consider the solvent (polar protic for SN1, polar aprotic for SN2) and nucleophile strength (weak for SN1, strong for SN2).
- 5In SN1 reactions, be vigilant for carbocation rearrangements (1,2-hydride or 1,2-alkyl shifts) if a more stable carbocation can be formed, as this dictates the final product structure.
⚠️ Common Mistakes
- ✕Confusing the roles of polar protic and polar aprotic solvents (e.g., expecting SN1 in polar aprotic solvents or SN2 in polar protic solvents).
- ✕Incorrectly predicting the stereochemical outcome, particularly assuming complete racemization for SN1 or retention for SN2.
- ✕Failing to consider and implement carbocation rearrangements in SN1 reactions, leading to incorrect major products.
- ✕Not recognizing the impact of steric hindrance on SN2 reactions, especially trying to force SN2 on a tertiary substrate.
📝 Practice Questions
See allQ52.Given below are two statements: Statement I: The conversion proceeds well in the less polar medium. HO− −CH3 −CH2 −CH2 −CH2 −Cl → CH3 −CH2 −CH2 −CH2 −OH + Cl(−) Statement II: The conversion proceeds well in the more polar medium. In the light of the above statements, choose the correct answer from the options given below (1) Both Statement I and Statement II are true (2) Statement I is true but Statement II is false (3) Statement I is false but Statement II is true (4) Both Statement I and Statement II are false
Q56.The products A and B in the following reactions, respectively are Ag−NO2 AgCN −A ⟷CH3 −CH2 −CH2 −Br → B (1) CH3 −CH2 −CH2 −NO2, CH3 −CH2 −CH2 −CN (2) CH3 −CH2 −CH2 −ONO, CH3 −CH2 −CH2 −NC (3) CH3 −CH2 −CH2 −ONO, CH3 −CH2 −CH2 −CN (4) CH3 −CH2 −CH2 −NO2, CH3 −CH2 −CH2 −NC
Q59.Given below are two statements : Statement I : CH3 −O −CH2 −Cl will undergo SN1 reaction though it is a primary halide. Statement II : will not undergo SN2 reaction very easily though it is a primary halide. In the light of the above statements, choose the most appropriate answer from the options given below : (1) Both Statement I and Statement II are incorrect (2) Both Statement I and Statement II are correct (3) Statement I is incorrect but Statement II is (4) Statement I is correct but Statement II is correct incorrect
Q65.The ascending order of relative rate of solvolysis of following compounds is : (1) (C) < (B) < (A) < (D) (2) (D) < (A) < (B) < (C) (3) (D) < (B) < (A) < (C) (4) (C) < (D) < (B) < (A) 2025 (23 Jan Shift 2) JEE Main Previous Year Paper
Q66.Following are the four molecules "P", "Q", "R" and "S". Which one among the four molecules will react with H −Br(aq) at the fastest rate? (1) R (2) P (3) Q (4) S
Q70.The correct increasing order of stability of the complexes based on Δo value is : I. [Mn(CN)6]3− II. [Co(CN)6]4− III. [Fe(CN)6]4− IV. [Fe(CN)6]3− (1) IV < III < II < I (2) I < II < IV < III (3) III < II < IV < I (4) II < III < I < IV
NCERT Chapters
- Class 12 Chemistry Part II Ch 10: Haloalkanes and Haloarenes