JEE Mains Chemistry · Haloalkanes and Haloarenes
SN1 and SN2: Mechanism, Kinetics and Stereochemistry
SN2 is one step, second order and inverts the carbon; SN1 ionises first to a planar carbocation, is first order in the halide alone, racemises a stereocentre and can rearrange.
Why this matters
Fourteen PYQs, one numerical, three from 2026. Seven ask which mechanism a reaction follows and what that means for its rate law, its solvent or a rearranged product; seven ask for the stereochemical result, whether inversion, racemisation or retention, or count how many optically active products form.
Concept 1 of 2: SN1 and SN2 compared: steps, rate law and conditions
In SN2 the nucleophile attacks the carbon from the side opposite the halogen while the halogen leaves, all in one step, so the rate depends on both. In SN1 the halogen leaves first, slowly, to give a carbocation; the nucleophile then adds fast, so the rate depends on the halide alone. Which path runs is set by the substrate, the nucleophile and the solvent together.
Definition
- SN2: rate = . Favoured by a strong nucleophile at high concentration, an uncrowded carbon (, 1°, unhindered 2°) and a polar aprotic solvent (acetone, DMSO, DMF).
- SN1: rate = . Favoured by a halide that gives a stable cation (3°, benzylic, allylic), a weak nucleophile (often the solvent itself, as in solvolysis) and a polar protic solvent (water, alcohols).
- A graph of SN1 rate against is a straight line through the origin; against it is flat.
- A carbocation can rearrange by a 1,2-hydride or 1,2-methyl shift to a more stable cation before the nucleophile adds. SN2 never rearranges.
- Solvent polarity (Hughes–Ingold): if charge is created on going to the transition state, as in , a more polar solvent speeds the reaction. If charge is spread out, as in , a less polar solvent speeds it.
| Feature | SN1 | SN2 |
|---|---|---|
| Steps | Two: slow ionisation, then fast attack | One, concerted |
| Rate law | rate = , first order | rate = , second order |
| Intermediate | Planar carbocation | None; a five-coordinate transition state |
| Stereochemistry at a chiral carbon | Racemisation (mostly) | Inversion (Walden inversion) |
| Substrate order | 3° > 2° > 1° > | > 1° > 2° > 3° |
| Nucleophile | Weak, often the solvent | Strong, at high concentration |
| Best solvent | Polar protic: water, alcohols | Polar aprotic: acetone, DMSO, DMF |
| Rearrangement | Possible, by a hydride or methyl shift | Never |
| Leaving group | I > Br > Cl > F | I > Br > Cl > F |
A secondary halide can go either way; the nucleophile and the solvent decide.
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
JEE Mains · 2024 · 30 January 2024 · Q123Moderate
Example 1 · Haloalkanes and Haloarenes · SN1 and SN2: Mechanism, Kinetics and Stereochemistry
Given below are two statements:
Statement - I: High concentration of strong nucleophilic reagent with secondary alkyl halides which do not have bulky substituents will follow mechanism.
Statement - II: A secondary alkyl halide when treated with a large excess of ethanol follows mechanism. In the light of the above statements, choose the most appropriate answer from the options given below:
A secondary halide does not have one fixed mechanism
A strong nucleophile at high concentration in an aprotic solvent pushes a secondary halide towards SN2; a weak nucleophile such as the solvent pushes it towards SN1. Read the conditions before choosing.
Adding more nucleophile does not speed SN1
The slow step of SN1 is ionisation of the halide, so rate = . Doubling the nucleophile leaves the rate unchanged.
Polar solvents do not speed every substitution
A polar solvent helps when the transition state carries more charge than the reactants. For attacking a neutral halide the charge is spread out in the transition state, so a less polar solvent is faster.
Concept 2 of 2: Stereochemistry of substitution: inversion, racemisation and retention
Backside attack in SN2 turns the carbon inside out, like an umbrella in the wind, so the product has the opposite arrangement. In SN1 the carbocation is flat and the nucleophile can add to either face, so a single enantiomer gives a nearly racemic product. A step that does not break any bond to the stereocentre leaves its arrangement unchanged.
Definition
- SN2: inversion of configuration (Walden inversion). It is stereospecific: one enantiomer gives one enantiomer.
- SN1: mostly racemisation, so an optically active halide gives a product with little or no optical rotation.
- Retention: converting an alcohol to its tosylate (TsCl, pyridine) breaks O–H, not C–O, so the carbon keeps its arrangement. An SN2 on the tosylate then inverts it: one inversion overall.
- Inversion describes the 3-D arrangement. The R/S label changes only if the new group takes the priority rank the leaving group held, so assign priorities again after substituting.
- To count optically active products, write every product and look for a carbon with four different groups.
Worked example
(R)-2-Bromobutane reacts with NaCN in DMSO. Name the product and give its configuration.
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
JEE Mains · 2024 · 27 Jan 2024 · Q49Moderate
Example 2 · Haloalkanes and Haloarenes · SN1 and SN2: Mechanism, Kinetics and Stereochemistry
The correct statement regarding nucleophilic substitution reaction in a chiral alkyl halide is ;
Inversion does not always change R to S
Inversion is a change in the 3-D arrangement. The R/S letter changes only when the incoming group has the priority rank of the leaving group. Assign the priorities of the product again before writing the label.
Tosylation keeps the configuration
TsCl reacts at the O–H bond, so the stereocentre is untouched. The inversion comes only in the next step, when a nucleophile displaces the tosylate.
Summary — formulas & gotchas at a glance
A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.
Reference tables (1)
SN1 and SN2 compared: steps, rate law and conditions9 rows
| Feature | SN1 | SN2 |
|---|---|---|
| Steps | Two: slow ionisation, then fast attack | One, concerted |
| Rate law | rate = , first order | rate = , second order |
| Intermediate | Planar carbocation | None; a five-coordinate transition state |
| Stereochemistry at a chiral carbon | Racemisation (mostly) | Inversion (Walden inversion) |
| Substrate order | 3° > 2° > 1° > | > 1° > 2° > 3° |
| Nucleophile | Weak, often the solvent | Strong, at high concentration |
| Best solvent | Polar protic: water, alcohols | Polar aprotic: acetone, DMSO, DMF |
| Rearrangement | Possible, by a hydride or methyl shift | Never |
| Leaving group | I > Br > Cl > F | I > Br > Cl > F |
A secondary halide can go either way; the nucleophile and the solvent decide.
Watch out for (5)
- A secondary halide does not have one fixed mechanism→ SN1 and SN2 compared: steps, rate law and conditions
- Adding more nucleophile does not speed SN1→ SN1 and SN2 compared: steps, rate law and conditions
- Polar solvents do not speed every substitution→ SN1 and SN2 compared: steps, rate law and conditions
- Inversion does not always change R to S→ Stereochemistry of substitution: inversion, racemisation and retention
- Tosylation keeps the configuration→ Stereochemistry of substitution: inversion, racemisation and retention
Test yourself on Haloalkanes and Haloarenes
20 past JEE Mains questions from this chapter, timed at 48 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.