Imagine you have a molecule with a leaving group (like a halogen) attached to a carbon. You want to replace that leaving group with a nucleophile (something that loves positive charge). There are two fundamentally different ways this can happen — like two different ways to replace a lightbulb.
SN2 is like unscrewing the old bulb and screwing in the new one in one smooth motion. SN1 is like first pulling the old bulb out completely, leaving an empty socket, and then putting the new bulb in.
That empty socket — the carbocation — is the key difference.
SN2: One Step, Backside Attack
The name says it all: Substitution, Nucleophilic, Bimolecular. "Bimolecular" means two molecules (the nucleophile and the substrate) are involved in the rate-determining step.
The Mechanism
The nucleophile attacks the carbon from the backside — directly opposite the leaving group. As the nucleophile approaches, the leaving group starts to leave. At the transition state, the nucleophile is partially bonded and the leaving group is partially detached. Then the leaving group departs completely, and the nucleophile is fully bonded.
SN2 mechanism in wedge-dash notation: hydroxide attacks the carbon of CH3Cl from the back side, passing through a five-coordinate transition state (partial HO and Cl bonds, both delta-minus) before chloride leaves
All of this happens in one step — no intermediate.
The Stereochemistry: Inversion
Because the nucleophile attacks from the back, the configuration at the carbon inverts — like an umbrella turning inside out in a strong wind. If you start with an R configuration, you get S (and vice versa). This is called Walden inversion.
Backside attack in the SN2 reaction (NCERT Fig 6.2): the incoming hydroxide (red) approaches the carbon opposite the outgoing halide (green), and the three hydrogens flip through like an umbrella inverting
What Favours SN2?
Primary carbon (least steric hindrance — the backside is wide open)
Strong nucleophile (needs to push its way in)
Good leaving group (but not too good — it needs to wait for the nucleophile)
Polar aprotic solvent (doesn't solvate the nucleophile too tightly)
Watch out
SN2 is impossible on tertiary carbons — the three bulky groups block the backside completely. The nucleophile simply cannot get close enough.
Steric effects in the SN2 reaction (NCERT Fig 6.3): relative rates fall from methyl (30) to ethyl (1) to isopropyl (0.02) to tert-butyl (0) as alkyl groups crowd the backside approach of the nucleophile
SN1: Two Steps, Carbocation Intermediate
Substitution, Nucleophilic, Unimolecular. "Unimolecular" means only one molecule (the substrate) is involved in the rate-determining step.
The Mechanism
Step 1 (slow, rate-determining): The leaving group leaves on its own, forming a carbocation (a carbon with only six electrons — positively charged and very unstable).
Step 2 (fast): The nucleophile attacks the carbocation. Since the carbocation is flat (trigonal planar), the nucleophile can attack from either side with equal probability.
The two steps of the SN1 mechanism for 2-bromo-2-methylpropane: slow reversible ionisation to the planar tert-butyl carbocation, then fast attack by hydroxide to give 2-methylpropan-2-ol
The Stereochemistry: Racemisation
Because the nucleophile can attack from either face of the flat carbocation, you get a racemic mixture — equal amounts of R and S. If the starting material is optically pure, the product will be optically inactive.
SN1 racemisation of 2-bromobutane: the planar carbocation is attacked by hydroxide on either face, giving equal amounts of (+)- and (-)-butan-2-ol — an optically inactive racemic mixture
Note
In practice, you often get slightly more inversion than retention (about 60:40) because the leaving group can partially block one face as it departs. But the key idea is loss of stereochemistry.
What Favours SN1?
Tertiary carbon (the carbocation is stabilised by three alkyl groups — hyperconjugation and inductive effect)
Weak nucleophile (doesn't need to force its way in — the carbocation is desperate for electrons)
Excellent leaving group (must be able to leave on its own)
Polar protic solvent (stabilises the carbocation and the leaving group)
Watch out
SN1 is impossible on primary carbons — a primary carbocation is so unstable it effectively doesn't exist. The leaving group would never leave on its own.
CH3CH2CH2Cl (primary) beats CH3CH(Cl)CH2CH3 (secondary) in SN2 because a primary carbon has less steric hindrance to backside nucleophilic attack.
Concept. The SN2 mechanism (CBSE Class-12 haloalkanes-and-haloarenes) is a one-step, concerted attack of the nucleophile from the side opposite the leaving group.
Q.In a nucleophilic substitution reaction, in which of the following cases is an intermediate carbocation produced?
(a) (CH3)3CCl
(b) (CH3)2CHCH2Cl
(c) CH3CH2Cl
(d) CH3CH2CH2Cl
›Reveal solutionSolution
A tertiary alkyl halide favours the SN1 pathway, which genuinely proceeds through a carbocation intermediate; the other, less-substituted halides here react by the concerted SN2 pathway with no carbocation ever forming.
Whether a nucleophilic substitution goes through a carbocation intermediate depends on how stable that carbocation would be and how crowded the carbon bearing the leaving group is.
(CH3)3CCl: the C–Cl carbon is tertiary (bonded to three methyl groups). Loss of Cl– gives the tertiary carbocation (CH3)3C+, stabilised by hyperconjugation and the +I effect of three alkyl groups. This stability lets the reaction proceed by SN1 — a two-step mechanism with the carbocation as a genuine intermediate. …
