medium difficulty8 min

Nucleophilic Substitution: SN1 and SN2 Mechanisms

Nucleophilic substitution is a reaction in which a nucleophile replaces a leaving group such as halide ion from a haloalkane. SN₁ occurs in two steps through a carbocation, while SN₂ occurs in one concerted step.

In one line: SN₁: one molecule in rate law, carbocation, racemisation. SN₂: two particles in rate law, backside attack, inversion.

Practice This Concept

Understand it

SN₁ reactions are favoured by tertiary substrates because the carbocation intermediate is more stable.
They may lead to racemisation when the carbocation is planar and the nucleophile attacks from either side.
SN₂ reactions are favoured by primary substrates because backside attack is less hindered; they usually give inversion of configuration.
Leaving group ability and solvent also affect the rate.

Example

Tert-butyl bromide undergoes hydrolysis mainly by SN₁ because it forms a stable tertiary carbocation. Methyl bromide reacts with OH^- mainly by SN₂ because backside attack is easy and no stable carbocation is needed.

Rule to remember

SN₁ rate law: rate = k[R-X].

SN₂ rate law: rate = k[R-X][Nu-].

Reactivity pattern for substrate: SN₁ generally 3° > 2° > 1°, while SN₂ generally methyl > 1° > 2° > 3°.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

Predict the major mechanism for hydrolysis of (CH₃)₃CBr.

1The substrate is tertiary, so carbocation formation is feasible.
2Water is a weak nucleophile in a polar medium, so the reaction follows SN₁ and forms tert-butyl alcohol.

Method you can reuse

Mark the carbon bonded to halogen, classify it as methyl, 1°, 2°, or 3°, check nucleophile and solvent, choose SN₁ or SN₂, then state rate law and stereochemical result.

Visual tip

A comparison diagram is strongly useful: show two-step SN₁ with carbocation and one-step SN₂ backside attack with transition state.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

Why does 2-bromobutane give inversion in an SN₂ reaction?

Think first, then show answer

In SN₂, the nucleophile attacks from the side opposite the leaving group, so the arrangement around the chiral carbon is inverted.

Common wrong answer

Saying tert-butyl bromide undergoes SN₂ with OH^- as the main path ignores steric hindrance at the tertiary carbon.

Don't mix this up

Students often think stronger nucleophile always means SN₂. Substrate structure is equally important; a tertiary haloalkane is too crowded for normal SN₂ attack.

Exam tip

For mechanism questions, check substrate class first, then nucleophile strength, solvent, and stereochemical outcome.

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

How CBSE asks it

Questions ask for mechanism comparison, rate law, order of reactivity, stereochemical result, and reasons for racemisation or inversion.

1
Enough for 1 mark
Nucleophilic substitution is a reaction in which a nucleophile replaces a leaving group such as halide ion from a haloalkane. SN₁ occurs in two steps through a carbocation, while SN₂ occurs in one concerted step.
2
Add for 2 marks
Example: Tert-butyl bromide undergoes hydrolysis mainly by SN₁ because it forms a stable tertiary carbocation. Methyl bromide reacts with OH^- mainly by SN₂ because backside attack is easy and no stable carbocation is needed.
3
Add for 3 marks
SN₁ reactions are favoured by tertiary substrates because the carbocation intermediate is more stable.

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