SN1 Reaction

SN1 stands for substitution nucleophilic unimolecular.

It is a nucleophilic substitution reaction in which the rate-determining step involves only one molecule: the substrate.

Overview

In an SN1 reaction, the leaving group first leaves to form a carbocation intermediate.

The nucleophile then attacks the carbocation to form the substitution product.

The reaction usually occurs in two main stages:

1. Ionisation — the leaving group leaves and a carbocation forms.
2. Nucleophilic attack — the nucleophile attacks the carbocation.

If the nucleophile is neutral, a final deprotonation step may also be needed.

Rate law

Rate = k[substrate]

The nucleophile does not appear in the rate law because the nucleophile attacks after the slow, rate-determining step.

Favoured conditions

SN1 reactions are favoured by:

• tertiary substrates
• good leaving groups
• polar protic solvents
• weak or neutral nucleophiles

Substrate effect

The stability of the carbocation strongly affects whether SN1 is likely.

General trend:

tertiary > secondary >> primary

Tertiary substrates are favoured because they form more stable tertiary carbocations.

Primary substrates are usually not suitable for SN1 because primary carbocations are too unstable.

Leaving group

A good leaving group is important because the first step requires the carbon–leaving group bond to break.

Common good leaving groups include:

• iodide
• bromide
• chloride
• tosylate
• mesylate

The better the leaving group, the easier it is to form the carbocation.

Solvent effect

Polar protic solvents favour SN1 reactions because they stabilise ions.

Examples include:

• water
• ethanol
• methanol
• acetic acid

These solvents can stabilise both the carbocation and the leaving group.

Mechanism

Step 1: Formation of carbocation

The carbon–leaving group bond breaks heterolytically.

The leaving group takes the bonding electron pair, producing a carbocation.

This is the slow step.

Step 2: Nucleophilic attack

The nucleophile attacks the carbocation.

Because the carbocation is planar, attack can occur from either side.

Step 3: Deprotonation

If the nucleophile is neutral, another molecule removes a proton to give the final neutral product.

Stereochemistry

The carbocation intermediate is planar.

Therefore, the nucleophile can attack from both sides.

This often leads to partial racemisation when the reacting carbon is chiral.

However, the product is not always perfectly 50:50 because the leaving group may partially block one side of the carbocation.

Rearrangement

Carbocations can rearrange if a more stable carbocation can be formed.

Common rearrangements include:

• hydride shift
• methyl shift

This is a major exam trap.

If an SN1 mechanism forms a secondary carbocation next to a tertiary carbon, always check whether rearrangement is possible.

Common exam traps

• Do not include the nucleophile in the rate law.
• Do not draw SN1 as a one-step backside attack.
• Do not assume complete inversion like SN2.
• Always check for carbocation rearrangement.
• Primary halides usually do not undergo SN1.
• Polar protic solvents favour SN1 more than polar aprotic solvents.

SN1 vs SN2

| Feature | SN1 | SN2 | |---|---|---| | Rate law | k[substrate] | k[substrate][nucleophile] | | Mechanism | Two-step | One-step | | Intermediate | Carbocation | No carbocation | | Best substrate | Tertiary | Primary | | Stereochemistry | Partial racemisation | Inversion | | Nucleophile | Can be weak | Usually strong | | Solvent | Polar protic | Polar aprotic |

Quick summary

SN1 is favoured when the substrate can form a stable carbocation.

The key ideas are:

• slow carbocation formation
• first-order rate law
• possible rearrangement
• partial racemisation
• favoured by tertiary substrates and polar protic solvents