R3.4.9 SN1 and SN2 mechanisms (Higher Level Only)

Nucleophilic Substitution Reactions: SN1 and SN2 Mechanisms

What Are Nucleophilic Substitution Reactions?

Definition

Nucleophilic substitution reactions

Nucleophilic substitution reactions occur when a nucleophile (an electron-rich species) attacks an electrophile (an electron-deficient species) and replaces a leaving group.

Example

When hydroxide ions react with bromoethane, the hydroxide ion replaces the bromine atom, forming ethanol and bromide ions:

${CH}_{3} {CH}_{2} Br + {OH}^{-} \to {CH}_{3} {CH}_{2} OH + {Br}^{-}$

In this reaction:

The hydroxide ion ( ${OH}^{-}$ ) is the nucleophile.
The bromine atom ( ${Br}^{-}$ ) is the leaving group.

The reaction can proceed via two distinct mechanisms: SN2(Substitution, Nucleophilic, Bimolecular) or SN1 (Substitution, Nucleophilic, Unimolecular).
The choice of mechanism depends on factors such as the type of halogenoalkane (primary, secondary, or tertiary) and the reaction conditions.

SN2 Mechanism: One-Step Reaction for Primary Halogenoalkanes

How It Works

The SN2 mechanismis a one-step, concerted process.
Here, the nucleophile attacks the electrophilic carbon atom directly from the opposite side of the leaving group.
This simultaneous attack and departure create a transition state, where the carbon is partially bonded to both the nucleophile and the leaving group.

Key Features

Bimolecular Reaction:
- The rate-determining step involves both the nucleophile and the halogenoalkane.
- The rate equation is: $rate = k [halogenoalkane] [nucleophile]$
Transition State:
- A high-energy, unstable arrangement forms, where the nucleophile and leaving group are partially bonded to the same carbon atom.
Inversion of Configuration:
- The nucleophile attacks at 180° to the leaving group, flipping the molecule’s stereochemistry (like an umbrella turning inside out).

Example

Reaction of Bromoethane with Hydroxide

The reaction between bromoethane ( ${CH}_{3} {CH}_{2} Br$ ) and hydroxide ions ( ${OH}^{-}$ ) is a classic SN2 reaction:

The hydroxide ion attacks the electron-deficient carbon atom in bromoethane.
A transition state forms, with partial bonds between the carbon, bromine, and hydroxide.
Bromine leaves as a bromide ion ( ${Br}^{-}$ ), and ethanol ( ${CH}_{3} {CH}_{2} OH$ ) forms.
Reaction equation: ${CH}_{3} {CH}_{2} Br + {OH}^{-} \to {CH}_{3} {CH}_{2} OH + {Br}^{-}$

Tip

SN2 reactions are favored by strong nucleophiles and polar aprotic solvents (e.g., acetone or DMSO) that do not form hydrogen bonds with the nucleophile.

Common Mistake

Do not confuse the transition state with an intermediate. The transition state is a fleeting, high-energy arrangement, whereas intermediates are more stable and exist for a measurable time.

SN1 Mechanism: Two-Step Reaction for Tertiary Halogenoalkanes

How It Works

The SN1 mechanism occurs in two steps.
First, the bond between the carbon atom and the leaving group breaks, forming a carbocation intermediate.
In the second step, the nucleophile attacks the positively charged carbocation, forming the final product.

Key Features

Unimolecular Reaction:
- The rate-determining step involves only the halogenoalkane.
- The rate equation is: $rate = k [halogenoalkane]$
Carbocation Intermediate:
- A positively charged carbon species forms after the leaving group departs.
Racemization:
- If the carbon is chiral, the planar structure of the carbocation allows the nucleophile to attack from either side, leading to a racemic mixture of products.

Example

Reaction of 2-Chloro-2-Methylpropane with Hydroxide

The reaction between 2-chloro-2-methylpropane ( ${C(CH}_{3})_{3} Cl$ ) and hydroxide ions ( ${OH}^{-}$ ) follows the SN1 mechanism:

Step 1 (Slow): The carbon-chlorine bond breaks, forming a carbocation ( ${C(CH}_{3})_{3}^{+}$ ) and a chloride ion ( ${Cl}^{-}$ ).
Step 2 (Fast): The hydroxide ion attacks the carbocation, forming 2-methylpropan-2-ol ( ${C(CH}_{3})_{3} OH$ ).
Reaction Equation: ${C(CH}_{3})_{3} Cl + {OH}^{-} \to {C(CH}_{3})_{3} OH + {Cl}^{-}$

Tip

SN1 reactions are favored by weak nucleophiles and polar protic solvents (e.g., water or ethanol) that stabilize the carbocation intermediate.

Common Mistake

Students often forget to include the carbocation intermediate in the SN1 mechanism. Always show the two distinct steps: formation of the carbocation and attack by the nucleophile.

Energy level profile for SN1 mechanism

1. Transition State 1 (Highest Potential Energy)

The reaction begins with the departure of the leaving group from the halogenoalkane, forming a carbocation intermediate.
This step requires the most energy due to bond breaking and the formation of a highly unstable carbocation.
It represents the rate-determining step since it is the slowest and most energy-demanding part of the mechanism.

2. Formation of the Carbocation Intermediate (Stable Relative to Transition State 1)

Once the leaving group departs, a carbocation remains.
This intermediate is more stable than the previous transition state but still reactive.

3. Transition State 2 (Lower Potential Energy)

The nucleophile attacks the carbocation, forming a bond.
The energy is lower compared to the first transition state as no bonds are breaking, only forming.

4. Final Product (Lowest Potential Energy)

The final step results in the formation of the substituted product with complete bond formation and the system at its most stable state.

Energy level profile for SN1 mechanism.

Hint

The rate-determining step is the formation of the carbocation due to the high energy required for bond cleavage.

Energy level profile for SN2 mechanism

The SN2 mechanism involves a single-step, concerted process where bond formation and bond breaking occur simultaneously, leading to a single transition state.

1. Transition State (Highest Potential Energy)

The reaction begins when the nucleophile approaches the electrophilic carbon from the opposite side of the leaving group.
During this transition state, the nucleophile and the leaving group are both partially bonded to the carbon atom.
The carbon is in a sp³ hybridized state, but it temporarily becomes a highly unstable, pentavalent structure as the nucleophile and leaving group interact simultaneously.

2. Final Product (Lowest Potential Energy)

As the nucleophile fully bonds to the carbon and the leaving group departs, the energy drops to its lowest point, forming a substituted product.
The final product shows inversion of configuration (stereochemical inversion) due to the backside attack by the nucleophile.

Energy level profile for SN2 mechanism.

Hint

The single transition state represents the highest energy point in the SN2 mechanism, and there is no formation of an intermediate.
The rate of the reaction depends on both the nucleophile and the substrate concentration, making it a second-order reaction.

Comparing SN1 and SN2 Mechanisms

Feature	SN2 Mechanism	SN1 Mechanism
Reaction Steps	Single-step (concerted)	Two-step (intermediate forms)
Rate Equation	$rate = k [halogenoalkane] [nucleophile]$	$rate = k [halogenoalkane]$
Nucleophile	Strong nucleophile required	Weak nucleophile sufficient
Solvent	Polar aprotic (e.g., acetone)	Polar protic (e.g., water)
Substrate	Primary halogenoalkanes	Tertiary halogenoalkanes
Stereochemistry	Inversion of configuration	Racemization (mixture of enantiomers)

Types of halogenoalkane and their suitable type of nucleophilic substitution reaction.

Reflection

Self review

What type of halogenoalkane undergoes SN2 reactions?
Why are tertiary halogenoalkanes more likely to follow the SN1 mechanism?
How does the solvent affect the choice of mechanism?

Theory of Knowledge

How do chemists decide which reaction mechanism to use in industrial processes? Consider the roles of cost, efficiency, and environmental impact.

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