Grade 12

Advanced

9 min

Lesson 4 of 12

Nucleophilic Substitution Mechanisms

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Detailed analysis of SN1 and SN2 reaction pathways, including kinetics and stereochemical outcomes.

Why does a chemical reaction sometimes flip a molecule inside out like an umbrella in the wind, while other times it creates a messy 50/50 mixture of mirror images? The answer lies in the hidden 'dance' of electrons during nucleophilic substitution.

Learning Objectives

1.Distinguish between SN1 and SN2 mechanisms based on their rate laws and molecularity.
2.Predict the stereochemical outcome (inversion vs. racemization) of a substitution reaction.
3.Evaluate how solvent polarity and nucleophile strength dictate the preferred reaction pathway.

The SN2 Mechanism: The One-Step Dance

The SN2 mechanism (Substitution Nucleophilic Bimolecular) is a concerted process, meaning bond-breaking and bond-forming happen at the exact same time. The nucleophile performs a backside attack on the electrophilic carbon, opposite to the leaving group. This spatial requirement causes the molecule to undergo Walden Inversion, much like an umbrella flipping inside out in a storm. Because two species are involved in the transition state, the reaction is second-order. The rate law is expressed as:

Rate = k[RX][Nu^-]

where

[RX]

is the substrate and

[Nu^-]

is the nucleophile.

Consider the reaction of methyl iodide with sodium hydroxide. If the concentration of methyl iodide is doubled and the concentration of sodium hydroxide is tripled, how does the rate change? 1. Identify the rate law: $Rate = k[CH_3I][OH^-]$ . 2. Apply the changes: $Rate_{new} = k(2 \times [CH_3I])(3 \times [OH^-])$ . 3. Calculate the factor: $2 \times 3 = 6$ . 4. Result: The reaction rate increases by a factor of 6.

Quick Check

In an SN2 reaction, if you use a chiral starting material with an (R) configuration, what is the typical configuration of the product?

Think about the 'backside attack' and the umbrella analogy.

Answer

The product will typically have an (S) configuration due to stereochemical inversion.

The SN1 Mechanism: The Two-Step Breakup

The SN1 mechanism (Substitution Nucleophilic Unimolecular) occurs in stages. First, the leaving group departs on its own, creating a high-energy carbocation intermediate. This is the rate-determining step (slow step). Because the carbocation is

sp^2

hybridized and planar, the nucleophile can attack from either the top or the bottom with equal probability. This leads to racemization, producing a mixture of enantiomers. Since only the substrate is involved in the slow step, the rate law is first-order:

Rate = k[RX]

The nucleophile's strength does not affect the speed of an SN1 reaction.

Predict the products when (S)-3-chloro-3-methylhexane reacts with ethanol ( $CH_3CH_2OH$ ) via an SN1 pathway. 1. The leaving group ( $Cl^-$ ) leaves, forming a planar carbocation at C3. 2. The ethanol nucleophile attacks from the 'top' face to form (S)-3-ethoxy-3-methylhexane. 3. The ethanol attacks from the 'bottom' face to form (R)-3-ethoxy-3-methylhexane. 4. Result: A racemic mixture (50:50) of the (R) and (S) enantiomers is formed.

Quick Check

Why doesn't increasing the concentration of the nucleophile speed up an SN1 reaction?

Recall which step is the 'bottleneck' of the reaction.

Answer

Because the nucleophile is not involved in the rate-determining step (the formation of the carbocation).

Choosing the Path: Solvents and Nucleophiles

Environmental factors dictate which 'lane' a reaction takes. SN2 reactions are favored by strong nucleophiles (usually negatively charged) and polar aprotic solvents (like DMSO or Acetone). These solvents do not hydrogen-bond with the nucleophile, leaving it 'naked' and highly reactive. In contrast, SN1 reactions are favored by tertiary substrates (which form stable carbocations) and polar protic solvents (like water or alcohols). Protic solvents stabilize the leaving group and the carbocation through ion-dipole interactions, lowering the activation energy for the first step.

A student reacts 2-bromobutane with sodium cyanide ( $NaCN$ ). In Scenario A, they use methanol ( $CH_3OH$ ). In Scenario B, they use dimethylformamide (DMF). Which scenario favors SN2, and why? 1. Analyze the substrate: 2-bromobutane is a secondary alkyl halide (can do both). 2. Analyze the nucleophile: $CN^-$ is a strong nucleophile (favors SN2). 3. Analyze the solvents: Methanol is polar protic; DMF is polar aprotic. 4. Conclusion: Scenario B (DMF) strongly favors SN2 because the aprotic solvent does not solvate the $CN^-$ ion, allowing it to attack the substrate more effectively.

Key Takeaways

1SN2 is a one-step concerted process featuring backside attack and stereochemical inversion.
2SN1 is a two-step process involving a planar carbocation intermediate and results in racemization.
3SN2 rates depend on both substrate and nucleophile ( $Rate = k[RX][Nu]$ ), while SN1 rates depend only on the substrate ( $Rate = k[RX]$ ).
4Polar aprotic solvents accelerate SN2, while polar protic solvents facilitate SN1 by stabilizing ions.

Quick Check

Multiple Choice

Which of the following would increase the rate of an SN1 reaction?

Multiple Choice

What is the characteristic stereochemical outcome of an SN2 reaction at a chiral center?

True/False

Polar protic solvents like water favor SN2 reactions by increasing the nucleophilicity of anions.

What's Next?

Review Tomorrow

In 24 hours, try to draw the transition state of an SN2 reaction and the intermediate of an SN1 reaction from memory.

Practice Activity

Create a table comparing SN1 and SN2 across four categories: Kinetics, Mechanism, Stereochemistry, and Preferred Solvent.

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Nucleophilic Substitution Mechanisms

Learning Objectives

The SN2 Mechanism: The One-Step Dance

The SN1 Mechanism: The Two-Step Breakup

Choosing the Path: Solvents and Nucleophiles

Key Takeaways

Quick Check

What's Next?

Nucleophilic Substitution Mechanisms

Learning Objectives

The SN2 Mechanism: The One-Step Dance

The SN1 Mechanism: The Two-Step Breakup

Choosing the Path: Solvents and Nucleophiles

Key Takeaways

Quick Check

What's Next?