SN2 Reaction

S_N2 Reaction is a Nucleophilic Substitution reaction (A class of reactions wherein the electron-rich nucleophile attacks a positively charged electrophile to replace a leaving group) in which two components are involved in the rate-determining step. With simultaneous bond-making and bond-breaking steps, S_N2 reactions are bimolecular.

Some of the examples of this reaction mechanism are Swarts Reaction, Finkelstein Reaction and Williamson Ether Synthesis Reaction.

What is S_N2 Reaction?

The S_N2 reaction is a type of reaction mechanism that breaks one bond and synchronously forms one bond, i.e. it is a single-step reaction. The S_N2 reaction is a substitution reaction, the name of which refers to the Hughes-Ingold symbol of the mechanism. “S_N” stands for “substitution nucleophilic“, and the “2” says that the rate-determining step is bimolecular.

S_N2 Reaction Mechanism

• There is a single transition state, according to the S_N2 process, since bond-breaking and bond-making happen simultaneously. Note that the nucleophile must approach from the backside of the carbon (Wherein the front side has the leaving group) for this to occur (so-called backside attack ).

• In an S_N2 reaction, there is no intermediate, just a transition state. A transition state does not have a real lifetime, as starting materials transition into products, it is the highest energy point on the reaction coordinate.

Stereochemistry of S_N2 Reaction

Theoretically, There are two ways the nucleophile can target the substrate stereo center:

1. A frontside attack, where the nucleophile attacks from the same side as the leaving group, resulting in the preservation of the product’s stereochemical configuration.
2. A backside attack, where the nucleophile attacks from the opposite side of the carbon-leaving group bond, resulting in the product’s stereochemical structure being reversed.

Since a pure S_N2 reaction shows 100% inversion in stereochemical configuration, it is clear that these reactions occur via a backside attack.

The stereocenter configuration is reversed during an S_N2 reaction if the halide leaving group is connected to the stereocenter. This is because the nucleophile joins as the departing group (‘backside attack’) from the opposite side of the molecule.

Inhibition by Steric Hindrance

Reactions following S_N2 mechanisms are particularly sensitive to steric factors since they are retarded by steric hindrance (crowding) at the site of reaction.

In general, the order of reactivity of alkyl halides in S_N2 reactions is:
methyl > 1° > 2°.

The 3° alkyl halides are so crowded that they do not generally react by an S_N2 reaction mechanism. 

Characteristics of S_N2 Reaction 

• Bond forming and bond breakage take place in the same step. 
• Bimolecular and follows second-order kinetics.  
• Tertiary are unreactive.
• Rate of the reaction depends on the concentration of the substrate.
• A polar aprotic solvent is used to enhance the reactivity.  

Factors Affecting S_N2 Reaction 

• A unhindered Substrate(the back of the substrate must be as unhindered as possible).
• A strong Nucleophile.
• A polar aprotic Solvent  with low dielectric constant.
• A weaker leaving group.

S_N1 Reaction Vs. S_N2 Reaction
For S_N1 reactions, the step determining the rate is unimolecular, while for S_N2 reactions it is bimolecular.
S_N1 is a system of two-stage, while S_N2 is a process of single stage.
During S_N1 reactions, the carbocation forms as an intermediate while it is not formed during S_N2 reactions.

Examples

Question 1. Is this S_N2 reaction? Explain.

Solution. This is an S_N2 reaction. When there is a methyl halide with a strong nucleophile, the nucleophile will force the halide group to leave. Strong nucleophiles dictate S_N2 mechanisms.

Question 2. Which of the following would most readily undergo an S_N2 mechanism?

Solution. I^– is a better leaving group than Cl^– because it is a larger molecule and can distribute the negative charge over a larger area. S_N2 works better with better leaving groups and with less-substituted carbons (methyl > primary > secondary > tertiary) so IV undergoes S_N2 mechanism most readily. 

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