Nucleophilic substitution is a fundamental reaction in organic chemistry in which a nucleophile attacks an electrophile, resulting in the replacement of a leaving group. This type of reaction occurs mainly with alkyl halides, where the leaving group is typically a halogen (such as Cl, Br, or I).
There are two primary mechanisms for nucleophilic substitution reactions:
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SN1 Mechanism (Unimolecular Nucleophilic Substitution):
- This mechanism involves two steps. First, the leaving group departs to form a carbocation intermediate. Then, the nucleophile attacks the carbocation, leading to the formation of the product.
- SN1 reactions are common with tertiarysubstrates where the carbocation is stabilized by surrounding alkyl groups.
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SN2 Mechanism (Bimolecular Nucleophilic Substitution):
- This mechanism involves a single concerted step with both the nucleophile and the substrate participating in the rate-determining step. The nucleophile attacks the electrophilic carbon from the opposite side of the leaving group, resulting in the inversion of configuration at that carbon center.
- SN2 reactions typically occur with primary and some secondary substrates where steric hindrance is minimal.
Example of Nucleophilic Substitution: SN2 Reaction
Reaction:
Let's consider the reaction of sodium hydroxide (NaOH) with bromoethane (ethyl bromide, C₂H₅Br):
\[ \text{C}_2\text{H}_5\text{Br} + \text{NaOH} \rightarrow \text{C}_2\text{H}_5\text{OH} + \text{NaBr} \]
Mechanism:
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Nucleophile Attack:
- Hydroxide ion (OH⁻) acts as the nucleophile. It attacks the carbon atom bonded to the bromine atom (the electrophile).
- As the nucleophile approaches, the C–Br bond begins to break.
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Transition State:
- An unstable transition state is formed where both the OH group and the Br group are partially bonded to the electrophilic carbon. This is referred to as a "tetrahedral transition state".
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Product Formation:
- The bromine atom leaves (since it is a good leaving group), and the bond between the carbon and the hydroxide ion is formed, resulting in the formation of ethanol (C₂H₅OH) and sodium bromide (NaBr).
Diagram of SN2 Mechanism:
Br
|
C2H5 --------> C2H5-OH
| \
O- Br
Conclusion:
In summary, nucleophilic substitution can occur via either the SN1 or SN2 mechanism depending on the structure of the substrate and the nature of the nucleophile. The example of bromoethane reacting with hydroxide to form ethanol illustrates the SN2 mechanism effectively.
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