A nucleophilic substitution reaction is a type of chemical reaction in which a nucleophile attacks an electrophile, resulting in the replacement of a leaving group in a molecule. This type of reaction is common in organic chemistry, particularly in the substitution of alkyl halides and similar compounds. There are two primary mechanisms for nucleophilic substitution: SN1 and SN2.
SN2 Mechanism
The SN2 (Substitution Nucleophilic Bimolecular) mechanism involves a single concerted step where the nucleophile attacks the electrophile at the same time the leaving group departs. This results in the inversion of stereochemistry at the carbon center being attacked.
Example of SN2 Reaction
Let’s consider the reaction of sodium hydroxide (NaOH) with bromoethane (C2H5Br):
-
Starting Materials: \[ \text{C}_2\text{H}_5\text{Br} + \text{NaOH} \rightarrow \text{C}_2\text{H}_5\text{OH} + \text{NaBr} \]
-
Mechanism:
- The oxygen atom of the hydroxide ion (OH⁻) acts as a nucleophile and attacks the electrophilic carbon atom of bromoethane (C2H5Br).
- The bond between the carbon and the bromine (the leaving group) breaks.
- This leads to the formation of ethanol (C2H5OH), while bromide (Br⁻) leaves.
The mechanism can be represented as follows:
\[ \text{Nucleophile:} \quad \text{OH}^- \quad \quad \quad \text{Electrophile:} \quad \text{C}_2\text{H}_5\text{Br} \]
\[ \begin{array}{c} \text{OH}^- \quad \quad \quad \quad \quad \ | \ C_2 H_5 \text{---} \text{Br} \ \rightarrow \ \text{C}_2\text{H}_5\text{OH} + \text{Br}^- \end{array} \]
SN1 Mechanism
The SN1 (Substitution Nucleophilic Unimolecular) mechanism involves two steps. The first step is the formation of a carbocation intermediate after the leaving group departs, followed by the nucleophilic attack.
Example of SN1 Reaction
Let’s consider the reaction of tert-butyl chloride (C4H9Cl) with water:
-
Starting Materials: \[ \text{C}_4\text{H}_9\text{Cl} + \text{H}_2\text{O} \rightarrow \text{C}_4\text{H}_9\text{OH} + \text{HCl} \]
-
Mechanism:
- Step 1: The leaving group (Cl⁻) departs, forming a stable carbocation (tert-butyl cation).
\[ \text{C}_4\text{H}_9\text{Cl} \rightarrow \text{C}_4\text{H}_9^+ + \text{Cl}^- \]
- Step 2: The water molecule acts as a nucleophile and attacks the carbocation, forming tert-butyl alcohol (C4H9OH).
\[ \text{C}_4\text{H}_9^+ + \text{H}_2\text{O} \rightarrow \text{C}_4\text{H}_9\text{OH} \]
Summary
- SN2 Reaction: A single concerted mechanism, resulting in inversion of stereochemistry. Example: bromoethane reacting with hydroxide ion.
- SN1 Reaction: A two-step mechanism involving a carbocation intermediate. Example: tert-butyl chloride reacting with water.
Each mechanism is favored in different conditions: SN2 is favored by primary alkyl halides and strong nucleophiles, while SN1 is favored by tertiary alkyl halides and weak nucleophiles due to the stability of the carbocation intermediate.
1 answer