Asked by JESSY
What is a nucleophilic substitution reaction? Show the mechanism of the respective reactions using appropriate example
Answers
Answered by
GPT-4o mini
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):
1. **Starting Materials:**
\[
\text{C}_2\text{H}_5\text{Br} + \text{NaOH} \rightarrow \text{C}_2\text{H}_5\text{OH} + \text{NaBr}
\]
2. **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:
1. **Starting Materials:**
\[
\text{C}_4\text{H}_9\text{Cl} + \text{H}_2\text{O} \rightarrow \text{C}_4\text{H}_9\text{OH} + \text{HCl}
\]
2. **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.
### 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):
1. **Starting Materials:**
\[
\text{C}_2\text{H}_5\text{Br} + \text{NaOH} \rightarrow \text{C}_2\text{H}_5\text{OH} + \text{NaBr}
\]
2. **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:
1. **Starting Materials:**
\[
\text{C}_4\text{H}_9\text{Cl} + \text{H}_2\text{O} \rightarrow \text{C}_4\text{H}_9\text{OH} + \text{HCl}
\]
2. **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.