The first steps of `SN^(1)` and `SN^(2)` recatiosns are, respectively

Options

A. Both exothermic
B. Both endotherimic
C. Endothermic and exothermic
D. Exothermic and endothermic

Explanation

To solve the question regarding the first steps of SN1 and SN2 reactions, we need to understand the mechanisms of these two types of nucleophilic substitution reactions.

### Step-by-Step Solution:

1. **Understanding SN1 Mechanism:**
- The SN1 (Substitution Nucleophilic Unimolecular) reaction involves two steps:
- **Step 1:** The leaving group departs, forming a carbocation intermediate. This step requires energy to break the bond between the carbon and the leaving group, making it **endothermic**.
- **Step 2:** The nucleophile attacks the carbocation, resulting in the formation of the product. This step releases energy, making it **exothermic**.

2. **Understanding SN2 Mechanism:**
- The SN2 (Substitution Nucleophilic Bimolecular) reaction involves a single concerted step:
- The nucleophile attacks the carbon atom simultaneously as the leaving group departs. This step is typically **exothermic** because it involves the formation of a new bond while breaking an old bond.

3. **Summarizing the Steps:**
- For **SN1**:
- First step: **Endothermic** (formation of carbocation)
- Second step: **Exothermic** (nucleophile attacks)
- For **SN2**:
- Only step: **Exothermic** (concerted reaction)

4. **Conclusion:**
- The first step of SN1 is endothermic, and the first step of SN2 is exothermic. Therefore, the correct answer to the question is that the first steps of SN1 and SN2 reactions are **endothermic and exothermic**, respectively.

### Final Answer:
The first steps of SN1 and SN2 reactions are **endothermic and exothermic**, respectively.

---