Select the molecules which has only one pi bond

Options

A. `CH-=CH`
B. `CH_(2)=CHCHO`
C. `CH_(3)=CH=CH_(2)`
D. `CH_(3)C-=C COOH`

Explanation

To determine which molecule has only one pi bond, we will analyze the given options step by step.

### Step 1: Understand the types of bonds
- A **sigma bond** is the first bond formed between two atoms and is a single bond.
- A **pi bond** is formed after a sigma bond and is present in double and triple bonds. A double bond consists of one sigma bond and one pi bond, while a triple bond consists of one sigma bond and two pi bonds.

### Step 2: Analyze each option
1. **First Option: Triple Bond (e.g., C≡C)**
- A triple bond consists of 1 sigma bond and 2 pi bonds.
- **Total Pi Bonds:** 2

2. **Second Option: CH2=CH-CHO**
- This molecule has:
- CH2=CH (double bond) contributes 1 sigma and 1 pi bond.
- C=O (in the carbonyl group) contributes 1 sigma and 1 pi bond.
- **Total Pi Bonds:** 2 (from double bond) + 1 (from carbonyl) = 3

3. **Third Option: CH3-CH=CH2**
- This molecule has:
- CH=CH (double bond) contributes 1 sigma and 1 pi bond.
- **Total Pi Bonds:** 1 (from the double bond)

4. **Fourth Option: CH3-C≡C-CHO**
- This molecule has:
- C≡C (triple bond) contributes 1 sigma and 2 pi bonds.
- C=O (in the carbonyl group) contributes 1 sigma and 1 pi bond.
- **Total Pi Bonds:** 2 (from triple bond) + 1 (from carbonyl) = 3

### Step 3: Conclusion
After analyzing all options, the only molecule that has exactly one pi bond is the **third option: CH3-CH=CH2**.

### Final Answer
The molecule that has only one pi bond is **CH3-CH=CH2**.

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