Which of the following compounds cannot exist in cis and trans forms?

Options

A. But-2-ene
B. But-2-ene-1, 4-dioic acid
C. 2-bromo-3-methylbut-2-ene
D. But-2-enoic acid

Explanation

To determine which of the given compounds cannot exist in cis and trans forms, we need to analyze the structural features of each compound. Cis and trans forms are types of geometric isomerism that occur in alkenes due to the restricted rotation around the double bond. For a compound to exhibit cis and trans isomerism, it must have two different substituents on each carbon of the double bond.

### Step-by-Step Solution:

1. **Identify the Compounds**:
- Option 1: But-2-ene
- Option 2: But-2-ene-1,4-dioic acid
- Option 3: 2-bromo-3-methyl-but-2-ene
- Option 4: But-2-enoic acid

2. **Analyze Each Compound**:
- **But-2-ene**:
- Structure: CH3-CH=CH-CH3
- Substituents on the double bond: Methyl (CH3) and Hydrogen (H) on each carbon.
- It can exist in both cis (both CH3 groups on the same side) and trans (CH3 groups on opposite sides) forms.

- **But-2-ene-1,4-dioic acid**:
- Structure: HOOC-CH=CH-COOH
- Substituents on the double bond: Carboxylic acid (COOH) and Hydrogen (H).
- It can also exist in both cis and trans forms.

- **2-bromo-3-methyl-but-2-ene**:
- Structure: CH3-CH=CH(CH3)-Br
- Substituents on the double bond: Methyl (CH3) and Bromine (Br).
- In this case, one carbon has a bromine and the other has two methyl groups. Since both substituents on one side are the same (methyl), it does not allow for distinct cis and trans forms.

- **But-2-enoic acid**:
- Structure: CH3-CH=CH-COOH
- Substituents on the double bond: Methyl (CH3) and Hydrogen (H).
- It can exist in both cis and trans forms.

3. **Conclusion**:
- The compound that cannot exist in cis and trans forms is **2-bromo-3-methyl-but-2-ene** (Option 3) because it has identical substituents on one side of the double bond.

### Final Answer:
The compound that cannot exist in cis and trans forms is **2-bromo-3-methyl-but-2-ene**.