Which one of the following molecules cannot serve as a monomer for an addition polymer ?

Options

A. `ClCH=CH_2`
B. `H_2C = CHCN`
C. `H_2C = CH-C_6H_5`
D. `CH_3overset(O)overset(||)(C) -OH`

Answer

To determine which molecule cannot serve as a monomer for an addition polymer, we need to analyze the given options based on their structures and the requirements for addition polymerization.

### Step-by-Step Solution:

1. **Identify the given molecules**:
- Vinyl chloride (C2H3Cl)
- Acrylonitrile (C3H3N)
- Styrene (C8H8)
- Ethanoic acid (C2H4O2)

2. **Understand the concept of addition polymers**:
- Addition polymers are formed from monomers that contain at least one double bond (alkenes).
- During polymerization, the double bond opens up, allowing the monomers to link together to form long chains.

3. **Analyze each molecule**:
- **Vinyl chloride**:
- Structure: CH2=CHCl
- Contains a double bond (alkene), so it can polymerize to form polyvinyl chloride (PVC).
- **Acrylonitrile**:
- Structure: CH2=CH-CN
- Contains a double bond (alkene), so it can polymerize to form polyacrylonitrile.
- **Styrene**:
- Structure: C6H5-CH=CH2
- Contains a double bond (alkene), so it can polymerize to form polystyrene.
- **Ethanoic acid**:
- Structure: CH3COOH
- Does not contain a double bond; it is a carboxylic acid, which means it cannot undergo addition polymerization.

4. **Conclusion**:
- Among the given molecules, ethanoic acid (acetic acid) does not serve as a monomer for an addition polymer because it lacks the necessary double bond for polymerization.
- Therefore, the answer is **ethanoic acid**.

### Final Answer:
**Ethanoic acid (option 4) cannot serve as a monomer for an addition polymer.**