In which compound, Cis-Trans nomenclature cannot be used?

Options

A. `CH_(3)-CH=CH-CH_(3)`
B. `CH_(3)-CH=CH-COOH`
C.
D. `C_(6)H_(5)-CH=CH-CHO`

Answer

To determine in which compound cis-trans nomenclature cannot be used, we need to analyze the conditions required for cis-trans isomerism (geometric isomerism). Here’s a step-by-step breakdown of the solution:

### Step 1: Understand the Conditions for Cis-Trans Isomerism
Cis-trans isomerism occurs in compounds with:
1. **Planarity**: The molecule must be planar, meaning it should lie flat in a single plane.
2. **Different Groups on sp² Carbons**: Each carbon atom involved in the double bond (sp² hybridized) must have two different groups attached to it.

### Step 2: Identify the Compounds
Let’s evaluate the compounds given in the options:
1. **Compound 1**: Contains a double bond with one hydrogen and one CH₃ group on one carbon, and two different groups on the other carbon. This can show cis-trans isomerism.
2. **Compound 2**: Has one hydrogen and one CH₃ on one carbon, and one hydrogen and one COH on the other carbon. This can also show cis-trans isomerism.
3. **Compound 3**: One carbon has bromine and chlorine (two different groups), and the other carbon has a methyl and an ethyl group (two different groups). However, all groups are different, which means it cannot show cis-trans isomerism.
4. **Compound 4**: One carbon has one hydrogen and one C₆H₅ group, and the other carbon has one hydrogen and one CHO group. This can show cis-trans isomerism.

### Step 3: Conclusion
From the analysis, **Compound 3** cannot exhibit cis-trans isomerism because it does not satisfy the requirement of having two identical groups on either side of the double bond.

### Final Answer
The compound in which cis-trans nomenclature cannot be used is **Option 3**.

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