`A underset(800^(@)C )overset(Delta)(rarr) CH_(2) =` C = O , Reactant ' A ' in the reaction is

Options

A. `CH_(3) CH_(2)` CHO
B. `CH_(3)`CHO
C. `CH_(3) - underset(O)underset(||)(C ) - CH_(3)`
D. `C_(2) H_(5)`OH

Explanation

To solve the question, we need to identify the reactant 'A' that, when heated at 800°C, produces the compound CH₂=C=O, which is known as ketene.

### Step-by-Step Solution:

1. **Identify the Product**: The product given is CH₂=C=O, which is ketene. Ketene has the general formula R₂C=C=O.

2. **Understand the Formation of Ketene**: Ketene is typically formed from aldehydes through a process called dehydrogenation. This involves the removal of hydrogen atoms from the aldehyde.

3. **Determine the Structure of the Precursor Aldehyde**: Since ketene has two carbon atoms (C=C), the precursor aldehyde must also have two carbon atoms. The simplest aldehyde with two carbon atoms is acetaldehyde (CH₃CHO).

4. **Reaction Mechanism**: When acetaldehyde is heated at a high temperature (800°C), it undergoes dehydrogenation:
- The reaction can be represented as:
\[ \text{CH}_3\text{CHO} \xrightarrow{800°C} \text{CH}_2=C=O + H_2 \]
- Here, one hydrogen atom is removed from the methyl group (CH₃) and one from the aldehyde (CHO), resulting in the formation of ketene (CH₂=C=O).

5. **Conclusion**: Therefore, the reactant 'A' that produces ketene when heated at 800°C is acetaldehyde (CH₃CHO).

### Final Answer:
The reactant 'A' is **acetaldehyde (CH₃CHO)**.

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