In the reaction sequences
`2CH_(3)CHO overset(OH^(-))toA overset(Delta)toB` the product B is

Options

A. `CH_(3)-CH_(2)-CH_2-CH_(2)-OH`
B. `CH_(3)-CH=CH-CHO`
C. `CH_(3)-CH_(2)-CH_(2)-CH_3`
D. `CH_(3)-overset(O)overset(||)(C)-CH_(2)`

Answer

To solve the problem, we need to analyze the reaction sequence given in the question, which involves the condensation of acetaldehyde (CH₃CHO) in the presence of hydroxide ions (OH⁻) and subsequent heating.

### Step-by-Step Solution:

1. **Identify the Initial Reactant:**
The initial reactant is acetaldehyde (CH₃CHO). We have 2 moles of it.

2. **Recognize the Reaction Type:**
The reaction involves the aldol condensation. In the presence of a base (OH⁻), two molecules of acetaldehyde will undergo a condensation reaction.

3. **Formation of Beta-Hydroxy Aldehyde (Product A):**
When 2 moles of acetaldehyde react, they form a beta-hydroxy aldehyde. The reaction can be represented as:
\[ 2 \, \text{CH}_3\text{CHO} \xrightarrow{\text{OH}^-} \text{CH}_3\text{CH(OH)CH}_2\text{CHO} \]
This product is known as 3-hydroxybutanal.

4. **Heating the Beta-Hydroxy Aldehyde:**
Upon heating, the beta-hydroxy aldehyde undergoes dehydration (loss of water) to form an alpha, beta-unsaturated aldehyde (Product B). The dehydration reaction can be represented as:
\[ \text{CH}_3\text{CH(OH)CH}_2\text{CHO} \xrightarrow{\Delta} \text{CH}_3\text{CH=CHCHO} + \text{H}_2\text{O} \]
The product formed is 2-butenal (or crotonaldehyde), which has the structure CH₃CH=CHCHO.

5. **Identify the Final Product (B):**
The final product B is therefore:
\[ \text{CH}_3\text{CH=CHCHO} \]
This corresponds to option B from the given choices.

### Conclusion:
The product B is CH₃CH=CHCHO, which is an alpha, beta-unsaturated aldehyde.