`underset("(1 e.q)")(CH_(3)-underset(O)underset(||)(C)-H)+underset("Excess")(H-underset(O)underset(||)(C)-H) overset(OH^(-))to ` product

Answer

To solve the given question, we will follow these steps:

### Step 1: Identify the Reactants
The reactants provided are:
- Acetaldehyde (CH₃CHO)
- Formaldehyde (HCHO)

### Step 2: Determine the Reaction Type
The reaction taking place is an **aldol condensation**. This occurs when two aldehydes react in the presence of a base, leading to the formation of a β-hydroxy aldehyde (aldol) which can further dehydrate to form an α,β-unsaturated aldehyde.

### Step 3: Write the Reaction Mechanism
1. **Formation of Enolate Ion**: In a basic medium, acetaldehyde (CH₃CHO) will form an enolate ion. The base (OH⁻) abstracts a proton from the alpha carbon of acetaldehyde.

\[ CH₃CHO + OH⁻ \rightarrow CH₂=CHO⁻ \]

2. **Nucleophilic Attack**: The enolate ion then acts as a nucleophile and attacks the carbonyl carbon of formaldehyde (HCHO).

\[ CH₂=CHO⁻ + HCHO \rightarrow CH₂(OH)CH₂CHO \]

3. **Formation of Aldol**: The product formed is a β-hydroxy aldehyde (aldol).

\[ CH₂(OH)CH₂CHO \quad \text{(3-hydroxybutanal)} \]

### Step 4: Dehydration to Form Alkene
4. **Dehydration**: The aldol product can undergo dehydration (loss of water) upon heating to form an α,β-unsaturated aldehyde.

\[ CH₂(OH)CH₂CHO \xrightarrow{\Delta} CH₂=CHCHO + H₂O \]

5. **Final Product**: The final product after dehydration is crotonaldehyde (CH₂=CHCHO).

### Summary of the Reaction
The overall reaction can be summarized as:
- Starting from acetaldehyde and formaldehyde in basic medium, the aldol condensation occurs, leading to the formation of 3-hydroxybutanal, which upon heating gives crotonaldehyde.

### Final Products
1. **Aldol Product**: 3-hydroxybutanal (CH₂(OH)CH₂CHO)
2. **Dehydrated Product**: Crotonaldehyde (CH₂=CHCHO)

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