in the following reaction
`"Aldeyde +ALcohol "overset( HCl) to " Acetal "`
`{:("Aldehyde ","Alcohol"),(HCHO,""^(t)BuOH),(CH_(3)CHO,MeOH):}`
the best combination is :

Options

A. HCHO and MeOH
B. `CH_3CHO and .^(t)BuOH`
C. `HCHO and .^(t)BuOH`
D. `CH_3CHO and MeOH`.

Explanation

To determine the best combination of aldehyde and alcohol that reacts to form an acetal in the presence of HCl, we can follow these steps:

### Step 1: Identify the Reactants
We have two types of reactants: aldehydes and alcohols. The options provided are:
- Aldehydes: HCHO (formaldehyde) and CH3CHO (acetaldehyde)
- Alcohols: t-BuOH (tertiary butyl alcohol) and MeOH (methanol)

### Step 2: Understand the Mechanism of Acetal Formation
In the formation of an acetal, the aldehyde acts as an electrophile and the alcohol acts as a nucleophile. The reaction involves:
1. Protonation of the carbonyl oxygen of the aldehyde by HCl.
2. Nucleophilic attack by the alcohol on the carbonyl carbon.
3. Formation of a hemiacetal intermediate.
4. Further reaction with another molecule of alcohol to form the acetal.

### Step 3: Evaluate the Electrophilicity of Aldehydes
- **Formaldehyde (HCHO)** has no alkyl groups attached to the carbonyl carbon, making it more electrophilic.
- **Acetaldehyde (CH3CHO)** has a methyl group, which donates electron density through the +I effect, making it less electrophilic than formaldehyde.

Thus, formaldehyde is the better choice for the reaction.

### Step 4: Evaluate the Nucleophilicity of Alcohols
- **Methanol (MeOH)** is a simple alcohol with no steric hindrance, allowing it to effectively attack the electrophilic carbonyl carbon.
- **Tertiary butyl alcohol (t-BuOH)** has three bulky methyl groups, which create steric hindrance, making it less effective as a nucleophile.

Therefore, methanol is the better nucleophile for this reaction.

### Step 5: Combine the Best Aldehyde and Alcohol
Based on the evaluations:
- The best aldehyde is **formaldehyde (HCHO)**.
- The best alcohol is **methanol (MeOH)**.

### Conclusion
The best combination for the reaction to form an acetal is:
**Formaldehyde (HCHO) + Methanol (MeOH) in the presence of HCl.**

### Final Answer
The best combination is: **(HCHO, MeOH)**.

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