By which of the following reactions can methylcyanide be prepared?

Options

A. `CH_3 Br underset("DMF") overset("KCN") (to)`
B. `CH_(3) NH_(2) + CHCI_(3) overset("KOH") (to)`
C. `CH_3 - CH = N - OH underset(Delta) overset(P_2 O_5) (to)`
D. `CH_3 - overset(overset(O)(||))C - NH_2 underset(Delta) overset(P_4 O_10) (to)`

Answer

To determine how methyl cyanide (also known as acetonitrile, CH3C≡N) can be prepared, we will analyze the given reactions one by one.

### Step-by-Step Solution:

1. **Identify Methyl Cyanide**:
Methyl cyanide is represented as CH3C≡N. It is a nitrile, which means it contains a cyano group (-C≡N).

2. **Evaluate Each Reaction**:
We will evaluate the four reactions provided to see which ones can produce methyl cyanide.

- **Reaction 1**:
\[ \text{CH}_3\text{Br} + \text{KCN} \rightarrow \text{CH}_3\text{C} \equiv \text{N} + \text{KBr} \]
This reaction is correct. Methyl bromide (CH3Br) reacts with potassium cyanide (KCN) to form methyl cyanide (CH3C≡N) and potassium bromide (KBr).

- **Reaction 2**:
\[ \text{CHCl}_3 + \text{KOH} + \text{amine} \rightarrow \text{isocyanide} \]
This reaction is incorrect for producing methyl cyanide. Instead, it leads to the formation of isocyanide, not the desired nitrile.

- **Reaction 3**:
\[ \text{CH}_3\text{OH} + \text{P}_2\text{O}_5 \rightarrow \text{CH}_3\text{C} \equiv \text{N} \]
This reaction is also correct. The dehydration of methanol (CH3OH) using phosphorus pentoxide (P2O5) can lead to the formation of methyl cyanide.

- **Reaction 4**:
\[ \text{CH}_3\text{NH}_2 + \text{P}_4\text{O}_{10} \rightarrow \text{CH}_3\text{C} \equiv \text{N} \]
This reaction is also correct. The reaction of methylamine (CH3NH2) with phosphorus pentoxide (P4O10) can yield methyl cyanide.

3. **Conclusion**:
The reactions that can prepare methyl cyanide are:
- Reaction 1
- Reaction 3
- Reaction 4

Therefore, the correct answer is options 1, 3, and 4.

### Final Answer:
Methyl cyanide can be prepared by the following reactions:
- Option 1
- Option 3
- Option 4