Question

How many molecules of methyl iodide are required to obtain tetramethyl ammonium iodide from dimethyl amine?

• A. 1
• B. 3
• C. 2
• D. 4

Hint:

Simply calculate the difference in the number of methyl groups between your reactant and your target product. Dimethyl amine has 2 methyl groups, and tetramethyl ammonium iodide has 4. To bridge this gap, you must add exactly $4 - 2 = 2$ methyl groups via methyl iodide.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
We are required to find the exact stoichiometric equivalent molecules of methyl iodide ($\text{CH}_3\text{I}$) consumed when converting a starting secondary amine, dimethyl amine ($(\text{CH}_3)_2\text{NH}$), into a quaternary ammonium salt, tetramethyl ammonium iodide ($[\text{TSMA}]^+\text{I}^-$).

Step 2: Detailed Explanation:
The reaction described follows the Hofmann's exhaustive alkylation mechanism, where an amine undergoes sequential nucleophilic substitution reactions with an alkyl halide:

Reaction Step 1: Dimethyl amine (a secondary amine) reacts with the first molecule of methyl iodide to displace a hydrogen halide and form trimethyl amine (a tertiary amine):
$$ (\text{CH}_3)_2\text{NH} + \text{CH}_3\text{I} \rightarrow (\text{CH}_3)_3\text{N} + \text{HI} $$

Reaction Step 2: The lone pair on the newly formed tertiary trimethyl amine molecule attacks a second molecule of methyl iodide to produce the final quaternary ammonium salt:
$$ (\text{CH}_3)_3\text{N} + \text{CH}_3\text{I} \rightarrow (\text{CH}_3)_4\text{N}^+\text{I}^- $$ Counting the active reaction steps shows that exactly two molecules of methyl iodide are consumed during this complete process.

Step 3: Final Answer: The total number of molecules required is 2, matching option (C).