Question

Identify the product formed when tertiary butyl bromide reacts with alcoholic $\text{NH}_3$ solution?

• A. 2-Methylpropene
• B. But-2-ene
• C. But-1-ene
• D. 2-Methylpropan-1-ol

Hint:

Whenever a tertiary ($3^\circ$) alkyl halide is treated with basic reagents (like alcoholic $\text{NH}_3$ or alcoholic KOH), steric hindrance makes nucleophilic substitution extremely difficult.
Instead, elimination wins out, and an alkene is always the principal product!

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
The problem asks for the major organic product when a tertiary alkyl halide, tertiary butyl bromide, is treated with a solution of alcoholic ammonia ($\text{NH}_3$).
We must analyze whether the reaction favors substitution ($S_N1$/$S_N2$) or elimination ($E1$/$E2$) pathways given the nature of the substrate and the reagent.

Step 2: Key Formula or Approach:
Tertiary butyl bromide is a highly sterically hindered tertiary ($3^\circ$) alkyl halide.
Alcoholic ammonia acts as a weak nucleophile but behaves as a base at elevated temperatures.
For tertiary substrates, substitution via an $S_N2$ mechanism is completely blocked due to intense steric hindrance, while $S_N1$ substitution competes with elimination ($E1$/$E2$). When heated in an alcoholic medium, elimination reactions strongly dominate over substitution.

Step 3: Detailed Explanation:
The structural formula of tertiary butyl bromide is $(\text{CH}_3)_3\text{C-Br}$.
When treated with alcoholic ammonia, the ethoxide ions/ammonia molecule abstract a $\beta$-hydrogen atom from one of the three equivalent methyl groups.
Simultaneously, the bromide leaving group departs, leading to a dehydrohalogenation reaction following an elimination mechanism.
Let's track the elimination process:
$$(\text{CH}_3)_3\text{C-Br} \xrightarrow{\text{alcoholic NH}_3} \text{CH}_2=\text{C}(\text{CH}_3)_2 + \text{HBr}$$
The resulting hydrocarbon contains a three-carbon principal chain with a double bond at position 1 and a methyl substituent at position 2.
According to IUPAC nomenclature, this compound is named 2-methylpropene (commonly known as isobutylene).

Step 4: Final Answer:
The dominant process is elimination, producing 2-methylpropene as the major product, which corresponds to option (A).