Question

What are X and Y respectively in the following reaction sequence? 
\(\text{Iso-pentane} \xrightarrow{\text{KMnO}_4} X \xrightarrow{\substack{20\% \text{H}_3\text{PO}_4 \\ 358\text{K}}} Y \)

 

• A. 1
• B. 2
• C. 3
• D. 4

Hint:

Strong oxidation of alkanes by $\text{KMnO}_4$ occurs most readily at the tertiary C-H bond, replacing it with C-OH. Dehydration of alcohols follows Zaitsev's rule, favoring the formation of the most stable (most substituted) alkene product.

Answer 1

The Correct Option is B

Step 1: Determine the structure of Isopentane and analyze the first reaction.
Isopentane is the common name for 2-Methylbutane. Its structure is $\text{CH}_3-\text{CH}(\text{CH}_3)-\text{CH}_2-\text{CH}_3$.
The reaction is $\text{Isopentane} \xrightarrow{\text{KMnO}_4} \text{X}$. $\text{KMnO}_4$ is a strong oxidizing agent.
Alkanes are generally resistant to oxidation, but strong oxidizing agents at high temperature can lead to a reaction, usually at the tertiary C-H bond.
In 2-Methylbutane, the tertiary carbon is at C2 ($\text{-CH}(\text{CH}_3)\text{-}$). The oxidation will replace the tertiary C-H bond with a C-OH bond.
\[ \text{CH}_3-\text{CH}(\text{CH}_3)-\text{CH}_2-\text{CH}_3 \xrightarrow{\text{KMnO}_4} \text{CH}_3-\text{C}(\text{OH})(\text{CH}_3)-\text{CH}_2-\text{CH}_3. \] The product X is 2-Methylbutan-2-ol, which corresponds to Structure 3.

Step 2: Analyze the second reaction: Dehydration of X.
The second reaction is $\text{X} \xrightarrow{\text{Acid catalyst, Heat}} \text{Y}$.
X is 2-Methylbutan-2-ol. The reagents $\text{H}_3\text{PO}_4$ (phosphoric acid) and heat cause the dehydration of the alcohol to form an alkene.
The reaction follows Zaitsev's rule: the most substituted (most stable) alkene is the major product.
The -OH group is on C2. The $\text{H}$ atom can be removed from C1 ($\text{-CH}_3$), C3 ($\text{-CH}_2\text{-}$), or the $\text{-CH}_3$ attached to C2.
Removal of $\text{H}$ from C3 gives 2-Methyl-2-butene (Zaitsev's product, most substituted).
Removal of $\text{H}$ from C1 gives 2-Methyl-1-butene (Hoffmann product).
\[ \text{CH}_3-\text{C}(\text{OH})(\text{CH}_3)-\text{CH}_2-\text{CH}_3 \xrightarrow{\text{Dehydration}} \text{CH}_3-\text{C}(\text{CH}_3)=\text{CH}-\text{CH}_3 + \text{H}_2\text{O}. \] The major product Y is 2-Methyl-2-butene, which corresponds to Structure 4.

Step 3: Conclude the final correct option.
X is 2-Methylbutan-2-ol (Structure 3), and Y is 2-Methyl-2-butene (Structure 4).
The pair (X, Y) is (Structure 3, Structure 4).