Question

Identify the reagent used in following conversion:
Pent-3-enenitrile $\xrightarrow{\text{A}}$ pent-3-enal

• A. $\text{AlH}(i\text{-Bu})_2 / \text{H}_3\text{O}^+$
• B. $\text{K}_2\text{Cr}_2\text{O}_7 / \text{H}_2\text{SO}_4$
• C. $\text{H}_2 / \text{Pd} \cdot \text{BaSO}_4$
• D. $\text{SnCl}_2 \cdot \text{HCl}$

Hint:

Whenever you see a conversion where a nitrile (-CN) needs to be neatly transformed into an aldehyde (-CHO) while keeping a sensitive double bond (C=C) completely safe, DIBAL-H ($\text{AlH}(i\text{-Bu})_2$) is almost always the designated textbook answer!

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
The question asks us to identify the specific chemical reagent needed to convert an unsaturated nitrile (pent-3-enenitrile) into an unsaturated aldehyde (pent-3-enal) without affecting the carbon-carbon double bond (C=C).

Step 2: Key Formula or Approach:
The selective reduction of nitriles ($\text{-C}\equiv\text{N}$) directly to aldehydes (-CHO) requires a specialized, sterically hindered reducing agent. Diisobutylaluminum hydride (DIBAL-H or $\text{AlH}(i\text{-Bu})_2$) at low temperatures selectively reduces nitriles to imines, which are then easily hydrolyzed by an acid ($\text{H}_3\text{O}^+$) to form aldehydes. Crucially, DIBAL-H does not reduce isolated carbon-carbon double bonds.

Step 3: Detailed Explanation:
Let's analyze the given options: 1.

$\text{AlH}(i\text{-Bu})_2 / \text{H}_3\text{O}^+$ (DIBAL-H): It coordinates with the nitrile nitrogen and transfers a single hydride ion to the carbon. This stops cleanly at the imine stage, which upon acid workup yields the corresponding aldehyde. It leaves the $\text{CH}_3\text{-CH=CH-CH}_2\text{-}$ chain completely untouched: $$\text{CH}_3\text{-CH=CH-CH}_2\text{-C}\equiv\text{N} \xrightarrow{\text{(i) DIBAL-H, (ii) }\text{H}_3\text{O}^+} \text{CH}_3\text{-CH=CH-CH}_2\text{-CHO}$$ 2.

$\text{K}_2\text{Cr}_2\text{O}_7 / \text{H}_2\text{SO}_4$: This is a powerful oxidizing agent, not a reducing agent. It would instead aggressively cleave or oxidize the double bond and cannot convert nitriles to aldehydes.
3.

$\text{H}_2 / \text{Pd} \cdot \text{BaSO}_4$: This is Rosenmund's catalyst structure, which is typically used for reducing acyl chlorides to aldehydes, not nitriles.
4.

$\text{SnCl}_2 \cdot \text{HCl}$: This combination belongs to the Stephen reduction. However, it is primarily effective for simple aliphatic and aromatic nitriles and is less suitable or selective when dealing with highly unsaturated systems compared to DIBAL-H.

Step 4: Final Answer:
The most appropriate selective reagent is $\text{AlH}(i\text{-Bu})_2 / \text{H}_3\text{O}^+$, which corresponds to option (A).