Step 1: Understanding the Concept:
DIBAL-H (\(\text{AlH}(i\text{-Bu})_2\)) is a selective reducing agent that can reduce nitriles (\(-\text{CN}\)) to aldehydes (\(-\text{CHO}\)) at low temperatures without affecting carbon-carbon double bonds.
Step 2: Key Formula or Approach:
\[ \text{R-CN} \xrightarrow{\text{DIBAL-H, then } \text{H}_3\text{O}^+} \text{R-CHO} \]
Step 3: Detailed Explanation:
The product is Pent-3-enal, which is an unsaturated aldehyde: \(\text{CH}_3\text{-CH=CH-CH}_2\text{-CHO}\).
Since the reagent converts a nitrile to an aldehyde, the starting material 'A' must be a nitrile with the same carbon skeleton and double bond position.
The corresponding nitrile is Pent-3-enenitrile (\(\text{CH}_3\text{-CH=CH-CH}_2\text{-CN}\)).
Step 4: Final Answer:
The substrate 'A' is Pent-3-enenitrile.