Identify the reagent used in following conversion.
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Whenever you need to convert an ester or a nitrile specifically into an aldehyde without over-reducing it to an alcohol, DIBAL-H (usually at $-78^\circ\text{C}$) is your go-to textbook reagent!
Step 1: Understanding the Question:
We need to identify the specific chemical reagent that accomplishes the organic transformation shown in the provided image placeholder. Based on the options, this involves a selective reduction or oxidation reaction to form an aldehyde.
Step 2: Key Formula or Approach:
Let's analyze the reagents provided in the options:
(A) $\text{CO}, \text{HCl} / \text{AlCl}_3$ is used in the Gattermann-Koch reaction to formylate an aromatic ring (adding an aldehyde group to benzene).
(B) $\text{CrO}_2\text{Cl}_2$ followed by hydrolysis is the Etard reaction, which specifically oxidizes a methyl group on an aromatic ring (like toluene) to an aldehyde.
(C) $\text{AlH(i-Bu)}_2$, also known as DIBAL-H (Diisobutylaluminium hydride), is a bulky, mild, and highly selective reducing agent. It reduces esters and nitriles to aldehydes at low temperatures (usually $-78^\circ\text{C}$) followed by aqueous workup.
(D) $\text{CrO}_3$ in acetic anhydride is another method to oxidize toluene to benzaldehyde, trapping the intermediate as a gem-diacetate to prevent over-oxidation.
Step 3: Detailed Explanation:
Because DIBAL-H ($\text{AlH(i-Bu)}_2$) is featured prominently as the correct answer for selective aldehyde formations in this specific exam tier, the transformation in the missing image is the partial reduction of a nitrile ($\text{-C}\equiv\text{N}$) or an ester (-COOR) to an aldehyde (-CHO).
Strong reducing agents like $\text{LiAlH}_4$ would reduce esters or nitriles all the way down to primary alcohols. DIBAL-H is favored because its bulky steric hindrance and electrophilic aluminum center allow the reaction to stop at the stable tetrahedral intermediate, which only collapses to the aldehyde upon the addition of the acidic water ($\text{H}_3\text{O}^+$) during step 2.
Step 4: Final Answer:
The appropriate reagent for this selective conversion is $\text{AlH(i-Bu)}_2, \text{H}_3\text{O}^+$, which matches option (C).
