Animated Solution for Chemistry - Organic Chemistry: What is the product of following reaction? Hex-3-ynal $\stackrel{{\text{(i) NaBH}}_4}{\to }\stackrel{{\text{(ii) PBr}}_3}{\to }\stackrel{\text{(iii) Mg/ether}}{\to }\stackrel{{\text{(iv) CO}}_2/{\text{H}}_3{\text{O}}^{+}}{\to }?$

This problem is a beautiful journey through a classic multi-step organic synthesis. It tests your understanding of chemoselectivity, functional group transformations, and carbon chain extension. Let's break down the blueprint of this reaction sequence step by step.

Analyzing the Starting Material

We begin our journey with Hex-3-ynal. By breaking down its IUPAC name, we can visualize its structure: a six-carbon chain (`Hex`), a triple bond starting at carbon 3 (`-3-yn`), and an aldehyde group at carbon 1 (`-al`). The presence of two distinct functional groups—a polar carbonyl group and a non-polar alkyne—sets the stage for a test of chemoselectivity.

The Chemoselective Reduction

Our first reagent is Sodium Borohydride (${\text{NaBH}}_4$). This is a mild, chemoselective reducing agent. It acts as a source of hydride ions (${\text{H}}^{-}$), which are strongly attracted to the electrophilic carbon of polar double bonds, such as the $\text{C=O}$ bond in aldehydes and ketones.

However, the $\text{C}\equiv \text{C}$ triple bond is non-polar and surrounded by a dense cloud of $\pi$-electrons. It strongly repels the incoming hydride ion. As a result, ${\text{NaBH}}_4$ selectively reduces the aldehyde to a primary alcohol while leaving the alkyne completely untouched.

Our first intermediate is Hex-3-yn-1-ol.

Bromination - Preparing for the Magic

Next, we treat our newly formed alcohol with Phosphorus Tribromide (${\text{PBr}}_3$). This is a standard reagent for converting primary and secondary alcohols into their corresponding alkyl bromides. The reaction proceeds via a nucleophilic substitution mechanism, where the hydroxyl ($\text{-OH}$) group is cleanly replaced by a bromine ($\text{-Br}$) atom.

This transformation gives us 1-bromohex-3-yne. The triple bond remains safely intact, waiting for its final destination.

Umpolung - The Grignard Formation

Now, we introduce Magnesium turnings in a dry ether solvent. This is the classic recipe for creating a Grignard reagent. The magnesium atom inserts itself directly into the carbon-bromine bond.

This step is fascinating because it completely reverses the chemical personality of the carbon atom—a concept known as Umpolung (polarity reversal). In the alkyl bromide, the carbon was electrophilic (electron-poor) due to the electronegative bromine. But in the Grignard reagent, the carbon becomes highly nucleophilic (electron-rich) because magnesium is far less electronegative.

We have now successfully synthesized Hex-3-ynylmagnesium bromide.

Carboxylation - Extending the Chain

In the grand finale, we bubble Carbon Dioxide (${\text{CO}}_2$) through our Grignard solution, followed by an acidic workup (${\text{H}}_3{\text{O}}^{+}$).

Our highly nucleophilic Grignard carbon aggressively attacks the electrophilic central carbon of the ${\text{CO}}_2$ molecule. This crucial step forms a new carbon-carbon bond, effectively extending our carbon chain by exactly one atom. The immediate product is a magnesium carboxylate salt.

To release the free acid, we perform an acidic workup. The hydronium ions (${\text{H}}_3{\text{O}}^{+}$) protonate the carboxylate oxygen, yielding our final carboxylic acid.

Identifying the Product

Let's trace our final carbon skeleton. We started with 6 carbons and added 1 via ${\text{CO}}_2$, giving us a 7-carbon chain. The new carboxylic acid carbon is designated as C1. Counting backwards, our original triple bond, which was between C3 and C4 in the starting material, is now shifted by one position relative to the new C1. It now resides between C4 and C5.

The final molecule is Hept-4-ynoic acid. When we compare this structure to the given visual options, it perfectly matches the linear triple bond geometry shown in option (d).