Animated Solution for Chemistry - Organic Chemistry: Among the following the number of reaction(s) that produce(s) benzaldehyde is –

Imagine you are an organic chemist tasked with synthesizing benzaldehyde, a crucial aromatic compound that smells wonderfully of bitter almonds. You are presented with four distinct chemical pathways and asked a simple question: Which of these reactions will successfully yield benzaldehyde?

To answer this, we must dive deep into the mechanisms and the specific roles of the reagents involved in each reaction. Let's analyze them one by one and uncover the beautiful chemistry behind them.

The Gattermann-Koch Synthesis

Our first candidate is the reaction of benzene with carbon monoxide ($\text{CO}$) and hydrogen chloride ($\text{HCl}$) in the presence of anhydrous aluminum chloride (${\text{AlCl}}_3$) and copper(I) chloride ($\text{CuCl}$).

This is the legendary Gattermann-Koch formylation. Benzene is a stable, electron-rich aromatic ring, and to attach a formyl group ($\text{-CHO}$) to it, we need a highly reactive electrophile. The combination of $\text{CO}$ and $\text{HCl}$ with the Lewis acid ${\text{AlCl}}_3$ generates the formyl cation (${\text{HCO}}^{+}$).

This powerful electrophile attacks the benzene ring via an electrophilic aromatic substitution mechanism, seamlessly yielding our target molecule, benzaldehyde.

Hydrolysis of a Gem-Dihalide

The second reaction involves heating dichloromethylbenzene (commonly known as benzal chloride) with water at $100^{\circ }\text{C}$.

This is a classic nucleophilic substitution reaction where the two chlorine atoms are replaced by hydroxyl ($\text{-OH}$) groups, forming an intermediate known as a gem-diol (${\text{C}}_6{\text{H}}_5{\text{CH(OH)}}_2$).

However, there is a fundamental rule in organic chemistry: having two highly electronegative hydroxyl groups on the exact same carbon atom creates immense steric and electronic repulsion. The gem-diol is incredibly unstable. To relieve this stress, it spontaneously eliminates a water molecule (${\text{H}}_2\text{O}$), collapsing into a much more stable carbon-oxygen double bond.

And just like magic, the unstable intermediate transforms into benzaldehyde.

The Rosenmund Reduction

Next, we look at the reaction of benzoyl chloride (${\text{C}}_6{\text{H}}_5\text{COCl}$) with hydrogen gas (${\text{H}}_2$) over a palladium catalyst supported on barium sulfate (${\text{Pd-BaSO}}_4$).

This is the famous Rosenmund reduction. Normally, catalytic hydrogenation is a vigorous process that would reduce the acid chloride all the way down to a primary alcohol. But we want to stop exactly halfway, at the aldehyde stage.

How do we hit the brakes? We use barium sulfate (${\text{BaSO}}_4$) as a catalytic poison. It intentionally deactivates the palladium catalyst just enough so that it can reduce the highly reactive acid chloride, but it lacks the power to reduce the resulting aldehyde.

Thanks to this clever poisoning technique, we successfully isolate benzaldehyde.

DIBAL-H Reduction of Esters

Finally, we examine the treatment of methyl benzoate (an ester) with Diisobutylaluminum hydride ($\text{DIBAL-H}$) at a freezing $-78^{\circ }\text{C}$, followed by aqueous workup.

$\text{DIBAL-H}$ is a bulky, electrophilic reducing agent. When it attacks the ester, it forms a stable tetrahedral aluminum intermediate. The extremely low temperature ($-78^{\circ }\text{C}$) is the secret ingredient here—it prevents this intermediate from collapsing prematurely.

If the intermediate were to collapse while excess reducing agent was still present, the resulting aldehyde would be rapidly reduced to an alcohol. By keeping it frozen in the tetrahedral state until we add water (which destroys the remaining $\text{DIBAL-H}$ and hydrolyzes the intermediate), we ensure that the reaction stops perfectly at the aldehyde stage.

Once again, we have successfully synthesized benzaldehyde.

After a thorough investigation, we have discovered that all four pathways—the Gattermann-Koch synthesis, the hydrolysis of benzal chloride, the Rosenmund reduction, and the DIBAL-H reduction—are perfectly valid, textbook methods for preparing benzaldehyde.

Therefore, the total number of reactions that produce benzaldehyde is exactly 4.