Q.(a) Explain the mechanism of Cannizaro reaction. OR

Clemmensen Reduction & Cannizzaro Reaction: Two Completely Different Reactions

These two reactions are often grouped together in textbooks because they both involve carbonyl compounds (C=O), but they do entirely different things. Let's take them one at a time.

Clemmensen Reduction

Intuition first. Imagine you have a ketone or aldehyde — a molecule with a C=O group. You want to remove that oxygen entirely and replace the C=O with two hydrogen atoms, turning it into a simple hydrocarbon chain. That's a reduction (adding hydrogen, removing oxygen). The Clemmensen reduction is a brute-force way to do this using a strongly acidic, reducing environment.

The precise reaction:

A ketone or aldehyde is heated with zinc amalgam (Zn-Hg) and concentrated hydrochloric acid (HCl). The C=O group is reduced to a CH₂ group.

Key points for exams:

• Works only for ketones and aldehydes that are stable in strong acid.
• Does not work for compounds that get destroyed by conc. HCl (e.g., acid-sensitive groups like esters, nitriles).
• The mechanism is complex and not usually tested in detail — just know it's a reductive removal of C=O.
• The product is always a saturated hydrocarbon (alkane).

Example:

Acetophenone ($\ce{C6H5-CO-CH3}$) → Ethylbenzene ($\ce{C6H5-CH2-CH3}$)

Cannizzaro Reaction

Intuition first. This is a disproportionation reaction — one molecule of aldehyde gets oxidised (to a carboxylic acid) while another gets reduced (to an alcohol). It happens only with aldehydes that have no alpha-hydrogen atoms (i.e., the carbon next to the C=O has no H). Why? Because if there were alpha-hydrogens, the aldehyde would undergo aldol condensation instead.

The precise reaction:

An aldehyde without $\alpha$-hydrogen is treated with concentrated aqueous or alcoholic base (NaOH/KOH). Two molecules of aldehyde react: one becomes a carboxylate salt, the other becomes a primary alcohol.

After acidification, the carboxylate salt gives the carboxylic acid.

Key points for exams:

• Only works for aldehydes with no $\alpha$-hydrogen: formaldehyde, benzaldehyde, trimethylacetaldehyde, etc.
• The base must be concentrated (dilute base won't work).
• Formaldehyde is the most common example — it gives formic acid (as formate) and methanol.
• Crossed Cannizzaro: When formaldehyde is mixed with another aldehyde (like benzaldehyde), formaldehyde is always the one that gets oxidised (to formate), and the other aldehyde gets reduced (to alcohol). This is because formaldehyde is the strongest reducing agent among aldehydes.

Example: …