Nucleophilic Addition Applications:

HCN Addition (Cyanohydrin Synthesis): $\text{CH}_3\text{CHO} + \text{HCN} \rightarrow \text{CH}_3\text{CH(OH)CN (lactonitrile)}$ Application: Extends carbon chain by one C; cyanohydrin can be hydrolyzed to alpha-hydroxy acid. Important in organic synthesis.

Grignard Addition — Classification of Product Alcohol: The type of alcohol (1°, 2°, 3°) is determined by the carbonyl compound reacted with the Grignard reagent. HCHO always gives primary alcohols. RCHO gives secondary alcohols (new branch on the OH carbon). R2CO gives tertiary alcohols (three C substituents on the OH carbon).

Hydride Reduction (NaBH4 / LiAlH4): Both reduce C=O to C-OH selectively. NaBH4 is mild and selective — does not reduce esters, carboxylic acids, or amides. LiAlH4 is powerful — reduces all carbonyl compounds including esters, acids, amides, and even nitro groups. For NEET: both give the same alcohol product from aldehydes and ketones.

Condensation Reaction Applications:

Aldol Condensation in Industry: Used to make important industrial solvents and intermediates. The crossed aldol of acetaldehyde with formaldehyde gives pentaerythritol (one application). Aldol condensation is the basis for the biological synthesis of sugars (via aldolase enzyme reactions in glycolysis/gluconeogenesis).

Haloform Reaction (Iodoform Test — Identification Tool): The iodoform test is used diagnostically to identify methyl ketones and compounds that can be oxidized to methyl ketones. Positive compounds include ethanol (important: common exam trap is methanol which is negative), isopropanol (gives acetone → iodoform), acetophenone (aryl methyl ketone), and any straight-chain methyl ketone (CH3CO-R where R is any group).

The Cannizzaro Reaction — Cross vs Self:

Self-Cannizzaro: $2\text{HCHO} \xrightarrow{\text{conc. NaOH}} \text{HCOONa} + \text{CH}_3\text{OH}$ $2\text{C}_6\text{H}_5\text{CHO} \xrightarrow{\text{conc. NaOH}} \text{C}_6\text{H}_5\text{COONa} + \text{C}_6\text{H}_5\text{CH}_2\text{OH}$

Crossed Cannizzaro (HCHO is always the reducing agent): $\text{HCHO} + \text{C}_6\text{H}_5\text{CHO} \xrightarrow{\text{conc. NaOH}} \text{HCOONa} + \text{C}_6\text{H}_5\text{CH}_2\text{OH}$ HCHO is preferentially oxidized because it is more reactive (no steric hindrance, most electrophilic).

Complete Deoxygenation Applications: Clemmensen and Wolff-Kishner are used in synthesis to convert a ketone (introduced by Friedel-Crafts acylation) to an alkyl group. Example: benzene → acetophenone (Friedel-Crafts) → ethylbenzene (Clemmensen). This is the indirect alkylation route that avoids carbocation rearrangements.