💎 Aldehydes & Ketones (Carbonyl Chemistry)

Carbonyl compounds undergo nucleophilic additions, $\alpha$-deprotonation condensations, and selective oxidation/reduction cascades. In competitive exams, mastery of Aldol vs Cannizzaro switch conditions, haloform test criteria ($C{H}_3-C=O$ or $C{H}_3-CH(OH)-$), and chemoselective reductions is crucial.

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1. 🎯 Nucleophilic Additions to the Carbonyl Group

Reactivity order toward nucleophilic addition:

$$\mathbf{HCHO}\mathbf{>}\mathbf{RCHO}\mathbf{>}{\mathbf{R}}_{\mathbf{2}}\mathbf{CO}\mathbf{>}\text{Aromatic Aldehydes}\mathbf{>}\text{Aromatic Ketones}$$

(Governed by steric hindrance and positive charge density on carbonyl carbon).

Key Addition Reactions:

1. $HCN$ Addition: Forms Cyanohydrins ($>C(OH)CN$), which on acid hydrolysis yield $\alpha$-hydroxy carboxylic acids.
2. $NaHS{O}_3$ Addition: Forms crystalline bisulfite adducts (used for purification of aldehydes and methyl ketones).
3. Grignard Reagents ($RMgX$):
   • $HCHO+RMgX\stackrel{H_3{O}^{+}}{\to }{\mathbf{1}}^{\circ }\text{ Alcohol}$
   • $R^{\prime }CHO+RMgX\stackrel{H_3{O}^{+}}{\to }{\mathbf{2}}^{\circ }\text{ Alcohol}$
   • $R^{\prime }CO{R}^{″}+RMgX\stackrel{H_3{O}^{+}}{\to }{\mathbf{3}}^{\circ }\text{ Alcohol}$

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2. 🔄 Named Carbonyl Condensation Reactions

2.1 Aldol Condensation (Requires at least one $\alpha$-Hydrogen)

Two molecules of an aldehyde or ketone containing $\alpha$-hydrogens react in dilute alkali ($NaOH/Ba(OH{)}_2$):

$$2C{H}_{3}CHO\stackrel{\text{dil. }NaOH}{\to }C{H}_3-CH(OH)-C{H}_2-CHO\stackrel{\mathrm{\Delta },-H_{2}O}{\to }{\mathbf{CH}}_{\mathbf{3}}\mathbf{-}\mathbf{CH}\mathbf{=}\mathbf{CH}\mathbf{-}\mathbf{CHO}\mathbf{(}\text{Crotonaldehyde}\mathbf{,}\alpha \mathbf{,}\beta \text{-unsaturated aldehyde}\mathbf{)}$$

2.2 Cannizzaro Reaction (Aldehydes with NO $\alpha$-Hydrogens)

Disproportionation of non-enolizable aldehydes ($HCHO,PhCHO,(C{H}_3{)}_{3}C-CHO$) in $50\mathrm{\%}\text{ conc. }KOH$:

$$2PhCHO+\text{conc. }KOH\stackrel{\mathrm{\Delta }}{\to }{\mathbf{PhCH}}_{\mathbf{2}}\mathbf{OH}\mathbf{(}\text{Benzyl alcohol}\mathbf{)}\mathbf{+}\mathbf{PhCOOK}\mathbf{(}\text{Potassium benzoate}\mathbf{)}$$

• Hydride Transfer ($H^{-}$) from gem-diol dianion to carbonyl carbon is the Slow Rate-Determining Step (RDS).

2.3 Haloform / Iodoform Reaction ($CH{I}_3$)

Reagents: $I_2+NaOH$ (or $NaOI$).

• Positive Test Criteria: Compounds containing either the Methyl Carbonyl group ($C{H}_3-CO-R$) or the $C{H}_3-CH(OH)-R$ group:
  • Positive: Acetaldehyde ($C{H}_{3}CHO$), Ethanol ($C{H}_{3}C{H}_{2}OH$), Acetone ($C{H}_{3}COC{H}_3$), 2-Propanol ($C{H}_{3}CH(OH)C{H}_3$), Acetophenone ($PhCOC{H}_3$), 2-Pentanone.
  • Negative: Methanol ($C{H}_{3}OH$), Formaldehyde ($HCHO$), 1-Propanol, 3-Pentanone, Benzaldehyde ($PhCHO$), Benzophenone ($PhCOPh$).
• Observation: Formation of bright yellow crystalline precipitate of Iodoform ($CH{I}_3$) with characteristic antiseptic smell (MP ${119}^{\circ }\text{C}$).

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3. 🛡️ Carbonyl Reductions Master Summary

Reduction Method	Reagents	Substrate $ o$ Product	Key Selectivity / Exam Trap
Clemmensen Reduction	$Zn-Hg+\text{conc. }HCl$	$>C=O\to >C{H}_2$ (Alkanes)	Cannot be used for acid-sensitive substrates (e.g. acetals, double bonds)!
Wolff-Kishner Reduction	$N{H}_{2}N{H}_2+KOH/\text{Ethylene glycol},\mathrm{\Delta }$	$>C=O\to >C{H}_2$ (Alkanes)	Cannot be used for base-sensitive substrates (e.g. alkyl halides)!
Lithium Aluminum Hydride ($LAH,LiAl{H}_4$)	$LiAl{H}_4\text{ in dry ether}\to H_3{O}^{+}$	Aldehydes $ o 1^circ$ alc Ketones $ o 2^circ$ alc Acids/Esters $ o 1^circ$ alc	Powerful reducing agent; reduces esters & amides, but does not reduce isolated $C=C$ (except cinnamyl systems).
Sodium Borohydride ($NaB{H}_4$)	$NaB{H}_4\text{ in }EtOH/H_{2}O$	Aldehydes $ o 1^circ$ alc Ketones $ o 2^circ$ alc	Mild reducing agent; does NOT reduce esters, amides, or carboxylic acids!