Exam Master Review Sheet - Aldehydes & Ketones

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EXAM MASTER REVIEW SHEET

Organic Chemistry: Aldehydes & Ketones

Comprehensive Notes for JEE Main, Advanced & NEET

1 Carbonyl Group: Structure & Reactivity

The carbonyl carbon is $sp^2$ hybridized. The $>C=O$ bond is highly polarized due to higher electronegativity of oxygen, making the carbon electrophilic.

Reactivity Order towards $N_A$

Aldehydes are generally more reactive than ketones towards Nucleophilic Addition ($N_A$) due to:

• Steric Hindrance: Ketones have two bulky alkyl groups that hinder the approach of a nucleophile to the carbonyl carbon.
• Electronic Factor (+I effect): Two alkyl groups in ketones release electrons (+I), reducing the electrophilicity (positive charge) on the carbonyl carbon more than one alkyl group in aldehydes.

$HCHO > RCHO > R_2CO$
Aliphatic > Aromatic (due to +R effect of benzene ring)

Physical Properties

Boiling Point: Higher than hydrocarbons and ethers of comparable mass (due to weak dipole-dipole interactions), but lower than alcohols (absence of intermolecular H-bonding).

Solubility: Lower aldehydes and ketones (up to 4 carbons) are miscible with water because they form H-bonds with water. Solubility drops rapidly with increasing alkyl chain size.

B.P. Order: Alkane < Ether < Aldehyde/Ketone < Alcohol

2 Key Preparation Methods

Rosenmund Reduction

Aldehydes Only

Catalytic hydrogenation of acid chlorides. Barium sulfate acts as a catalytic poison to stop further reduction to alcohol.

$R-COCl + H_2 \xrightarrow{Pd / BaSO_4} R-CHO + HCl$

Stephen Reaction

Aldehydes Only

Reduction of nitriles with stannous chloride and HCl, followed by hydrolysis of the intermediate imine.

$R-C \equiv N \xrightarrow{SnCl_2 + HCl} R-CH=NH \cdot HCl \xrightarrow{H_3O^+} R-CHO$

Etard Reaction

Benzaldehyde

Oxidation of toluene using chromyl chloride ($CrO_2Cl_2$). Forms a brown chromium complex which hydrolyzes to benzaldehyde.

$Ph-CH_3 + 2CrO_2Cl_2 \xrightarrow{CS_2} \text{Complex} \xrightarrow{H_3O^+} Ph-CHO$

Ozonolysis of Alkenes

Ald/Ket

Reductive cleavage of $C=C$ double bonds using ozone followed by Zinc dust and water.

$>C=C< \xrightarrow{(i) O_3 \ (ii) Zn/H_2O} >C=O + O=C<$

JEE Pro-Tip: Hydration of alkynes ($dil. H_2SO_4 / HgSO_4$) always yields ketones (via Markovnikov's addition and tautomerism), except ethyne which yields acetaldehyde.

3 Nucleophilic Addition Reactions

The planar carbonyl carbon is attacked by a nucleophile from above or below, changing hybridization from $sp^2$ to $sp^3$ (tetrahedral intermediate).

Reagent / Nucleophile	Product Formed	Mechanism / Key Observation
Hydrogen Cyanide ($HCN$)	Cyanohydrin	Base-catalyzed reaction (base generates the strong $CN^-$ nucleophile). Useful for ascending the carbon chain.
Sodium Bisulphite ($NaHSO_3$)	Bisulphite Addition Compound	Forms a white crystalline precipitate. Used for separation and purification of aldehydes (and sterically unhindered ketones).
Grignard Reagent ($RMgX$)	Alcohols	$HCHO \rightarrow 1^\circ$ Alcohol Other Aldehydes $\rightarrow 2^\circ$ Alcohol Ketones $\rightarrow 3^\circ$ Alcohol
Alcohols ($ROH / H^+$)	Acetals & Ketals	Aldehydes form hemiacetals then acetals. Ketones form cyclic ketals with ethylene glycol (used as a protecting group in organic synthesis).
Ammonia Derivatives ($NH_2-Z$)	Imines / Hydrazones / Oximes	Requires strict pH control (~3.5). 2,4-DNP test (Brady's reagent) yields an Orange/Yellow/Red precipitate, confirming presence of carbonyl group.

4 Reactions of $\alpha$-Hydrogen

Aldol Condensation

Requirement: Must have at least one $\alpha$-Hydrogen atom.
Reagent: Dilute alkali ($dil. NaOH$, $Ba(OH)_2$).

Two molecules condense to form a $\beta$-hydroxy aldehyde (Aldol) or ketone (Ketol). Upon heating, they readily dehydrate to form $\alpha,\beta$-unsaturated carbonyl compounds.

$2CH_3CHO \xrightarrow{dil. OH^-} CH_3-CH(OH)-CH_2-CHO \xrightarrow{\Delta} CH_3-CH=CH-CHO$

Cannizzaro Reaction

Requirement: Aldehydes with NO $\alpha$-Hydrogen atoms ($HCHO$, $PhCHO$).
Reagent: Concentrated alkali ($50\% \ NaOH$ or $KOH$).

A disproportionation (self-oxidation-reduction) reaction. One molecule is oxidized to carboxylic acid salt, the other is reduced to an alcohol.

$2HCHO \xrightarrow{Conc. KOH} CH_3OH \text{ (Methanol)} + HCOO^-K^+ \text{ (Formate)}$

5 Distinguishing Tests & Reductions

Reduction to Hydrocarbons (Alkanes)

Clemmensen Reduction (Acidic Medium)

Uses Zinc amalgam and concentrated HCl.

$>C=O \xrightarrow{Zn-Hg / \text{Conc. } HCl} >CH_2 + H_2O$

Wolff-Kishner Reduction (Basic Medium)

Uses Hydrazine followed by heating with KOH/Glycol.

$>C=O \xrightarrow{NH_2NH_2} \text{Hydrazone} \xrightarrow{KOH / \text{Glycol}, \Delta} >CH_2 + N_2$

Chemical Tests (Lab Identification)

• 🪞

  Tollens' Test (Silver Mirror Test)

  Reagent: Ammoniacal Silver Nitrate. Only Aldehydes (both aliphatic & aromatic) reduce Tollens' reagent to a bright silver mirror. Ketones fail.

• 🔴

  Fehling's Test

  Reagent: Fehling A (aq. $CuSO_4$) + Fehling B (Rochelle salt). Only Aliphatic Aldehydes give a reddish-brown ppt of $Cu_2O$. Aromatic aldehydes (Benzaldehyde) and ketones fail.

• 🟡

  Haloform (Iodoform) Test

  Reagent: $NaOI$ (or $NaOH + I_2$). Compounds with a methyl ketone group ($CH_3-CO-$) or those oxidized to it ($CH_3-CH(OH)-$) give a yellow ppt of Iodoform ($CHI_3$).

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