Quick Revision Notes & Formula Sheet

Class: 12th Chemistry
Chapter: Aldehydes, Ketones and Carboxylic Acids
Board: MP Board (NCERT Based)

1. Introduction & Structure of Carbonyl Group

Carbonyl Group (>C=O): The carbon-oxygen double bond is characterized by a $sp^2$ hybridized carbon atom, forming a trigonal planar geometry with bond angles of approximately $120^\circ$.
Polarity: Due to the higher electronegativity of oxygen, the bond is highly polarized (>C^{\delta+} = O^{\delta-}), making carbonyl compounds susceptible to Nucleophilic Addition Reactions.

2. Preparation of Aldehydes and Ketones

A. Oxidation of Alcohols

Primary Alcohol $\rightarrow$ Aldehyde: Use mild oxidizing agents like Pyridinium chlorochromate (PCC) or $\mathrm{CrO_3}$. ($R-CH_2OH \xrightarrow{PCC} R-CHO$)
Secondary Alcohol $\rightarrow$ Ketone: Use $\mathrm{CrO_3}$, $\mathrm{KMnO_4}$, or PCC. ($R_2CH-OH \xrightarrow{Oxidation} R_2C=O$)

B. Dehydrogenation of Alcohols

Passing alcohol vapors over heavy metal catalysts (like Cu at $573\text{ K}$).
- $1^\circ$ Alcohol $\rightarrow$ Aldehyde
- $2^\circ$ Alcohol $\rightarrow$ Ketone

C. Hydrocarbon Conversions

Ozonalysis of Alkenes: Cleavage of alkenes using $\mathrm{O_3}$ followed by $\mathrm{Zn/H_2O}$ yields aldehydes and/or ketones.
Hydration of Alkynes: Addition of water to ethyne gives acetaldehyde (via tautomerization), and other alkynes give ketones (Markovnikov's rule, using $\mathrm{HgSO_4/H_2SO_4}$).

D. Specific Preparations for Aldehydes

Rosenmund Reduction: Hydrogenation of acyl chlorides over poisoned palladium catalyst ($\mathrm{Pd/BaSO_4}$).
- R-COCl + H_2 \xrightarrow{Pd/BaSO_4} R-CHO + HCl
Stephen Reaction: Reduction of alkyl cyanides (nitriles) using stannous chloride ($\mathrm{SnCl_2}$) and $\mathrm{HCl}$ followed by hydrolysis.
- R-CN + SnCl_2 + HCl \rightarrow [R-CH=NH] \xrightarrow{H_3O^+} R-CHO
Etard Reaction: Oxidation of toluene to benzaldehyde using chromyl chloride ($\mathrm{CrO_2Cl_2}$).

E. Specific Preparations for Ketones

Friedel-Crafts Acylation: Reaction of benzene with acyl chloride in the presence of anhydrous $\mathrm{AlCl_3}$.
- Benzene + R-COCl \xrightarrow{Anhyd. AlCl_3} Acetophenone (R-CO-C_6H_5) + HCl

3. Physical Properties

Boiling Point: Aldehydes and ketones have higher boiling points than non-polar hydrocarbons/ethers of comparable molecular mass due to dipole-dipole interactions, but lower than alcohols due to the absence of intermolecular hydrogen bonding.
Solubility: Lower members (formaldehyde, acetaldehyde, acetone) are miscible with water due to hydrogen bonding with water molecules. Solubility decreases with an increase in the hydrophobic alkyl chain.

4. Chemical Reactions of Aldehydes and Ketones

A. Nucleophilic Addition Reactions (`>C=O + Nu^- \rightarrow`)

Reactivity order: Aldehydes > Ketones (due to steric and electronic factors: $+I$ effect of alkyl groups in ketones decreases positive charge on carbonyl carbon).

Addition of Hydrogen Cyanide (HCN): Forms Cyanohydrins (>C(OH)(CN)).
Addition of Sodium Bisulphite ($\mathrm{NaHSO_3}$): Forms crystalline bisulphite addition products (used for separation/purification).
Addition of Grignard Reagents ($\mathrm{RMgX}$):
- HCHO + $\mathrm{RMgX} \rightarrow 1^\circ$ Alcohol
- Other Aldehydes + $\mathrm{RMgX} \rightarrow 2^\circ$ Alcohol
- Ketones + $\mathrm{RMgX} \rightarrow 3^\circ$ Alcohol
Addition of Alcohols:
- Aldehyde + Alcohol $\rightarrow$ Hemiacetal $\xrightarrow{\text{Alcohol}}$ Acetal
- Ketone + Glycol $\rightarrow$ Cyclic Ketal

B. Reduction Reactions

Reduction to Alcohols:
- $\mathrm{NaBH_4}$ or $\mathrm{LiAlH_4}$ converts aldehydes to $1^\circ$ alcohols and ketones to $2^\circ$ alcohols.
Reduction to Hydrocarbons:
- Clemmensen Reduction: $\mathrm{Zn-Hg} / \mathrm{Conc. HCl}$ (>C=O \rightarrow >CH_2)
- Wolff-Kishner Reduction: $\mathrm{NH_2NH_2}$ followed by $\mathrm{KOH}$ in glycol (>C=O \rightarrow >CH_2)

C. Oxidation Reactions

Aldehydes are easily oxidized to carboxylic acids with the same number of carbon atoms using mild oxidizing agents. Ketones resist oxidation and require strong oxidizing agents ($\mathrm{HNO_3}$, $\mathrm{KMnO_4}$) at high temperatures, resulting in cleavage of C-C bonds.
Tollens' Test (Silver Mirror Test): Aldehyde + $\mathrm{[Ag(NH_3)_2]^+} \rightarrow$ Silver Mirror (forms shining silver layer). Ketones give negative test.
Fehling's Test: Aldehyde + $\mathrm{Cu^{2+}}$ (Fehling's solution A & B) $\rightarrow$ Red-brown precipitate of $\mathrm{Cu_2O}$. Aromatic aldehydes do not respond.

D. Reactions Involving $\alpha$-Hydrogens

Aldol Condensation: Aldehydes and ketones having at least one $\alpha$-hydrogen, in the presence of dilute alkali ($\mathrm{NaOH}$), form $\beta$-hydroxy aldehydes (aldol) or $\beta$-hydroxy ketones.
Cross Aldol Condensation: Condensation between two different aldehydes/ketones.
Cannizzaro Reaction: Aldehydes without $\alpha$-hydrogen (e.g., $\mathrm{HCHO}$, $\mathrm{C_6H_5CHO}$) undergo self-oxidation and reduction (disproportionation) on treatment with concentrated alkali ($\mathrm{NaOH}$/$\mathrm{KOH}$) to yield an alcohol and a salt of carboxylic acid.

5. Carboxylic Acids ($\mathrm{R-COOH}$)

A. Preparation Methods

From Primary Alcohols and Aldehydes: Direct oxidation using strong oxidizing agents like $\mathrm{KMnO_4}$ in neutral, acidic, or alkaline media, or $\mathrm{K_2Cr_2O_7 / H^+}$.
From Alkyl Benzenes: Oxidation of side-chain alkyl groups with acidic or alkaline $\mathrm{KMnO_4}$ yields benzoic acid (irrespective of chain length).
From Nitriles and Amides: Acid or alkaline hydrolysis of cyanides (R-CN) or amides (R-CONH_2) gives carboxylic acids.
From Grignard Reagents: Reaction of Grignard reagents with solid carbon dioxide (dry ice) followed by acid hydrolysis.
- R-MgX + CO_2 \rightarrow R-COOMgX \xrightarrow{H_3O^+} R-COOH + Mg(OH)X

B. Physical Properties

Boiling Points: Much higher than aldehydes, ketones, and corresponding alcohols of comparable molecular masses due to extensive association through intermolecular hydrogen bonding, existing even as dimers in the vapor phase.
Solubility: Lower aliphatic carboxylic acids (up to 4 carbons) are miscible with water due to hydrogen bonding with water. Solubility decreases as the hydrophobic hydrocarbon chain increases.

C. Chemical Reactions

Acidity: Carboxylic acids dissociate in water to give resonance-stabilized carboxylate ions (R-COO^-), making them stronger acids than alcohols and phenols.
- Electron-withdrawing groups (-I, -R) increase acidity.
- Electron-donating groups (+I, +R) decrease acidity.
Formation of Anhydrides: Heating with $\mathrm{P_2O_5}$ dehydrates acids to form acid anhydrides.
Esterification: Reacting carboxylic acids with alcohols in the presence of concentrated $\mathrm{H_2SO_4}$ forms esters (R-COOR').
Reaction with $\mathrm{PCl_5, PCl_3, \text{ and } SOCl_2}$: Replaces the $-\mathrm{OH}$ group with $-\mathrm{Cl}$ to form acyl chlorides (R-COCl).
Decarboxylation: Heating sodium salts of carboxylic acids with soda lime ($\mathrm{NaOH} + \mathrm{CaO}$ at $630\text{ K}$) eliminates $\mathrm{CO_2}$ to form alkanes.
- R-COONa + NaOH \xrightarrow{\Delta, CaO} R-H + Na_2CO_3
Hell-Volhard-Zelinsky (HVZ) Reaction: Carboxylic acids having a $\alpha$-hydrogen are halogenated at the $\alpha$-position on treatment with chlorine or bromine in the presence of red phosphorus to form $\alpha$-halo carboxylic acids.

Quick Memory Tips for MP Board Exams

Tollens' Test: Used to distinguish between Aliphatic/Aromatic Aldehydes and Ketones.
Cannizzaro Reaction: Remember No $\alpha$-H (Formaldehyde $\mathrm{HCHO}$ and Benzaldehyde $\mathrm{C_6H_5CHO}$).
Aldol Condensation: Remember Presence of $\alpha$-H is mandatory.
HVZ Reaction: Focus on red phosphorus and halogen ($\mathrm{Cl_2/Br_2}$) for $\alpha$-substitution in carboxylic acids.

Aldehydes, Ketones and Carboxylic Acids

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