Comprehensive Notes on Aldehydes, Ketones, and Carboxylic Acids

Introduction to Carbonyl and Carboxyl Compounds

• Definition of Carbonyl Group: The carbonyl group consists of a carbon-oxygen double bond (>C=O). It is considered one of the most important functional groups in organic chemistry.

• Classification:

  • Aldehydes: The carbonyl group is bonded to at least one hydrogen atom and one carbon atom (except for formaldehyde, where it is bonded to two hydrogens).

  • Ketones: The carbonyl group is bonded to two carbon atoms.

  • Carboxylic Acids: The carbonyl carbon is bonded to a hydroxyl group ($-OH$), represented as $-COOH$.

  • Acyl Derivatives: Substituents of the carbonyl group include nitrogen ($-NH_2$ for amides), halogens (for acyl halides), and oxygen-linked groups (for esters and anhydrides).

• Significance: Carbonyl compounds are widespread in nature (plants and animals) and are essential for biochemical processes. They provide fragrance and flavor (e.g., vanillin, salicylaldehyde, cinnamaldehyde) and are used in fabrics, plastics, drugs, and solvents (e.g., acetone).

Nomenclature of Aldehydes and Ketones

Common Names

• Aldehydes: Derived from the common names of corresponding carboxylic acids by replacing the suffix "-ic acid" with "-aldehyde." Source-based names are often used (e.g., Formic acid $\rightarrow$ Formaldehyde).

• Ketones: Named by listing the two alkyl or aryl groups attached to the carbonyl group. The simplest ketone is known as acetone. Alkyl phenyl ketones are named by adding the acyl group as a prefix to "-phenone."

• Substituent Labeling: Locations are indicated by Greek letters ($\alpha$, $\beta$, $\gamma$, $\delta$, etc.). The $\alpha$-carbon is the one directly attached to the carbonyl group.

IUPAC Names

• Aldehydes: Suffix "-al" replaces "-e" in the parent alkane name. The longest chain is numbered starting from the aldehyde carbon.

• Ketones: Suffix "-one" replaces "-e" in the parent alkane name. Numbering starts from the end nearer the carbonyl group.

• Cyclic Compounds:

  • When the $-CHO$ group is attached to a ring, the suffix "carbaldehyde" is added to the full name of the cycloalkane.

  • The simplest aromatic aldehyde is benzenecarbaldehyde (IUPAC), though "benzaldehyde" is industry-accepted.

• Numerical Examples:

  • $HCHO$: Methanal (Formaldehyde)

  • $CH_3CHO$: Ethanal (Acetaldehyde)

  • $(CH_3)_2CHCHO$: 2-Methylpropanal (Isobutyraldehyde)

  • $CH_2=CHCHO$: Prop-2-enal (Acrolein)

  • $CH_3COCH_2CH_2CH_3$: Pentan-2-one (Methyl n-propyl ketone)

Structure of the Carbonyl Group

• Hybridization: The carbonyl carbon is $sp^2$-hybridized. It forms three sigma ($\sigma$) bonds and one pi ($\pi$) bond through the overlap of its p-orbital with the p-orbital of oxygen.

• Geometry: It is a trigonal coplanar structure with bond angles of approximately $120^\circ$. The $\pi$-electron cloud is located above and below the plane.

• Polarity: The carbon-oxygen double bond is highly polarized due to oxygen's higher electronegativity ($3.5$) compared to carbon ($2.5$).

  • Carbon is the electrophilic (Lewis acid) center.

  • Oxygen is the nucleophilic (Lewis base) center.

• Resonance: The group is represented by a neutral structure and a dipolar structure where oxygen carries a negative charge and carbon carries a positive charge.

Preparation of Aldehydes and Ketones

General Methods

1. Oxidation of Alcohols:

   • Primary alcohols $\rightarrow$ Aldehydes.

   • Secondary alcohols $\rightarrow$ Ketones.

2. Dehydrogenation of Alcohols: Alcohol vapors are passed over heavy metal catalysts ($Ag$ or $Cu$). Used industrially for volatile alcohols.

3. From Hydrocarbons:

   • Ozonolysis of Alkenes: Alkenes react with $O_3$ followed by $Zn/H_2O$ to give aldehydes, ketones, or mixtures.

   • Hydration of Alkynes: Adding water to ethyne with $H_2SO_4$ and $HgSO_4$ yields acetaldehyde; other alkynes yield ketones.

Specific Preparation of Aldehydes

• Rosenmund Reduction: Hydrogenation of acyl chlorides over palladium catalyst supported on barium sulphate ($Pd-BaSO_4$).

• Stephen Reaction: Reduction of nitriles with stannous chloride ($SnCl_2$) in $HCl$ to imines, followed by hydrolysis.

• DIBAL-H Reduction: Nitriles and esters are selectively reduced to imines/aldehydes by diisobutylaluminium hydride ($DIBAL-H$) at low temperatures.

• Oxidation of Methylbenzenes:

  • Etard Reaction: Use of chromyl chloride ($CrO_2Cl_2$) to form a chromium complex, which hydrolyzes to benzaldehyde.

  • Chromic Oxide ($CrO_3$): Toluene is treated with $CrO_3$ in acetic anhydride to form benzylidene diacetate, then hydrolyzed.

• Gatterman-Koch Reaction: Benzene treated with $CO$ and $HCl$ in the presence of anhydrous $AlCl_3$ or $CuCl$ yields benzaldehyde.

Specific Preparation of Ketones

• From Acyl Chlorides: Treatment with dialkylcadmium ($R_2Cd$).

• From Nitriles: Reaction with Grignard reagents followed by hydrolysis.

• Friedel-Crafts Acylation: Benzene treated with acid chloride in the presence of anhydrous $AlCl_3$.

Physical Properties of Aldehydes and Ketones

• Physical State: Methanal is a gas; Ethanal is a volatile liquid; others are liquids or solids.

• Boiling Points:

  • Higher than hydrocarbons and ethers of similar mass due to dipole-dipole interactions.

  • Lower than alcohols because they cannot form intermolecular hydrogen bonds.

  • Comparison (Mol. Mass 58-60): n-Butane ($273\,K$) < Methoxyethane ($281\,K$) < Propanal ($322\,K$) < Acetone ($329\,K$) < Propan-1-ol ($370\,K$).

• Solubility: Lower members (methanal, ethanal, propanone) are miscible with water via H-bonding. Solubility decreases as the alkyl chain length increases.

• Odors: Lower aldehydes have pungent odors; higher members are more fragrant and used in perfumes.

Chemical Reactions: Nucleophilic Addition

Mechanism and Reactivity

• Mechanism: A nucleophile attacks the electrophilic carbon perpendicular to the carbonyl plane. Hybridization changes from $sp^2$ to $sp^3$. A tetrahedral alkoxide intermediate is formed, which then captures a proton.

• Reactivity Order: Aldehydes > Ketones.

  • Steric Reason: Two large groups in ketones hinder the nucleophile's approach.

  • Electronic Reason: Two alkyl groups reduce the carbon's electrophilicity via inductive effect ($+I$).

Examples of Addition Reactions

1. HCN Addition: Forms cyanohydrins. Catalyzed by base to generate the stronger $CN^-$ nucleophile.

2. Sodium Hydrogensulphite ($NaHSO_3$): Forms crystalline addition products. Used for purification as they can be hydrolyzed back.

3. Alcohol Addition:

   • Aldehyde + 1 Alcohol $\rightarrow$ Hemiacetal.

   • Aldehyde + 2 Alcohols $\rightarrow$ Acetal (gem-dialkoxy compound).

   • Ketones + Ethylene glycol $\rightarrow$ Cyclic ethylene glycol ketals.

4. Ammonia Derivatives ($H_2N-Z$): Formation of imines ($Z=H$), oximes ($Z=OH$), hydrazones ($Z=NH_2$), and semicarbazones. $2,4$-DNP derivatives (yellow/orange/red solids) are used for characterization.

Reduction and Oxidation Reactions

Reduction

• To Alcohols: Using $NaBH_4$, $LiAlH_4$, or catalytic hydrogenation.

• To Hydrocarbons:

  • Clemmensen Reduction: $Zn-Hg$ and conc. $HCl$.

  • Wolff-Kishner Reduction: Hydrazine ($NH_2NH_2$) followed by heating with $KOH$ in ethylene glycol.

Oxidation

• Aldehydes: Easily oxidized to acids by $HNO_3$, $KMnO_4$, or mild agents.

  • Tollens' Test: Ammoniacal silver nitrate forms a silver mirror ($Ag$ metal).

  • Fehling's Test: Aqueous copper sulphate (A) + Rochelle salt (B) forms a reddish-brown precipitate ($Cu_2O$). Note: Aromatic aldehydes do not respond.

• Methyl Ketones: Haloform reaction occurs with sodium hypohalite ($NaOX$). The methyl group is converted to haloform ($CHX_3$). Iodoform ($CHI_3$) test identifies $CH_3CO-$ groups.

Other Important Reactions

• Aldol Condensation: Occurs in molecules with $\alpha$-hydrogen in dilute alkali. Forms $\beta$-hydroxy aldehydes/ketones, which dehydrate to $\alpha,\beta$-unsaturated carbonyl compounds.

• Cross Aldol Condensation: Reaction between two different carbonyl compounds; can yield a mixture of four products if both have $\alpha$-hydrogens.

• Cannizzaro Reaction: Aldehydes without $\alpha$-hydrogen (e.g., formaldehyde, benzaldehyde) undergo self-oxidation/reduction in conc. alkali to form an alcohol and a carboxylic acid salt.

• Electrophilic Substitution: The carbonyl group is meta-directing and deactivating.

Carboxylic Acids: Nomenclature and Structure

• Nomenclature: Suffix "-oic acid" replaces "-e" in alkanes. Common names often reflect natural sources (Formica for ant, Acetum for vinegar).

• Structure: The carboxyl carbon is $sp^2$-hybridized. It is less electrophilic than a carbonyl group due to resonance stabilization with the adjacent oxygen.

• Physical Properties:

  • Highest boiling points among comparable organics due to extensive H-bonding (forming dimers).

  • Solubility: Simple acids ($C_1-C_4$) are miscible in water. Solubility decreases with increased hydrocarbon chain length.

Acidity of Carboxylic Acids

• Acid Strength: Measured by $pK_a$. Smaller $pK_a$ means a stronger acid.

• Resonance Stability: Carboxylate ions are more stable than phenoxide ions because the negative charge is delocalized over two equivalent electronegative oxygen atoms.

• Substituent Effects:

  • Electron Withdrawing Groups (EWG): Increase acidity (e.g., CF_3COOH > CCl_3COOH > CH_3COOH).

  • Electron Donating Groups (EDG): Decrease acidity.

  • Acidity order: Ph < I < Br < Cl < F < CN < NO_2 < CF_3.

Reactions of Carboxylic Acids

1. Reduction: To primary alcohols using $LiAlH_4$ or diborane ($B_2H_6$). Diborane does not reduce esters or nitro groups.

2. Decarboxylation: Heating sodium salts with sodalime ($NaOH$ and $CaO$) removes $CO_2$ to give hydrocarbons.

3. Hell-Volhard-Zelinsky (HVZ) Reaction: Halogenation at the $\alpha$-position with $Cl_2/Br_2$ and red phosphorus.

4. Esterification: Reaction with alcohols/phenols in the presence of $H_2SO_4$ or $HCl$ gas.

5. Acyl Halide Formation: Treatment with $PCl_5$, $PCl_3$, or $SOCl_2$.

Questions & Discussion

• Example 8.1: Write the names of reagents for transformations. Solution: (i) Hexan-1-ol to hexanal: PCC. (vi) But-2-ene to ethanal: $O_3 / H_2O-Zn$.

• Example 8.2: Arrange in increasing B.P.: n-Pentane < Ethoxyethane < Butanal < Butan-1-ol.

• Example 8.3: Reactivity of benzaldehyde vs. propanal. Benzaldehyde is less reactive due to resonance reducing the electrophilicity of the carbonyl carbon.

• Example 8.4: Identifying $C_8H_8O$ (Ketone A). It forms 2,4-DNP, gives iodoform test (methyl ketone), does not reduce Tollens'.drastic oxidation yields benzoic acid. Result: Acetophenone.

• Intext 8.8: Which is stronger? (i) CH_2FCO_2H > CH_3CO_2H due to the inductive effect of fluorine.