Oxidation: Primary alcohols $\xrightarrow{\text{Oxidation}}$ Aldehydes; Secondary alcohols $\xrightarrow{\text{Oxidation}}$ Ketones.
Dehydrogenation: Alcohol vapours over heavy metal catalysts (Ag or Cu). Primary alcohols $\xrightarrow{\text{Cu/Ag, Heat}}$ Aldehydes; Secondary alcohols $\xrightarrow{\text{Cu/Ag, Heat}}$ Ketones.

Ozonolysis of Alkenes: Alkenes $\xrightarrow{\text{1. O}_3 \text{ 2. Zn/H}_2\text{O}}$ Aldehydes/Ketones.
Hydration of Alkynes: Ethyne $\xrightarrow{\text{H}_2\text{SO}_4/\text{HgSO}_4}$ Acetaldehyde. Other alkynes $\xrightarrow{\text{H}_2\text{SO}_4/\text{HgSO}_4}$ Ketones.

From Acyl Chlorides (Rosenmund Reduction): $\text{RCOCl} \xrightarrow{\text{H}_2/\text{Pd-BaSO}_4}$ $\text{RCHO}$.
From Nitriles and Esters:
- Stephen Reaction: $\text{RCN} \xrightarrow{\text{1. SnCl}_2/\text{HCl 2. H}_2\text{O}}$ $\text{RCHO}$.
- Using DIBAL-H: $\text{RCN} \xrightarrow{\text{1. DIBAL-H 2. H}_2\text{O}}$ $\text{RCHO}$; $\text{RCOOR}' \xrightarrow{\text{1. DIBAL-H 2. H}_2\text{O}}$ $\text{RCHO}$.
From Hydrocarbons (Aromatic Aldehydes):
- Oxidation of Methylbenzene:
  - Using Chromyl Chloride (Etard Reaction): $\text{Toluene} \xrightarrow{\text{1. CrO}_2\text{Cl}_2/\text{CS}_2 \text{ 2. H}_3\text{O}^+}$ Benzaldehyde.
  - Using Chromic Oxide: $\text{Toluene} \xrightarrow{\text{1. CrO}_3/(\text{CH}_3\text{CO})_2\text{O} \text{ 2. H}_3\text{O}^+}$ Benzaldehyde.
- Side Chain Chlorination: $\text{Toluene} \xrightarrow{\text{Cl}_2/\text{hv}} \text{Benzal chloride} \xrightarrow{\text{H}_2\text{O}/\text{Heat}}$ Benzaldehyde.
- Gatterman-Koch Reaction: $\text{Benzene} \xrightarrow{\text{CO, HCl}/\text{Anhyd. AlCl}_3/\text{CuCl}}$ Benzaldehyde.

From Acyl Chlorides: $\text{2R'COCl} + \text{R}_2\text{Cd} \rightarrow \text{2R'COR} + \text{CdCl}_2$. ($\text{R}_2\text{Cd}$ from $\text{2RMgX} + \text{CdCl}_2$)
From Nitriles: $\text{RCN} \xrightarrow{\text{1. R'MgX 2. H}_2\text{O}}$ $\text{RCOR}'$.
From Benzene (Friedel-Crafts Acylation): $\text{Benzene} + \text{RCOCl} \xrightarrow{\text{Anhyd. AlCl}_3}$ $\text{C}_6\text{H}_5\text{COR}$.

Methanal is a gas, ethanal is a volatile liquid. Others are liquids or solids.
Higher boiling points than hydrocarbons/ethers of comparable molecular mass due to dipole-dipole interactions.
Lower boiling points than alcohols of comparable molecular mass due to absence of intermolecular H-bonding.
Lower members (methanal, ethanal, propanone) are miscible with water due to H-bonding. Solubility decreases with increasing alkyl chain length.
Lower aldehydes have sharp pungent odours; larger molecules are less pungent, more fragrant.

Aldehydes and ketones undergo nucleophilic addition.
Mechanism: Nucleophile attacks electrophilic carbonyl carbon (from $sp^2$ to $sp^3$ hybridisation). Tetrahedral alkoxide intermediate forms, which captures a proton.
Reactivity: Aldehydes are generally more reactive than ketones due to:
- Steric Reasons: Less steric hindrance around carbonyl carbon in aldehydes.
- Electronic Reasons: Alkyl groups reduce electrophil