Key Points

The carbonyl group consists of a carbon-oxygen double bond ($>C=O$). Due to the higher electronegativity of oxygen, the bond is polar, making the carbonyl carbon electrophilic and the carbonyl oxygen nucleophilic.

In the IUPAC system, aldehydes are named by replacing '-e' of the parent alkane with '-al'. Ketones are named with '-one', and carboxylic acids are named with '-oic acid'.

Aldehydes are prepared by Rosenmund reduction of acyl chlorides using $H_2$ over a $Pd/BaSO_4$ catalyst. They can also be formed from nitriles via the Stephen reaction ($SnCl_2/HCl$ followed by hydrolysis).

Aromatic ketones are prepared by Friedel-Crafts acylation, where an aromatic ring reacts with an acyl chloride ($RCOCl$) or anhydride in the presence of a Lewis acid catalyst like $AlCl_3$.

Aldehydes and ketones undergo nucleophilic addition reactions at the carbonyl carbon. Aldehydes are generally more reactive than ketones due to less steric hindrance and greater electrophilicity of the carbonyl carbon.

Aldehydes and ketones with at least one $\alpha$-hydrogen undergo aldol condensation in the presence of a dilute base to form $\beta$-hydroxy aldehydes (aldols) or $\beta$-hydroxy ketones (ketols).

When aldol condensation occurs between two different aldehydes or ketones, it is called cross aldol condensation. If both reactants contain $\alpha$-hydrogens, a mixture of four different products is formed.

Aldehydes that lack an $\alpha$-hydrogen atom undergo self-oxidation and reduction (disproportionation) when treated with a concentrated alkali. One molecule is reduced to an alcohol, and the other is oxidized to a salt of a carboxylic acid.

Aldehydes and ketones are reduced to primary and secondary alcohols, respectively, using reagents like $NaBH_4$ or $LiAlH_4$. The carbonyl group can be reduced to a methylene group ($CH_2$) by Clemmensen ($Zn-Hg/HCl$) or Wolff-Kishner reduction ($N_2H_4/KOH$).

Aldehydes are oxidized by Tollens' reagent (ammoniacal silver nitrate solution) to produce a carboxylate ion and elemental silver, which deposits as a 'silver mirror' on the test tube. Ketones do not react. The reaction is: $RCHO + 2[Ag(NH_3)_2]^+ + 3OH^- \rightarrow RCOO^- + 2Ag(s) + 4NH_3 + 2H_2O$.

Aliphatic aldehydes are oxidized by Fehling's solution (alkaline solution of $Cu^{2+}$ complexed with tartrate ions) to give a red-brown precipitate of copper(I) oxide ($Cu_2O$). Aromatic aldehydes do not give this test.

Aldehydes and ketones containing a methyl group attached to the carbonyl carbon ($CH_3CO-$ group) give a positive haloform test. They react with sodium hypohalite to form a haloform ($CHX_3$) and the sodium salt of a carboxylic acid with one less carbon atom.

Carboxylic acids are more acidic than alcohols and phenols because their conjugate base, the carboxylate ion ($RCOO^-$), is stabilized by resonance, delocalizing the negative charge over two oxygen atoms. Electron-withdrawing groups increase acidity.

Carboxylic acids react with alcohols in the presence of an acid catalyst (e.g., concentrated $H_2SO_4$) to form esters. This reversible reaction is known as Fischer esterification: $RCOOH + R'OH \rightleftharpoons RCOOR' + H_2O$.

Carboxylic acids with at least one $\alpha$-hydrogen react with chlorine or bromine in the presence of red phosphorus to give $\alpha$-halocarboxylic acids. This reaction is known as the Hell-Volhard-Zelinsky (HVZ) reaction.

Carboxylic acids lose carbon dioxide to form hydrocarbons when their sodium salts are heated with sodalime ($NaOH$ and $CaO$). The reaction is: $R-COONa \xrightarrow{NaOH/CaO, \Delta} R-H + Na_2CO_3$.