Functional groups are specific atoms or arrangements of atoms within molecules that give rise to characteristic chemical reactions. Understanding functional group reactivity is fundamental to organic chemistry and essential for predicting reaction outcomes in synthesis and biochemical pathways.

Alcohols (R-OH)

1. Oxidation: Primary alcohols oxidize to aldehydes and then to carboxylic acids using reagents such as KMnO4, K2Cr2O7, or PCC (pyridinium chlorochromate). Secondary alcohols oxidize to ketones; tertiary alcohols resist oxidation.
2. Esterification: Alcohols react with carboxylic acids in the presence of an acid catalyst (H2SO4) to form esters and water via Fischer esterification.
3. Dehydration: Concentrated H2SO4 or H3PO4 at elevated temperatures eliminates water from alcohols to yield alkenes.

Carbonyl Compounds (C=O)

1. Nucleophilic Addition: Aldehydes and ketones undergo addition reactions with Grignard reagents, hydride reducing agents (NaBH4, LiAlH4), and hydrogen cyanide to form alcohols, cyanohydrins, and other adducts.
2. Aldol Condensation: Enolates of aldehydes or ketones react with carbonyl groups to form β-hydroxy carbonyl compounds, which can dehydrate to α,β-unsaturated systems.
3. Acetal Formation: Aldehydes and ketones react with alcohols in the presence of an acid catalyst to form acetals, which serve as protecting groups for carbonyls.

Carboxylic Acids and Derivatives (R-COOH)

1. Nucleophilic Acyl Substitution: Carboxylic acids form acyl chlorides (with SOCl2 or PCl5), anhydrides, esters, and amides through substitution at the acyl carbon.
2. Reduction: LiAlH4 reduces carboxylic acids to primary alcohols, while borane (BH3) selectively reduces acids without affecting esters.
3. Decarboxylation: Heating carboxylic acids with a base can remove CO2, forming alkanes or alkenes, particularly for β-keto acids.

Amines (R-NH2)

1. Basicity: Amines act as bases, with aliphatic amines (pKa ~10-11) being stronger bases than aromatic amines (pKa ~4-5).
2. Acylation: Amines react with acyl chlorides or anhydrides to form amides (Schotten-Baumann reaction) in the presence of a base.
3. Diazotization: Primary aromatic amines react with NaNO2 and HCl at 0-5°C to form diazonium salts, which are key intermediates for azo dyes and substitution reactions.

Alkenes (C=C)

1. Electrophilic Addition: Alkenes react with halogens (Br2, Cl2), hydrogen halides (HX), and water (H2O/H+) to form dihalides, alkyl halides, and alcohols respectively.
2. Hydrogenation: H2 gas with a metal catalyst (Pd/C, PtO2) reduces alkenes to alkanes.
3. Polymerization: Alkenes undergo chain-growth polymerization via radical, cationic, or anionic mechanisms to form polyolefins such as polyethylene and polypropylene.