Organic Reactions: Substitution, Addition and Combustion

One atom or group is swapped for another while the carbon skeleton stays intact. Alkanes undergo free-radical halogenation with a halogen under UV light, and haloalkanes are substituted by nucleophiles such as OH⁻ to give alcohols.

For methane chlorination under UV light, CH₄ + Cl₂ → CH₃Cl + HCl shows substitution: one C–H is replaced by C–Cl. Further substitution can occur, so the first equation is not a promise of one pure product. In aqueous OH⁻ substitution of a simple haloalkane, R–Br + OH⁻ → R–OH + Br⁻, the leaving group is replaced. Reagent, substrate and solvent matter; calling a molecule saturated does not mean substitution is its only possible reaction.

A pi bond opens and two fragments add across it, so a double bond becomes single. Hydrogenation adds H₂ over a metal catalyst; halogenation adds Br₂; hydration adds water with an acid catalyst; hydrohalogenation adds HX. Addition across a multiple bond is characteristic of unsaturated substrates; the specific bond and conditions determine the reaction.

For the unsymmetrical additions of H₂O and HBr, the usual major-product pattern places H on the carbon already bearing more H, and OH or Br on the more substituted carbon. This Markovnikov pattern has conditions and exceptions: peroxide-initiated HBr addition follows a different radical pathway, and rearrangements can matter for other substrates. Bromine water can produce a bromohydrin instead of the simple dibromide, so a product prediction should include its reaction medium. Alkynes can add successive equivalents, giving different products after one versus two additions.

A condensation joins species while eliminating a small molecule. In acid-catalyzed esterification, CH₃COOH + CH₃OH ⇌ CH₃COOCH₃ + H₂O forms methyl ethanoate. It is reversible: excess reactant or removal of water can favor ester formation. Acid-catalyzed hydrolysis uses water to regenerate acid and alcohol; base hydrolysis instead gives carboxylate plus alcohol under the usual conditions.

Amide bonds can be made by suitable condensation or activated-acyl chemistry. Simply mixing a carboxylic acid with an amine usually first gives an ammonium carboxylate by proton transfer; a direct high-yield amide is not guaranteed without appropriate activation or dehydration conditions. Peptide bonds are amide linkages, but their controlled formation requires more than identifying N and COOH groups.

Complete combustion in plenty of oxygen gives CO₂ and H₂O and releases a lot of energy. Limited oxygen gives incomplete combustion, producing CO or soot along with water. Balance combustion by carbons first, then hydrogens, then oxygens last.

For hydrocarbon CxHy, complete combustion is CxHy + (x + y/4)O₂ → x CO₂ + (y/2)H₂O. Start by counting C and H, then oxygen, and multiply the whole equation if integer coefficients are wanted. For ethane: 2 C₂H₆ + 7 O₂ → 4 CO₂ + 6 H₂O.

Incomplete combustion is not one unique reaction because product proportions depend on oxygen supply and conditions. If the problem specifies CO only as the carbon product, balance 2 C₂H₆ + 5 O₂ → 4 CO + 6 H₂O. If it specifies soot, 2 C₂H₆ + 3 O₂ → 4 C + 6 H₂O. A mixture of CO₂, CO and soot requires its stated composition; do not assume one from the phrase “limited oxygen” alone. State water's phase when using a combustion enthalpy.