Types of Organic Reactions and Their Mechanisms

Types of organic reactions

• Substitution: an atom or group is replaced. CH₄ + Cl₂ → CH₃Cl + HCl
• Addition: atoms add across a C=C or C=O; two molecules become one. CH₂=CH₂ + HCl → CH₃CH₂Cl
• Elimination: a small molecule (H₂O, HCl) leaves and a double bond forms. CH₃CH₂OH → CH₂=CH₂ + H₂O (hot conc. acid)
• Condensation: two molecules join and lose a small molecule, usually water. Acid + alcohol → ester + water.
• Hydrolysis: water breaks a bond; the reverse of condensation.
• Polymerisation: many small monomers join into a long chain (addition polymers like polythene; condensation polymers like nylon, proteins).

How bonds break and the particles that react

A mechanism is the step-by-step path of a reaction, showing which bonds break and form and how electrons move. A curly arrow shows a pair of electrons moving (from a bond or lone pair to an atom).

Bond fission

• Homolytic: each atom takes one electron → two free radicals (with a dot, like Cl•). Needs UV light or heat.
• Heterolytic: one atom takes both electrons → a cation and an anion. Happens in polar bonds.

Attackers

• Nucleophile: electron-pair donor, has a lone pair or negative charge: OH⁻, H₂O, NH₃, CN⁻, the O of an alcohol.
• Electrophile: electron-pair acceptor, positive or δ+: H⁺, NO₂⁺, the C of C=O, Br in a polarised Br₂.
• Radical: has one unpaired electron; very reactive.

Intermediates: carbocations (C⁺), carbanions (C⁻) and radicals (C•).

Three classic mechanisms

1. Free-radical substitution: chlorination of methane

1. Initiation: Cl₂ → 2Cl• (UV light)
2. Propagation: Cl• + CH₄ → HCl + •CH₃; •CH₃ + Cl₂ → CH₃Cl + Cl•
3. Termination: two radicals join, e.g. Cl• + •CH₃ → CH₃Cl

The same radical chain explains H₂ + Cl₂ → 2HCl in light. Further substitution can give CH₂Cl₂, CHCl₃, CCl₄.

2. Electrophilic addition: HCl to ethene

The C=C is electron-rich. H of H–Cl (δ+) is the electrophile. The π electrons attack H⁺, forming a carbocation CH₃–CH₂⁺ and Cl⁻; then Cl⁻ (nucleophile) bonds to C⁺ → chloroethane.

3. Electrophilic substitution: chlorination of benzene

Benzene's ring is very stable, so it substitutes rather than adds. A catalyst (FeCl₃ or AlCl₃) makes Cl⁺: Cl₂ + FeCl₃ → Cl⁺ + FeCl₄⁻. Cl⁺ attacks the ring, then H⁺ is lost so the ring stays aromatic: C₆H₆ + Cl₂ → C₆H₅Cl + HCl.

Esters: condensation, hydrolysis and reversibility

Esterification: carboxylic acid + alcohol ⇌ ester + water, with a little conc. H₂SO₄ as catalyst and warming. CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O (ethyl ethanoate).

Mechanism in simple steps

1. H⁺ adds to the C=O oxygen, making the carbon more δ+ (a better electrophile).
2. The alcohol O (nucleophile) attacks that carbon.
3. A proton moves and water leaves.
4. H⁺ is lost again: the catalyst is regenerated.

Reversibility

All steps can go back, so an equilibrium forms. Removing water or using excess alcohol gives more ester (Le Chatelier).

Hydrolysis

• Acid hydrolysis: ester + water ⇌ acid + alcohol (reversible).
• Base (alkaline) hydrolysis: ester + OH⁻ → carboxylate + alcohol. OH⁻ attacks the C=O carbon. It goes to completion because the acid becomes a salt. With fats this is saponification (soap).

Properties and uses

Small esters smell fruity, are volatile, have lower boiling points than acids of similar size (no O–H hydrogen bonding between ester molecules) and dissolve poorly in water. Uses: flavours, perfumes, solvents (nail-polish remover), plastics (polyesters). Aspirin is made by esterifying the –OH of salicylic acid (with ethanoic anhydride); it slowly hydrolyses back in damp air or the body. Cellulose can be esterified to cellulose acetate (film, fibres).

Condensation and hydrolysis in living things

• Peptide bond: –COOH of one amino acid + –NH₂ of another → –CO–NH– + H₂O. Proteins are chains made by condensation.
• Sugars: glucose and fructose (C₆H₁₂O₆) join by condensation into sucrose + H₂O. In water, glucose closes into a ring: the –OH on carbon 5 (nucleophile) adds to the C=O carbon (electrophile), a nucleophilic addition.
• Keto-enol tautomerism: a C=O next to a C–H can shift an H to form C=C–OH (enol) and back. This lets glucose and fructose change into each other in base.
• Enzymatic hydrolysis in digestion: amylase splits starch into maltose; proteases split proteins into amino acids; lipases split fats into fatty acids and glycerol. Enzymes are catalysts at body temperature.

Try it at home

Smell some fruits: banana, pineapple, orange. Each smell is mostly a different ester. Then mix a few drops of vinegar (ethanoic acid) with baking soda: that is an acid reaction, not esterification. Why can't you make an ester at home safely? (It needs a strong acid catalyst and heat.)

Key formulas and definitions

• Substitution: CH₄ + Cl₂ → CH₃Cl + HCl (UV light)
• Addition: CH₂=CH₂ + HCl → CH₃CH₂Cl
• Elimination: CH₃CH₂OH → CH₂=CH₂ + H₂O (conc. H₂SO₄, heat)
• Esterification: RCOOH + R′OH ⇌ RCOOR′ + H₂O (H⁺ catalyst)
• Base hydrolysis: RCOOR′ + NaOH → RCOONa + R′OH
• Peptide bond: –COOH + H₂N– → –CO–NH– + H₂O
• Homolytic: A–B → A• + B•; Heterolytic: A–B → A⁺ + B⁻

Worked examples

Common mistakes

• Drawing curly arrows from the atom that receives electrons. Arrows always start at the electrons (a bond or lone pair).
• Thinking benzene adds like ethene. Benzene usually substitutes, to keep its stable ring.
• Forgetting that acid-catalysed esterification is reversible and stops at equilibrium.
• Naming esters backwards: CH₃COOC₂H₅ is ethyl ethanoate (alcohol part 'ethyl' first), not ethanoyl ethyl.