Introduction to Organic Chemistry

The three reaction types you need to know

• Substitution — one atom (or group) is replaced by another atom or group
• Addition — two reactant molecules merge into one product molecule and nothing else is given off
• Combustion — the organic compound burns in oxygen to give carbon-containing oxides plus water

Substitution reactions

• A characteristic feature: an existing functional group (often a single C–H or C–X bond) is swapped for a new one, and a small by-product (usually HX) is released
• Example: chlorination of ethane in ultraviolet light

C₂H₆(g) + Cl₂(g) → C₂H₅Cl(g) + HCl(g)

ethane + chlorine → chloroethane + hydrogen chloride

• One hydrogen on the ethane has been swapped for a chlorine atom; the displaced hydrogen pairs up with the second chlorine atom from Cl₂ to make HCl

Addition reactions

• A characteristic feature: two or more molecules combine into one product molecule, with nothing else released
• The molecule that accepts the new atoms typically has a C=C double bond, which "opens up" to single bonds and accommodates the incoming atoms
• Example: hydrogenation of propene (margarine manufacture)

CH₃−CH=CH₂(g) + H₂(g) → CH₃−CH₂−CH₃(g)

propene + hydrogen → propane

• Example: bromination of propene (also the test for an alkene)

CH₃−CH=CH₂(g) + Br₂(l) → CH₃−CHBr−CH₂Br(l)

propene + bromine → 1,2-dibromopropane

• Notice that both examples have one product, not two

Combustion reactions

• All organic compounds burn in oxygen; whether the products are carbon dioxide or carbon monoxide depends on how much oxygen is available
• Complete combustion (plenty of oxygen): products are carbon dioxide and water only

2 C₂H₆(g) + 7 O₂(g) → 4 CO₂(g) + 6 H₂O(l) (ethane)

2 C₄H₁₀(g) + 13 O₂(g) → 8 CO₂(g) + 10 H₂O(l) (butane)

• Incomplete combustion (limited oxygen): one or more carbons end up as carbon monoxide (a colourless, odourless poison) or as soot (unburnt carbon particles)

2 C₂H₆(g) + 5 O₂(g) → 4 CO(g) + 6 H₂O(l) (ethane — incomplete)

• Incomplete combustion is why gas heaters need good ventilation: a build-up of CO is invisible and lethal
• All combustion reactions are exothermic — they are why fossil fuels are used as energy sources

Quick recognition guide