Alkanes and alkenes

MDCAT Chemistry · Chemistry · Hydrocarbons. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

Alkanes (C_nH_2n+2) have only strong, non-polar C–C and C–H σ bonds, so acids, alkalis and oxidising agents do not attack them. They react with chlorine or bromine in light by free-radical substitution, a chain reaction with initiation, propagation and termination steps.

Alkenes (C_nH_2n) have a C=C bond. Its π electrons are exposed, so electrophiles attack them and add across the double bond. This is electrophilic addition.

For names, choose the longest carbon chain and give the branch or the double bond the lowest number. Boiling points rise as the chain gets longer and fall as branching increases.

Rules to remember

Initiation: Cl₂ → 2Cl• (light). Propagation: Cl• + CH₄ → •CH₃ + HCl, then •CH₃ + Cl₂ → CH₃Cl + Cl•. Termination: two radicals join
Reactivity of halogens with alkanes: F₂ > Cl₂ > Br₂ > I₂
Markovnikov's rule: when HX adds to an unsymmetrical alkene, H goes to the carbon that already has more H atoms, because the more stable carbocation forms
Reactivity of HX with alkenes: HI > HBr > HCl (the weakest H–X bond breaks most easily)
Tests for C=C: bromine turns from red-brown to colourless; cold, dilute, alkaline KMnO₄ (Baeyer's reagent) loses its purple colour and a diol forms
Alkenes are made by dehydrating alcohols (concentrated H₂SO₄ at about 170 °C, or hot Al₂O₃), by heating alkyl halides with alcoholic KOH, or from vicinal dihalides with zinc dust
More branching gives a lower boiling point; a cycloalkane boils higher than the straight-chain alkane with the same number of carbon atoms

Common mistake

Students add H to the wrong carbon of an unsymmetrical alkene. Draw both possible carbocations first and choose the more stable one (tertiary > secondary > primary); the halide ion then joins that carbon.

Worked example

But-1-ene, CH₃CH₂CH=CH₂, reacts with HBr with no peroxide present. What is the main product?

The electrophile H⁺ adds first. Carbon 1 has two H atoms and carbon 2 has one, so H⁺ joins carbon 1.
This leaves the positive charge on carbon 2: CH₃CH₂CH⁺CH₃, a secondary carbocation, more stable than the primary one.
Br⁻ attacks carbon 2, giving CH₃CH₂CHBrCH₃.

Answer: 2-Bromobutane

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