Reactions of Hydrocarbons: Combustion, Substitution and Addition

What hydrocarbons do

Alkanes are sometimes called paraffins, from Latin words meaning "little affinity", because they react with very little at room temperature. Alkenes and alkynes are far more reactive: their double and triple bonds are rich in electrons and easy to attack. This lesson covers the four big reaction types of hydrocarbons:

1. Combustion (all hydrocarbons)
2. Substitution (alkanes and aromatics)
3. Addition (alkenes and alkynes)
4. Tests that use these differences to identify an unknown

Remember from Grade 11: in Chemical Reactions you balanced combustion equations and compared complete and incomplete combustion. We start there.

1. Combustion

Complete combustion (plenty of oxygen) gives carbon dioxide and water:

C₃H₈ + 5 O₂ → 3 CO₂ + 4 H₂O

Incomplete combustion (limited oxygen) also gives carbon monoxide (CO) and soot (C):

2 CH₄ + 3 O₂ → 2 CO + 4 H₂O

Carbon monoxide is colourless, odourless and deadly, because it binds to hemoglobin in place of oxygen. That is why Ontario law requires carbon monoxide alarms in homes with a fuel-burning appliance, a fireplace or an attached garage, and why you should never run a generator, barbecue or car in a closed space.

2. Substitution

Alkanes react with chlorine or bromine only when energy is supplied as ultraviolet light (or high heat). One H atom is replaced by a halogen atom, and the H leaves with the other halogen atom as HX:

CH₄ + Cl₂ —(UV light)→ CH₃Cl + HCl

The reaction does not stop neatly. CH₃Cl can react again to give CH₂Cl₂, CHCl₃ and CCl₄, and larger alkanes give mixtures of position isomers, so this is a messy way to make one pure product.

Benzene also undergoes substitution, not addition, because addition would destroy its stable ring of delocalized electrons. It needs a catalyst such as FeBr₃:

C₆H₆ + Br₂ —(FeBr₃)→ C₆H₅Br + HBr

How to recognize a substitution: two reactants → two products, and one of the products is usually a small molecule such as HCl or HBr.

3. Addition to alkenes and alkynes

In an addition reaction, the double bond opens and one new atom bonds to each carbon of the former double bond. Two reactants → one product. The four main additions are:

• Hydrogenation, H₂: metal catalyst (Pt, Pd or Ni) → alkane. CH₂=CH₂ + H₂ → CH₃CH₃
• Halogenation, Br₂ or Cl₂: room temperature → dihaloalkane. CH₂=CH₂ + Br₂ → CH₂BrCH₂Br
• Hydrohalogenation, HCl or HBr: → haloalkane. CH₂=CH₂ + HBr → CH₃CH₂Br
• Hydration, H₂O: acid catalyst (H₂SO₄) → alcohol. CH₂=CH₂ + H₂O → CH₃CH₂OH (This is how much industrial ethanol is made.)

An alkyne can add twice: the triple bond becomes a double bond, then a single bond.

HC≡CH + 2 H₂ —(Pt)→ CH₃CH₃

Markovnikov's rule

When H–Br adds to a symmetrical alkene such as ethene or but-2-ene, there is only one possible product. But propene is unsymmetrical: the H could go to C1 or to C2. Experiments show that one product strongly dominates. The Russian chemist Vladimir Markovnikov summarized the pattern in 1869:

When HX or H–OH adds to an unsymmetrical alkene, the H atom goes to the double-bond carbon that already has more H atoms, and the X (or OH) goes to the other carbon.

So propene + HBr gives mainly 2-bromopropane, and propene + H₂O gives mainly propan-2-ol. In university organic chemistry you will learn the reason: the reaction passes through a positively charged intermediate, and a positive charge on the more substituted carbon is more stable.

4. Testing for unsaturation: a lab you can describe

In a school lab, a few drops of a hydrocarbon are added to bromine water, which is orange-brown.

• An alkene or alkyne decolourizes it immediately: the Br₂ adds across the multiple bond and is used up.
• An alkane gives no change in the dark. (In bright sunlight it slowly reacts by substitution.)
• Benzene gives no change: without a catalyst, it does not react.

Potassium permanganate solution is another test: an alkene turns the purple solution brown (a brown precipitate of MnO₂ forms), while alkanes and benzene leave it purple.

Safety (A1.4): bromine water is toxic and corrosive, and its vapour irritates the lungs. The test is done in a fume hood, with safety goggles, gloves and a lab coat. Hydrocarbons such as hexane and cyclohexene are flammable: no open flames in the room. Check the WHMIS label and the safety data sheet (SDS) for each chemical before you begin, and pour all organic waste into the labelled waste container, never down the sink. Do not try this test at home.

Addition in your kitchen: hydrogenation and trans fats

Vegetable oils are liquids because their long carbon chains contain cis C=C bonds, which put kinks in the chains so they cannot pack closely. Adding H₂ with a nickel catalyst (hydrogenation) converts some C=C bonds to C–C bonds, so the chains straighten and the fat becomes solid, like shortening or hard margarine.

Partial hydrogenation has a side effect: some of the remaining double bonds flip from cis to trans. Trans fats raise the risk of heart disease, so Canada banned partially hydrogenated oils in foods in September 2018. This is a real example of B1: understanding an organic reaction helped shape a public health law.