Some reactions of hydrocarbons

Substitution of alkanes Aqueous bromine is put in a test tube, then cyclohexane poured carefully on top. The hydrocarbon layer floats on top of the aqueous bromine solution.

When mixed, the non-polar bromine moves from the aqueous layer into the hydrocarbon layer, turning the alkane orange. A small amount of bromine remains in the aqueous layer, making it very pale orange. No chemical reaction has occurred at this stage.

Put the test tube in bright light for ten minutes.

After 10 minutes sitting in the bright light of an overhead projector, the bromine in the hydrocarbon layer has decolourised. The colour of the aqueous layer is unaffected.

The reaction between bromine and alkanes is a SUBSTITUTION reaction. The reaction is slow. It requires UV light. Two products are formed: a bromoalkane and hydrogen bromide. Any hydrogen in the chain could be replaced by a bromine atom, so in fact there are many different organic products formed. CH3CH3 + Br2 → CH3CH2Br + HBr

Addition of alkenes Aqueous bromine is put in a test tube, then cyclohexene poured carefully on top. The hydrocarbon floats on the more dense aqueous solution.

When the test tube is shaken, the bromine enters the organic layer and is immediately decolourised. More bromine from the aqueous layer moves into the organic layer, until all the bromine has reacted.

Bromine water added dropwise to cyclohexene is also instantly decolourised.

The reaction between bromine and an alkene is an ADDITION reaction. The reaction is fast. Light is not required. Only one product is formed. cyclohexene + bromine → 1, 2-dibromocyclohexane CH2=CH2 + Br2 → CH2Br—CH2Br

Acidified potassium permanganate and hydrocarbons. Compounds containing no double or triple bonds are called saturated. Alkanes are saturated hydrocarbons. Compounds containing one or more C=C or C≡C bonds are unsaturated because these bonds can be broken to add in more hydrogen (or anything else). Purple potassium permanganate solution can be used to distinguish between saturated and unsaturated hydrocarbons.

Alkane Alkene Saturated Unsaturated The hydrocarbons float on top of the aqueous solution of permanganate.

Alkane Alkene When the test tubes are shaken, the alkene decolourises the acidified permanganate. There is no reaction with the alkane.

If acid is not added to the permanganate solution, the purple solution will change to a brown precipitate.

The permanganate oxidises the double bond to form a diol.