Some reactions of hydrocarbons

The hydrocarbon layer floats on top of the aqueous bromine solution.

Aqueous bromine is put in a test tube, then cyclohexane poured carefully on top.

Substitution of alkanes

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 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.

The reaction between bromine and alkanes is a SUBSTITUTION reaction.

CH3CH3 + Br2 → CH3CH2Br + HBr

Cracking alkanes

Long-chain alkane molecules can be cracked into a shorter-chain alkane and an alkene:

C12H26 → C8H18 + C4H8

Paraffin—a mixture of liquid alkanes—can be cracked in the laboratory using a steel wool catalyst.

Here the catalyst is Al2O3.

In industry a nickel catalyst is used.

Aqueous bromine is put in a test tube, then cyclohexene poured carefully on top.

The hydrocarbon floats on the more dense aqueous solution.

Addition of alkenes

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 hydrocarbonsCompounds 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

The hydrocarbons float on top of the aqueous solution of permanganate.

Unsaturated

Saturated

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.

Alkenes undergo addition reactions with several different reagents:

• with hydrogen to form alkanes (hydrogenation). This occurs during margarine manufacture or at the oil refinery where hydrogen is added to stop alkenes forming in petrol.

• with bromine to form a dibromoalkane (or chlorine)

• with HBr or (HCl) to form haloalkanes

• with H2O to form alcohols. An acid catalyst is required. Typically a 50% H2SO4 solution or conc H3PO4 with

water is used.

• with permanganate to form diols.

Markovnikov’s rule

When an unsymmetric reagent, such as HBr or H2O, is added to an unsymmetric alkene, two different products will be formed. In these cases one product will be formed in a greater amount than the other.

The rule is that the hydrogen atom is given to the carbon atom which already has the greater number of hydrogen atoms. In summary: the rich get richer.

H—C—C = C—H + HBr → H—C—C—C—H

H

H H H

H

H H H

C

rich

C

poor

HBrMajor product

H—C—C—C—H

H H H

H H Br

Minor product

Another look at alkenes with bromine

The experiment shown in the movie which follows this slide is extremely dangerous and should only be done by very experienced chemistry teachers if they know how to handle liquid bromine safely.

Bromine vapour causes deep and painful burns to the skin which can take many months to heal – even through gloves…

— which is why we made a movie of the experiment so you can see it in safety.

Special thanks to Rudi Jansen from Middleton Grange School who did this reaction for you.

Click on the movie to start.

It’s a big file that may take a

while to load.

In the test tube there is some bromine water to which we are now going to add an alkene. You will notice the alkene does not mix with bromine water, alkene being non-polar and the bromine being in aqueous solution. There's not much reaction there. If we were to shake it the colour fairly quickly is removed from the bromine, as you can see, and the hydrocarbon continues to sit at the top.

We use this as a test for unsaturation but it's not particularly impressive, so what we want to do is to show how reactive bromine really is with the alkene by adding molecular bromine directly to the alkene.

So this is one of the two liquid elements, bromine itself (you can see the vapour -- maybe) and we will add some drops of (bromine to the cyclohexene). You will notice the reaction is fairly vigorous and bromine is immediately decolourised and, over time you may see a change in the meniscus of the alkene...as the reaction proceeds.

This should impress you that the reaction of bromine with alkenes is certainly rapid, and you can notice too, that the material has now sunk to the bottom.

There is a small amount of bromine still present and if we were to shake this up now you will see the colour quite disappear.

If you write the formula for this and check out the molar mass of the cyclohexene and then the cyclohexene with two bromines added, you will notice a very marked difference in the molar masses which should account for the change in behaviour from the layer sitting on top to now at the bottom of the aqueous layer.

The audio is a little hard to follow in places, so it is written for you here.

Cyclohexene = C6H10 M(C6H12) = 82 g mol–1

Bromine = Br2 M(Br2) = 160 g mol–1

1, 2-dibromocyclohexane = C6H10Br2

M(C6H10Br2) = 242 g mol–1

So when two bromine atoms are added to cyclohexene the molar mass triples.

Although cyclohexene is less dense than water, the haloalkane formed is more dense than water and sinks.