Hydrocarbons (1): Alkanes vs alkenes: which are more reactive?

Alkanes vs alkenes: which are more reactive?

Alkanes contain only single chemical bonds.

But alkenes contain single and double chemical bonds

You can see this from their general formulas

Alkanes CnH2n+2   but alkenes CnH2n

We fit four bonds to every carbon atom and one bond to every hydrogen atom

So hexane has this formula C6H14   but hexene has this formula C6H12

Here are the displayed formulas for each hydrocarbon:

hexane

Molecules with only single bonds we call saturated (not to be confused with sponges and water!!)

hexene

Molecules with multiple bonds like hexene we call unsaturated (no marks for guessing that!!)

The hexane is more reactive because it has the double bond between two of its carbon atoms.

You can distinguish the two hydrocarbons using a reagent like bromine water as here.

Here is the equation for the reaction with bromine:

The red/orange bromine water turns colourless (don't use the word: clear that's a mistake many students make, a solution could be red and clear or see-through, the point is the solution turns colourless) when it reacts with the hexene.

The reactive part of the hexene is its double bond because one of the two bonds is much weaker than all the others in the molecule.

The weaker of the two bonds in the double bond is the only bond broken at room temperature when hexene comes into contact with the bromine molecule.

Why does red/orange bromine loses its colour?

This is because the bond between the two bromine atoms in the bromine molecule is broken.  

The two bromine atoms attach to the hexene at the hexene double bond.

The final result in this case is a molecule with no double bonds.

The reactivity of alkenes allows them to be used to produce many other petrochemicals such as polymers (see the posts on polymers).

Here are some equations for the reactions of ethene C2H4 with simple reagents:

(This is the sort of list you'll need to know for a GCSE Chemistry course.)

All these reaction below are called addition reactions.

  

You can probably guess why that is-a molecule is added to the alkene across the double bond.  

But look at the result the new molecule is saturated.

This is a template for the three reactions below:

1)

You will need to remember the name of the product 1,2-dibromoethane

Notice too that this product is now saturated hence the name ending has changed to -ane.

2)

Here's a bit of alkene history as the caption includes here the old name for ethene: ethylene

3)

Here is how an unsaturated alkene molecule is turned into saturated alkane molecule.  

Here ethene turns into ethane.

Last one is the polymer reaction: polymerisation

polymerisation

n can be in the thousands e.g. 1500 but its never specific because it varies with every polymer chain.

Here is a typical question on this area of chemistry:

Ethene is used as a starting material for the production of many other substances,

including ethanol.

Ethene is produced when hydrocarbons are cracked. To do this hydrocarbons are

heated to vaporise them. The vapours are then passed over a hot catalyst. The

symbol equation shows the reaction for one hydrocarbon.

C28H58   →  C11H24 + C11H22 + C4H8 + C2H4

One of the products is a different type of hydrocarbon to the other products.

Complete the sentences.

The formula of the product that is a different type of hydrocarbon is ............................. .

The chemical structure of this product is different to the other products because

................................................................................................

................................................................................................

Ethanol is produced when ethene reacts with steam in the presence of a hot catalyst.

Draw the missing bonds and atom symbols to complete the displayed structures in the equation.

ethene      +      steam       →       ethanol

New Pages on the "Mole" and "Using the Mole" in chemical calculations are here