Alkanes and isomers

Sterioisomers

Geometrical and optical

geometrical isomers

CIs and trans, only takes place in compunds with restricted rotation on the c-c bond

optical isomers

takes place at chiral centres, each ciral centre give two enatiomers giving rise to a mirror image

Production of alkanes

hydrogenation and cracking

Hydrogenation

Catalyst:pt/ni
addition of hydrogen to an alkene

Catalytic cracking

Catalyst:Al2O3
Breaking down of a large hydrocarbons to form smaller hydrocarbons such as alkanes and alkenes
Compounds formed in layers are dependent on the Mr for the order of layer from top to down

Incomplete combustion of Alkanes

When hydrocarbons are burned in insufficient oxygen to give CO and H2O

Complete combustion of Alkanes

Sufficient oxygen
CnH2n+2 + (3n+1)/2O + nCO2 + n+1 H2O

Free radical substitution

Intiation
termination
propogation

Intiation 

When the Cl-Cl bond is broken in the presence of UV
Each Cl takes 1 electron from the bond pair (homolytic)
This forms a radical

Propogation

The free radical attacks the unreactive alkanes
1) Alkyl radical formed
2) Chlorine radical regenerated by the alkyl attacking Cl2 bond
3)If there is enough halogen present, all the hydrogens in the alkane will eventually get substituted

Termination

The chain reaction stops due to two free radicals reacting

Alkyl group

An alkyl group is a functional group derived from an alkane by removing one hydrogen atom.

Unreactivity of Alkanes

• Unless a lot of heat is supplied, it is difficult to break these strong C-C and C-H covalent bonds

• The electronegativities of the carbon and hydrogen atoms in alkanes are almost the same

• This means that both atoms share the electrons in the covalent bond almost equally

Volatile Organic Compounds (VOCs)

• Formation:

  • Unburnt hydrocarbons from fuels

  • Oxides of these hydrocarbons formed in car engines

• Environmental consequence:

  • React with oxides of nitrogen in the atmosphere to form PAN

PAN (peroxyacyl nitrate)

• Formation:

  • From the photochemical reaction of VOCs and nitrogen oxides in the atmosphere

• Environmental consequence:

  • Contributes to photochemical smog