ALKANES

I know the exams are close and you're all sick of it all but we need to finish it. Here's something to cheer you up: 

Alkanes are hydrocarbons, which means that they are a compound made of hydrogen and carbon only. Common sense, eh??

You know from GCSE that crude oil is used as a source of hydrocarbons. During fractional distillation you are able to separate the fuels because each chain has a different boiling point.

Moving on to more difficult part is the effect of branching and chain length on boiling points.

CHAIN LENGTH: As the length increases, there are more electrons and hence more points of contact. This means more Van der Waals's forces form and more energy is required to break them. So as chain length increases the boiling point increases.

BRANCHING: Branched molecules have fewer points of contact (same number of electrons but different shape - isomerism, remember?). they are also further apart as molecules cannot get close together. This means less Van der Waal's forces and less energy required to break them. So as a molecule becomes more branched it's boiling point decreases.

When producing alkanes, you should keep in mind that they are fuels. Therefore they will be combusted. 

In real life complete combustion is not possible, especially since most cars have limited supply of oxygen. This results in incomplete combustion, which produces dangerous gases.

C4H10 (l) + 9/2 O2 (g) ----> 4 CO + 5 H2O

Carbon monoxide is poisonous, it prevents haemoglobin from binding with oxygen in red blood cells, so body tissues become deprived of oxygen. This can lead to death. Simply put.

It is much easier to combust branched alkanes than straight chains of alkanes. This is because oxygen only attacks the outside of the chain, so when the alkane is branched it is easier to attack the molecule and more of the molecule will be combusted.

Because of this companies crack the long chains of alkanes into smaller ones, produce branched alkanes or cyclic hydrocarbons, which combust more efficiently.

Time to step up the game now. It's radical substitution time! Yeah! Am I the only one happy? Nevermind. 

• We use ultraviolet light in a process called INITIATION to break Cl2, Br2, etc. into two radicals by homolytic fission (rings a bell?).

Important to note that radicals are very aggressive (reactive in chemistrian speak), so they will attack anything to form a bond.

Usually in a reaction bonds are first broken, using energy and then reformed releasing energy. In this case the radicals do not require any energy as their bonds are already broken.

• During PROPAGATION the radical attacks an alkane, steals a hydrogen and donates it's extra electron to the left over molecule, to form a HCl (or whatever halogen is there) and that radical alkane, minus one hydrogen.
• That radical alkane now wishes to attack everything, when it chances upon a Cl2 molecule it steals one Cl atoms and donates it's extra electron to the other Cl atom forming a Cl radical. We have formed the radical we started off with a ChloroAlkane (or whatever halogen used).
• This will go on to form many compounds, until no reactants are left. The problem is compounds may form that you do not wish to form.
• Final step is TERMINATION. During this process two radicals react together and share their extra electrons to form a covalent bond (shared pair of electrons, NO??) 

Cl. + Cl. ----> Cl2

CH3. +CH3. ----> C2H6

CH3. + Cl. ----> CH3Cl

Here are termination products when dealing with methane.

Ensure you understand that radical substitution forms a mixture of products.