Chapter 10: Organic Chemistry
Alkanes are non-polar molecules, with London forces between the molecules, meaning they are volatile (evaporate easily). As they are non-polar, they are insoluble in water.
Reactions of Alkanes:
Alkanes undergo combustion very easily.
- Complete combustion of alkanes requires bountiful oxygen and produces carbon dioxide and water.
- Incomplete combustion takes place in insufficient oxygen- not all of the carbon reacts and carbon monoxide and/or particulates are formed (soot).
Unreactivity of Alkanes
Apart from combustion, alkanes are very unreactive. The reasons for this are:
- The C-C and C-H which means that it generally energetically unfavourable to break them for the reaction
- C-C and C-H bonds are non-polar so don’t attract polar molecules or ions.
Alkane Reactions with Halogens:
Alkanes can react with Halogens in the presence of sunlight or UV light.
CH4+Cl2 ⎯→ CH3Cl + HCl
(In presence of UV- will not react at room temperature in the dark)
A substitution reaction is one where an atom or group is replaced by another one.
CH4+Cl2 → HCl + CH3Cl
This is an example of a mono-substitution reaction. One halogen atom replaces one hydrogen atom.
Free Radical Substitution
For example, the reaction between chlorine and methane: CH4+Cl2 → CH3Cl+HCl
- The first stage involves breaking apart chlorine atoms into separate chlorine atoms.
- CL2 → 2Cl•
- The energy that is required to do this is provided via UV light. the Cl-Cl bond is weaker than the C-H bond, therefore the Cl-Cl bond is the one that is broken.
- Chlorine atoms only have 7 electrons in their outer shell, so each has an unpaired electron represented by •
- Chlorine atoms are described as free radicals.
Free Radicals are species (atoms or multiple atoms) with an unpaired electron– this makes them very reactive.
- The first step of the reaction is to increase number of free radicals- this is called the initiation step.
- This process is homolytic fission. When the atoms are split up, an electron being shared covalently returns to each atom.
A chlorine free radical is a very reactive species, meaning when it collides with a methane molecule in the reaction mixture, it will bond with an H atom to pair up its unpaired electron.
Cl•+CH4 → HCl + •CH3
A highly reactive methane free radical is produced, and will react with a CL2 molecule to form a C-Cl bond.
•CH3+Cl2 → CH3Cl+Cl•
The Cl• generated in this step can go on to react with another methane molecule, thus the propagation cycle starts again. This is a chain reaction.
In the reaction mixture, free radicals exist in low concentrations therefore the chance of them colliding is low. However, this happens and this brings the chain to an end.
Several possible termination reactions:
Cl•+ Cl• → Cl2
Cl•+•CH3 → CH3Cl
•CH3+•CH3 → C2H6
Each termination reaction causes a decrease in the number of free radicals.
10.2.2 Reactions of Alkenes and Alkynes: