10.6 – Reaction Pathways

10.6 – Reaction Pathways

The different reactions that are possible for organic compounds are explained in more detail in previous sections. Some are outlined in this section too. It is important to be able to manipulate the reactions and create combinations that will produce the desired product. Remember that the IB will never ask you to deduce a reaction pathway that involves more than two steps.

10.6.1 – Deduce reaction pathways given the starting materials and the product

Reactions of Alkanes

Combustion

The general equation for the complete combustion of alkanes is:

However, if there is insufficient oxygen available, the alkanes undergo incomplete combustion to produce a mixture of CO₂, CO and C. For example:

Substitution

Alkanes will undergo substitution reactions with halogens if there is a mechanism present (such as UV light or heat). During these reactions, the halogens will form free radicals – these have no charge, but have an unpaired electron, making them highly reactive. During the reaction, the halogen will undergo homolytic fission to make the radicals.

This reaction takes place in a few steps:

Initiation

This is when the radicals are formed.

This step happens first because the bonds between the halogen molecules are weaker and have a lower enthalpy than the C-H or C-C bonds.

Propagation

In this step, the radicals are used, and new radicals are formed.

In the second stage of propagation, the CH₃ radical reacts with the chlorine molecule to make CH₃Cl

Termination

The free radicals are removed by reacting together

The methane and chlorine radicals can also be removed:

The methane radicals may react together:

In reality, there will be a mixture of products, depending on the concentration of the reactants. These products can then be separated using distillation processes.

Alkane to Alcohol

This reaction must take place in two steps. In the first step, UV light is used to create chlorine radicals, and the reaction continues to form a halogenoalkane.

The second step is an S_N2 reaction, with the OH^– ion attacking the carbon-chlorine bond. A reactive intermediate is formed, which then loses the Cl^–.

Formation of a Halogenoalkane

This is a substitution reaction using a free radical mechanism