7.2 Addition, elimination and substitution reactions

Addition, substitution and elimination reactions

Addition reactions

An addition reaction occurs when two or more reactants combine to form a final product. This product will contain all the atoms that were present in the reactants. The following is a general equation for this type of reaction:

Notice that C is the final product with no A or B remaining as a residue.

The following are some examples.

1. The reaction between ethene and bromine to form 1,2-dibromoethane ( [link] ). ${\mathrm{C}}_2{\mathrm{H}}_4+{\mathrm{Br}}_2\to {\mathrm{C}}_2{\mathrm{H}}_4{\mathrm{Br}}_2$

   

2. Polymerisation reactions In industry, making polymers is very important. A polymer is made up of lots of smaller units called monomers . When these monomers are added together, they form a polymer. Examples of polymers are polyvinylchloride (PVC) and polystyrene. PVC is often used to make piping, while polystyrene is an important packaging and insulating material. Polystyrene is made up of lots of styrene monomers which are joined through addition reactions ( [link] ). 'Polymerisation' refers to the addition reactions that eventually help to form the polystyrene polymer.

   

3. The hydrogenation of vegetable oils to form margarine is another example of an addition reaction. Hydrogenation involves adding hydrogen (H ${}_2$ ) to an alkene. An alkene is an organic compound composed of carbon and hydrogen. It contains a double bond between two of the carbon atoms. If this bond is broken, it means that more hydrogen atoms can attach themselves to the carbon atoms. During hydrogenation, this double bond is broken, and more hydrogen atoms are added to the molecule. The reaction that takes place is shown below. Note that the 'R' represents any side-chain or the rest of the molecule. A side-chain is simply any combination of atoms that are attached to the central part of the molecule. ${\mathrm{RCHCH}}_2+{\mathrm{H}}_2\to {\mathrm{RCH}}_2{\mathrm{CH}}_3$
4. The production of the alcohol ethanol from ethene. Ethanol (CH ${}_3$ CH ${}_2$ OH) can be made from alkenes such as ethene (C ${}_2$ H ${}_4$ ), through a hydration reaction like the one below. A hydration reaction is one where water is added to the reactants.
   $$C_2{H}_4+H_{2}O\to C{H}_{3}C{H}_{2}OH$$
   A catalyst is needed for this reaction to take place. The catalyst that is most commonly used is phosphoric acid.

Elimination reactions

An elimination reaction occurs when a reactant is broken up into two products. The general form of the equation is as follows:

The examples below will help to explain this:

1. The dehydration of an alcohol is one example. Two hydrogen atoms and one oxygen atom are eliminated and a molecule of water is formed as a second product in the reaction, along with an alkene.

   

   ${\mathrm{CH}}_3{\mathrm{CH}}_2\mathrm{OH}\to {\mathrm{CH}}_2{\mathrm{CH}}_2+{\mathrm{H}}_2\mathrm{O}$
2. The elimination of potassium bromide from a bromoalkane . ${\mathrm{CH}}_3{\mathrm{CH}}_2\mathrm{Br}+\mathrm{KOH}\to {\mathrm{CH}}_2{\mathrm{CH}}_2+\mathrm{KBr}+{\mathrm{H}}_2\mathrm{O}$

   

3. Ethane cracking is an important industrial process used by SASOL and other petrochemical industries. Hydrogen is eliminated from ethane (C ${}_2$ H ${}_6$ ) to produce an alkene called ethene (C ${}_2$ H ${}_4$ ). Ethene is then used to produce other products such as polyethylene. You will learn more about these compounds in Grade 12. The equation for the cracking of ethane looks like this: ${\mathrm{C}}_2{\mathrm{H}}_6\to {\mathrm{C}}_2{\mathrm{H}}_4+{\mathrm{H}}_2$