Everything Science Grade 11

CHAPTER 8. TYPES OFREACTIONS 8.3

Elimination reactions ESBCF

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

A→ B + C

The examples below will help to explain this:

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

H C

H

H

C

H

OH

H

H

H

C C

H

H

+

H

O

H

CH 3 CH 2 OH→ CH 2 CH 2 + H 2 O

2. The elimination of potassium bromide from a bromoalkane.

CH 3 CH 2 Br + KOH→ CH 2 CH 2 + KBr + H 2 O

H

H

C C

H

H

H C

H

H

C

Br

H

H+KOH + KBr + H 2 O

3. Ethane cracking is animportant industrial process used by SASOL and other petrochemical in-
   dustries. 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 forthe cracking of ethane looks like this:

C 2 H 6 → C 2 H 4 + H 2

Substitution reactions ESBCG

A substitution reaction occurs when an exchange of elements in the reactants takes place. Theinitial
reactants are transformed or ’swopped around’ to give a final product. Asimple example of a reaction
like this is shown below:

AB + CD→ AC + BD

Some simple examples of substitution reactions are shown below:

CH 4 + Cl 2 → CH 3 Cl + HCl