TYPES OF ORGANIC REACTIONS
Organic reaction can be classified into different types. Many of these you
learned about last year. This paper summarises them for you.
Combustion:
All hydrocarbons and some other organic chemicals (e.g. alcohols) undergo
complete combustion in a sufficient supply of oxygen to produce carbon
dioxide and water. If there is insufficient oxygen the combustion is incomplete
and the products are carbon (soot), carbon monoxide and water.
e.g. CH4(g) + O2(g)
CO2 (g) + H2O (l)
Substitution:
In these reactions one atom is replaced by another. There are two products.
Last year you saw that alkanes undergo a slow substitution reaction in the
presence of UV. We will meet some more substitution reaction this year.
e.g. CH3CH3 (g) + Br2 (l)
UV light
CH3CH2Cl (g) + HBr (g)
Addition:
In addition reactions a double bond is broken and atoms are added to a
molecule. These reactions are normally fast and only one product is formed.
e.g. CH2=CH2 (g) + Br2(l)
CH2BrCH2Br (g)
When the molecule that is being added (the addition reagent) is not
symmetrical, two products may be possible, as the two carbon atoms will
bond with different atoms.
e.g. CH3CH=CH2 (g) + HBr
CH3CH2CH2Br (g) + CH3CHBrCH3(g)
Minor product
Major product
One product will be there in greater amounts than the other and is called the
major product (the other is called the minor product). To decide which is the
major product, Markovnikov’s rule is used:
The hydrogen atom of the addition
reagent goes to the carbon atom of the
double bond, that is attached to the
greatest number of hydrogen atoms.
“the rich get richer”
Condensation:
In this type of reaction two molecules join together and expel a small molecule
(usually water or HCl).An ester if formed when a carboxylic acid undergoes a
condensation reaction with an alcohol.
e.g. CH3COOH (l) + CH3CH2OH (l)
Conc. H2SO4
CH3COOCH2CH3 (l) + H2O (l)
The formation of proteins involves a series condensation reactions of amino
acids.
Elimination:
This is the reverse of addition reactions. Something is removed from a
molecule and a double bond forms. E.g. the dehydration of an alcohol to form
an alkene.
CH3CH2OH
Conc. H2SO4
CH2 = CH2 + H2O
If the molecule is longer, then the reverse of Markonikov’s rule (Saytzeff’s
rule) applies:
The hydrogen atom tends to be
eliminated from the carbon atom joined
to the least number of hydrogen atoms.
“the poor get poorer”
Conc. H2SO4
CH3CH2CH(OH)CH3
CH3CH = CHCH3 + CH3CH2CH = CH2 + H2O
Major product
poor
Minor product
rich
Oxidation and Reduction
Oxidising agents e.g. MnO4- /H+, Cr2O72- / H+ are used to oxidise organic
chemicals such as alcohols. A primary alcohol can be oxidised to an
aldehydes and then to a carboxylic acid.
Cr2O72-/ H+
e.g. CH3CH2OH
CH3CHO
Cr2O72-/ H+
CH3COOH