Grade 10 Introduction to Organic Chemistry
Organic Chemistry is that branch of Chemistry that deals with carbon compounds
(except for CO2, CO and CO32- compounds, which are considered to be Inorganic
compounds). While all Organic compounds contain carbon, hydrogen is usually
found as well, with oxygen, nitrogen, sulphur and halogens in many cases.
With its four valence electrons, carbon has unique bonding abilities. Each carbon
atom can form 4 covalent bonds with other atoms. These include other carbon
atoms, hydrogen, oxygen, halogens, sulphur, etc.. The molecules (always covalently
bonded) often consist of long carbon chains, and branches often occur.
The atoms attached to carbon sometimes lie in the same plane, but generally the
molecules are three dimensional. We will, however, depict them as two dimensional
structures, using Couper diagrams, while bearing in mind that the diagrams actually
represent three dimensional structures.
C
E.g.:
C
C
C
C
C
C
C
C
C
(The H atoms have not been shown here, but usually they must be shown.)
The Aliphatic Compounds
These are hydrocarbons with mainly open chain structures. They appear in three main
‘families’, the properties of which are determined to a large extent by the type of
bond (called a functional group) found between carbon atoms:
Functional group
Single bonds: ALKANES
C
C
Double bonds: ALKENES
C
C
Triple bonds: ALKYNES
C
C
2
The Alkanes
The alkanes form a homologous series with the general formula CnH2n+2 where n is
the number of C atoms in a molecule. The name of each member of the series has the
suffix –ane, while the prefix is determined by the number of C atoms:
n
1
2
3
4
5
6
7
8
9
10
Formula
CH4
C2H6
C3H8
C4H10
C5H12
C6H14
C7H16
C8H18
C9H20
C10H22
Name
Methane
Ethane
Propane
Butane
Pentane
Hexane
Heptane
Octane
Nonane
Decane
Phase at room temp.
Gas
Gas
Gas
Gas
Liquid
Liquid
Liquid
Liquid
Liquid
Liquid
Alkanes with 17 or more C atoms are waxy solids at room temperature.
Note that only single covalent bonds appear in the alkanes. Such molecules are called
saturated, which means that the maximum number of atoms are bonded to each C
atom.
Alkanes with branched chain structures
Branches consist of alkyl groups, which are alkanes with one fewer H atom than
usual:
CH3
C2H5
C3H7
methyl
ethyl
propyl
etc.
To name the alkane according to the IUPAC naming system, follow the steps outlined
below:
1. Choose the longest continuous C chain.
2. Number the chain so that the first branch appears at the C atom with the
lowest number.
3. The total number of C atoms in the longest chain determines the name of the
alkane.
4. The branches are named in alphabetical order (e.g. ethyl before methyl), with
the number of its position in the chain given before the alkyl name, e.g.
2-methylbutane. There must be no gap between the alkyl and alkane names. If
two of the same alkyl group appear, the prefix di- is used, e.g. dimethyl. For
3, use tri-, and for 4 use tetra-.
3
Examples:
C
1.
C
C
C
C
C
2-methylpentane
C
2.
C
C
C
2,2-dimethylbutane
C
C
C
3.
C
C
C
C
2,3-dimethylbutane
C
The examples above are called structural isomers. This means that they are
molecules that have the same molecular formula (C6H14 in each case), but different
structural formulae. Isomers have different physical properties, like boiling points
and melting points.
Write structural formulae for the isomers of pentane, and name each one:
4
The Alkenes
This series of compounds is unsatutated, since double covalent bonds appear
between two C atoms. The general formula is CnH2n. The suffix –ene is added to the
same prefixes as we use for the alkanes:
n
2
3
4
5
6
7
Formula
C2H4
C3H6
C4H8
C5H10
C6H12
C7H14
Name
Ethene
Propene
Butene
Pentene
Hexene
Heptene
Phase at room temp.
Gas
Gas
Gas
Liquid
Liquid
Liquid
IUPAC names
The same procedure is followed as for the alkanes, but the longest chain must contain
the double bond and be numbered from the end closest to the double bond. The
position of the double bond is shown by the lower number of its two C atoms.
E.g.:
C
C
C
C
C
C
C
but-1-ene
but-2-ene
C
C
C
C
C
C
C
3-methylpent-2-ene
C
C
3,4-dimethylpent-2-ene
C
C
C
C
C
5
The Alkynes
The alkynes are also unsaturated, with a triple covalent bond between two C atoms.
The general formula is CnH2n-2. Naming a molecule follows the same route as that
for alkenes, with the suffix –yne used.
E.g.:
C
ethyne
C
propyne
C
C
C
C
C
C
C
but-1-yne
C
C
C
C
but-2-yne