Download 9. Introducing Organic Chemistry

1 of 30
© Boardworks Ltd 2009
2 of 30
© Boardworks Ltd 2009
Molecular and empirical formulae
There are many ways of representing organic compounds
by using different formulae.
The molecular formula of a
compound shows the number
of each type of atom present in
one molecule of the compound.
The empirical formula of a
compound shows the simplest
ratio of the atoms present.
Molecular
formula
C2H6
Empirical
formula
CH3
C6H12O6
CH2O
C2H4O2
CH2O
Neither the molecular nor empirical formula gives information
about the structure of a molecule.
3 of 30
© Boardworks Ltd 2009
Displayed formula of organic compounds
The displayed formula of a compound shows the
arrangement of atoms in a molecule, as well as all the bonds.
Single bonds are represented
by a single line, double
bonds with two lines and
triple bonds by three lines.
The displayed formula can show the different structures of
compounds with the same molecular formulae.
ethanol (C2H6O)
4 of 30
methoxymethane (C2H6O)
© Boardworks Ltd 2009
Structural formula of organic compounds
The structural formula of a compound shows how the
atoms are arranged in a molecule and, in particular, shows
which functional groups are present.
Unlike displayed formulae, structural formulae do not show
single bonds, although double/triple bonds may be shown.
CH3CHClCH3
H2C=CH2
CH3C≡N
2-chloropropane
ethene
ethanenitrile
5 of 30
© Boardworks Ltd 2009
Displayed and structural formula activity
6 of 30
© Boardworks Ltd 2009
Skeletal formula of organic compounds
The skeletal formula of a
compound shows the
bonds between carbon
atoms, but not the atoms
themselves. Hydrogen
atoms are also omitted, but
other atoms are shown.
7 of 30
© Boardworks Ltd 2009
Types of formulae
8 of 30
© Boardworks Ltd 2009
Functional groups and homologous series
A functional group is an atom or group of atoms
responsible for the typical chemical reactions of a molecule.
A homologous series is a group of molecules with the same
functional group but a different number of –CH2 groups.
methanoic acid
(HCOOH)
ethanoic acid
(CH3COOH)
propanoic acid
(CH3CH2COOH)
Functional groups determine the pattern of reactivity of a
homologous series, whereas the carbon chain length
determines physical properties such as melting/boiling points.
9 of 30
© Boardworks Ltd 2009
Functional groups
10 of 30
© Boardworks Ltd 2009
Homologous series and general formulae
The general formula of a homologous series can be used
to calculate the molecular formula of any member of the
series by substituting n for the number of carbon atoms.
For example, the general formula of a halogenoalkane is
CnH2n+1X, where X is a halogen.
Example: what is the molecular formula of chloroethane?
1. Write down the general formula:
CnH2n+1X
2. Write down the value of n:
n=2
3. Substitute n into the general formula:
C2H5Cl
11 of 30
© Boardworks Ltd 2009
Homologous series
12 of 30
© Boardworks Ltd 2009
13 of 30
© Boardworks Ltd 2009
What is isomerism?
Isomers are molecules with the same molecular formula
(i.e. the same number and type of atoms) but in which the
atoms are arranged in a different way.
There are two main categories of isomerism: structural
isomerism and stereoisomerism.

Structural isomers have different structural formulae.
Three types of structural isomerism are chain isomerism,
positional isomerism and functional group isomerism.

Stereoisomers have the same structural formula, but
the 3D arrangement of atoms is different. Two types
are cis–trans isomerism and optical isomerism.
14 of 30
© Boardworks Ltd 2009
Chain isomerism in alkanes
In chain isomers, the carbon chain is arranged differently.
For example, hexane has several chain isomers, all with the
molecular formula C6H14:
hexane
2,3-dimethylbutane
3-methylpentane
15 of 30
© Boardworks Ltd 2009
Positional isomerism
In positional isomers, the functional group is attached to
a different carbon atom.
For example,
chloropentane has
several positional
isomers, all with
the molecular
formula C5H11Cl:
1-chloropentane
2-chloropentane
3-chloropentane
16 of 30
© Boardworks Ltd 2009
Positional isomerism in alkenes
Positional isomerism also exists in alkenes with four or more
carbon atoms.
hex-1-ene
For example,
hexene has
several positional
isomers, all with
the molecular
formula C6H12:
hex-2-ene
hex-3-ene
17 of 30
© Boardworks Ltd 2009
Functional group isomerism
Functional group isomers contain different functional groups
and so are members of different homologous series.
For example, both alcohols and ethers have the general
formula CnH2n+2O so they may be functional group isomers:
propanol (C3H8O)
an alcohol
18 of 30
methoxyethane (C3H8O)
an ether
© Boardworks Ltd 2009
Structural isomers activity
19 of 30
© Boardworks Ltd 2009
20 of 30
© Boardworks Ltd 2009
Rotation around the C=C bond in alkenes
Molecules can rotate freely around single C-C covalent
bonds, but not around C=C double bonds.
This leads to type of stereoisomerism called cis–trans
isomerism, in which isomers differ in the arrangement of
the groups attached to the carbons in the double bonds.
is not the
same as
These isomers cannot be superimposed on each other
because the arrangement of the methyl groups is different.
21 of 30
© Boardworks Ltd 2009
Cis–trans isomerism
If an alkyl group or atom other than hydrogen is attached
to each carbon then the isomers can be named either
cis (‘on the same side’) or trans (‘on the opposite side’).
cis-but-2-ene
cis-1,2-dichloroethene
22 of 30
trans-but-2-ene
trans-1,2-dichloroethene
© Boardworks Ltd 2009
Limitations of cis–trans isomerism
In more complex organic compounds, in which multiple
hydrogens have been substituted by different groups,
isomers cannot be defined using the cis–trans notation.
For example, is it possible to identify which of these
halogenoalkanes is the cis isomer and which is the
trans isomer?
Instead, a different system is used for these type of
molecules: E–Z notation.
23 of 30
© Boardworks Ltd 2009
E–Z isomerism
The E–Z notation is used to identify stereoisomers that
cannot be called cis or trans.
Isomers are identified as either E or Z depending on what
‘priority’ is given to the groups attached to the carbon
atoms in the double bond. The priority of these groups is
determined by a complex series of rules.

E represents the German word ‘entgegen’, and
corresponds to trans isomers. The highest priority
groups are on the opposite side of the double bond.

Z represents the German word ‘zusammen’, and
corresponds to cis isomers. The highest priority
groups are on the same side of the double bond.
24 of 30
© Boardworks Ltd 2009
Optical isomerism
Another form of stereoisomerism is optical isomerism, in
which a molecule can exist as two isomers that are nonsuperimposable, mirror images of each other, just like a left
hand and right hand.
optical isomers of the
amino acid alanine
Optical isomers have the same physical properties, but they
rotate polarized light in opposite directions.
25 of 30
© Boardworks Ltd 2009
Stereoisomerism: true or false?
26 of 30
© Boardworks Ltd 2009
27 of 30
© Boardworks Ltd 2009
Glossary
28 of 30
© Boardworks Ltd 2009
What’s the keyword?
29 of 30
© Boardworks Ltd 2009
Multiple-choice quiz
30 of 30
© Boardworks Ltd 2009