Unit 2 Mod 1 Intro to organic chemistry page 1 of 8
Carbon can form vast numbers of organic compounds.
This occurs because of carbon atoms’ ability to bond to themselves in
long chains. This ability is called catenation. Catenation occurs
because of the tetravalency of carbon (i.e. its valency is 4 and carbon
being a small atom, therefore it forms strong bonds with itself)
These chains can be straight chains:-
branched chains:-
and rings
Carbon can form 4 covalent bonds, but this is not apparent when
analysing its electronic configuration of 1s2 2s2 2p2. Only the last 2
electrons are unpaired which can be used to form covalent bonds.
However only TWO covalent bonds can be formed!!!
Unit 2 Mod 1 Intro to organic chemistry page 2 of 8
In order to form 4 covalent bonds, it must have FOUR unpaired
electrons.
One of the 2s electrons would absorb energy and be promoted to the vacant 2p
orbital to form a new EXCITED electronic configuration 1s2 2s1 2p3! The 3
electrons in the 2p subshell and the 1 electron in the 2s orbital are all now
unpaired, so now carbon can form 4 covalent bonds.
This happens spontaneously since the energy released by forming 4 bonds
compensates for the energy absorbed in promoting an electron to a higher
energy level. Also the formation of 4 bonds is more favourable than forming 2
covalent bonds.
The combination of the usage of the ONE s electron and the THREE “p”
electrons in bond formation results in a hybrid and the process is called
hybridisation.
Organic compounds are placed in groups called homologous series.
Homologous series have several characteristics
1. All compounds have the same general formula
2. Same functional group
3. Similar chemical properties
4. Graduating physical properties as the number of carbon atoms increases
5. The masses of consecutive members differ by 14 g or by a –CH2
group.
A functional group is an atom or a group of atoms responsible for the chemical
properties of a compound
Bonds must be first broken before a chemical reaction can take place. In
organic chemistry, bonds can be broken in two ways.
a) homolytic fission and b) heterolytic fission
Homolytic fission :- breakage of a covalent bond with atoms of similar or same
electronegativity resulting in the formation of free radicals. Each atom retain
one electron from the covalent bond.
NB Free radicals are highly reactive species with an unpaired electron
Heterolytic fission :- breakage of a covalent bond with atoms of different
electronegativities with the formation of a cation and an anion. The more
electronegative atom retains both electrons from the covalent bond.
Unit 2 Mod 1 Intro to organic chemistry page 3 of 8
Terminology
 Electrophile:- an electron loving species to negatively charged
centres e.g. H+, Cl+, Br+, I-, NO2+

Nucleophile:- a nucleus loving species i.e. attracted to positively
charged centres e.g. OH-, H2O, NH3, CN-, halide ion

Hydrolysis – the splitting of a molecule using water or in some cases
the reaction of a molecule with water as opposed to simple
dissolution
Molecules can be illustrated in several ways, via empirical formula,
molecular formula, structural or displayed formula, condensed
formula or skeletal formula.
1. Empirical formula refers to the simplest whole number ratio of atoms
in a compound. e.g. CH2
2. Molecular formula refers to the actual number of atoms in a
compound e.g. C2H4
3. Structural / displayed formula refers to the diagrammatic
representation of an organic molecule showing the arrangement of atoms
e.g.
4. Condensed formula refers to the groups of atoms arranged around
each carbon atom in the compound. It does not show the arrangement of
the atoms. The condensed formula can be determined from the structural
formula e.g. butane is C4H10 and the structural formula
Therefore the condensed formula is CH3CH2CH2CH3
Unit 2 Mod 1 Intro to organic chemistry page 4 of 8
5. Skeletal formula refers to the bare structure of the compound where
carbon and single hydrogen atoms are not shown. It is shown by a series
of straight lines and bends. An open end represents a –CH3 group which
a “bend” represents a –CH2 group. If a double bond is present, the
number of hydrogen atoms would decrease appropriately. Other
functional groups are shown explicitly.
butane is shown as
and butan-2-ol is represented by
Isomerism
There are two main types:- structural isomerism and stereoisomerism
Structural isomerism occurs when 2 or more compounds have the same
molecular formulae but their atoms are arranged differently
There are 3 categories of structural isomerism
a) chain b) functional c) positional
CHAIN ISOMERISM
Chain isomers have same molecular formula but differ in the length of
the longest carbon atom chain
Example:
2-methylpropane
POSITION ISOMERIS
Positional isomerism is caused by the difference in the position of
functional group in the same chain is termed as position isomerism.
Example:
1,2-dichlorobenzene
1,3- dichlorobenzene
Unit 2 Mod 1 Intro to organic chemistry page 5 of 8
ONAL ISOMERISM
Functional isomers have same molecular formula but different functional
group.
Example:
Ethyl Alcohol
Dimethyl Ether
Stereoisomerism occurs when 2 or more compounds have the same
molecular and structural formulae but the atoms are arranged differently
in space
Stereoisomerism can be either a) geometric / cis-trans isomerism or
b) optical isomerism
Geometric isomerism occurs in unsaturated compounds
With the presence of the multiple bond, rotation about the C=C bond is
difficult without breaking the bonds between them
Example
Both (a) and (b) have the molecular formula CHClCHCl but the atoms
are arranged differently in space
In (a) similar atoms either the Cl or the H are on OPPOSITE sides of
THE DOUBLE BOND. This configuration is called “cis”. While in (b)
similar atoms are on the SAME side of THE DOUBLE BOND, this
configuration is called “trans”
So compound (a) is trans-1,2-dichloroethene and
compound (b) is cis-1,2-dichloroethene
Unit 2 Mod 1 Intro to organic chemistry page 6 of 8
Optical isomerism occurs when there is no symmetry about a carbon
atom in a compound
For example Imagine a carbon atom with four different atoms or groups
of atoms attached to it
Please note the diagram shows a 3
dimensional arrangement. The
“wedge” shows the bond coming
out of the plane of the paper
towards the observer and the
dotted line shows the bond
coming out of the plane of the
paper away from the observer.
There is no line of symmetry one can draw for this molecule
Therefore the molecule is asymmetric. Since there is no line of
symmetry about the central carbon atom, that carbon atom is said to be
CHIRAL.
Mirror images of compounds with chiral carbon atoms are said to be
non-superimposable. Think of trying to fit your left hand directly on top
of your right hand. You would realise that they don’t fit neatly on each
other.
Mirror images of compounds with chiral carbon atoms are called
enantiomers. The dotted line represents the mirror line.
Note When showing chiral carbon atoms they MUST be indicated
with an asterix next to it. (*)
Unit 2 Mod 1 Intro to organic chemistry page 7 of 8
Enantiomers are identical in every way in terms of chemical and physical
properties EXCEPT that one rotates plane polarised light to the left and
the other rotates plane polarised light to the right by the same degree.
If there are equal concentrations of both enantiomers in a mixture, there
would be no observed optical activity and the mixture would called a
racemic mixture or a racematate.
One final concept to understand is that some atoms or groups of atoms
are electron withdrawing (pull electrons towards themselves) and others
are electron donating (push electrons away from themselves). Atoms or
groups of atoms with electronegative atoms are electron withdrawing and
thus have a negative inductive effect (-I), while the others like alkyl
groups are electron donating and have a positive inductive effect (+I).
Mesomeric effect is the same as the inductive effect but it occurs in
multiple bonds, while inductive effect occurs in single bonds.
Worksheet on intro to organic chemistry
Write your answers on the spaces provided or circle the most suitable
letter
1. What is tetravalency?
………………………………………………………………………
2. How many structural isomers have the molecular formula C4H10?
(a) 1 (b) 2 (c) 3 (4)
3.
a) Draw the fully displayed formula corresponding to CH2ClCHClCH3
b) On the fully displayed formula you drew above, indicate the chiral
carbon atom with an asterix.
c) Draw the two enantiomers resulting from the chiral carbon atom you
indicated above in part b)
Unit 2 Mod 1 Intro to organic chemistry page 8 of 8
4. Using this condensed molecular formulae CH2ClCH2CH3
Which type of structural isomerism can it exhibit?
………………………….…………………
5 i). Which compounds would exhibit optical isomerism?
a) CH4
b) C2H6
c) CH3CHClBr
(d) CH3CH2Cl
ii) Draw the optical isomers for the compound you have chosen in 5 i)
6. But-2-ene is said to show “stereoisomerism”. Draw the cis and trans
isomers of but-2-ene
7.