Week 5 - ORGANIC CHEMISTRY (2) - Updated

ORGANIC CHEMISTRY
Organic chemistry is often described as the chemistry of carbon-based
compounds that consist primarily of carbon and hydrogen.
The unique chemistry of carbon
• Carbon atoms have the ability to form four strong covalent bonds
• Carbon undergoes a process known as hybridization which produces four available bonding
sites ( see “process of hybridization”)
• Carbon atoms bond with other carbon atoms to form chains or ring structures. This is called
catenation These chains can be thousands of atoms long.
• Carbon has the ability to make single, double and triple bonds with itself
Catenation is described as the ability of carbon atoms to bond with themselves to form chain or
ring structures

The process of hybridisation
A carbon atom in the ground state:
C
A carbon atom in the “excited” state:
4 x sp3 hyhrid sub-orbitals
able to accept one electron
each
A process called orbital mixing now occurs where the 2s and 2p orbital’s
now mix together to produce four sub-orbitals of equal energy. There suborbitals
are known as sp 3 hybrid orbital’s and it is these hybrid orbital’s
that provide the four available bonding sites

Classification of organic compounds

THE HYDROCARBONS……are organic compounds containing carbon and
hydrogen only
H
H H
Alkanes
H
C
H
H
Saturated compound – compounds in which all bonds between the carbon atoms
are single bonds.
Alkenes
H
H
C
H
C
H
Unsaturated compound – compounds in which there is at least one double and/or
triple bond between carbon atoms.
H
C
H
C
H
H

Homologous Series and Functional groups
Alkanes C n H 2n+2
H
H
H
H
H
H
H
H
H
H
C
C
H
H
C
C
C H H C
C
C
C
H
H
H
H
H
H
H
H
H
H
Alkenes
C 2 H 6 C 3 H 8
C 4 H 10
C n H 2n
H H
H C C
H H H
H H
H
H C C C H
H C C
C 2 H C 4 3 H 6 H
C 4 H 8
H
C
H
H
C
H
H

Homologous Series and Functional Groups
Functional group - a bond, atom or group of atoms that form the centre of chemical
activity in the organic compound. ( also identifies to which Homologous Series an organic
compound belongs
carbon – carbon
single bond
or…..halo group

Representing organic compounds
C 4 H 10 C 4 H 8

Organic nomenclature
Straight chained hydrocarbon molecules
The functional group determines the suffix of the
name:
Alkanes (single bonds) end in
Alkenes (double bond) end in
-ane.
-ene.
ethane
ethene
Numerous organic molecules are found to have carbon based side chains
attached to a main chain within the structure. These side chains are simply
known as side branches or more scientifically correct – Alkyl Substituents
Alkyl substituent – a carbon based “side chain” which
is attached to the longest continuous carbon chain in an
organic molecule.
Branched hydrocarbon molecules

Naming branched chained alkanes

Naming unsaturated hydrocarbons (alkenes)

Exercise 1 : Name the following hydrocarbons
1. 2.
3.
4.
5. 6.

Memorandum
1. 2-methylbutane
2. 2,3-dimethylbutane
3. 3-ethyl-3-methylpentane
4. 3-methylbut-1-ene
5. 3,3-dimethylbut-1-ene
6. 6-methylhept-3-ene

Hydrocarbons with more than one double bond in the chain (dienes)
The rules work exactly the same in all nomenclature, the only difference is that
now two positions must be stated in the naming. These positions must still be at
the lowest substituted position in the molecule
For euphonic purposes, the vowel "a" must be added to the name before
the suffix for dienes, e.g. buta−1,3−diene is correct, and not but−1,3−diene.
2- methylpenta -1,3 - diene 2 – methylhexa – 1,4 - diene
CH 3
CH 2 CH 2
2-methylpent-1-ene

Haloalkanes (Alkyl halides) General formula : C n H 2n+1 X
Functional group = X where X = F, Cl, Br, I ( halo functional group)
The position of the halogen is specified by the lowest substituted carbon to which that halogen is
attached.
Naming : The haloalkane is named with the lowest substituted carbon having the halogen
attached placed first in the naming sequence
3

Alcohols: General formula : C n H 2n+1 OH or C n H 2n+2 O
Ethanol is used widely as a solvent in paints, glues, perfumes,
aftershaves and any other household products.
The strong hydrogen bonds in alcohols result in alcohols having
higher melting and boiling points than hydrocarbons of similar
size.
Functional group:
OH
Pentane -2,3 - diol

Carboxylic Acid : General formula C n H 2n O 2

Esters : C n H 2n O 2

Ester formation

Isomerism
Isomers – organic molecules which have the same molecular formula but different structural
formulae.
C 6 H 14
2,2 – dimethylbutane 2 – methylpentane
There are various other types of structural isomers that can be found in organic chemistry
• Chain isomers – these are isomers that will have different chain lengths. The examples seen above
are those of chain isomers
• Positional isomers – these are isomers that have a different position of the same functional group
CH 2 = CH – CH 2 –CH 2 – CH 3 CH 3 – CH = CH – CH 2 – CH
C 3
5 H 10
pent – 1 – ene
pent – 2 – ene
• Functional isomers – isomers that contain different functional groups (eg) carboxylic acids and esters
O
O
C 5 H 10 O 2
CH 3 – CH 2 – CH 2 – CH 2 – C – OH CH 3 – CH 2 – O – C – CH 2 – CH 3
pentanoic acid
ethyl propanoate

Summary of Isomerism

Exercise
1. 2. 3.
4. 5. 6.
CH 2 – CH – CH 2
O-H
O-H
O-H
7.

Memorandum
1. 1,2-dichloropropane
2. 1,1-dichloro-2,2-difluoroethane
3. Butan-2-ol
4. 2-methylbutan-2-ol
5. Propane-1,2,3-triol
6. Butyl propanoate
7. 3-methylpropa-1,2-diene

Practice Example 2

Memorandum 2

Organic Chemical Reactions
Organic chemical reactions can be classified into FIVE different types of reactions based on
how the molecule is able to react under certain reaction conditions
• Combustion • Addition
• Substitution • Elimination
• Esterification - seen already
Combustion
Alkanes, alkenes and alcohols burn in oxygen and form carbon dioxide and water.
The reaction is exothermic and a great deal of energy is released.
Propane is the gas used in Bunsen burners in the laboratory
C 3 H 8 + 5O 2 3CO 2 + 4H 2 O
Butane burns in oxygen
C 4 H 10 + 6½O 2 4CO 2 + 5H 2 O
(x 2) 2C 4 H 10 + 13O 2 8CO 2 + 10H 2 O
Ethanol burns in oxygen
C 2 H 6 O + 3O 2 2CO 2 + 3H 2 O

Addition - when a double bond is broken and new molecular fragments are added to both
ends of the bond
In the double bond, one of the bonds is very weak
and will break under reaction conditions to form two
unpaired electrons on each carbon. These radicals are
highly reactive and will this bond to form an electron
pair
Hydrogenation
hydrohalide (HCl)
Halogenation
water (H 2 O)
Hydrohalogenation
Hydration

Substitution - “swapping” reactions
• Halogenation (Free radical substitution) – alkanes to haloalkanes
• Hydrolysis – haloalkanes to alcohols

Elimination This is the opposite of addition where functional groups are removed to form an
alkene
• Dehydrohalogenation
H H Cl H
H - C - C - C - H H - C - C = C - H + HCl
H H H H H H
• Dehydration
H H OH H
H - C - C - C - H H - C - C = C - H + H 2 O
H H H H H H
• Cracking - the breaking up of large hydrocarbon molecules into smaller, more useful molecules
and

Past examination Question

Memorandum