Organic Chemistry II - Reactions I
ChemFactsheet
April 2001 Number 16
1
Exam Hint - Unfamiliar compounds are used in questions but the
secret is to leam how the family of compounds reacts. You identify the
family the unfamiliar compound belongs to, remember how the family
reacts and then apply this to the compound. Again, being able to name
organic compounds is essential.
Reaction Mechanisms
When compounds or elements react then bonds are broken and bonds are
formed in the chemical reaction.
The reaction mechanism is the method used to show the bond making and
breaking processes by explaining what happens to the electrons involved
in bonding.
N.B. A2 Units require a much more detailed use of reaction mechanisms.
To succeed with this topic you need to
••
••
•Be able to name and draw organic molecules (Factsheet 15)
••
••
•have revisited Factsheet 5 (‘Bonding’) and Factsheet 06
(‘Structure of elements and compounds’) so you understand the
bonding and shapes of organic molecules.
After working through this Factsheet you will
••
••
•Know the reactions of the families alkanes, alkenes and
halogenoalkanes.
••
••
•Know the conditions and reagents for the reactions
••
••
•Understand how the bonding in the compounds influences their
reactions
••
••
•Know some of the terms used in reaction mechanisms.
Term Definition Example
in the example above
Definition of Terms
Reactions in organic chemistry
Substitution – When an atom or ‘group’ in a molecule is replaced by
another atom or ‘group’
Addition – When two molecules react to form a single product.
Elimination – When a simple molecule e.g. HCl, HBr, H2O, is removed
from a molecule and not replaced.
Hydrolysis - When water reacts with a molecule and the molecule is
split into two parts.
Alkanes
First let us revise what we covered in Factsheet number 15 about alkanes.
••
••
•Alkanes are hydrocarbons (made of carbon and hydrogen only).
••
••
•In an homologous series (differ from one another by only –CH2).
••
••
•Alkanes have a general formula of CnH2n+2.
••
••
•Alkanes are saturated compounds (Contain only single carbon bonds
i.e. C–C).
Reactions
Alkanes are relatively unreactive because of the strength of the C – C and
C – H bonds they contain. The two reactions all alkanes undergo are :
1. Burning in excess oxygen to form carbon dioxide and water
C2H6 + 3½O2 → 2CO2 + 3H2O
2C2H6 + 7O2 → 4CO2 + 6H2O)
Reaction type : combustion
2. React with Cl2 or Br2 in the presence of UV light/sunlight to form a
complex mix of halogenoalkanes and hydrocarbons.
Cl2 + C2H6
UV Light C2H5Cl, C2H4Cl2, C2H3Cl3, etc + HCl
Reaction type: substitution
Mechanism: free radical
Alkenes
Unsaturated hydrocarbons (i.e. C=C) with a general formula of CnH2n.
The C=C bond is
The double bond contains a π-bond and a σ-bond. The σ– bond is strong
(the bond pair is in the plane between the two carbon nuclei) But the π-
bond is weaker (the bond pair of electrons lies outside the plane of the
nuclei). The π-bond will break so other atoms / groups add on to the C–C link.
CX
.
X
.C
Free
Radical A species with
a single unpaired
electron
Cl2 → Cl. + Cl.
the . on the Cl
represents the unpaired
electron
Heterolytic
Fisson When a bond breaks
and both electrons go
to one atom (forming
ions)
AB
x
.
A
+
B
x
.
_+
Nucleophile
(literally 'liking
the nucleus/
positive charge')
A donator of a lone pair
of electrons (which so
forms a new covalent
bond)
HO:
_
HO C
C+
Electrophile
(literally 'liking
electrons/
negative charge')
An acceptor of
a lone pair of
electrons (so
forming a new
covalent bond)
The C+
Homolytic
Fission
When a bond breaks
and one electron goes
to each atom (forming
free radicals)
AB
x
.
Ax+.B
e.g.
Or
