Types of Reaction in Organic
Chemistry
8 main types of Organic
Reaction to learn
•
•
•
•
•
•
•
•
Substitution Reactions
Esterification Reactions
Addition Reactions
Polymerisation Reactions
Elimination Reactions
Redox Reactions
Reactions as Acids
Organic Synthesis
Substitution Reactions
• A substitution reaction is a chemical
reaction where an atom or group of atoms
in a molecule is replaced by an atom or
another group of atoms
• A very common example of substitution
reactions is the reaction between alkanes
and halogens (chlorine/Bromine) in the
presence of UV light
Mechanism for reaction
• The mechanism of a reaction tells you the
detailed step by step processes that occur
in a reaction.
• For example in the following reaction
CH4 + Cl2
CH3Cl + HCl
What exactly happened between the
molecules when they were reacting?
• We know roughly what has happened and
we can come up with a simple mechanism
Methane
Chloromethane
H
+
Cl
Cl
Chlorine
=
Cl
+
H
Cl
Hydrogen
Chloride
Simple mechanism
Chloromethane
Methane
H
Hydrogen and
Chlorine have
swapped places
Substitution
Hydrogen
Chlorine
Chloride
Cl
Cl
We need to find out the
following
• How does this happen?
• What causes it?
• How do we know that what we
suggest is correct?
Stage 1
Initiation
Getting Started
Both species are the same
Ultra violet light breaks the bond
Called Homolytic
Fission
Chlorine molecule
Cl2
2 Chlorine radicals each
with an unpaired electron
i.e. 7 electrons in outside
shell
Stage 2
Propagation
Keeping it going
Methane
H
Methyl
radical
Chlorine radical
Cl
The chlorine radical pulls
the hydrogen and one
electron across to it.
Hydrogen
chloride
Let’s put in the 2 electrons in this bond
The methyl radical is now free to
react with a chlorine molecule
Chlorine
radical
Cl
Cl
Methyl
Chlorine
Chloromethane
radical
Chlorine radical can now go and
react with a methane molecule
Methane
H
Methyl
radical
Chlorine radical
Cl
The chlorine radical pulls
the hydrogen and one
electron across to it.
Hydrogen
chloride
Let’s put in the 2 electrons in this bond
The methyl radical is now free to
react with a chlorine molecule
Chlorine
radical
Cl
Cl
Methyl
Chlorine
Chloromethane
radical
Chlorine radical can now go and
react with a methane molecule
And on and on and on and on
Stage 3
Termination
Grinding to a halt
• Three different ways this can happen
Cl
Cl
Chlorine molecule
Chlorine
Chlorine
radical
radical
Reaction stops
No free radicals to keep it going
Cl
Chlorine
Methyl
Chloromethane
forms
radical
radical
Reaction stops
Because there are no free radicals to
keep it going
Ethane
Methyl stops
Methyl
Reaction
because no free radicals
radical
radical
to keep it going
The formation of ethane proves that this
is the mechanism
Reaction speeded up by sources of free
radicals such as tetraethyl lead.
Remember the evidence that
proves this mechanism
1. Small amounts of ethane are found in the
products of the reaction
2. The amount of chloromethane produced
is proportional to the amount of UV light
absorbed
3. If tetra methyl lead is added to the
mixture there is a marked increase in the
rate of reaction, we know that tetra methyl
lead decomposes to form methyl radicals
which feed into the system to speed it up
4. If ethane is used instead of methane then
butane will be formed rather than ethane
(as 2 ethyl radicals combine)
• The chlorination of methane is commonly
called a Free Radical Substitution
Reaction
Esterification
• Esters are formed when an alcohol and
carboxylic acid react together
• This is called a condensation reaction as
water is formed
• It could also be called a substitution
reaction since the H of the OH group of
the carboxylic acid molecule is replaced by
an alkyl group
• Formation of esters is Esterification
• It is important to remember the reaction
can go backwards also ie. The ester
formed can react with the water to form
the carboxylic acid and alcohol
• This reverse reaction is called Hydrolysis
• Hydrolysis will happen very easily in the
presence of a base such as NaOH
• This base hydrolysis of esters results in
the formation of the sodium salt of the
carboxylic acid
• NB see fig 23.9 p 364
• This reaction is very important in the
manufacture of soap and is commonly
referred to as Saponification
Substitution Reactions
Tetrahedral
Planar
Tetrahedral
Planer
S ubs titutio n R e actio ns
a) H alogen ation of an Alkan e
Cl 2
A lk an e
H aloalk an e
U .V . lig h t
b) E sterification
A cid + A lc o ho l
E ste r + Wa ter
c) S ap on ification
L ard/O liv e O il + So diu m H y dro xid e
Soa p + G ly cerol
Learning Check
• What is a substitution reaction?
• Can you describe in 4 steps and using
diagrams the mechanism of a substitution
reaction?
• What is this mechanism called?
• What is a radical?
• What causes the formation of radicals?
Learning Check
• Can you describe how 3 pieces of
evidence support the theory of free radical
substitution?
• Can you name 3 types of substitution
reaction?
• What 2 molecules react in an
esterificateion raection
Learning Check
• What type of reaction is an esterification
raection
• What is the reverse of an esterification
raection called?
• What is the product of this type of
raectiopn?
Addition Reactions
• An addition reaction is one in which two
substances react together to form a single
substance
• Addition reactions are very important in
industry as a whole range of new products
can be formed
Consider the following reactions
• You may notice that in these addition
reactions double bonds were broken and
replaced with single bonds
• An important feature of addition reactions
is that saturated compounds are formed
from unsaturated compounds
Margarine
• Hydrogenation of C=C double bonds is used in the
manufacture of margarine
• It is believed that unsaturated fats are better for health.
• Butter contains many saturated fats whereas sunflower
oils etc don’t
• However oils are not good for spreading on bread
• By controlling the degree of saturation in margarines we
can manipulate their melting points to make them
spreadable and control the amount of saturation to try to
make them healthier
Mechanism for Addition
Reactions
• The reaction between ethene and bromine
can occur in the dark indicating it is
different to free radical substitution
• There are 4 overall steps in this
mechanism
Step 1
• The C=C double bond is a region of high
negative charge
• The Br2 molecule approaches the double
bond and becomes polarised
Step 2
• The polarisation becomes so strong the
Br2 molecule splits in Br+ and Br –
• As two different species are firmed this is
called Heterolytic fission
Step 3
• The Br+ species in order to gain the two
electrons it needs to give it a stable octet
of electrons attacks the C2H4 molecule
• It forms a covalent bond with one of the
carbon atoms
• The other carbon atom is left with a
positive charge and is now called a
carbonium ion
• There is evidence to suggest the
carbonium ion has a cyclic structure
known as a cyclic bromonium ion
• Because bromine has a relatively large
size it allows this to form
Step 4
• The carbonium ion is attacked by the Brion
• This forms 1,2-dibromoethane
Ionic Addition
• This mechanism is often called ionic
addition since ions add across the C=C
double bond
Evidence for this mechanism
• When ethene reacts with bromine water in
in the presence of sodium chloride solution
2 other compounds (1-bromo-2-chloro
ethane) and ( 2- bromo ethanol) are
formed as well as 1,2 di-bromoethane
• 1-bromo-2-chloro ethane is formed when
the carbonium ion is attacked by the Clion
• This is evidence for the existence of the
carbonium ion
• 2- bromo ethanol is formed when the
carbonium ion is attacked by the water
molecule
• Again this is evidence for the presence of
the carbonium ion
Addition of Cl2 and HCl to
Ethene
• The same mechanism applies
• In the case of Cl2 the Cl+ species attacks
the C=C double bond then the Cl- attacks
to form 1,2 di-chloroethane
• In the case of HCl the H+ ion attacks the
C=C double bond then the Cl- attacks to
form chloroethane
• NB, a cyclic intermediate is not formed in
these cases as the Cl and H atoms are too
small
Addition Reactions
Planar
Tetrahedral
Tetrahedral
Carbons
C3H8
A ddition R eactions
Alkanes
H 2 /N i
B r2
H aloalkanes
H 2O
A lken es
H 2 /N i
P olym ers
Alk ynes
Alcohols
Learning check
• What is an addition reaction?
• Describe the mechanism of an addition
reaction for Br2 and Ethene
• What is the evidence for this mechanism?
• What is formed in the reaction between
Br2 and Ethene but not in the reaction
between Cl2 and HCl and Ethene?
• Why is this not formed?
• What is a catalyst for addition reactions?
• Name some addition reactions
Polymerisation Reactions
• It is possible for molecules containing
double bonds to undergo addition
reactions among themselves
• Molecules of ethene for example may link
with each other to form a long chain of
carbon atoms
• The compound Polythene which is formed
is an example of a polymer
• Polymers are long chains of molecules
made by joining together many small
molecules
• Polymers have a repeating structure and
can have a molecular mass of many
thousand
• The small unit which make up polymers
are called monomers
• Ethene is a monomer used to make
polythene
• Polythene is a very useful material and is
used to make plastic bags, bowls etc.
• Polypropene is another important
polymer used in toys, beakers, chairs etc.
• These polymers are called Addition
Polymers because they are made from
addition reactions
• Crude oil is the raw material for these
polymers!
Learning Check
•
•
•
•
What is the starting material for polymers?
What type of reaction is polymerisation?
How do polymers form?
What are the repeating units in a polymer
called?
• Name some common polymers
• Give a use for these polymers
Elimination Reactions
• When we made ethene in the lab we used
ethanol as our starting material and
removed a water molecule from it
• An elimination reaction is one in which
a small molecule is removed from a
larger molecule to leave a double bond
in the larger molecule
• The removal of water from an alcohol is
the only type of elimination reaction you
need to know
• Since water is removed it is also known as
a Dehydration Reaction
Elimination Reaction
Tetrahedral
Planar
E lim inatio n R e actio n
D ehy dratio n o f a n A lc oh ol
A l2O 3
Alcohols
Alkenes
- H2 O
Learning Check
• What is an elimination reaction?
• Describe an elimination reaction you
performed in the school laboratory
• What is another name for this type of
reaction?
• What catalyst was used?
Redox Reactions
• When a primary alcohol reacts with an
oxidising agent it is converted to an
aldehyde
• Suitable oxidising agents are Acidified
Sodium Dichromate or Acidified
Potassium Permanganate
• If the oxidising agent is in excess the
aldehyde is further oxidised to a carboxylic
acid
• If a secondary alcohol reacts with an
oxidising agent a ketone will be formed
• A bottle of wine left
open in air will soon
become oxidised and
the ethanol will
change into a dilute
solution of ethanoic
acid (vinegar)
• Your liver oxidises
ethanol to ethanal in
your body
• It can oxidise about
8g of ethanol an hour
• Excessive quantities
of ethanol can cause
cirrhosis of the liver
• The ethanal is
eventually oxidised to
carbon dioxide +
water
• Sodium Dichromate
was used in older
breathalyser tests as
it changes from
orange to green when
it oxidises alcohol
• It is important to note that in the
preaparation of aldehydes the acidified
sodium dichromate is added slowly and
the aldehyde is distilled off immediately to
prevent further oxidation to carboxylic
acids
• In the production of carboxylic acids the
reaction is refluxed as we want to achieve
complete oxidation of the alcohol
• Carboxylic acids can be reduced to
aldehydes and then to alcohols in the
presence of a nickel catalyst using
hydrogen to add to the molecules
R edo x R e actio ns
A lcoh ol
O xid ation
R ed u ction
H 2 /N i
N a 2 C r 2O 7
and H+
A ld eh y d e
O xid ation
R ed u ction
N a 2 C r 2O 7
and H+
H 2 /N i
A cid
Learning Check
• What is does redox stand for?
• What will a primary alcohol become
oxidised to?
• What will a secondary alcohol become
oxidised to?
• What will an aldehyde become oxidised
to?
• Why does wine turn acidic if left open?
• Name an oxidising agent used in the
laboratory
Reactions as Acids
• If a piece of sodium is added to ethanol
hydrogen gas is released
• A hydrogen atom in the ethanol is
replaced with a sodium atom
• The ethanol is thus behaving as an acid as
it is losing a H+ ion
• Alcohols are much weaker acids than
water
Acid Nature of The Carboxylic
Acid Group
• What makes it possible for the hydrogen
atom of the carboxyl group to break off as
a proton?
• 2 reasons .................
1. Inductive Effect
• In the carboxyl functional group the H
atom is attached to an oxygen atom in a
C-H- O chain
• The carbon atom is slightly positive and
tends to attract electrons from the Oxygen
atom in the OH group
• This electron pulling effect is called the
inductive effect
• This drawing away of electrons facilitates
the ionisation of the H atom in the O-H
bond
• This means the COOH group can lose a
proton quite easily and hence show acidic
properties
2. Stability of Carboxylate Ion
• When the carboxyl group loses a proton it
forms a negative ion called the carboxylate
ion
• Chemists have found that the 2
Carbon/Oxygen bonds are the same
length suggesting the actual structure is
an intermediate of the image below
• The negative charge is not localised on
one oxygen but is spread over 3 atoms
giving it extra stability
• This stability means it will lose the proton
easily
R eaction as A cid s
a ) A lcoho l and rea ctive M etal
Alcohol + S odium
S odium S alt + H ydrogen
b) R eactio n o f A cids
S odium S alt + H ydrogen
Na
N aO H
A cid
S odium S alt + Water
N a 2 CO 3
S odium S alt + C a rbon D ioxide + Water
Learning Check
• What is an acid?
• There are 2 reasons carboxylic acids readily donate a
proton what are they?
• When an alcohol reacts with a sodium ion what is
formed?
• When a carboxylic acid reacts with a sodium ion what is
formed?
• When a carboxylic acid reacts with sodium hydroxide
what is formed?
• When a carboxylic acid reacts with sodium carbonate
what is formed?
Organic Synthesis
• Organic Synthesis is the process of
making organic compounds from simpler
starting materials
• Chemists specialising in organic synthesis
have to plan steps and design
experiments for making new and useful
substances
• The new molecule being made is called
the Target Molecule
• Many medicines and other useful
substances are made in organic synthesis
Learning Check
• What is meant by organic synthesis?
• When chemists are trying to make
something ne what is the molecule they
are trying to make called?
• In the pharmaceutical industry new
organic compounds are continuously
being designed
• Organic synthesis involves bond breaking
and bond making
• The following is a summary of some of the
steps to make new substances out of
others and back again!