Chpt. 23: Types
of Reactions In
Organic
Chemistry
So far in organic chemistry we have
studied fuels and the various
families of organic compounds. In
this final organic chapter we will
study some of the reactions that
these organic families undergo!!!!!
This chapter involves a study of the :
1) various types of chemical reactions
that organic compounds undergo
2) synthesis of organic compounds
3) methods used to analyse for the
presence of various organic
substances
Part1: Types of chemical
reactions that organic
compounds undergo
Organic reactions can be classified as follows:
- Substitution Reactions
- Addition Reactions
- Polymerisation Reactions
- Elimination Reactions
- Redox Reactions
- Reactions as Acids
Substitution Reactions
Apart from combustion alkanes are rather unreactive.
However, in addition to combustion reactions ALKANES
also undergo SUBSTITUTION reactions
Definition:
A substitution reaction is a chemical reaction in which
an atom or group of atoms in a molecule is replaced by
another atom or group of atoms.
Types of Substitution Reactions:
- Halogenation of Alkanes (Free Radical Substitution)
- Esterification (previously discussed)
- Saponification (previously discussed)
Halogenation of Alkanes (Halogen
replaces H):
Ordinary Level
This involves the reaction of the alkanes with
halogens in the presence of light e.g.
1) Methane reacts with chlorine in the presence of
ultraviolet light to yield (mono)chloromethane
and HCl:
CH4 + Cl2 → CH3Cl + HCl
*Space for diagram
2) Ethane reacts with chlorine in the presence of
ultraviolet light to form (mono)chloroethane:
C2H6 + Cl2 → C2H5Cl + HCl
*Space for diagram
Higher Level
Must understand the mechanism of this reaction i.e. the
detailed step by step description of how the overall
reaction occurs.
As discussed preciously ( chemical bonding,
thermochemistry) chemical reactions involve the
breaking and forming of bonds. When such bonds are
broken and formed it involves the rearrangement of
electrons and this is what needs to be considered when
studying reaction mechanisms.
Reaction Mechanism Halogenation
STAGE 1: INITATION (Getting it started)
- UV light breaks down the chlorine molecule into
two chlorine atoms – HOMOLYTIC FISSION
Cl
uv
Cl → Cl
Covalent Bond
+ Cl
Unpaired electron
- this is called a photochemical reaction i.e. a rxn that
is brought about by light. This is proven to be a light
dependent reaction as it does not occur in the
dark at room temperature
STAGE 2: PROPAGATION 1 (Keep it going)
- a free chlorine atom attacks a methane molecule to
form HCl and a methyl free radical
(Free radical is any atom or group of atoms with an
unpaired electron)
Cl
+ CH4
→ HCl + CH3
Unstable
- due to an incomplete outer shell the methyl free
radical is very reactive
STAGE 3: PROPAGATION 2 (Keep it going)
- this methyl free radical attacks a chlorine molecule
to form chloromethane and a free Cl atom:
CH3 + Cl2 → CH3Cl + Cl
- the chlorine atom produced during propagation 2 can
then react with another molecule of methane as in
propagation 1:
Cl
+ CH4
→ HCl + CH3
- thus a chain reaction is started!!!!!!
- Evidence for this chain reaction - chemists
compared the amount of light falling on this
reaction with the amount of light emitted. They
were then able to determine the number of
photons that were absorbed by the reactants and
found that thousands of molecules of
chloromethane were produced for every photon
that was absorbed.
STEP 4: TERMINATION (Grinding to a Halt)
- The chain reaction comes to an end when – the
various free radicals combine with each other to
form un-reactive molecules i.e. (not free radicals)
- Some possible combinations (Chlorine & Methane):
CH3 + Cl → CH3Cl Formation of chloromethane
Cl
+ Cl → Cl2
CH3 + CH3 → C2H6
Formation of chlorine molecule
Formation of ethane
Evidence for this Free Radical Mechanism:
• Reaction requires UV light of energy high enough to
homolyse chlorine to initiate.
For every photon absorbed very many molecules of a
product are formed.
Therefore when a mixture of an alkane and chlorine
are irradiated wIth UV light for even a short period , a
chain reaction occurs – RXN STOPS IN THE DARK
• Formation of products such as ethane,
chloroethane, butane etc. is evidence for the
termination steps.
• If radical promoters such as tetramethyl lead,
Pb(CH3)4, or tetraethyl lead are added to the reaction
mixture, there is a marked increase in the rate of the
reaction.
Pb(CH3)4 → Pb + 4CH3
Syllabus requires that you know the mechanism of free
radical substitution for both methane and ethane.
Free Radical Substitution of Ethane:
• Propagation 1: Radical formed will be ethyl
instead of methyl:
C2H6 + Cl → HCl + C2H5
• Tetraethyl lead, Pb(C2H5)4, is needed to increase
the rate of the reaction
• Termination stage – butane formed instead of ethane
and some other possible combinations (Chlorine &
Ethane):
Cl
+ Cl → Cl2
Formation of chlorine molecule
C2H5 + Cl → C2H5Cl Formation of chloroethane
C2H5 + C2H5 → C4H10 Formation of butane
*Points to Note:
- almost all organic compounds burn to form
carbon dioxide and water
- exceptions to this rule are the fully halogenated
alkanes, CBrClF2
- these type of compounds do not support
combustion and are denser than air, hence, they
are added to fabrics to reduce their tendency to
catch fire.
- known as FLAME RETARDANTS
Esterfication ( H atom of carboxylic acid
replaced by alkyl group):
Esterification (ester formation) is an example of a
substitution reaction, with the carboxyl hydrogen in an
acid replaced by the alkyl group of an alcohol
This is also know as a CONDENSATION REACTION
Definition:
A condensation reaction is one in which two small
molecules react to form a larger one with the
elimination of a smaller one like water.
Ethanoic acid + Ethanol
H
O
C
H
C
+
H
O-H
HYDROLYSIS
H+
H
H
O
C
C
H
H-O
H+
Ethyl Ethanoate +
H
H
C
C
H
H
Water
H
ESTERFICSATION
O
H
H
C
C
H
H
O
H
+
H
H
Formation of an ester is also a reversible reaction i.e. a
rxn in which the products react to form back the
product and vice versa.
In this case the reverse reaction is called HYDROLYSIS
Definition:
Hydrolysis is the chemical decomposition of a
substance by water, the water itself also being
decomposed.
Base Hydrolysis of Esters (Na/K replaces
alkyl group):
Esters can also be hydrolysed very effectively in the
presence of a base like sodium hydroxide or potassium
hydroxide.
The base hydrolysis of esters using sodium hydroxide
forms the sodium salt of the carboxylic acid (SOAP)
rather than the carboxylic acid itself and alcohol.
It is called BASE HYDROLYSIS OF AN ESTER or
SAPONIFICATION reaction
Basic Saponification of an Ester:
Ethyl
+ Sodium
Ethanoate Hydroxide
Sodium
+
Ethanoate
Ethanol
Sodium salt of
carboxylic acid
*Important Saponification of an Ester:
Glyceryl +
Tristearate
3NaOH
3Sodium + Glycerol
Stearate
(Soap)
Sodium salt of
carboxylic acid
*Syllabus requires that
students are able to draw
the structures of the
reactants and products
for soap manufacture!!!
O
C17H35
H
C
O
O
C
H
+ NaOH
O
C
H
+ NaOH
C
H
+ NaOH
C17H35 C
O
C17H35 C
O
H
Triester – Glyceryl
Tristearate (fat)
Sodium
Hydroxide
O
C17H35
H
C
C
H
H-O
C
H
H-O
C
H
H-O
O
O
Na
+
C17H35 C
O
Na
O
C17H35 C
O
Na
Sodium Stearate
(Soap)
H
Glycerol
Elimination
Reactions
Definition:
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.
• Alkenes can be formed from their corresponding
alcohols using elimination reactions.
• Since water is removed this particular type of
elimination reaction is known as a DEHYDRATION
REACTION
• The change in structure is from tetrahedral carbon
(alcohol) to planar carbon (alkene)
Elimination Reaction – Preparation of Ethene Gas!!!
Ethanol
Saturated
-H2O
→
Ethene
+
Unsaturated
Water
Elimination Reaction – Preparation of Ethene Gas!!!!
H
H
H
C
C
H
H
H
C
H
Ethanol is a primary alcohol
H
OH
The functional group of an
alcohol is O-H the hydroxyl
group.
H
C
OH
H
A water molecule is
eliminated
H
H
+
C C
H
O
H
H
H
A double bond is formed between the
carbons
Ethene an unsaturated compound is formed
The laboratory preparation of ethene involves
elimination:
X=Ethanol.
Y=Hot aluminum oxide (Catalyst)
Question:
Name two features of elimination reactions:
Answer:
- remove small molecule
- make a double bond
*Note: the removal of water from an alcohol to form an
alkene is the only type of elimination reaction on the
course!!!
ADDITION
REACTIONS
Alkenes are much more reactive than
alkanes and their characteristic
reactions are ADDITION REACTIONS
where the double bond opens up
allowing various substances to add on
and produce saturated compounds
The addition of bromine to a sample of ethene
causes bromine to add across the C=C double
bond to form 1,2-dibromoethane. This is an
example of an addition reaction
H
H
C = C + Br-Br
H
H
Br Br
H C-C H
HH
If double bonds are stronger than single bonds why do
alkenes react so readily with bromine???
• Double bond consists of a sigma and a pi bond.
• Pi bonds are weaker than sigma bonds (less
overlapping of orbitals)
• When bromine adds across the double bond in ethene
the energy required to break the pi bond is released
when two single bonds to the bromine atoms are
formed.
• Products are more stable than reactants
Definition:
An addition reaction is one in which two substances
react together to form a single substance
In general, an addition reaction involves a change in
structure from planar to tetrahedral
Addition Reactions involving Ethene
(Must be aware of the industrial importance of the addition reactions of
ethene)
a) Addition of Hydrogen (H2):
The addition of hydrogen to alkenes is known as
HYDROGENATION
• Sunflower oil, palm oil etc. are said to be
polyunsaturated (C=C) and are thought to be
less damaging to our health than the saturated
fats found in dairy products.
• Adding hydrogen to some of the C=C bonds
in these oils changes them into soft solids.
Thus hydrogenation is used in industry to
convert vegetable oils into solid saturated
materials used in margarine and dairy spreads
• By controlling the degree of hydrogenation, the
margarine can be made as hard or as soft as
needed
* Animal fats – saturated
* Vegetable Fats unsaturated
H
H
H
C
C
+ H2
H
H
Ethene
Catalyst
H
H
C
C
H
H
Heat
Ethane
H
B) Addition of Chlorine (Cl2):
• The reaction between chlorine and ethene
results in the product, 1,2-dichloroethane
• 1,2-dichloroethane, is used in industry to make
chloroethene, the raw material for the
manufacture of the plastic PVC polyvinylchloride
H
H
H
C
H
H
+ Cl2
C
H
H
C
C
Cl
Cl
H
1,2-dichloroethane
C) Addition of Bromine (Br2):
• This reaction is used to test for unsaturation
(preparation of ethene)
• Used as an additive in leaded petrol
H
H
H
C
H
H
+ Br2
C
H
H
C
C
Br
Br
H
1,2-dibromoethane
D) Addition of water (H2O):
• Addition of water is known as a HYDRATION
reaction
• Reaction used in the manufacture of ethanol (a
widely used solvent in industry)
H
H
H
C
H
H
+ HOH
C
H
H
C
C
H
OH
Ethanol
H
E) Addition of Hydrogen Chloride (HCl):
H
H
H
C
H
H
+ HCl
C
H
H
C
C
H
Cl
H
Chloroethane
• Main modern use of chloroethane is the
manufacture of ethylcellulose, a thickening agent
and binder in paints and cosmetics.
In each of the reactions (a) – (e)
the structure changes from planar
to tetrahedral
Ionic Addition – Reaction Mechanism
(Higher Level Only)
H
H
• 4 electrons in total.
C =C
H
• The double bond is made
up of 2 bond pairs.
H
• One pi bond and one sigma
bond.
• It is an electron rich region
with a slightly negative
charge.
STAGE 1: Polarisation
• A bromine molecule is a nonpolar molecule
H
H
C

Br - Br.
C
H

H
• However, as it approaches
the double bond the high
concentration of negative
charge in the C=C bond
causes the approaching
Br-Br molecule to become
polarised.
STAGE 2: Heterolytic Fission
Br
Br
Br+ + Br_
• The induced polarisation becomes so great the Br2
molecule splits into Br+ and Br- species. This is
known as heterolytic fission.
*Heterolytic Fission:
The breaking of a bond so that the bonding
electrons(two) end up on one atom is known as
heterolytic fission
STAGE 3: Carbonium Ion Formation
• The Br+ species in order to gain the electrons
needed for a full outer shell attacks the C2H4
molecule.
• The Br+ ion forms a covalent bond with one of the
carbon atoms.
• The other carbon atom has lost an electron and
so becomes positively charged – CARBONIUM ION
H
C
Br+
H
H
C
Br
C
H
H
C+
H
H
H
*N.B. Note: As there is a shortage of electrons in this
intermediate species, C2H4Br, it has an overall positive
charge. Modern evidence shows that rather than
bonding to one of the carbons, the Br+ is attached to
both in a bridged structure. This cyclic structure is known
as cyclic bromonium ion

H
C
H
H
C
H
The relatively large size of the bromine atom
allows the formation of this three-membered
ring structure
STAGE 4: Bromide ion attack on carbonium ion
• The presence of the carbonium ion makes the
substance unstable and it quickly combines with
the bromide ion, Br-, to form 1,2-dibromoethane
H
H
Br
C
H
Br
C
Br
C
H
Br-
C+
H
H
H
H
1,2-dibromoethane
Evidence for Ionic Addition
• When ethene reacts with bromine in water in the
presence of sodium chloride a number of compounds
are formed:
- 1,2-dibromoethane
- 1-bromo-2-chloroethane – formed when the
carbonium ion is attacked by the Cl- ion.
- 2-bromoethanol – formed when the carbonium ion
is attacked by the water molecule
The syllabus requires you also know the mechanism
of ionic addition of Cl2 and HCl to ethene(Bk. Pg 369)
*Note: a cyclic intermediate (stage 3) is not formed
in the case of addition of Cl2 or HCl to ethene. This is
because the Cl atom and the H atom are to small to
form a ring compound.