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Free Radical Substitution
Homolytic Fission
Substitution Rxn
(free radical substitution)
• Is a chemical reaction in which an atom or
group of atoms in a molecule is replaced
by another atom or group of atoms
Mechanism of reaction
• Is the detailed step by step description of
how the overall reaction occurs
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
Stage 1
Initiation
Getting Started
Ultra violet light breaks the bond
Chlorine molecule
Cl2
2 Chlorine radicals each
with an unpaired electron
Both species are the same
Called Homolytic Fission
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
Lets 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
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
produced to keep it going
The formation of ethane proves that this
is the mechanism
Reaction speeded up by sources of free
radicals such as tetramethyl lead.
Proof of mechanism
•Small amount of ethane detected
•Not initiated (start) in dark needs UV light
•Tetra methyl lead decomposes to form methyl
free radicals, if Tetra methyl lead added it
increases rate of reaction
Pb (CH ) = Pb + 4 CH º
THERFORE Methyl radicals are used in reaction
•Halogenated alkanes are used as flame
retardants
3
4
3
Step1: Initiation UV light stimulates rxn.
Cl-Cl molecule splits
equally (homolytic fission)
Step 2.
Free Cl° atoms (RADICAL)
Propagation
attacks methane and
forms HCL
Step 3.
Methyl free radical
Propagation
attacks a Cl-Cl molecule
and forms chloromethane.
Chain rxn continues.
Step 4:
Cl°+ Cl° =Cl-Cl
Termination
Cl°+ °CH3= CH3Cl
CH3+ CH3= C2H6
Tetramethyl lead is added to
speed up the rxn
• It supplies the solution with methl free
radicals.
• Evidence for free radicals comes from
small amounts of ethane being found in
the solution
• Halogenation of alknaes makes them
more flame resistant
Addition Reaction (pg. 367)
• When two substances react together to
form a single substance
Addition Reaction
Mechanism
and evidence for
Heterolytic Fission
Step 1
Polarising of the bond
in bromine
Ethene
Concentration of negative charge
Because 4 electrons in this area
Bromine Br2
δ+
δ-
Moves in this direction
At this point the negative
charge of the double
bond in ethene forces the
electrons to the right Br
and it becomes δ− and
other one becomes δ+
Step 2
Heterolytic Fission
Occurs
The two electrons of the bond
have been forced across to the
right Br making it Br- while the
other is Br+
Br+
Br -
δ+
δ-
At this point the negative
of thebeen
double
Thecharge
Br2 has
split into
bond in ethene forces the +
2 different
species
i.e.
Br
electrons to the right Br
andand
Brit- becomes
this is called
δ− and
other one becomes
Heterolytic
Fission δ+
Step 3
Formation of the
Carbonium Ion
At
a lotofof
Thethis
twopoint
electrons
thethings
bond
have beenatforced
acrosstime
to the
happen
the same
Br
Br+
+
Carbonium ion
Cyclic
bromium ion
right Br making it Br- while the
•other
Theis two
Br+ electrons are pulled to
the Br+
us put in the two
Br
• A bondLet
is formed
electrons of this bond
• one of the bonds between the
two carbons disappears
• The lower carbon becomes
The
split
+veBr
because
it has lost
aninto
2 has been
electron species i.e. Br+
2 different
- this
• TheBr
two
hydrogens
on the
and
is called
upper carbon
move to make
Heterolytic
Fission
way for the Br
Step 4
Attack on
carbonium ion
by Br
The negative bromide
ion is attracted by the
positive carbonium ion
Br
+
Br
Br -
Carbonium ion
1,2 dibromoethane
Called Ionic Addition
because the species are ions
when they add on
The two electrons of
the bromide ion are
used to form the bond
The two hydrogen
atoms move round to
allow the Br in
The negative and
positive cancel each
other out
Step 5
Proof of
mechanism
Br
+
Br -
Cl
Cl Br
Proof of the mechanism is that if
there are Cl- in the environment then
some 1-bromo, 2-chloroethane will
be formed.
This can be identified by its different
Relative Molecular Mass
Step1:
σ+Br-σ-Br-
Step 2.
Ionic addition
Step 3.
Ionic addition
Step 4: Termination
Carbon double bond is
region of high edensity. Br2 becomes
polar as comes close
Br2 splits into
ions.heterolytic
fission because Br+
and Br- created
Br+ molecule attacks
double bond and
forms cyclic
bromonium ion/
carbonium ion
Br- now attacks the
carbonium ion
Hydrogenation
• Adding of hydrogen's into a molecule
(addition)
• Occurs in manufacture of margarine
• Add hydrogen into double bonds causes
oils to become solid
• Unsaturated fats are better for you that
saturated fats
Evidence for the carbonium ion
• When bromine and chlorine ions present
• Ethene forms 1-bromo-2-chloroethane
as well as 1, 2 dibromoethane
Polymerisation rxns
• Molecules that contain double bonds
undergo addition to become less
unsaturated (addition polymers such as
polythene and polypropene)
Polymerisation Reactions
• Example of an addition reaction
• Ethene molecules add together
• Polymers are long chain molecules made
by joining together many small molecules
+
=
Polymers
• Commonly reffered to as plastics
• Polyethene used for plastic bags, bowls,
lunch boxes, bottles etc
• Polypropene is used in toys, jugs, chairs
etc
• Crude oil is raw material for their
manufacture
Elimination reactions
Elimination reactions
• When a small molecule is removed from a
larger molecule to leave a double bond in
the larger molecule
Elimination rxns
• a compound breaks down into 2 or more simpler substances
• Double bond created
• only one reactant
AB  A + B
Elimination reactions
• Ethene is made from ethanol from
removing water using AlO as catalyst
• Elimination reaction is one in which a
small molecule is removed from a larger
molecule to leave a double bond in the
larger molecules
• Dehydration reaction
• Only need to know dehydration of alcohol
Elimination reaction
• Dehydration of an alcohol is an example of
an elimination reaction
• In this reaction, a larger alcohol molecule
reacts to form a smaller alkene molecule
and an even smaller water molecule
• The change in structure is from tetrahedral
to planar
Dehydration of ethanol
• Ethanol is dehydrated to ethene
• This reaction is used in the preparation of
ethene
Dehydration of ethanol to
ethene
Reaction conditions
• Heat
• Aluminium oxide catalyst
Preparation of ethene
Elimination rxn
• Is when a small molecule is removed from a
larger molecule to leave a double bond in the
larger molecule
• Alcohol =water + alkene
• Dehydration reaction since water is removed
• Ethanol=ethene + water
• 2 methanol +sulphuric acid =methoxymethane ether +water
C. Decomposition
2 H2O(l)  2 H2(g) + O2(g)
Redox reactions
Redox reactions
• These reactions involves oxidation and
reduction reactions
• The removal or addition of lectrons
from the molecule
-3 -2 -1
0
Reduction
Receives
electrons
Reducing agents
give electrons
1
2
3
Oxidation
Looses
electrons
Oxidation agents
take electrons
Redox reactions of primary
alcohols
• Primary alcohols react with oxidising
agents such as potassium manganate(VII)
or sodium dichromate(VI), forming the
corresponding aldehyde
• For example, ethanol reacts forming
ethanal
• Ethanal is also formed in the metabolism
of ethanol in the human body
Redox reaction
• Primary alcohol oxidised to an aldehyde
• Oxidising agent: sodium dichromate or
potassium permanganate
• The oxidising agent must be limited to
prevent the aldehyde from being further
oxidised to an carboxylic acid
Reaction of ethanol with sodium
dichromate(VI)
Reaction of ethanol with sodium
dichromate(VI)
• This reaction is used in the preparation of
ethanal
• Reaction conditions: heat, excess ethanol,
acidified sodium dichromate(VI) solution
• The aldehyde is distilled off as it is formed
in order to prevent further oxidation to
ethanoic acid
Preparation of ethanal
Oxidation of primary alcohols
• Primary alcohols such as ethanol are
oxidised to the corresponding aldehydes,
which can be further oxidised to the
corresponding carboxylic acids.
Oxidation of ethanol
H
H
H
C
C
H
OH
H
+
H
Na2Cr2O7 / H
H
C
H
O
C
H
Na2Cr2O7 / H+
H
H
C
H
O
C
OH
Reaction of ethanol with sodium
dichromate(VI)
• This reaction is used in the preparation of
ethanoic acid
• Reaction conditions: heat, excess acidified
sodium dichromate(VI) solution
• The reaction mixture is refluxed in order to
bring about oxidation to ethanoic acid
Preparation of ethanoic acid
Reflux
followed by
Distillation
Oxidation of secondary alcohols
• Secondary alcohols such as propan-2-ol
are oxidised to the corresponding ketones,
such as propanone
• Unlike aldehydes, ketones are not easily
oxidised, and so no further oxidation takes
place
Oxidation of propan-2-ol
OH
O
+
H3C
C
CH3
H
Na2Cr2O7 / H
H3C
C
CH3
Combustion of organic
compounds
• Most organic compounds burn in air,
forming carbon dioxide and water
• The structure of the compounds’
molecules is completely destroyed, with
the carbon and hydrogen atoms in each
molecule being oxidised
• Combustion is exothermic, and ethanol is
used as a fuel where it can be produced
cheaply
Non-flammable organic
compounds
• Fully halogenated alkanes such as
bromochlorodifluoromethane are nonflammable
• Because of this they can be used in fire
extinguishers and as flame retardants
• For environmental reasons, the use of
many of these substances is being phased
out
Reduction of aldehydes and
ketones
• Aldehydes and ketones can be reduced to
the corresponding alcohols, using
hydrogen passed over the heated surface
of a nickel catalyst
• For example, ethanal is reduced to ethanol
Reduction of ethanal to ethanol
Reduction of propanone to
propan-2-ol
ENERGY PROFILE
one step reaction
ONE STEP
transition
state TS energy maximum
E
N
E
R
G
Y
activation
energy Ea
obtained from
heat (collisions)
heat of
reaction
DH exothermic
starting
material
(releases heat)
product
REACTION COORDINATE
( follows the progress of the reaction )
opposite is
endothermic
Reactions as acids
Reactions of alcohols with
sodium
• Alcohols react with the reactive metal
sodium, forming a sodium salt and
hydrogen
• For example, ethanol reacts with sodium
forming sodium ethoxide and hydrogen
Reaction of ethanol with sodium
Acidic nature of the carboxylic
acid group
• Ethanoic acid is a far stronger acid than
ethanol
• This is because its anion is much more
stable than that of ethanol
• This enables it to lose a hydrogen ion
more readily
• The stability of the ethanoate ion is due to
electron delocalisation (as in benzene)
Reactions of carboxylic acids as
acids
• Carboxylic acids react with:
• Magnesium, forming a magnesium salt
and hydrogen
• Sodium hydroxide, forming a sodium salt
and hydrogen
• Sodium carbonate, forming a sodium salt ,
carbon dioxide and water
Reaction of ethanoic acid with
magnesium
• Acid + metal →
salt + hydrogen
2 CH3COOH + Mg → (CH3COO)2Mg +
H2
ethanoic acid
ethanoate
magnesium
Reaction of ethanoic acid with
sodium hydroxide
• Acid + Base
Water
→
Salt
+
CH3COOH + NaOH → CH3COONa +
H2O
ethanoic acid
sodium ethanoate
Reaction of ethanoic acid with
sodium carbonate
Acid + Carbonate → Salt + Water + Carbon dioxide
2CH3COOH + Na2CO3 → 2CH3COONa + H2O + CO2
ethanoic acid
sodium ethanoate