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Organic Mechanisms
Chapter 23
Free Radical Substitution
CH4 + Cl2  CH3Cl + HCl
An example of a substitution reaction is the
chlorination of methane.
A chlorine atom replaces an atom of Hydrogen
in a molecule of methane.
Free radical Substitution
The mechanism involved in the chlorination of
Methane is believed to consist of the following
steps.
Initiation
Cl2
uv light
Cl*
+
Cl*
• The reaction mechanism begins with the homolytic
fission of the chlorine molecule by UV light.
• Two atoms of chlorine with unpaired electrons are
formed. These are very reactive and, as stated
above, are called free radicals.
Propagation
CH4
+ Cl *
CH3* + Cl2
CH3*
+
HCl
CH3CL + Cl*
• A chlorine atom attacks the methane molecule to form Hydrogen
chloride and a methyl free radical. The methy free radical
attacks a chlorine molecule and gives us one of the desired
products, CH3Cl. In so doing it yields another chlorine free
radical. If this follows the same pathway it will yield more
products and more free radicals.
• We now have a chain reaction initiated by chlorine radicals and
ending with new chlorine radicals. This also explains why a
large number of chloromethane molecules are produced for
every photon absorbed.
Termination
As the number of free radicals is increasing and the concentrations
of methane and chlorine are falling. A single free radical has
caused many thousands of methane and chlorine molecules to be
broken down.
Eventually, the probability of one of these reactions occurring
increases.
2Cl· .... Cl2
CH3· + Cl· .... CH3Cl
CH3· + CH3· .... CH3CH3
Evidence
* Tetramethyl-lead greatly speeds up the reaction.
* Molecular oxygen slows down the reaction.
Studies have shown that tetramethyl-lead, Pb(CH3)4, decomposes to
give lead, Pb, and four CH3· radicals. This would greatly increase
the concentration of methyl radicals, thus increasing the reaction
rate, i.e it serves as an accelerator.
On the other hand oxygen, O2, combines with methyl radicals, CH3·, to
form the less reactive peroxymethyl radical, CH3OO·. This slows
down the reaction as a single oxygen molecule prevents thousands
of CH3Cl molecules being formed. Oxygen is an inhibitor and the
slowing down of a reaction by small amounts of a substance is a
sure indication that a chain reaction is involved.
Evidence for free radical
substitution
• Free Radical Substitution Mechanism
Halogenation reactions with alkanes involve replacement of one or all of the
hydrogens in the alkane. These reactions may produce many products due to
the high reactivity of the free radical species. The substitution reaction needs
energy to be supplied before the reaction can proceed. Heating or shining
ultraviolet light on the reaction mixture may supply this energy.
(a) Chlorination of Methane
Evidence for the mechanism occurs at all steps
• For the initiation step
1. The reaction will not occur in the dark at room temperature. It will occur at
room temperature if ultraviolet light is shone on the reactants.
2. The energy supplied is not sufficient to break a C-H bond. Sufficient energy
isupplied to break a Cl-Cl bond however. The energy of the radiation needs to
be at least that required to homolytically spilt the chlorine molecule.
3. No molecular hydrogen produced – hence no hydrogen free radicals have been
formed.
For the propagation steps
1. Thousands of chloromethane molecules are produced for every one photon of
light used. This suggests a chain reaction consistent with theproposed
mechanism.
2. No molecular hydrogen produced – hence no hydrogen free radicals have been
formed.
For the termination steps
1. Ethane is produced in small amounts. Its occurrence can only be explained by
CH3+ CH3
CH3CH3
If the reaction is left run with excess chlorine and uv light di- tri- and tetra-chloro
methane are produced as are minute amounts of a range of chloroethanes.
2. The presence of tetramethyl-lead greatly speeds up the reaction as it a source of
methyl free radicals
Ionic Addition
An addition reaction is one in
which 2 substances react
together to form a single
substance.
The mechanism involved is
different from that between
methane and chlorine
C
ELECTROPHILIC ADDITION OF BROMINE
Reagent
Bromine. (Neat liquid or dissolved in tetrachloromethane, CCl4 )
Conditions
Room temperature. No catalyst or UV light required!
Equation
C2H4(g) + Br2(l)
——>
CH2BrCH2Br(l)
1,2 - dibromoethane
Mechanism
It is surprising that bromine
should act as an electrophile
as it is non-polar.
CONVERSIONS
Ionic Mechanism of
Bromination of Ethene
Step 1
The first stage in the mechanism involves a
bromine molecule becoming momentarily
polarised on approach to the region of high
electron density of the double bond. The
bromine molecule undergoes heterolytic
fission (unequal splitting), forming a
bromonium ion (Br+) and a bromide ion(Br–),
Step 2
The Br+, in order to gain the 2
electrons it needs, attacks the
C2H4 molecule.
Carbonium ion
The Br+ forms a covalent bond
with one of the carbon atoms.
The other carbon atom is left with
a positive charge since it lost
one of its outer electrons. This
positively charged atom is
called a carbonium ion.
Step3
The carbonium ion is then attacked by the Br- ion. This
results in the formation of 1,2-dibromoethane.
Evidence of ionic addition
Evidence: addition using bromine water gives 2-bromoethanol
(CH2BrCH2OH)
OR
addition with bromine water containing a chloride (sodium chloride)
gives 1-bromo-2-chloroethane (Allow 1-chloro-2-bromoethane)
(CH2BrCH2Cl)
OR
Another specified anion / chlorine water / HCl in water (HCl(aq),
hydrochloric acid)
Product where that anion has added in place of the chlorine (e.g. 2chloroethanol for chlorine water, and ethanol for HCl(aq))
ELECTROPHILIC ADDITION OF HCl
B
Reagent
Hydrogen Chloride... it is electrophilic as the H is slightly positive
Condition
Room temperature.
Equation
C2H4(g) + HCl(g) ———> C2H5Cl(l)
chloroethane
Mechanism
Step 1
As the HCl nears the alkene, one of the carbon-carbon bonds breaks
The pair of electrons attaches to the slightly positive H end of H-Cl.
The HCl bond breaks to form a chloride ion.
A carbocation (positively charged carbon species) is formed.
Step 2
The chloride ion behaves as a nucleophile and attacks the carbocation.
Overall there has been addition of HCl across the double bond.
CONVERSIONS
Esterification-Formation of an
Ester
An Ester is formed when an alcohol and a carboxylic acid react together.
This is called a condensation reaction.
Alcohol + Carboxylic Acid  Ester + Water
The reverse reaction is called a Hydrolysis.
Esters may be Hydrolysed easily in the presence of a Base like NaOH or
KOH.
Ethyl Ethanoate + Sodium Hydroxide  Sodium Ethanoate + Ethanol
CH3COOC2H5 +
NaOH
 CH3COONa
+ C2H5OH
Soap formation
• Soaps are salts of fatty acids (long chain carboxylic acids). Fats are
esters formed by the condensation of fatty acids and glycerol (propane1,2,3-triol).
• Soaps are manufactured by the base hydrolysis of these fats (esters). In
this experiment the fat is hydrolysed using sodium hydroxide in ethanol
solution. The ethanol is then removed by distillation.
• Soaps are formed by the hydrolysis of fatty acid esters to produce salts
of the fatty acids.
How Soap Works
The hydrocarbon end
of the molecule is
hydrophobic (water
repelling) and the
carboxylate end is
hydrophilic (water
attracting). The
hydrophobic end
dissolves in grease and
the hydrophilic end
dissolves in the water.
Soap
Soaps are formed by the hydrolysis of fatty acid esters to
produce salts of the fatty acids.
Glycerine TriSterate + NaOH  Sodium Sterate +
3C17H35COOCH2 + 3NaOH
3C17H35COONa
Glycerol
Preparation of Soap
Reflux apparatus used in the
preparation of Soap
The ethanol solvent is removed
by distillation
Polymerisation reactions
• Polymers are long chain molecules made by
joining together many small molecules
called monomers.
• The polymers that we study are Addition
polymers because their manufacture
involves addition reactions.
POLYMERISATION OF ALKENES
ADDITION POLYMERISATION
Process • during polymerisation, an alkene undergoes an addition reaction with itself
• all the atoms in the original alkenes are used to form the polymer
• long hydrocarbon chains are formed
the equation shows the original monomer and the repeating unit in the polymer
n represents a
large number
ethene
poly(ethene)
MONOMER
POLYMER
POLYMERISATION OF ALKENES
EXAMPLES OF ADDITION POLYMERISATION
ETHENE
PROPENE
CHLOROETHENE
POLY(ETHENE)
POLY(PROPENE)
POLY(CHLOROETHENE)
POLYVINYLCHLORIDE
TETRAFLUOROETHENE
PVC
POLY(TETRAFLUOROETHENE)
PTFE
“Teflon”
L
ELIMINATION OF WATER (DEHYDRATION)
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.
Example. The removal of water from an alcohol is an example of an elimination
reaction
Product
Equation
alkene
e.g. C2H5OH(l) ————> CH2 = CH2(g) + H2O(l)
CONVERSIONS
Redox reactions
• When a primary alcohol reacts with an
oxidising agent the primary alcohol is
converted to an aldehyde.
• When a secondary alcohol reacts with an
oxidising agent the secondary alcohol is
converted to a ketone.
OXIDATION OF PRIMARY ALCOHOLS
N
Primary alcohols are easily oxidised to aldehydes
e.g.
———>
CH3CH2OH(l) + [O]
CH3CHO(l) + H2O(l)
it is essential to distil off the aldehyde before it gets oxidised to the acid
CH3CHO(l) + [O]
———>
OXIDATION TO
ALDEHYDES
DISTILLATION
CH3COOH(l)
OXIDATION TO
CARBOXYLIC ACIDS
REFLUX
Aldehyde has a lower boiling point so
distils off before being oxidised further
Aldehyde condenses back into the
mixture and gets oxidised to the acid
CONVERSIONS
O
OXIDATION OF ALDEHYDES
Aldehydes are easily oxidised to carboxylic acids
e.g. CH3CHO(l) + [O]
•
•
•
•
———>
CH3COOH(l)
one way to tell an aldehyde from a ketone is to see how it reacts to mild oxidation
ALDEHYES are EASILY OXIDISED
KETONES are RESISTANT TO MILD OXIDATION
reagents include TOLLENS’ REAGENT
and FEHLING’S SOLUTION
TOLLENS’ REAGENT
Reagent
ammoniacal silver nitrate solution
Observation
a silver mirror is formed on the inside of the test tube
Products
silver + carboxylic acid
Equation
Ag+ + e- ——> Ag
FEHLING’S SOLUTION
Reagent
a solution of a copper(II) complex
Observation
a red precipitate forms in the blue solution
Products
copper(I) oxide + carboxylic acid
Equation
Cu2+ + e- ——> Cu+
CONVERSIONS
OXIDATION OF SECONDARY ALCOHOLS
Secondary alcohols are easily oxidised to
ketones
e.g.
CH3CHOHCH3(l) + [O]
———> CH3COCH3(l) + H2O(l)
Propan-2-ol is oxidised to propanone
CONVERSIONS
REDUCTION OF ALDEHYDES
R
Reagent
H2 / Nickel catalyst
Conditions
Product
Equation
primary alcohol
e.g.
CH3CHO(l) + 2[H]
———>
Ethanal is reduced to Ethanol
CONVERSIONS
C2H5OH(l)
Q
REDUCTION OF CARBOXYLIC ACIDS
Reagent/catalyst
H2 Nickel catalyst
Conditions
reflux in ethoxyethane
Product
aldehyde
Equation
e.g.
CH3COOH(l)
+ 2[H]
———>
CONVERSIONS
CH3CHO(l) + H2O(l)
S
REDUCTION OF KETONES
Reagent
H2 / Nickel catalyst
Conditions
warm in water or ethanol
Product
secondary alcohol
Equation
e.g. CH3COCH3(l) + 2[H]
———>
CH3CH(OH)CH3(l)
Propanone is reduced to Propan-2-ol
CONVERSIONS
ESTERS
Structure
Substitute an organic group for the H in carboxylic acids
Nomenclature
first part from alcohol, second part from acid
e.g. methyl ethanoate CH3COOCH3
METHYL ETHANOATE
ETHYL METHANOATE
ESTERS
Structure
Substitute an organic group for the H in carboxylic acids
Nomenclature
first part from alcohol, second part from acid
e.g. methyl ethanoate CH3COOCH3
METHYL ETHANOATE
ETHYL METHANOATE
Preparation
From carboxylic acids or acyl chlorides
Reactivity
Unreactive compared with acids and acyl chlorides
ESTERS
Structure
Substitute an organic group for the H in carboxylic acids
Nomenclature
first part from alcohol, second part from acid
e.g. methyl ethanoate CH3COOCH3
METHYL ETHANOATE
ETHYL METHANOATE
Preparation
From carboxylic acids or acyl chlorides
Reactivity
Unreactive compared with acids and acyl chlorides
Isomerism
Esters are structural isomers of carboxylic acids
STRUCTURAL ISOMERISM – FUNCTIONAL GROUP
Classification
Functional Group
Name
CARBOXYLIC ACID
ESTER
R-COOH
R-COOR
PROPANOIC ACID
METHYL ETHANOATE
Physical properties
O-H bond gives rise
to hydrogen bonding;
get higher boiling point
and solubility in water
No hydrogen bonding
insoluble in water
Chemical properties
acidic
reacts with alcohols
fairly unreactive
hydrolysed to acids
PREPARATION OF ESTERS - 1
Reagent(s)
alcohol + carboxylic acid
Conditions
reflux with a strong acid catalyst (e.g. conc. H2SO4 )
Equation
Notes
e.g. CH3CH2OH(l) + CH3COOH(l)
ethanol
ethanoic acid
CH3COOC2H5(l) + H2O(l)
ethyl ethanoate
Conc. H2SO4 is a dehydrating agent - it removes water
causing the equilibrium to move to the right and thus
increases the yield of the ester
For more details see under ‘Reactions of carboxylic acids’
HYDROLYSIS OF ESTERS
Hydrolysis is the opposite of esterification
ESTER + WATER
CARBOXYLIC ACID + ALCOHOL
HCOOH
METHANOIC
ACID
ETHYL METHANOATE
+
C2H5OH
ETHANOL
HYDROLYSIS OF ESTERS
Hydrolysis is the opposite of esterification
ESTER + WATER
CARBOXYLIC ACID + ALCOHOL
HCOOH
METHANOIC
ACID
ETHYL METHANOATE
METHYL ETHANOATE
+
C2H5OH
ETHANOL
HYDROLYSIS OF ESTERS
Hydrolysis is the opposite of esterification
ESTER + WATER
CARBOXYLIC ACID + ALCOHOL
HCOOH
+
METHANOIC
ACID
C2H5OH
ETHANOL
ETHYL METHANOATE
CH3COOH
ETHANOIC
ACID
METHYL ETHANOATE
+
CH3OH
METHANOL
HYDROLYSIS OF ESTERS
Hydrolysis is the opposite of esterification
ESTER + WATER
CARBOXYLIC ACID + ALCOHOL
The products of hydrolysis depend on the conditions used...
acidic
CH3COOCH3
+ H2 O
alkaline
CH3COOCH3 + NaOH
CH3COOH
+
CH3OH
——> CH3COO¯ Na+ + CH3OH
HYDROLYSIS OF ESTERS
Hydrolysis is the opposite of esterification
ESTER + WATER
CARBOXYLIC ACID + ALCOHOL
The products of hydrolysis depend on the conditions used...
acidic
CH3COOCH3
+ H2 O
alkaline
CH3COOCH3 + NaOH
CH3COOH
+
CH3OH
——> CH3COO¯ Na+ + CH3OH
If the hydrolysis takes place under alkaline conditions,
the organic product is a water soluble ionic salt
HYDROLYSIS OF ESTERS
Hydrolysis is the opposite of esterification
ESTER + WATER
CARBOXYLIC ACID + ALCOHOL
The products of hydrolysis depend on the conditions used...
acidic
CH3COOCH3
+ H2 O
alkaline
CH3COOCH3 + NaOH
CH3COOH
+
CH3OH
——> CH3COO¯ Na+ + CH3OH
If the hydrolysis takes place under alkaline conditions,
the organic product is a water soluble ionic salt
The carboxylic acid can be made by treating the salt with HCl
CH3COO¯ Na+ +
HCl
——>
CH3COOH
+
NaCl
NATURALLY OCCURING ESTERS - TRIGLYCERIDES
• triglycerides are the most common component of edible fats and oils
• they are esters of the alcohol
glycerol (propane-1,2,3-triol)
CH2OH
CHOH
CH2OH
Saponification
•
•
•
•
alkaline hydrolysis of triglycerol esters produces soaps
a simple soap is the salt of a fatty acid
as most oils contain a mixture of triglycerols, soaps are not pure
the quality of a soap depends on the oils from which it is made
Hydrolysis of Esters to produce
soap
Soaps
Soaps are formed by the hydrolysis of fatty acid
esters to produce salts of the fatty acids. The
hydrocarbon end of the molecule is hydrophobic
(water repelling) and the carboxylate end is
hydrophilic (water attracting). The hydrophobic end
dissolves in grease and the hydrophilic end
dissolves in the water.