Organic%20Mechanisms

1
Organic Mechanisms

• Chapter 23

2
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.

3
Free radical Substitution

• The mechanism involved in the chlorination of
  Methane is believed to consist of the following
  steps.

4
Initiation

• uv light
• Cl2 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.

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Propagation

• CH4 Cl CH3
  HCl
• CH3 Cl2 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.

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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

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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.

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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.

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• 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

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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

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ELECTROPHILIC ADDITION OF BROMINE
C
Reagent Bromine. (Neat liquid or
dissolved in tetrachloromethane, CCl4
) Conditions Room temperature. No catalyst
or UV light required! Equation C2H4(g)
Br2(l) gt CH2BrCH2Br(l) 1,2 -
dibromoethane Mechanism It is surprising
that bromine should act as an
electrophile as it is non-polar.
CONVERSIONS
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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),

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Step 2

• The Br, in order to gain the 2 electrons it
  needs, attacks the C2H4 molecule.
• 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.

Carbonium ion
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Step3

• The carbonium ion is then attacked by the Br-
  ion. This results in the formation of
  1,2-dibromoethane.

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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. 2-chloroethanol for chlorine
  water, and ethanol for HCl(aq))

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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) gt 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
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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

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Soap formation

• Soaps are salts of fatty acids (long chain
  carboxylic acids). Fats are esters formed by the
  condensation of fatty acids and glycerol
  (propane-1,2,3-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.

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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.
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Soap
Soaps are formed by the hydrolysis of fatty acid
esters to produce salts of the fatty acids.

• Glycerine TriSterate NaOH ? Sodium
  Sterate Glycerol
• 3C17H35COOCH2 3NaOH 3C17H35COONa

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Preparation of Soap
The ethanol solvent is removed by distillation
Reflux apparatus used in the preparation of Soap
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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.

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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
ethene poly(ethene)
MONOMER POLYMER
n represents a large number
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POLYMERISATION OF ALKENES
EXAMPLES OF ADDITION POLYMERISATION
ETHENE
POLY(ETHENE)
PROPENE
POLY(PROPENE)
CHLOROETHENE
POLY(CHLOROETHENE) POLYVINYLCHLORIDE PVC
POLY(TETRAFLUOROETHENE) PTFE Teflon
TETRAFLUOROETHENE
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ELIMINATION OF WATER (DEHYDRATION)
L
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 alkene Equation e.g.
C2H5OH(l) gt CH2 CH2(g) H2O(l)
CONVERSIONS
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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.

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OXIDATION OF PRIMARY ALCOHOLS
N
Primary alcohols are easily oxidised to
aldehydes e.g. CH3CH2OH(l)
O gt CH3CHO(l) H2O(l)
it is essential to distil off the aldehyde before
it gets oxidised to the acid
CH3CHO(l) O gt CH3COOH(l)
OXIDATION TO ALDEHYDES DISTILLATION
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
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OXIDATION OF ALDEHYDES
O

• Aldehydes are easily oxidised to carboxylic acids
• e.g. CH3CHO(l) O
  gt 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
  FEHLINGS 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- gt Ag
• FEHLINGS SOLUTION
• Reagent a solution of a copper(II) complex
• Observation a red precipitate forms in the blue
  solution
• Products copper(I) oxide carboxylic acid

CONVERSIONS
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OXIDATION OF SECONDARY ALCOHOLS
Secondary alcohols are easily oxidised to
ketones e.g. CH3CHOHCH3(l)
O gt CH3COCH3(l) H2O(l)
Propan-2-ol is
oxidised to propanone
CONVERSIONS
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REDUCTION OF ALDEHYDES
R
Reagent H2 / Nickel catalyst
Conditions Product primary alcohol Equation
e.g. CH3CHO(l) 2H gt
C2H5OH(l)
Ethanal is reduced to Ethanol
CONVERSIONS
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REDUCTION OF CARBOXYLIC ACIDS
Q
Reagent/catalyst H2 Nickel catalyst
Conditions reflux in ethoxyethane Product aldehy
de Equation e.g. CH3COOH(l)
2H gt CH3CHO(l) H2O(l)
CONVERSIONS
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REDUCTION OF KETONES
S
Reagent H2 / Nickel catalyst
Conditions warm in water or ethanol Product seco
ndary alcohol Equation e.g.
CH3COCH3(l) 2H gt
CH3CH(OH)CH3(l)
Propanone is reduced to Propan-2-ol
CONVERSIONS
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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
ETHYL METHANOATE
METHYL ETHANOATE
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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 Preparation From
carboxylic acids or acyl chlorides Reactivity Unr
eactive compared with acids and acyl chlorides
ETHYL METHANOATE
METHYL ETHANOATE
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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 Preparation From
carboxylic acids or acyl chlorides Reactivity Unr
eactive compared with acids and acyl
chlorides Isomerism Esters are structural
isomers of carboxylic acids
ETHYL METHANOATE
METHYL ETHANOATE
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STRUCTURAL ISOMERISM FUNCTIONAL GROUP
Classification CARBOXYLIC ACID
ESTER Functional Group R-COOH
R-COOR Name PROPANOIC ACID
METHYL ETHANOATE Physical properties O-H bond
gives rise No hydrogen bonding to
hydrogen bonding insoluble in water get
higher boiling point and solubility in
water Chemical properties acidic
fairly unreactive reacts with alcohols
hydrolysed to acids
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PREPARATION OF ESTERS - 1
Reagent(s) alcohol carboxylic
acid Conditions reflux with a strong acid
catalyst (e.g. conc. H2SO4 ) Equation
e.g. CH3CH2OH(l) CH3COOH(l)
CH3COOC2H5(l) H2O(l) ethanol
ethanoic acid ethyl ethanoate Notes 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
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HYDROLYSIS OF ESTERS
Hydrolysis is the opposite of esterification
ESTER WATER CARBOXYLIC ACID
ALCOHOL
HCOOH C2H5OH METHANOIC
ETHANOL ACID
ETHYL METHANOATE
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HYDROLYSIS OF ESTERS
Hydrolysis is the opposite of esterification
ESTER WATER CARBOXYLIC ACID
ALCOHOL
HCOOH C2H5OH METHANOIC
ETHANOL ACID
ETHYL METHANOATE
METHYL ETHANOATE
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HYDROLYSIS OF ESTERS
Hydrolysis is the opposite of esterification
ESTER WATER CARBOXYLIC ACID
ALCOHOL
HCOOH C2H5OH METHANOIC
ETHANOL ACID
ETHYL METHANOATE
CH3COOH CH3OH ETHANOIC METHANOL
ACID
METHYL ETHANOATE
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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
H2O CH3COOH CH3OH alkaline
CH3COOCH3 NaOH gt CH3COO Na
CH3OH
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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
H2O CH3COOH CH3OH alkaline
CH3COOCH3 NaOH gt CH3COO Na
CH3OH If the hydrolysis takes place
under alkaline conditions, the organic
product is a water soluble ionic salt
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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
H2O CH3COOH CH3OH alkaline
CH3COOCH3 NaOH gt 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
gt CH3COOH NaCl
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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) S
aponification 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
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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.