Organic reaction mechanisms

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Organic reaction mechanisms
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Homolytic
Free Radical Substitution
Free Radical Addition
Heterolytic
Electrophilic Addition
SN1
SN2
Nucleophilic Substitution
Electrophilic Substitution
Alkylation
Nitration
Br2
Acylation
Nucleophilic Addition
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Free radical substitution
chlorination of methane
i.e. homolytic breaking of covalent bonds
Overall reaction equation
CH4 Cl2
CH3Cl HCl
Conditions
ultra violet light
excess methane
to reduce further substitution
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Free radical substitution mechanism
ultra-violet
initiation step
two propagation steps
termination step
minor termination step
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Further free radical substitutions
Overall reaction equations
CH3Cl Cl2
CH2Cl2 HCl
CH2Cl2 Cl2
CHCl3 HCl
CHCl3 Cl2
CCl4 HCl
Conditions
ultra-violet light
excess chlorine
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Free radical addition
addition polymerisation of ethene
i.e. homolytic breaking of covalent bonds
Overall reaction equation
n H2CCH2
polyethene
ethene
Conditions
free radical source
(a species that generates free radicals that
allow the polymerisation of ethene molecules)
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Free radical addition mechanism
initiation step
chain propagation steps
Addition of H2CCH2 repeats the same way until
termination step
polyethene
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Electrophilic addition
bromine with propene
mechanism
CH3CHCH2
Br2
CH3CHBrCH2Br
1,2-dibromopropane
hydrogen bromide with but-2-ene
mechanism
CH3CHCHCH3
HBr
CH3CH2CHBrCH3
2-bromobutane
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bromine with propene
Electrophilic addition mechanism
reaction equation
Cyclic intermediate
1,2-dibromopropane
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hydrogen bromide with trans but-2-ene
Electrophilic addition mechanism
reaction equation
Carbonium ion
2-bromobutane
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Nucleophilic substitution
hydroxide ion with bromoethane
mechanism
CH3CH2Br
OH-
CH3CH2OH Br-
(aqueous)
ethanol
hydroxide ion with 2-bromo,2-methylpropane
mechanism
(CH3)3CBr
OH-
(CH3)3COH Br-
(aqueous)
2-methylpropan-2-ol
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hydroxide ion with bromoethane (SN2)
Nucleophilic substitution mechanism
ethanol
SN2
reaction equation
2(species reacting in the slowest step)
S (substitution)
N(nucleophilic)
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OH- ion with 2-bromo,2-methylpropane (SN1)
Nucleophilic substitution mechanism
2-methylpropan-2-ol
SN1
reaction equation
1(species reactingin the slowest step)
S (substitution)
N(nucleophilic)
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Nucleophilic substitution
cyanide ion with iodoethane
mechanism
CH3CH2I (ethanol)
CN-(aq)
CH3CH2CN I-
propanenitrile
cyanide ion with 2-bromo,2-methylpropane
mechanism
(CH3)3CBr
(ethanol) CN-
(aqueous)
(CH3)3CCN Br-
2,2-dimethylpropanenitrile
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cyanide ion with iodoethane (SN2)
Nucleophilic substitution mechanism
propanenitrile
SN2
2(species reacting in the slowest step)
S (substitution)
N(nucleophilic)
reaction equation
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CN- ion with 2-bromo,2-methylpropane (SN1)
Nucleophilic substitution mechanism
SN1
2,2-dimethylpropanenitrile
1(species reactingin the slowest step)
S (substitution)
N(nucleophilic)
reaction equation
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Nitration of benzene
Where an H atom attached to an aromatic ring is
replaced by an NO2 group of atoms
C6H6
HNO3
C6H5NO2
H2O
Conditions / Reagents
concentrated HNO3
and concentrated H2SO4
50oC
mechanism
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electrophilic substitution mechanism (nitration)
the nitronium ion (nitryl cation)
HNO3
H2SO4
HSO4- H2O
2. nitronium ion attack on benzene
3. Forming the product
H
H reacts to reform catalyst
reaction equation
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Bromination of benzene
C6H6
Br2
C6H5Br
HBr
Conditions / Reagents
Br2
and CATALYST Fe, FeBr3 or anhydrous AlBr3
25oC
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Electrophilic substitution mechanism
1. Formation of the bromine cation
FeBr3
2. Attack on benzene
3. Forming the organic product
Forming the minor product and re-forming the
catalyst
FeBr3
bromobenzene
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Alkylation of benzene
Where an H atom attached to an aromatic ring is
replaced by a C atom
electrophilic substitution
C6H6
RCl
C6H5R
HCl
R alkyl group
Conditions / Reagents
RCl (haloakane)
and anhydrous AlCl3
0 - 25oC
to prevent further substitution
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Alkylation example
With chloroethane
overall reaction equation
C6H6 CH3CH2Cl
C6H5CH2CH3
HCl
Three steps in electrophilic substitution
mechanism
1. Formation of the electrophile (a carbocation)

AlCl3
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Alkylation electrophilic substitution mechanism 2
2. Electrophilic attack on benzene
3. Forming the product
and re-forming the catalyst
AlCl3
ethylbenzene
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Acylation of benzene
An H atom attached to an aromatic ring is
replaced by a C atom where C is part of CO
electrophilic substitution
C6H6
RCOCl
C6H5COR
HCl
Conditions / Reagents
RCOCl (acyl chloride)
and anhydrous AlCl3
50 oC
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Acylation example
With ethanoyl chloride
overall reaction equation
C6H6 CH3COCl
C6H5COCH3
HCl
Three steps in electrophilic substitution
mechanism
1. Formation of the electrophile (an acylium
ion)
AlCl3
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Acylation electrophilic substitution mechanism 2
2. Electrophilic attack on benzene
3. Forming the products
and re-forming the catalyst
AlCl3
phenylethanone
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Nucleophilic Addition
addition of hydrogen cyanide to carbonyls to
form hydroxynitriles
HCN
RCOR
RC(OH)(CN)R
RCH(OH)CN
HCN
RCHO
Conditions / Reagents
NaCN (aq)
and H2SO4(aq)
supplies H
supplies the CN- nucleophile
Room temperature and pressure
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Nucleophilic Addition Mechanism
hydrogen cyanide with propanone
HCN
CH3COCH3
CH3C(OH)(CN)CH3
NaCN (aq) is a source of cyanide ions
H
from H2SO4 (aq)
H
2-hydroxy-2-methylpropanenitrile
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Advice
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References
Steve Lewis for the Royal Society of Chemistry