AQA organic reaction mechanisms
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AS
Free Radical Substitution
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Electrophilic Addition
Nucleophilic Substitution
(nitrile hydrolysis reaction)
Elimination of HX from haloalkanes
Elimination of water from alcohols
A2
Electrophilic Substitution
Friedel-Crafts Alkylation
Acylation
Nucleophilic Addition
Nucleophilic Addition Elimination
Original slide
prepared for the
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
Original slide
prepared for the
Free radical substitution mechanism
ultra-violet
Cl2
Cl + Cl
initiation step
two
propagation
steps
CH4 + Cl
CH3 + HCl
CH3 + Cl2
CH3Cl + Cl
CH3 + Cl
CH3Cl
termination step
CH3 +
CH3CH3
minor
termination step
CH3
Original slide
prepared for the
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
Original slide
prepared for the
Electrophilic addition
bromine with propene
CH3CH=CH2 + Br2
mechanism
CH3CHBrCH2Br
1,2-dibromopropane
hydrogen bromide with but-2-ene
CH3CH=CHCH3+ HBr
mechanism
CH3CH2CHBrCH3
2-bromobutane
concentrated sulphuric acid with but-2-ene
CH3CH=CHCH3+ HOSO3H
mech
CH3CH2CH(OSO3H)CH3
2-butylhydrogensulphate
Original slide
prepared for the
Electrophilic addition mechanism
bromine with propene
H
reaction equation
H
C C
CH3
H
+
Br
Br
-
H
H
carbocation
CH3 C
+
C
H
BrBr
Br
H
H
CH3 C
C
Br 1,2-dibromopropane
Br
H
Br
Original slide
prepared for the
Electrophilic addition mechanism
hydrogen bromide with trans but-2-ene
reaction equation
CH3
H
C C
CH3
H
H
+
Br
-
H
H
carbocation
CH3 C
+
C
CH3
Br-
H
H
H
CH3 C
C
Br
H
CH3
2-bromobutane
Original slide
prepared for the
Electrophilic addition mechanism
concentrated H2SO4 with cis but-2-ene
H
H
C C
CH3
CH3
H
+
reaction equation
H
CH3 C
H
H
carbocation
C CH3
+
OSO3H
OSO3H
-
H
H
CH3 C
C
2-butylhydrogensulphate
H
CH3
OSO3H
Original slide
prepared for the
Nucleophilic substitution
hydroxide ion with bromoethane
CH3CH2Br + OH- (aqueous)
CH3CH2OH + Brethanol
cyanide ion with iodoethane
CH3CH2I (ethanol) + CN-(aq)
ammonia with bromoethane
CH3CH2Br + 2 NH3
mechanism
mechanism
CH3CH2CN + Ipropanenitrile
mechanism
CH3CH2NH2 + NH4+Braminoethane
Original slide
prepared for the
Nucleophilic substitution mechanism
hydroxide ion with bromoethane
H
+
CH3 C
H
OH-
Br
H
CH3 C
OH
H
Br
ethanol
reaction equation
Original slide
prepared for the
Nucleophilic substitution mechanism
cyanide ion with iodoethane
H
+
CH3 C
H
CN-
Br
H
CH3 C
CN
H
Br
propanenitrile
reaction equation
Original slide
prepared for the
Nucleophilic substitution mechanism
ammonia with bromoethane
H
+
CH3 C
H
Br
H
CH3 C
H
-
Br
+
NH2
H
NH3
NH3
H
CH3 C
aminoethane
H
NH2
reaction equation
H
NH3+Br Original slide
prepared for the
Nitrile hydrolysis
Acid hydrolysis of nitriles:
CH3CH2CN +2 H2O + H+
Reflux with strong acid
CH3CH2COOH + NH4+
propanoic acid
eg HCl (aq)
Original slide
prepared for the
Elimination of HX from haloalkanes
Elimination of HBr from 2-bromopropane
CH3CHBrCH3 + OHCH3CH=CH2 + H2O + Br(in ethanol)
H
H
CH3 C
C
Br
H
H
H
H
C
CH3
OH -
propene
C
H
Br -
H
OH
acting as a base
Original slide
prepared for the
Haloalkanes and hydroxide ions
nucleophilic substitution
alcohol
RCH3CH2OH + Br-
+ OH- (aqueous) hydroxide acts as a nucleophile
RCH2CH2X
+ OH- (ethanol)
elimination
hydroxide acts as a base
RCH=CH2 + H2O + Xalkene
Original slide
prepared for the
Elimination of water from alcohols
Elimination of H2O from propan-1-ol
CH3CH2CH2OH
CH3CH=CH2 + H2O
Heat with concentrated H2SO4
Original slide
prepared for the
Elimination mechanism – water from propan-1-ol
H
H
CH3 C
C
H
H
H
CH3 C
H
+
H
H
OH
H
CH3 C
H
C
OH
+
H H
protonated alcohol
OH
C+
H
H
C
H
carbocation
H H
+
H
C
CH3
reaction equation
H
H
OH
propene
Original slide
prepared for the
Electrophilic Substitution
Nitration of benzene
C6H6
+ HNO3
C6H5NO2
+ H2O
Conditions / Reagents
concentrated HNO3 and concentrated H2SO4
50oC
mechanism
Original slide
prepared for the
electrophilic substitution mechanism (nitration)
+
1. Formation of NO2 the nitronium ion
+
HNO3 + 2H2SO4
NO2
+ 2HSO4- + H3O+
2. Electrophilic attack on benzene
+
NO2
+
NO2
H
-
O SO3H
3. Forming the product
and re-forming the catalyst
reaction equation
NO2
H O SO3H
Original slide
prepared for the
Friedel-Crafts alkylation
Where an H atom attached to an aromatic ring
is replaced by a C atom
Alkylation of benzene
C6H6
+ RCl
electrophilic substitution
C6H5R
+ HCl
R = alkyl group
Conditions / Reagents
RCl (haloakane)
0 - 25oC
and anhydrous AlCl3
to prevent further substitution
Original slide
prepared for the
Alkylation example
With chloroethane
C6H6 + CH3CH2Cl
overall reaction equation
C6H5CH2CH3 + HCl
Three steps in electrophilic substitution mechanism
1. Formation of the electrophile (a carbocation)
CH3CH2
Cl
AlCl3
+
CH3CH2
Cl
-
AlCl3
Original slide
prepared for the
Alkylation electrophilic substitution mechanism 2
2. Electrophilic attack on benzene
+
CH3CH2
CH3CH2
H
+
-
Cl
AlCl3
3. Forming the product
and re-forming the catalyst
CH3CH2
H
AlCl3
Cl
ethylbenzene
Original slide
prepared for the
Friedel-Crafts acylation
An H atom attached to an aromatic ring
is replaced by a C atom where C is part of C=O
Acylation of benzene
C6H6
+ RCOCl
electrophilic substitution
C6H5COR + HCl
Conditions / Reagents
RCOCl (acyl chloride) and anhydrous AlCl3
50 oC
Original slide
prepared for the
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)
O
CH3C
Cl
+
CH3C O
AlCl3
Cl
-
AlCl3
Original slide
prepared for the
Acylation electrophilic substitution mechanism 2
2. Electrophilic attack on benzene
O
+
CH3C O
CH3C
+
H
Cl
3. Forming the product
and re-forming the catalyst
O
CH3C
-
AlCl3
H
Cl
AlCl3
phenylethanone
Original slide
prepared for the
Nucleophilic Addition
Reduction of carbonyls
RCHO
RCOR
+ 2[H]
+ 2[H]
RCH2OH
RCH(OH)R
primary
alcohol
secondary
alcohol
Conditions / Reagents
NaBH4 and H2SO4(aq)
Room temperature and pressure
Original slide
prepared for the
Nucleophilic Addition Mechanism
RCOR
+ 2[H]
RCH(OH)R
alcohol
reduction of propanone
NaBH4 is a source of hydride ions H
+
CH3 C
O
-
CH3
H
H+ from H2SO4 (aq)
H+
O
CH3 C
CH3
H
O
H
CH3 C
H
CH3
propan-2-ol
Original slide
prepared for the
Nucleophilic Addition
addition of hydrogen cyanide to carbonyls
to form hydroxynitriles
RCOR
+ HCN
RCHO + HCN
RC(OH)(CN)R
RCH(OH)CN
Conditions / Reagents
NaCN (aq) and H2SO4(aq) supplies H+
supplies the CN- nucleophile
Room temperature and pressure
Original slide
prepared for the
Nucleophilic Addition Mechanism
hydrogen cyanide with propanone
CH3COCH3 + HCN
CH3C(OH)(CN)CH3
NaCN (aq) is a source of cyanide ions
+
CH3 C
O
-
CH3
CN
H+ from H2SO4 (aq)
+
H
O
CH3 C
CH3
CN
C N
O
CH3 C
H
CN
CH3
2-hydroxy-2-methylpropanenitrile
Original slide
prepared for the
Nucleophilic Addition Elimination
Acylation of water to give carboxylic acids
RCOCl + H2O
RCOOH
+ HCl
carboxylic acid
RC
Conditions
O
OH
room temperature and pressure
Original slide
prepared for the
Formation of ethanoic acid
ethanoyl chloride
CH3COCl
+ H 2O
CH3COOH + HCl
ethanoic acid
O
CH3 C
OH
mechanism
Original slide
prepared for the
Nucleophilic Addition Elimination Mechanism
+ O
CH3 C
nucleophilic
addition
O
CH3 C
+
OH
Cl H
Cl
OH
elimination
H
CH3
O
C
CH3
O
C +
OH
H
OH
H
Cl
Cl
reaction equation
Original slide
prepared for the
Acylation of primary amines to N-alkyl amides
RCOCl + 2 R’NH2
RCONHR’ + R’NH3+ClN-alkylamide
RC
Conditions
O
NHR’
room temperature and pressure
N-propylethanamide
CH3 C
O
NHCH2CH2CH3
Original slide
prepared for the
Formation of N-propyl ethanamide
from
ethanoyl chloride
CH3COCl
1-aminopropane
+ 2 CH3CH2CH2NH2
CH3CONHCH2CH2CH3
+ CH3CH2CH2NH3+Cl-
N-propylethanamide
mechanism
Original slide
prepared for the
Nucleophilic Addition Elimination Mechanism
nucleophilic
O
O
+
+
addition
CH3 C
CH3 C NHCH2CH2CH3
Cl
Cl H
NHCH2CH2CH3
elimination
H
CH3
H
O
C
CH3
NHCH2CH2CH3
Cl
+ NH2CH2CH2CH3
O
C +
NHCH2CH2CH3
H
Cl
NH2CH2CH2CH3
reaction equation
Original slide
prepared for the
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Original slide
prepared for the
References
Steve Lewis for the Royal Society of Chemistry
Original slide
prepared for the