Download - Organic Mechanisms
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Original slideprepared for the
Free Radical Substitution
AQA organic reaction mechanisms
Click a box below to go to the mechanism
Electrophilic Addition
Nucleophilic Substitution
Elimination of HX from haloalkanes
Elimination in alcohols (mechanism no longer required)
Electrophilic Substitution
Nucleophilic Addition
Nucleophilic Addition Elimination
Friedel-Crafts Alkylation Acylation
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here foradvice
AS
A2
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Original slideprepared for the
CH4 + Cl2 CH3Cl + HCl
Overall reaction equation
Conditions
ultra violet lightexcess methane
i.e. homolytic breaking of covalent bonds
to reduce further substitution
Free radical substitution
chlorination of methane
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Original slideprepared for the
CH4 + Cl CH3 + HCl
Cl2 Cl + Cl
CH3 + Cl2 CH3Cl + Cl
CH3ClCH3 + Cl
initiation step
twopropagation
steps
termination step
ultra-violet
CH3CH3CH3 + CH3 minortermination step
Free radical substitution mechanism
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Original slideprepared for the
CH3Cl + Cl2 CH2Cl2 + HCl
Overall reaction equations
Conditions ultra-violet light
CH2Cl2 + Cl2 CHCl3 + HCl
CHCl3 + Cl2 CCl4 + HCl
excess chlorine
Further free radical substitutions
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Original slideprepared for the
Electrophilic addition
CH3CH=CH2+ Br2
CH3CHBrCH2Br
bromine with propene
hydrogen bromide with but-2-eneCH3CH=CHCH3+ HBr
CH3CH2CHBrCH3
2-bromobutane
1,2-dibromopropane
concentrated sulphuric acid with but-2-ene
CH3CH=CHCH3+ HOSO3H CH3CH2CH(OSO3H)CH3
2-butylhydrogensulphate
mechanism
mechanism
mech
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Original slideprepared for the
Br
Br
Br
Br
Electrophilic addition mechanism
CH3
H H
H
CC
+
-
CH3
H H
HCC
Br
+
Br-
carbocation
CH3
H H
HCC
Br Br1,2-dibromopropane
bromine with propene reaction equation
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Original slideprepared for the
Electrophilic addition mechanism
CH3
H CH3
H
CC
CH3
H H
CH3CC
H
+
carbocation
CH3
H H
CH3CC
Br H
2-bromobutane
hydrogen bromide with trans but-2-ene
-
+
Br
H
reaction equation
Br-
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Original slideprepared for the
CH3
H
CH3
H
CC
OSO3H
H+
-
OSO3H-
carbocation
2-butylhydrogensulphate
CH3
H H
CC
H
+CH3
CH3H H
CC
H OSO3H
CH3
Electrophilic addition mechanism
concentrated H2SO4 with cis but-2-ene
reaction equation
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Original slideprepared for the
hydroxide ion with bromoethane
ethanol
CH3CH2Br+ OH- CH3CH2OH + Br
-(aqueous)
Nucleophilic substitution
propanenitrile
CH3CH2I + CN- (ethanolic) CH3CH2CN + I
-
cyanide ion with iodoethane
ammonia with bromoethane
ethylamine
CH3CH2Br + NH3 CH3CH2NH22 + NH4
+Br-
mechanism
mechanism
mechanism
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Original slideprepared for the
+ -CH3
H
BrC
H
-OH
CH3
H
OHC
H Br-
hydroxide ion with bromoethane
Nucleophilic substitution mechanism
ethanol
reaction equation
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Original slideprepared for the
+ -CH3
H
BrC
H
CN-
CH3
H
CNC
H Br-
cyanide ion with iodoethane
Nucleophilic substitution mechanism
propanenitrile
reaction equation
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Original slideprepared for the
ammonia with bromoethane
Nucleophilic substitution mechanism
ethylamine
+ -CH3
H
BrC
H
Br-
H
CH3
H
NH2C
H
+
CH3H
NH2C
H
NH3
NH4+Br-
reaction equation
NH3
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Original slideprepared for the
Elimination of HBr from 2-bromopropane
CH3
H H
H
CC
OH-
CH3
H H
HCC
Br H
propene
H OHBr -
CH3CHBrCH3 + OH- CH3CH=CH2 + H2O + Br
-(in ethanol)
acting as a base
Elimination of HX from haloalkanes
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Original slideprepared for the
elimination
+ OH-
RCH=CH2 + H2O + X-
(ethanol)
nucleophilic substitutionalcohol
+ OH-
RCH3CH2OH + X-
(aqueous)
RCH2CH2X
alkene
hydroxide acts as a base
hydroxide acts as a nucleophile
Haloalkanes and hydroxide ions
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Original slideprepared for the
Elimination of H2O from propan-1-ol
CH3CH2CH2OH CH3CH=CH2 + H2O
Elimination of water from alcohols
Heat with concentrated H2SO4 orconcentrated H3PO4
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Original slideprepared for the
CH3
H H
H
CCpropene
H OH
H+
CH3H H
OHCC
H H
CH3
H H
OHCC
H H
H
+
CH3
H H
CC
H H
+ H OH
H+
protonated alcohol
carbocation
Elimination mechanism water from propan-1-ol
reaction equation
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Original slideprepared for the
Electrophilic Substitution
Nitration of benzene
C6H6 + HNO3 C6H5NO2 + H2O
Conditions / Reagents
concentrated HNO3 and concentrated H2SO4
50oC
mechanism
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Original slideprepared for the
electrophilic substitution mechanism (nitration)
2. Electrophilic attack on benzene
NO2
NO2+
3. Forming the product
+
NO2
H
the nitronium ion
HNO3 + H2SO4 HSO4- + NO2 + H2O
1. Formation of NO2
+
reaction equation
+
+ H+and re-forming thecatalyst (H+ reforms
H2SO4 with HSO4-
)
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Original slideprepared for the
Friedel-Crafts alkylation
Alkylation of benzene
C6H6 + RCl C6H5R + HCl
Conditions / Reagents
RCl (haloakane) and anhydrous AlCl3
0 - 25oC
Where an H atom attached to an aromatic ringis replaced by a C atom
electrophilic substitution
to prevent further substitution
R = alkyl group
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Original slideprepared for the
Alkylation example
Three steps in electrophilic substitution mechanism
1. Formation of the electrophile (a carbocation)
AlCl3
-
Cl AlCl3CH3CH2
+
With chloroethane overall reaction equation
C6H6 + CH3CH2Cl C6H5CH2CH3 + HCl
CH3CH2 Cl
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Original slideprepared for the
CH2CH3+
Alkylation electrophilic substitution mechanism 2
2. Electrophilic attack on benzene
CH2CH3
ethylbenzene
3. Forming the product
CH2CH3
+H
+ H+
and re-forming thecatalyst (H+ reforms AlCl3
and HCl with AlCl4-)
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Original slideprepared for the
Friedel-Crafts acylation
Acylation of benzene
C6H6 + RCOCl C6H5COR + HCl
Conditions / Reagents
RCOCl (acyl chloride) and anhydrous AlCl3
50 oC
An H atom attached to an aromatic ringis replaced by a C atom where C is part of C=O
electrophilic substitution
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Original slideprepared for the
CH3CCl
O
Acylation example
Three steps in electrophilic substitution mechanism
1. Formation of the electrophile (an acylium ion)
AlCl3-
Cl AlCl3
With ethanoyl chloride overall reaction equation
C6H6 + CH3COCl C6H5COCH3 + HCl
+
CH3C O
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Original slideprepared for the
Acylation electrophilic substitution mechanism 2
2. Electrophilic attack on benzene
phenylethanone
3. Forming the product
+
CH3C O
CH3C
O
+H
C
OCH3
+ H+
and re-forming thecatalyst (H+ reforms AlCl3and HCl with AlCl4
-)
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Original slideprepared for the
Nucleophilic Addition
Reduction of carbonyls
RCHOaldehyde
+ 2[H] RCH2OH
Conditions / Reagents
NaBH4 (aq)
Room temperature and pressure
primaryalcohol
RCORketone
+ 2[H] RCH(OH)R
secondary
alcohol
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Original slideprepared for the
+
-
Nucleophilic Addition Mechanism
reduction of propanone
H+
NaBH4 is a source ofhydride ions
RCOR + 2[H] RCH(OH)R alcohol
CH3
CH3
C
O
CH3
CH3
C
OH
H
propan-2-ol
H
CH3
CH3
C
O
H
H
H+ fromH2SO4 (aq)
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Original slideprepared for the
Nucleophilic Addition
RCHOaldehyde
+ HCN RCH(OH)CN
Conditions / Reagents
HCN (aq)
Room temperature and pressure
RCORketone
+ HCN RC(OH)(CN)R
supplies H+
and the CN-
nucleophile
addition of hydrogen cyanide to carbonylsto form hydroxynitriles
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Original slideprepared for the
Nucleophilic Addition Mechanism
hydrogen cyanide with propanone
H+
HCN (aq) is a source ofcyanide ions
CH3COCH3 + HCN CH3C(OH)(CN)CH3
CH3
CH3
C
O
CH3
CH3
C
OH
CN
H+
2-hydroxy-2-methylpropanenitrile
fromH2SO4 (aq)+
-
CN
C N
CH3
CH3
C
O
CN
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Original slideprepared for the
Nucleophilic Addition Elimination
4 main types of reaction mechanism to consider1. acyl chloride + water carboxylic acid2. acyl chloride + alcohol ester3. acyl chloride + ammonia acid amide4. acyl chloride + amine N-substituted amideHCl is a secondary product in all of the above
reactions.Two of the 4 mechanisms are shown on the next few
slides, but all 4 versions must be known for theexam. Mechanisms for acid anhydrides need not belearned (but equations should, giving a carboxylic
acid in place of HCl)
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Original slideprepared for the
Nucleophilic Addition Elimination
Acylation of water to give carboxylic acids
RCOCl + H2O RCOOH
Conditions room temperature and pressure
carboxylic acid
+ HCl
OHR C
O
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Original slideprepared for the
Formation of ethanoic acid
+ H2O CH3COOH + HClCH3COCl
ethanoyl chloride
ethanoic acid
mechanism
OHCH3 C
O
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Original slideprepared for the
Nucleophilic Addition Elimination Mechanism
CH3
Cl
CO+-
OH
H
CH3
Cl
C
O
OH
H
+
CH3 C
O
OH
H
+
CH3 C
O
OH
+ HCl
Cl
nucleophilic
addition
elimination
reaction equation
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Original slideprepared for the
Acylation of primary amines to N-alkyl amides
RCOCl + RNH2 RCONHR
Conditions room temperature and pressure
N-alkylamide
+ HCl
NHRR C
O
N-propylethanamideNHCH2CH2CH3
CH3 CO
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Original slideprepared for the
Formation of N-propyl ethanamide
from
+ CH3CH2CH2NH2
CH3CONHCH2CH2CH3 + HCl
CH3COCl
ethanoyl chloride 1-aminopropane
N-propylethanamide
mechanism
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Original slideprepared for the
+-
Nucleophilic Addition Elimination Mechanism
CH3
Cl
C
O
NHCH2CH2CH3
H
CH3
Cl
C
O
NHCH2CH2CH3
H
+
CH3 C
O
NHCH2CH2CH3
H
+
+ HCl
CH3
C
O
NHCH2CH2CH3 Cl
nucleophilic
addition
elimination
reaction equation
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Original slide
Steve Lewis for the Royal Society of Chemistry
Adapted by Mark Smith for Turton ChemistryDepartment
References