ib chemistry on nucleophilic substitution, sn1, sn2 and protic solvent
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Prepared by Lawrence Kok
Tutorial on Nucleophilic Substitution, SN1 and SN2 reactions
Class Functional gp
Suffix
Example
Formula
Alkane C - C - ane ethane CnH2n+2
H H ׀ ׀ H - C – C – H ׀ ׀ H H
H ׀ H - C – H ׀ H
H H H ׀ ׀ ׀ H - C – C – C – H ׀ ׀ ׀ H H H
H H H H ׀ ׀ ׀ ׀ H - C – C – C – C – H׀ ׀ ׀ ׀ H H H H
Number carbon
Word IUPAC name
Structure formula
Molecular formula
1 Meth Methane CH4 CH4
2 Eth Ethane CH3CH3 C2H6
3 Prop Propane CH3CH2CH3 C3H8
4 But Butane CH3(CH2)2CH3 C4H10
5 Pent Pentane CH3(CH2)3CH3 C5H12
6 Hex Hexane CH3(CH2)4CH3 C6H14
7 Hept Heptane CH3(CH2)5CH3 C7H16
8 Oct Octane CH3(CH2)6CH3 C8H18
9 Non Nonane CH3(CH2)7CH3 C9H20
10 Dec Decane CH3(CH2)8CH3 C10H22
methane ethane propane butane
Saturated hydrocarbon (C – C single bond)
Chemical rxn Alkane Reactivity for AlkanesCombustion rxn
Complete combustion – produce CO2 + H2O• C2H6 + 7/2O2 → 2CO2 + 3H2O
Incomplete combustion – produce C, CO, CO2, H2O• 2C3H8 + 7O2 → 2C + 2CO + 8H2O + 2CO2
Free Radical Substitution rxn
Free Radical Substitution Mechanism
- Homolytic fission- bond break by radical form. - Covalent bond split, each atom obtain one electron (unpair e)- UV needed
- Radical react with molecule
- Radical + radical → molecule
CH4 + CI2 → CH3CI + HCI
• Low reactivity - Strong stable bond bet C - C, C - H • Low reactivity - Low polarity of C - H bond• Saturated hydrocarbon – Non polar bond
Initiation
Propagation
Radical (dot)
Termination
homolytic fission
Radical recycle again
1
2
H H ׀ ׀ C = C ׀ ׀ H H
H H H ׀ ׀ ׀ C = C – C - H ׀ ׀ H H
H H H H ׀ ׀ ׀ ׀ C = C – C – C - H ׀ ׀ ׀ H H H
Unsaturated hydrocarbon (C = C double bond)
H H H H H ׀ ׀ ׀ ׀ ׀ C = C – C – C – C - H ׀ ׀ ׀ ׀ H H H H
ethene propene butene pentene
Reactivity for Alkene
- High reactivity - Unstable bond bet C = C - High reactivity – Weak pi bond overlap bet p orbital- Unsaturated hydrocarbon – ᴨ bond overlap
Combustion rxn
Chemical rxn Alkane
Complete combustion – produce CO2 + H2OC2H4 + 3O2 → 2CO2 + 2H2O
Incomplete combustion – produce C, CO, CO2, H2O2C2H4 + 7/2O2 → 2C + CO + 4H2O + CO2
CH2 = CH2 + Br2 → CH2BrCH2BrCH2 = CH2 + HCI → CH3CH2CICH2 = CH2 + H2O → CH3CH2OH
Addition rxn
Addition CI2
Addition Br 2
Addition HCIAddition H2 O catalyst
nickel, H3 PO
4 / 300C
H H ׀ ׀ C = C ׀ ׀ H H
H H ׀ ׀ H - C – C – H ׀ ׀ CI CI
Class Functional
Suffix Example Formula
Alkene Alkenyl - ene ethene CnH2n
H H ׀ ׀ H - C – C – H ׀ ׀ Br Br
H H ׀ ׀ H - C – C – H ׀ ׀ H CI
H H ׀ ׀ H - C – C – H ׀ ׀ H OH
1
2
Polymerization (Addition rxn)3
Polymers are long chains molecules (plastics)• Join repeat units call monomers• Addition and condensation polymerization
• Monomers double bond (unsaturated)• Repeat units join together by covalent bond without loss of
any molecule
ethene polyethene
add monomer
polymer
propene polypropylene
add monomer
H CH3 H CH3
monomer
monomer
chloroethene polychloroethene (PVC)
tetrafluoroethene polytetrafluoroethene (PTFE)
H CI H CI
F F
F F
F F
F F
polymerization
polymer
Alkene decolourize brown liq Br2
OH׀ CH3-C – CH3 + [O] No product ׀ CH3
OH O׀ ‖CH3- C–CH3 + [O] CH3- C – CH3 + H2O H
׀ CH3 – C – OH ׀ H
Class Functional
Suffix Example Formula
Alcohol
Hydroxyl - ol methanol CnH2n+1OH
Number carbon
IUPAC name
Structure formula Molecular formula
1 Methanol CH3OH CH3OH
2 Ethanol CH3CH2OH C2H5OH
3 Propanol CH3CH2CH2OH C3H7OH
4 Butanol CH3(CH2)2CH2OH C4H9OH
methanol ethanol propanol butanol
H ׀ H - C – OH ׀ H
H H ׀ ׀ H - C – C – OH ׀ ׀ H H
H H H ׀ ׀ ׀ H - C – C – C – OH ׀ ׀ ׀ H H H
H H H H ׀ ׀ ׀ ׀ H - C – C – C – C – OH׀ ׀ ׀ ׀ H H H H
Hydrocarbon skeleton Functional gp
Chemical rxn Alcohol Reactivity for Alcohol
Primary 1 0
1 alkyl /R gp bond to C attach to OH
CH3 H ׀ ׀ CH3 – C – C – OH ׀ ׀ CH3 H
Combustion rxnComplete combustion–produce CO2 + H2OC2H6OH + 3O2 → 2CO2 + 3H2O
Incomplete combustion-produce C, CO, CO2, + H2O2C2H5OH + 4O2 → C + 2CO + 6H2O + CO2Oxidation rxn
Secondary 2 0
2 alkyl/R gp bond to C attach to OH
H ׀ CH3 – C – OH ׀ CH3
H H H ׀ ׀ ׀ H - C – C – C – H ׀ ׀ ׀ H OH H
Tertiary 3 0
3 alkyl/R gp bond to C attach to OH
CH3 ׀ CH3 – C – OH ׀ CH3
R ׀ R – C – OH ׀ R
H׀ CH3-CH2-OH + [O] CH3- C = O + H2O
MnO4- /H+
K2Cr2O7/H+
Primary 10 – Oxidised to Aldehyde and Carboxylic acid
H OH׀ ׀CH3- C= O + [O] CH3-C= O Secondary 20 - Oxidised to Ketone
Tertiary 30 - Cannot be Oxidised
MnO4- /H+
K2Cr2O7/H+
MnO4- /H+
K2Cr2O7/H+
MnO4- /H+
K2Cr2O7/H+
1
1
Esterification rxn3
O H׀ ‖ H - C – O – C – H + H2O׀ H
H ׀ H- O – C – H ׀ H
O ‖ H - C – O-H +
Chemical rxn Alcohol
Oxidation rxn – oxidized carbon attach to OH
Primary 10 – Oxidised to Aldehyde and Carboxylic acid
Secondary 20 - Oxidised to Ketone Tertiary 30 - Cannot be Oxidised
OH׀ CH3-C – CH3 + [O] No product ׀ CH3
MnO4- /H+
K2Cr2O7/H+ MnO4
- /H+
K2Cr2O7/H+
MnO4- /H+
K2Cr2O7/H+
Alcohol to Aldehyde (Distillation)1. Acidified dichromate(VI)/permanganate(VII)
2. Warm it , collect distillate (Distillation)
AldehydeCarboxylic acid
-1 + 1
ON carbon increaseAlcohol
H OH׀ ׀CH3- C= O + [O] CH3- C =O
H H׀ ׀CH3- C -O-H + [O] CH3- C = O ׀ H
+ 1 + 3
ON carbon increaseAldehyde
Primary 10 – Oxidised to Aldehyde and Carboxylic acid
Alcohol to Carboxylic acid (Reflux)1. Acidified dichromate(VI)/permanganate(VII)
2. Warm it , collect distillate (Distillation)
Alcohol oxidize to Aldehyde• MnO4
- reduce from purple (Mn7+) to pink (Mn2+)• Cr2O7
2- reduce from orange (Cr6+) to green (Cr3+)
0 + 2
ON carbon increaseAlcohol Ketone
Alcohol to Ketone (Reflux)1. Acidified dichromate(VI)/permanganate(VII)
2. Warm it , collect distillate (Distillation) Click here oxidation alcohol
RCH2OH + [O] → RCHO + H2O
RCH2OH + 2[O] → RCOOH + H2O
RCH(OH)R + [O] → RCOR + H2O
Oxidation eqn (addition of O)
AldehydeAlcohol
Alcohol
Alcohol
Carboxylic acid
Ketone
Alcohol oxidize to Carboxylic acid• MnO4
- reduce from purple (Mn7+) to pink (Mn2+)• Cr2O7
2- reduce from orange (Cr6+) to green (Cr3+)
distillation
reflux
Aldehyde turn to carboxylic acid
AldehydeAlcohol
reflux
Alcohol turn to ketone
OH O׀ ‖CH3- C – CH3 + [O] CH3- C – CH3 + H2O
Class Functional
Suffix Formula
Ester Ester - oate R –COO-R
Number carbon
IUPAC name Structure formula Molecular formula
1 Methyl methanoate
HCOOCH3 R–COO-R
2 Methyl ethanoate
CH3COOCH3 R–COO-R
3 Methyl propanoate
CH3CH2COOCH3 R–COO-R
4 Methyl butanoate
CH3CH2CH2COOCH3
R–COO-R
methyl methanoate methyl ethanoate methyl propanoate
O H׀ ‖ H - C – O – C - H׀ H
H O H ׀‖ ׀ H - C - C – O - C - H׀ ׀ H H
H H O H ׀‖׀ ׀ H - C – C – C – O - C - H ׀ ׀ ׀ H H H
Hydrocarbon skeleton Functional gp
Esterification
O ‖ H - C – O-H
H ׀ H- O – C – H ׀ H
O H׀ ‖ H - C – O – C – H + H2O׀ H
Ester
Condensation rxn
↔+
Methanoic acid Methanol Methyl methanoate
Esterification (reversible rxn)After reflux – reach equilibrium
Acid and alcohol (reflux)Conc H2SO4 (catalyst) used
Water produced
condensation
reflux
Ester purified and distillClick here ester preparation
H O H ׀‖ ׀ H - C - C – O - C – H + H2O׀ ׀ H H
H ׀ H- O – C – H ׀ H
H O ‖ ׀ H - C - C – OH ׀ H
CH3COOH + CH3OH → CH3COOCH3 + H2O
H O H H ׀ ׀ ‖ ׀ H – C – C– O - C–C-H׀ ׀ ׀ H H H
+
Ethanoic acid Methanol Methyl ethanoate
↔ H H ׀ ׀ H- O- C– C – H ׀ ׀ H H
H O ‖ ׀ H – C – C - OH ׀ H
condensation
CH3COOH + CH3CH2OH → CH3COOCH2CH3 + H2O
+condensation
↔
Ethanoic acid Ethanol Ethyl ethanoate
+ H2O
H ׀ CH3 – C – CI ׀ H
H ׀ H - C – CI ׀ H
H H ׀ ׀ H - C – C – CI ׀ ׀ H H
H H H ׀ ׀ ׀ H - C – C – C – CI ׀ ׀ ׀ H H H
Hydrocarbon skeleton Functional gp
Primary 1 0
1 alkyl /R gp bond to C attach to CISecondary 2 0
2 alkyl/R gp bond to C attach to CI
H ׀ CH3 – C – CI ׀ CH3
H H H ׀ ׀ ׀ H - C – C – C – H ׀ ׀ ׀ H CI H
Tertiary 3 0
3 alkyl/R gp bond to C attach to CI
CH3 ׀ CH3 – C – CI ׀ CH3
R ׀ R – C – CI ׀ R
Reactivity for Halogenoalkane
Class Functional
Prefix Example
Halogenoalkane
F, CI, Br, I - chloro chloroethane
Number carbon
IUPAC name Structure formula Molecular formula
1 chloromethane
CH3CI CH3CI
2 chloroethane CH3CH2CI C2H5CI
3 chloropropane
CH3CH2CH2CI C3H7CI
4 chlorobutane CH3(CH2)2CH2CI C4H9CI
chloromethane chloroethane chloropropane
Reactivity for halogenoalkane• Carbon bond to halogen – F, CI,
Br, I• High electronegativity on
halogen gp• High reactivity – due to polarity
of C+- Br -
Nucleophile– species with lone pair electron – donate electron pair (Lewis base)
Chemical rxn Halogenoalkane
C - Brᵟ+ ᵟ-
electron
Electron deficient carbon
O–H ....
ᵟ- ᵟ+
Cᵟ+
Substitution rxn
CH3CH2CI + OH- → CH3CH2OH + CI-
H H ׀ ׀ H - C – C – CI ׀ ׀ H H
+ OH- ᵟ+ ᵟ-
H H ׀ ׀ H - C – C – OH + CI- ׀ ׀ H H
H Br H ׀ ׀ ׀ H - C – C – C – H ׀ ׀ ׀ H H H
CH3CHBrCH3 + OH- → CH3CHOHCH3 + Br-
+ OH-
H OH H ׀ ׀ ׀ H - C – C – C – H + Br- ׀ ׀ ׀ H H H
ᵟ+
ᵟ-
CH3 H ׀ ׀ CH3 – C – C – CI ׀ ׀ CH3 H
Electrophile - Electron deficient - Accept lone pair- Positive charge- Lewis Acid
C - Br
Reactivity for halogenoalkane• Carbon bond to halogen – F, CI,
Br, I• High electronegativity on
halogen gp• High reactivity – due to polarity
of C+- CI -C - Brᵟ+ ᵟ-
electron
Electron deficient carbon
OH ..ᵟ-ᵟ+
Nucleophilic Substitution rxn
CH3CH2CI + OH- → CH3CH2OH + CI-
H H ׀ ׀ H - C – C – CI ׀ ׀ H H
+ OH- ᵟ+ ᵟ-
H H ׀ ׀ H - C – C – OH + CI- ׀ ׀ H H
H Br H ׀ ׀ ׀ H - C – C – C – H ׀ ׀ ׀ H H H
CH3CHBrCH3 + OH- → CH3CHOHCH3 + Br-
+ OH-
H OH H ׀ ׀ ׀ H - C – C – C – H + Br- ׀ ׀ ׀ H H H
ᵟ+ ᵟ-
Nucleophile and SubstitutionElectrophile and Addition
vs Reactivity of Alkene- High reactivity - Unstable bond bet C = C - High reactivity – Weak pi bond overlap bet p orbital- Unsaturated hydrocarbon – ᴨ bond overlap
C = CElectron rich π electron
ᵟ- ᵟ-
H ᵟ+
C = Cᵟ-ᵟ-
E ᵟ+
E+ Electron deficientNu ᵟ-
ᵟ-
Nucleophile– Lone pair electron – Donate electron pair- Lewis Base
H H ׀ ׀ C = C ׀ ׀ H H
CH2=CH2 + Br2 → CH2BrCH2Br
+ Br – Br ᵟ- ᵟ+
H H ׀ ׀ H - C – C – H ׀ ׀ Br Br
vs
CH2=CH2 + HCI → CH3CH2CI H H ׀ ׀ C = C ׀ ׀ H H
ᵟ- + H – CI ᵟ+
H H ׀ ׀ H - C – C – H ׀ ׀ H CI
Electrophilic Addition rxn
ᵟ-
Electron rich region
Electrophilic Substitution rxn
C6H6 + Br2 C6H5Br + HBr
+ Br-Br ᵟ+
+ NO2+
ᵟ+
Electrophile and SubstitutionElectrophile and Addition
vs
C = CElectron rich π electron
ᵟ- ᵟ-
ᵟ+
C = Cᵟ-ᵟ-
E ᵟ+
E+ Electron deficient
E ᵟ+
H H ׀ ׀ C = C ׀ ׀ H H
CH2=CH2 + Br2 → CH2BrCH2Br
+ Br – Br ᵟ- ᵟ+
H H ׀ ׀ H - C – C – H ׀ ׀ Br Br
vs
CH2=CH2 + HCI → CH3CH2CI
H H ׀ ׀ C = C ׀ ׀ H H
ᵟ- + H – CI ᵟ+
H H ׀ ׀ H - C – C – H ׀ ׀ H CI
Electrophilic Addition rxn
E
Electrophile - Electron deficient - Accept lone pair- Positive charge- Lewis Acid
ᵟ++H E
+ H
Electron rich region
H Br + HBr
C6H6 + HNO3 C6H5NO2 + HCI
AICI3 dry ether
warm/Conc H2SO4
H NO2
Reactivity of Alkene- High reactivity - Unstable bond bet C = C - High reactivity – Weak pi bond overlap bet p orbital- Unsaturated hydrocarbon – ᴨ bond overlap
Reactivity of Benzene (Unreactive)- Delocalization of electron in ring- Stability due to delocalized π electron- Substitution instead of Addition
C6H6 – no reaction with brown Br2(I)
ethene decolourize brown Br2(I)
benzene –stable (unreactive) toward addition rxn
Electrophile - Electron deficient - Accept lone pair- Positive charge- Lewis Acid
H
Electrophile - Electron deficient - Accept lone pair- Positive charge- Lewis Acid
C - Br OH ..ᵟ-ᵟ+
NucleophileElectrophile
ᵟ+C = Cᵟ-
Nucleophile– Lone pair electron – Donate electron pair- Lewis Base
Organic Rxn
Addition rxn Substitution rxn
Nucleophilic Substitution
Free Radical Substitution
Electrophilic Substitution
Electrophilic Addition rxn
Free radicle
CI CICI CI. .
:
Radical (unpair electron)
uv radiation
H H ׀ ׀ C = C ׀ ׀ H H
+ Br – Br
H H ׀ ׀ H - C – C – H ׀ ׀ Br Br
ᵟ+
ᵟ-
H H ׀ ׀ H - C – C – CI ׀ ׀ H H
+ OH-
H H ׀ ׀ H - C – C – OH + CI- ׀ ׀ H H
ᵟ-ᵟ+H
E + + H E ᵟ+
H H ׀ ׀ C = C ׀ ׀ H H
H H ׀ ׀ H - C – C – H ׀ ׀ CI CI
H H ׀ ׀ H - C – C – H ׀ ׀ H CI
H H ׀ ׀ H - C – C – H ׀ ׀ H OH
Add CI- C
I
Add HCI
Add H2 O nickel,
H3 PO
4 / 300C
CI2 / UV
H H ׀ ׀ H - C – C – CI ׀ ׀ H H
H H ׀ ׀ H - C – C – OH + CI-
׀ ׀ H H
H H ׀ ׀ H - C – C – NH2 + CI-
׀ ׀ H H
H H ׀ ׀ H - C – C – CN + CI-
׀ ׀ H H
NH3
OH-
CN-
H ׀ H - C – H ׀ H
H ׀ H - C – CI + H ׀ H
CI2 → 2 CI•
CH3• + CI2 → CH3CI + CI•
CI• + CH4 → HCI + CH3•
H
Electrophile - Electron deficient - Accept lone pair- Positive charge- Lewis Acid
C - Br OH ..ᵟ-ᵟ+
NucleophileElectrophile
H ᵟ+C = Cᵟ-
Nucleophile– Lone pair electron – Donate electron pair- Lewis Base
Free radicle
CI CICI CI. .
:
Radical (unpair electron)
uv radiation
H H ׀ ׀ C = C ׀ ׀ H H
H H ׀ ׀ H - C – C – H ׀ ׀ CI CI
H H ׀ ׀ H - C – C – H ׀ ׀ H CI
H H ׀ ׀ H - C – C – H ׀ ׀ H OH
Add CI- C
I
Add HCI
Add H2 O nickel,
H3 PO
4 / 300C
CI2 / UV
H H ׀ ׀ H - C – C – CI ׀ ׀ H H
H H ׀ ׀ H - C – C – OH + CI-
׀ ׀ H H
H H ׀ ׀ H - C – C – NH2 + CI-
׀ ׀ H H
H H ׀ ׀ H - C – C – CN + CI-
׀ ׀ H H
NH3
OH-
CN-
H ׀ H - C – H ׀ H
H ׀ H - C – CI + H ׀ H
CI2 → 2 CI•
CH3• + CI2 → CH3CI + CI•
CI• + CH4 → HCI + CH3•
Alkene – Addition rxn Halogenoalkane – Substitution rxn Alkane - Radical substitution
H OH ׀ ׀ H - C – C – H ׀ ׀ H H
H O ‖ ׀ H - C – C – H ׀ H
H O ‖ ׀ H - C – C – OH ׀ H
H O H ׀‖׀ H - C – C – C – H ׀ ׀ H H
H OH H ׀ ׀ ׀ H - C – C – C – H ׀ ׀ ׀ H H H
H OH H ׀ ׀ ׀ H - C – C – C – H ׀ ׀ ׀ H CH3 H
Alcohol – Oxidation rxn
10 alcohol 20 alcohol 30 alcohol
carboxylic acid aldehyde ketone
no reaction
׀ ׀ C- C –OH ׀ ׀
O ‖ C – C – C
O ‖C – C – H
O ‖ C – C – OH
O ‖C –C – C– O – C – C
O H׀ ‖ C – C – N – C – C
No reaction
1o alcohol[O]/Cr2O7/H
+
AldehydeKetone Carboxylic Acid
Free radical substitutionCI2/ UV
Halogenoalkane
Alkane
2 o alcohol[O]/ Cr
2 O7 /H +
[O]/ Cr2O7/H+
3o alcohol[O]/ Cr2O7/H
+
Substitutionwarm / OH-
Alcohol
Substitutio
n / NH 3
Substitution / CN-
Substitution / OH -
Amine
Nitrile
Alcohol
Condensation
Amide
Hydrogenation/Reduction
H 2 /Nickel
Amine
Acid Hydrolysis Carboxylic Acid
Alkene
Elimination 100C /Conc alcoholic OH-
Alkane Halogenoalkane Dihalogenoalkane
Condensation
Ester
Addi
tion
HCI
Addition Br2
Addition CI2Addition H 2
AdditionPolymerisation
X
׀ ׀ C – C – CI ׀ ׀
׀ ׀
C = C
׀ ׀
׀ ׀ ׀ ׀ C – C – C – C ׀ ׀ ׀ ׀
׀ ׀ C – C
׀ ׀ H CI
׀ ׀ C – C ׀ ׀ CI CI
׀ ׀ C – C ׀ ׀ Br Br
׀ – C ׀C ׀
׀
׀ ׀ C – C – OH ׀ ׀
׀ ׀ C – C – CN ׀ ׀
׀ ׀ C – C – NH2 ׀ ׀
׀ ׀ ׀ C – C – C –NH2 ׀ ׀ ׀
׀ ׀ C – C – COOH ׀ ׀
Start here
PolyAlkene
׀ ׀ C – C
׀ ׀ H H
H ׀ CH3 – C – Br ׀ H
CH3 H ׀ ׀ CH3 – C – C – Br ׀ ׀ CH3 H
Reactivity for halogenoalkane• Carbon bond to halogen – F, CI,
Br, I• High electronegativity on
halogen • High reactivity – polarity of C+-
Br -
Nucleophile– Lone pair electron – Donate electron pair - (Lewis base)
Chemical rxn Halogenoalkane
C - Brᵟ+ ᵟ-
electron
Electron deficient carbon
O–H ....
ᵟ-C
ᵟ+
H H ׀ ׀ H - C – C – Br ׀ ׀ H H
+ OH- ᵟ+ ᵟ-
H H ׀ ׀ H - C – C – OH + Br-
׀ ׀ H H
Nucleophilic Substitution
Primary 10 - SN2
Primary 10 - SN2
- Experimentally rate expression = k [CH3CH2Br][OH-]- Rate dependent on conc- CH3CH2Br and OH-
- Molecularity = 2- No bulky alkyl gp, less steric effect - Allow nucleophile to attack electron deficient carbon from opposite site (Inversion of configuration)
CH3CH2Br + OH- → CH3CH2OH + Br-
SN2Substitution Bimolecular collision
bet 2 molecule
Nucleophilic
Bimolecular Nucleophilic Substitution
OH- + CH3CH2Br [ HO---CH2(CH3)---Br]- CH3CH2OH + Br-
HO-
Bond breaking and making in transition state
+ Br-
One step mechanism – Bond break and making in transition state
nucleophile attack
leaving gp
Click here to view SN2
slow step (RDS)
fast step
slow step (RDS) fast step
✓1ₒ SN2
Hydrolysis bromoethane (1o)
H׀ OH- + CH3 – C – Br ׀ H
Bond Breaking and Making at transition state Br leaving gp substituted with OH-
H H׀ ׀ CH3 - C – Br + OH- CH3 – C –OH + Br - ׀ ׀ H H
Nucleophile collide with bromoethane
CH3CH2Br + OH- → CH3CH2OH + Br- Single step
Nucleophilic Substitution
Click here view SN2
SN2Substitution
Nucleophilic
Bimolecular Nucleophilic Substitution
Bimolecular collision bet 2 molecule
- Experimentally rate expression = k [CH3CH2Br][OH-]- Rate dependent on conc = CH3CH2Br and OH-
- Molecularity = 2- No bulky alkyl gp, less steric effect - Allow nucleophile to attack electron deficient carbon from the opposite site (Inversion of configuration)
Formation of ethanol
1 step mechanism (concerted)
SN21ₒ
Nucleophile– Lone pair electron – Donate electron pair - (Lewis base)
CH3 ׀ CH3 – C – Br ׀ CH3
CH3 ׀ CH3 – C – Br ׀ CH3
R ׀ R – C – Br ׀ R
Reactivity for halogenoalkane• Carbon bond to halogen gp – F,
CI, Br, I• High electronegativity on
halogen gp• High reactivity – polarity of C+-
Br -
Chemical rxn Halogenoalkane
C - Brᵟ+ ᵟ-
electron
Electron deficient carbon
O–H ....
ᵟ-C
ᵟ+
+ OH- ᵟ+ ᵟ-
Nucleophilic Substitution
Tertiary 30 – SN1
Tertiary 30 – SN1
- Experimentally rate expression = k [(CH3)3CBr]- Rate dependent on conc - (CH3)3CBr - Molecularity = 1- 3 Bulky alkyl gp, Steric hindrance effect - 30 carbocation more stable due to inductive effect• 3 alkyl gp stabilize carbocation by inductive effect push electron to carbocation (reducing positive charge) making it more stable
SN1Substitution
Unimolecular (1 molecule)
Nucleophilic
Unimolecular Nucleophilic Substitution
+ :OH-
carbocation (Intermediate)
+ Br-
1st step mechanism – carbocation formation
nucleophile attack
Click here to view SN1
(CH3)3CBr + OH- → (CH3)3COH + Br-
CH3 ׀ CH3 – C – OH + Br -
׀ CH3
slow step (RDS)
heterolytic fission Br leaving gp
fast step
2nd step mechanism – OH attack carbocation
(CH3)3CBr → (CH3)3C+ + Br- 1st step (slow)
(CH3)3C+ + OH- → (CH3)3COH 2nd step (fast)
✓3ₒ SN1
Formation of 2 methylpropan-2-ol
Hydrolysis 2-bromo- 2- methylpropane (3o) CH3 │ CH3 - C – Br │ CH3
Carbocation formation (Intermediate) Nucleophile OH- attack carbocation
Heterolytic fission - Carbocation and Br- form
(CH3)3CBr → (CH3)3C+ + Br- 1st step (slow)
(CH3)3C+ + OH- → (CH3)3COH 2nd step (fast)
CH3 CH3
׀ ׀ CH3 - C – Br + OH- CH3 –C – OH + Br - ׀ ׀ CH3 CH3
Nucleophilic Substitution
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- 3 Bulky alkyl gp - Steric hindrance effect - 30 carbocation more stable due to inductive effect• 3 alkyl gp stabilize carbocation by inductive
effect push electron to carbocation (reducing positive charge) making it more stable
SN1 Unimolecular (1 molecule)
Substitution
Nucleophilic
Unimolecular Nucleophilic Substitution
2 step mechanism
3ₒ SN1
H Br H ׀ ׀ ׀ H - C – C – C – H ׀ ׀ ׀ ׀׀ H H H
+ :OH- ᵟ+
Nucleophilic Substitution
Secondary 20 - SN1 and SN2
- Experimentally rate expression = k [CH3CHBrCH3][OH-]- Rate dependent conc = CH3CHBrCH3 and OH-
- Molecularity = 2- No bulky alkyl gp, less steric effect - Allow nucleophile to attack electron deficient carbon from opposite site (Inversion of configuration)
SN2Substitution Bimolecular collision
bet 2 molecule
Nucleophilic
Bimolecular Nucleophilic Substitution
HO-
Bond breaking and making in transition state
+ Br-
One step mechanism – Bond break and making in transition state
nucleophile attack
leaving gp
slow step (RDS)
fast step
CH3CHBrCH3 + OH- → CH3CH(OH)CH3 + Br-
H OH H ׀ ׀ ׀ H - C – C – C – H + Br -
׀ ׀ ׀ H H H
CH3 CH3 CH3
SN1Substitution
Nucleophilic
Unimolecular (1 molecule)
Unimolecular Nucleophilic Substitution
heterolytic fission Br leaving gp
slow step (RDS)
carbocation (Intermediate)
+ Br-
nucleophile attack
+ :OH-
CH3
1st step mechanism – carbocation formation
fast step
+
+
2nd step mechanism – OH attack carbocation
CH3
Click here SN1 vs SN2
1 step mechanism (concerted)
CH3CHBrCH3 → CH3CH+ CH3 + Br- 1st step (slow)
CH3CH+ CH3 + OH- → CH3CHOHCH3 2nd step (fast)2 step mechanism CH3CHBrCH3 + OH- →
CH3CH(OH)CH3 + Br-
Click here SN1 vs SN2 Khan academy
✓2ₒ SN1SN2
Electrophile - Electron deficient - Accept lone pair- Positive charge- Lewis Acid
C - Br OH ..ᵟ-ᵟ+
NucleophileElectrophile
H ᵟ+C = Cᵟ-
Nucleophile– Lone pair electron – Donate electron pair- Lewis Base
Free radicle
CI CICI CI. .
:
Radical (unpair electron)
uv radiation
H+ Br+ NO2+ :OH- :CN- H2O: :NH3
Homolytic fission Heterolytic fission
CI CI:uv radiation
CI CI..fish hook arrowSingle electron movement
A B:
A B:
A – B A + :B
Double headed arrowpair electron movement
Control by electronic
factor (charges)
vs vs
vs
Nucleophilic Substitution
Primary 10 - SN2 Secondary 20 -SN1 and SN2 Tertiary 30 – SN1
SN1
SN2
Control by steric factor (alkyl gp)
SN2 SN1Favour 10 30
Nature mechanis
m
1 step (transition
state)
2 step
(carbocation)
Rate lower higher
Solvent Polar aprotic Polar protic
Reaction profile
Click here SN1 vs SN2
Factor affecting Rate of Nucleophilic Substitution
• Bond polarity decrease ↓• Bond strength decrease ↓
• Rate fastest (Halogen leave easily)
Iodo > Bromo > Chloro > Fluoro
Nucleophilic Substitution
• SN 1 > SN 2 mechanism• 3o > 2o > 1o
• 3o – SN 1 - Carbocation - faster
• 1o - SN 2 – Transition state - slower
Nature of solvent
Nature of Halogen
CH3 ׀ CH3 – C – Br ׀ CH3
H ׀ CH3 – C – Br ׀ CH3
H ׀ CH3 – C – Br ׀ H
> > CH3CH2 – I >
CH3CH2 – CI >
CH3CH2 – F fastest slowest
weak bond strong bond
C - Br OH Nucleophile
ᵟ-
H bond to O or NH2 bonding/donate H+
H2O, NH3 CH3OH, CH3CH2OHAble to solvate cation and anion
Polar protic Polar aprotic
Lack acidic H, no H2 BondingAcetone/CH3COCH3, DMSO, CH3CN
Solvate cation–nucleophile free for SN2
H H ׀ ׀ H - C – C – OH ׀ ׀ H H
H ׀ H -– C – OH ׀ H
ᵟ+
Nature of Halogenoalkane
SN1
polar + H2 bonding
:O: ‖ CH3 – C – CH3
:O: ‖ CH3 – S – CH3
polar only
SN2
Rate of hydrolysis of halogenoalkane
C4H9CI + H2O → C4H9OH + H+ + CI-
C4H9Br + H2O → C4H9OH + H+ + Br-
C4H9I + H2O → C4H9OH + H+ + I-
Reaction Time ppt to appear
Observation
1-chlorobutane slowest white ppt 1-bromobutane cream ppt1-iodobutane fastest yellow pptMethod:
- Prepare 3 test tube contain 2 ml of ethanol each- Pipette 0.1ml of chloro, bromo and iodobutane to each test tube- Leave 3 test tube in 60C bath.- Add 1ml AgNO3, mix and record time ppt to form
Ag+ react CI- → AgCI (white ppt)Ag+ react Br- → AgBr (cream ppt)Ag+ react I- → AgI (yellow ppt)
fastest slowest
1-iodobutane 1-chlorobutane✓
+ Ag+
Factor affecting Rate of Nucleophilic Substitution
Click here protic/aprotic solvent
Nucleophilic Substitution
Nature of solvent
H bond to O or NH2 bonding/donate H+
H2O, NH3 CH3OH, CH3CH2OHAble to solvate cation and anion
+ Br-
Polar protic Polar aprotic
Lack acidic H, no H2 BondingAcetone/CH3COCH3, DMSO
Solvate cation–nucleophile free for SN2
NaOH → Na+ + OH-
SN1 SN2
H2O solvate carbocation and Br- formStabilize it – exist in intermediate state
H H ׀ ׀ H - C – C – Br ׀ ׀ H H
+ OH-
H H ׀ ׀ H - C – C – OH + Br-
׀ ׀ H H
H H ׀ ׀ H - C – C – OH ׀ ׀ H H
CH3 │ CH3 - C – Br │ CH3
carbocation solvated by H2O
anion solvated by H2O
H ׀ H -– C – OH ׀ H
Acetone solvate cation – nucleophile free for SN2No H2 bond- unable to solvate anion/nucleophile
:O: ‖ CH3 – C – CH3
: O
:
‖
CH
3 – C
– C
H3
CH3 – C – CH3
‖ :O:
Na+ solvated by CH3COCH3
nucleophile free to attack
C - Br OH Nucleophile
ᵟ+ ᵟ-
Click here protic/aprotic solvent
:O: ‖ CH3 – C – CH3
:O: ‖ CH3 – S – CH3
Click here expt protic/aprotic solvent
Acknowledgements
Thanks to source of pictures and video used in this presentation
Thanks to Creative Commons for excellent contribution on licenseshttp://creativecommons.org/licenses/
Prepared by Lawrence Kok
Check out more video tutorials from my site and hope you enjoy this tutorialhttp://lawrencekok.blogspot.com
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