ib chemistry on nucleophilic substitution, sn1, sn2 and factors affecting rate of halogenoalkane...
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IB Chemistry on Nucleophilic Substitution, SN1, SN2 and factors affecting rate of hydrolysis of halogenoalkane.TRANSCRIPT
http://lawrencekok.blogspot.com
Prepared by Lawrence Kok
Tutorial on Nucleophilic Substitution SN1, SN2 and Factors affecting rate of hydrolysis of Halogenoalkane.
Nucleophilic Substitution
Factors affecting Rate of Nucleophilic Substitution
Nature of Nucleophile• SN2 mechanism (only)• Nucleophile – electron pair donor • anion more reactive > neutral species due to electron charge density(negative charged)• order of reactivity/better nucleophile• CN - > OH- > NH3 > H2O
Nucleophilic Substitution
Factors affecting Rate of Nucleophilic Substitution
Nature of Nucleophile• SN2 mechanism (only)• Nucleophile – electron pair donor • anion more reactive > neutral species due to electron charge density(negative charged)• order of reactivity/better nucleophile• CN - > OH- > NH3 > H2O
H H │ │
CH3 - C – Br + OH- > CH3 –C –Br + H2O │ │ H H
SN2 (1o) mechanism OH - > H2O
CH3 CH3
│ │CH3 - C – Br + OH- = CH3 –C –Br + H2O │ │ CH3 CH3
SN1 (3o) mechanism No effect!
Faster ↑
No effect
Nucleophilic Substitution
Factors affecting Rate of Nucleophilic Substitution
Nature of Nucleophile• SN2 mechanism (only)• Nucleophile – electron pair donor • anion more reactive > neutral species due to electron charge density(negative charged)• order of reactivity/better nucleophile• CN - > OH- > NH3 > H2O
H H │ │
CH3 - C – Br + OH- > CH3 –C –Br + H2O │ │ H H
Nature of Halogen• SN1 and SN2 mechanism• polarity of bond decrease ↓• bond strength decreases ↓• halogen leaves easily• rate hydrolysis fastest iodo > bromo > chloro compounds
SN2 (1o) mechanism OH - > H2O
CH3 CH3
│ │CH3 - C – Br + OH- = CH3 –C –Br + H2O │ │ CH3 CH3
SN1 (3o) mechanism No effect!
Faster ↑
No effect
Nucleophilic Substitution
Factors affecting Rate of Nucleophilic Substitution
Nature of Nucleophile• SN2 mechanism (only)• Nucleophile – electron pair donor • anion more reactive > neutral species due to electron charge density(negative charged)• order of reactivity/better nucleophile• CN - > OH- > NH3 > H2O
H H │ │
CH3 - C – Br + OH- > CH3 –C –Br + H2O │ │ H H
Nature of Halogen• SN1 and SN2 mechanism• polarity of bond decrease ↓• bond strength decreases ↓• halogen leaves easily• rate hydrolysis fastest iodo > bromo > chloro compounds
H H H │ │ │CH3- C –I > CH3 - C – Br > CH3 – C - CI │ │ │ H H H CH3 CH3 CH3
│ │ │CH3 - C – I > CH3 –C –Br > CH3 –C –CI │ │ │ CH3 CH3
CH3
SN2 (1o) mechanism I - > Br - > CI -
SN2 (1o) mechanism OH - > H2O
CH3 CH3
│ │CH3 - C – Br + OH- = CH3 –C –Br + H2O │ │ CH3 CH3
SN1 (3o) mechanism No effect!
Faster ↑
No effect
SN1 (3o) mechanism I - > Br - > CI -
Nucleophilic Substitution
Factors affecting Rate of Nucleophilic Substitution
Nature of Nucleophile• SN2 mechanism (only)• Nucleophile – electron pair donor • anion more reactive > neutral species due to electron charge density(negative charged)• order of reactivity/better nucleophile• CN - > OH- > NH3 > H2O
H H │ │
CH3 - C – Br + OH- > CH3 –C –Br + H2O │ │ H H
Nature of Halogen• SN1 and SN2 mechanism• polarity of bond decrease ↓• bond strength decreases ↓• halogen leaves easily• rate hydrolysis fastest iodo > bromo > chloro compounds
H H H │ │ │CH3- C –I > CH3 - C – Br > CH3 – C - CI │ │ │ H H H CH3 CH3 CH3
│ │ │CH3 - C – I > CH3 –C –Br > CH3 –C –CI │ │ │ CH3 CH3
CH3
SN2 (1o) mechanism I - > Br - > CI -
SN2 (1o) mechanism OH - > H2O
CH3 CH3
│ │CH3 - C – Br + OH- = CH3 –C –Br + H2O │ │ CH3 CH3
SN1 (3o) mechanism No effect!
Faster ↑
No effect
Nature of Halogenoalkane• SN1 > SN2 mechanism• 3o > 2o > 1o
• 3o – by SN1 - carbocation - faster • 1o - by SN2 – transition state -
slower
SN1 (3o) mechanism I - > Br - > CI -
Nucleophilic Substitution
Factors affecting Rate of Nucleophilic Substitution
Nature of Nucleophile• SN2 mechanism (only)• Nucleophile – electron pair donor • anion more reactive > neutral species due to electron charge density(negative charged)• order of reactivity/better nucleophile• CN - > OH- > NH3 > H2O
H H │ │
CH3 - C – Br + OH- > CH3 –C –Br + H2O │ │ H H
Nature of Halogen• SN1 and SN2 mechanism• polarity of bond decrease ↓• bond strength decreases ↓• halogen leaves easily• rate hydrolysis fastest iodo > bromo > chloro compounds
H H H │ │ │CH3- C –I > CH3 - C – Br > CH3 – C - CI │ │ │ H H H CH3 CH3 CH3
│ │ │CH3 - C – I > CH3 –C –Br > CH3 –C –CI │ │ │ CH3 CH3
CH3
SN2 (1o) mechanism I - > Br - > CI -
SN2 (1o) mechanism OH - > H2O
CH3 CH3
│ │CH3 - C – Br + OH- = CH3 –C –Br + H2O │ │ CH3 CH3
SN1 (3o) mechanism No effect!
Faster ↑
No effect
Nature of Halogenoalkane• SN1 > SN2 mechanism• 3o > 2o > 1o
• 3o – by SN1 - carbocation - faster • 1o - by SN2 – transition state -
slower
SN1 (3o) mechanism I - > Br - > CI -
CH3 CH3 H │ │ │CH3 - C – Br > CH3 –C –Br > CH3 –C –Br │ │ │ CH3 H
H
3o > 2o > 1o
SN1 (3o) > SN2 (1o)
Hydrolysis of 2-Bromo-2-Methylpropane (3o) by SN1 mechanism
CH3 │ CH3 - C – Br │ CH3
(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
Hydrolysis of 2-Bromo-2-Methylpropane (3o) by SN1 mechanism
CH3 │ CH3 - C – Br │ CH3
Heterolytic fission - Carbocation and Br- formation
(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
Hydrolysis of 2-Bromo-2-Methylpropane (3o) by SN1 mechanism
CH3 │ CH3 - C – Br │ CH3
Carbocation formation (Intermediate)
Heterolytic fission - Carbocation and Br- formation
(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
Hydrolysis of 2-Bromo-2-Methylpropane (3o) by SN1 mechanism
CH3 │ CH3 - C – Br │ CH3
Carbocation formation (Intermediate) Nucleophile OH- attacking the carbocation
Heterolytic fission - Carbocation and Br- formation
(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
Formation of 2 methylpropan-2-ol
Tertiary carbocation are more stable due to inductive effect• Three alkyl gp stabilises the carbocation by inductive effect – pushing electrons to carbocation (reducing positive charge) making it more stable
Hydrolysis of 2-Bromo-2-Methylpropane (3o) by SN1 mechanism
CH3 │ CH3 - C – Br │ CH3
Carbocation formation (Intermediate) Nucleophile OH- attacking the carbocation
Heterolytic fission - Carbocation and Br- formation
(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
3o Halogenoalkane by SN1
Nucleophilic Substitution
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Hydrolysis of Bromoethane (1o) by SN2 mechanism
H │ OH- + CH3 – C –Br │ H
H H │ │CH3 - C – Br + OH- CH3 – C –OH + Br - │ │ H H
CH3CH2Br + OH- → CH3CH2OH + Br- Single step
Nucleophilic Substitution
Hydrolysis of Bromoethane (1o) by SN2 mechanism
H │ OH- + CH3 – C –Br │ H
H H │ │CH3 - C – Br + OH- CH3 – C –OH + Br - │ │ H H
Nucleophile colliding with bromoethane
CH3CH2Br + OH- → CH3CH2OH + Br- Single step
Nucleophilic Substitution
Hydrolysis of Bromoethane (1o) by SN2 mechanism
H │ OH- + CH3 – C –Br │ H
Bond Breaking and Making at transition state
H H │ │CH3 - C – Br + OH- CH3 – C –OH + Br - │ │ H H
Nucleophile colliding with bromoethane
CH3CH2Br + OH- → CH3CH2OH + Br- Single step
Nucleophilic Substitution
Hydrolysis of Bromoethane (1o) by SN2 mechanism
H │ OH- + CH3 – C –Br │ H
Bond Breaking and Making at transition state Bromine substituted with OH- group
H H │ │CH3 - C – Br + OH- CH3 – C –OH + Br - │ │ H H
Single step mechanism – Bond breaking and Bond making in transition state• Involve collision between 2 molecules• no bulky alkyl gp, less steric effect – • allows nucleophile to attack the electron deficient carbon from the opposite site
Nucleophile colliding with bromoethane
CH3CH2Br + OH- → CH3CH2OH + Br- Single step
1o Halogenoalkane by SN2
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Nucleophilic Substitution
Reaction of Halogenoalkanes
Reactivity for halogenoalkanes• Carbon bonded to halogen group – F, CI, Br, I• High electronegativity on halogen group• High reactivity – due to polarity of C+- CI -, C+-Br -
• Nucleophile – species with lone pair electron – donate to carbon center
• Reaction for Halogenoalkanes• Substitution reactionTypes of halogenoalkanePrimary 10 – One or NO alkyl gp on C attach to halogen gpSecondary 2o – Two alkyl gp on C attach to halogen gpTertiary 3o – Three alkyl gp on C attach to halogen gp
H │ CH3 - C – Br │ H
H │ H - C – Br │ H
Primary halogenoalkane 10 - SN2
Single Step
Nucleophilic Substitution SN2• Undergo SN2 mechanism, Bimolecular Nucleophilic Substitution
CH3CH2Br + OH- → CH3CH2OH + Br-
• Single step mechanism – Bond breaking and Bond making in transition state• Involve collision between 2 molecules• Rate is dependent on concentration of CH3CH2Br and OH-
• Molecularity = 2• Experimentally rate expression = k [CH3CH2Br][OH-]
Transition state
Bond making and bond breaking
Nucleophile OH attack
Br2 leaving group
Single step
OH- + CH3CH2Br → [ HO---CH2(CH3)---Br ] → CH3CH2OH + Br-
CH3CH2Br + OH- → CH3CH2OH + Br-
SN2
Reaction of Halogenoalkanes
Tertiary halogenoalkane 30 – SN1
Nucleophilic Substitution SN1• Undergo SN1 mechanism, Unimolecular Nucleophilic Substitution(CH3)3CBr + OH- → (CH3)3COH + Br-
• Two steps mechanism 1st step – slow step, rate determining step, formation of carbocation by heterolysis(CH3)3CBr → (CH3)3C
+ + Br-
2nd step – fast step, OH- reacting with carbocation forming product(CH3)3C
+ + OH- → (CH3)3COH • Rate is dependent on concentration of (CH3)3CBr • Molecularity = 1• Experimentally rate expression = k [(CH3)3CBr]
CH3
│ CH3 - C – Br │ CH3
(CH3)3CBr → (CH3)3C+ + Br- 1st step (slow)
(CH3)3C+ + OH- → (CH3)3COH 2nd step (fast)
Reactivity for halogenoalkanes• Carbon bonded to halogen group – F, CI, Br, I• High electronegativity on halogen group• High reactivity – due to polarity of C+- CI -, C+-Br -
• Nucleophile – species with lone pair electron – donate to carbon center
• Reaction for Halogenoalkanes• Substitution reaction
(CH3)3CBr + OH- → (CH3)3COH + Br-
SN1
Types of halogenoalkanePrimary 10 – One or NO alkyl gp on C attach to halogen gpSecondary 2o – Two alkyl gp on C attach to halogen gpTertiary 3o – Three alkyl gp on C attach to halogen gp
Reaction of Halogenoalkanes
Types of halogenoalkanePrimary 10 – One or NO alkyl gp on C attach to halogen gpSecondary 2o –Two alkyl gp on C attach to halogen gpTertiary 3o – Three alkyl gp on C attach to halogen gp
Secondary halogenoalkane 20 -SN1 and SN2
Nucleophilic Substitution SN2• Undergo SN2 mechanism, Bimolecular Nucleophilic SubstitutionCH3CH(CH3)Br + OH- → CH3CH(CH3)OH + Br-
• Single step mechanism – Bond breaking and Bond making in transition state• Involve collision of 2 molecules• Rate is dependent on concentration of CH3CH(CH3)Br and OH-
• Molecularity = 2• Experimentally rate expression = k [CH3CH(CH3)Br][OH-]
CH3 │ CH3 - C – Br │
H
Nucleophilic Substitution SN1• Undergo SN1 mechanism, Unimolecular Nucleophilic SubstitutionCH3CH(CH3)Br + OH- → CH3CH(CH3)OH + Br-
• Two steps mechanism 1st step – slow step, rate determining step, formation of carbocation by heterolysis CH3CH(CH3)Br → CH3CH(CH3)
+ + Br-
2nd step – fast step, OH- reacting with carbocation forming productCH3CH(CH3)
+ + OH- → CH3CH(CH3)OH • Rate is dependent on concentration of CH3CH(CH3)Br• Molecularity = 1• Experimentally rate expression = k [CH3CH(CH3)Br]
Reactivity for halogenoalkanes• Carbon bonded to halogen group – F, CI, Br, I• High electronegativity on halogen group• High reactivity – due to polarity of C+- CI -, C+-Br -
• Nucleophile – species with lone pair electron – donate to carbon center
• Reaction for Halogenoalkanes• Substitution reaction
AND
CH3CH(CH3)Br + OH- -> CH3CH(CH3)OH + Br-
SN1
SN2
Questions on Nucleophilic Substitution
Which Rate of hydrolysis is faster ?
H H │ │
CH3 - C – Br + OH- CH3 –C –OH + Br -
│ │ H H
H H │ │
CH3 - C – Br + H2O CH3 –C –OH + Br -
│ │ H H
CH3 CH3
│ │CH3 - C – Br + OH- CH3 –C –OH + Br-
│ │ CH3 CH3
CH3 CH3
│ │CH3 - C – Br + H2O CH3 –C –OH + Br-
│ │ CH3 CH3
CH3 – CH2 – I + OH- CH3 –CH2 –OH + I -
CH3 – CH2 –Br + OH- CH3 –CH2 –OH + Br -
CH3 CH3
│ │CH3 - C – Br + OH- CH3 –C –OH + Br-
│ │ H H
CH3 – CH2 –I + OH- CH3 –CH2 –OH + I -
CH3 CH3
│ │CH3 - C – Br + OH- CH3 –C –OH + Br-
│ │ CH3 CH3
CH3 –CH2 –Br + OH- CH3 –CH2 –OH + Br -
or
or
or
or
or
A
A
A
A
A
B
B
B
B
B
Questions on Nucleophilic Substitution
Which Rate of hydrolysis is faster ?
H H │ │
CH3 - C – Br + OH- CH3 –C –OH + Br -
│ │ H H
H H │ │
CH3 - C – Br + H2O CH3 –C –OH + Br -
│ │ H H
CH3 CH3
│ │CH3 - C – Br + OH- CH3 –C –OH + Br-
│ │ CH3 CH3
CH3 CH3
│ │CH3 - C – Br + H2O CH3 –C –OH + Br-
│ │ CH3 CH3
CH3 – CH2 – I + OH- CH3 –CH2 –OH + I -
CH3 – CH2 –Br + OH- CH3 –CH2 –OH + Br -
CH3 CH3
│ │CH3 - C – Br + OH- CH3 –C –OH + Br-
│ │ H H
CH3 – CH2 –I + OH- CH3 –CH2 –OH + I -
CH3 CH3
│ │CH3 - C – Br + OH- CH3 –C –OH + Br-
│ │ CH3 CH3
CH3 –CH2 –Br + OH- CH3 –CH2 –OH + Br -
or
or
or
or
or
√
√
√
√
A
A
A
A
A
B
B
B
B
B
Nature of Nucleophile, OH- > H2O
Nature of Halogen , I - better leaving gp > Br -
Nature of Halogen , I - better leaving gp > Br -
Nature of Halogenoalkane, 3o > 1o
Rate the same3o Halogenoalkane NOT affected by nucleophile
Rate the same
H H │ │CH3 CH2- C- Br + OH- CH3 CH2-C –OH + Br - │ │ H H
CH3 CH3 │ │CH3 C-Br + OH-
CH3 C-OH │ │ CH3
CH3
CH3 CH3 │ │ CH3 C- Br + OH- CH3 C- OH │ │ H H
Questions on Nucleophilic Substitution
Primary halogenoalkane 10 - SN2
Tertiary halogenoalkane 30 - SN1
Secondary halogenoalkane 20 - SN2 and SN1
SN1
SN2
H H │ │CH3 CH2- C- Br + OH- CH3 CH2-C –OH + Br - │ │ H H
CH3 CH3 │ │CH3 C-Br + OH-
CH3 C-OH │ │ CH3
CH3
CH3 CH3 │ │ CH3 C- Br + OH- CH3 C- OH │ │ H H
Questions on Nucleophilic Substitution
Primary halogenoalkane 10 - SN2
Tertiary halogenoalkane 30 - SN1
Secondary halogenoalkane 20 - SN2 and SN1
Single step mechanism - Bond breaking + Bond making in transition state
Two step mechanism – Formation of carbocation
OH
OH
OH
OH
Single step mechanism - Bond breaking + Bond making in transition state
Two step mechanism - Formation of carbocation
OHOH
OHOH
SN1
SN2
SN1
SN2
SN2
SN1
Acknowledgements
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Thanks to Creative Commons for excellent contribution on licenseshttp://creativecommons.org/licenses/
Prepared by Lawrence Kok
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