IB Chemistry on Nucleophilic Substitution, SN1, SN2 and factors affecting rate of hydrolysis of...
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Transcript of IB Chemistry on Nucleophilic Substitution, SN1, SN2 and factors affecting rate of hydrolysis of...
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)
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
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 reaction
Types of halogenoalkane Primary 10 – One or NO alkyl gp on C attach to halogen gp
Secondary 2o – Two alkyl gp on C attach to halogen gp
Tertiary 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
CH3 │ CH3 - C – Br │ CH3
CH3 │ CH3 - C – Br │
H
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 reaction
Types of halogenoalkane Primary 10 – One or NO alkyl gp on C attach to halogen gp
Secondary 2o – Two alkyl gp on C attach to halogen gp
Tertiary 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 halogenoalkane Primary 10 – One or NO alkyl gp on C attach to halogen gp
Secondary 2o – Two alkyl gp on C attach to halogen gp
Tertiary 3o – Three alkyl gp on C attach to halogen gp
Reaction of Halogenoalkanes
Types of halogenoalkane Primary 10 – One or NO alkyl gp on C attach to halogen gp
Secondary 2o –Two alkyl gp on C attach to halogen gp
Tertiary 3o – Three alkyl gp on C attach to halogen gp
Secondary halogenoalkane 20 -SN1 and SN2
Nucleophilic Substitution SN2 • Undergo SN2 mechanism, Bimolecular Nucleophilic Substitution
CH3CH(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 Substitution
CH3CH(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 product
CH3CH(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
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 same 3o 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
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
OH OH
OH OH
SN1
SN2
SN1
SN2
SN2
SN1