ib chemistry on nucleophilic substitution and elimination reaction

http://lawrencekok.blogspot.com

Prepared by Lawrence Kok

Tutorial on Nucleophilic Substitution and Elimination Reaction.

http://lawrencekok.blogspot.com/


Nucleophilic Substitution reaction with nucleophile• Hydroxide ion (OH-)• Cyanide ion (CN -)• Ammonia (NH3)

Nucleophilic Substitution

H H │ │CH3 CH2- C- Br + OH- CH3 CH2-C –OH + Br - │ │ H H

SN2

H H │ │CH3 CH2- C- Br + CN- CH3 CH2-C –CN + Br - │ │ H H

SN2

H H │ │CH3 CH2- C- Br + NH3 CH3 CH2-C –NH2 + Br - │ │ H H

SN2

Nucleophilic Substitution reaction with nucleophile• Hydroxide ion (OH-)• Cyanide ion (CN -)• Ammonia (NH3)

Nucleophilic Substitution

H H │ │CH3 CH2- C- Br + OH- CH3 CH2-C –OH + Br - │ │ H H

SN2 SN2OH- OH-

H H │ │CH3 CH2- C- Br + CN- CH3 CH2-C –CN + Br - │ │ H H

SN2 SN2CN- CN

H H │ │CH3 CH2- C- Br + NH3 CH3 CH2-C –NH2 + Br - │ │ H H

SN2 SN2NH3 NH2

Nucleophilic Substitution with nucleophile• Nucleophile (electron pair donor)• Attack the electron deficient carbon • Bond breaking and Bond Making in

transition state• Single step mechanism• SN2 mechanism

Single StepNucleophilic 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

HO:- + CH3CH2Br → [HO---CH2(CH3)---Br] - → CH3CH2OH + Br-


SN2

Nucleophilic Substitution with Hydroxide OH -

Mechanism for Nucleophilic Substitution

HO: -

H │ OH- + CH3 – C –Br │ H

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

CH3CH2Br + OH- → CH3CH2OH + Br- Single step through transition state

Nucleophilic Substitution by OH - SN2



H │ OH- + CH3 – C –Br │ H



Nucleophile colliding with bromoethane





H │ OH- + CH3 – C –Br │ H

Bond Breaking and Making at transition state








H │ OH- + CH3 – C –Br │ H

Bond Breaking and Making at transition state Bromine substituted with OH- group






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http://www.youtube.com/watch?v=h5xvaP6bIZI&feature=related


CH3CH2Br + CN- → CH3CH2CN + 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 CN-

• Molecularity = 2• Experimentally rate expression = k [CH3CH2Br][CN-]

Transition state


Nucleophile CN- attack

Br2 leaving group

Single step

CN- + CH3CH2Br → [NC---CH2(CH3)---Br]- → CH3CH2CN + Br-


SN2

Nucleophilic Substitution with Cyanide ion CN-


NC: - NCNC



• Single step mechanism – Bond breaking and Bond making in transition state• Involve collision between 2 molecules• Rate is dependent on concentration of CH3CH2Br and CN-

• Molecularity = 2• Experimentally rate expression = k [CH3CH2Br][CN-]

Transition state


Nucleophile CN- attack

Br2 leaving group

Single step

CN- + CH3CH2Br → [NC---CH2(CH3)---Br]- → CH3CH2CN + Br-


SN2



NC: - NCNC

Use to increase the length of carbon• propanenitrile reduced using hydrogen with nickel catalyst to amine• propanenitrile hydrolysed using acid to carboxylic acid

Reduction

H2/ nickel CH3CH2CH2NH2

CH3CH2COOH

CH3CH2Br + CN- → Br- + CH3CH2CN Propanenitrile Acid Hydrolysis

H │ :CN- + CH3 – C –Br │ H

H H │ │CH3 - C – Br + :CN- CH3 – C –CN + Br - │ │ H H


CH3CH2Br + :CN- → CH3CH2CN + Br- Single step through transition state

Nucleophilic Substitution by CN- SN2



H │ :CN- + CH3 – C –Br │ H








H │ :CN- + CH3 – C –Br │ H

Bond Breaking and Making at transition state Bromine substituted with CN group





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CH3CH2Br + NH3 → CH3CH2NH2 + 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 NH3

• Molecularity = 2• Experimentally rate expression = k [CH3CH2Br][NH3]

Transition state


Nucleophile NH3 attack

Br2 leaving group

Single step

:NH3 + CH3CH2Br → [H3N---CH2(CH3)---Br] → CH3CH2NH2 + H+ + Br-


SN2

Nucleophilic Substitution with Ammonia NH3


NH3 :NH3 NH2



• Single step mechanism – Bond breaking and Bond making in transition state• Involve collision between 2 molecules• Rate is dependent on concentration of CH3CH2Br and NH3

• Molecularity = 2• Experimentally rate expression = k [CH3CH2Br][NH3]

Transition state


Nucleophile NH3 attack

Br2 leaving group

Single step

:NH3 + CH3CH2Br → [H3N---CH2(CH3)---Br] → CH3CH2NH2 + H+ + Br-


SN2



NH3 :NH3 NH2

Nucleophile Product becomes nucleophile Product - Tertiary amine formed

Nucleophilic Substitution forming Primary, Secondary and Tertiary Amine

H │ :NH3 + CH3 – C –Br │ H

H H │ │CH3 - C – Br + :NH3 CH3 – C –NH2 + Br - │ │ H H


CH3CH2Br + :NH3 → CH3CH2NH2 + Br- Single step through transition state



Nucleophilic Substitution by NH3 SN2

H │ :NH3 + CH3 – C –Br │ H








H │ :NH3 + CH3 – C –Br │ H

Bond Breaking and Making at transition state Bromine substituted with NH3 group





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H H │ │ H - C - C – H │ │ H Br

Substitution vs Elimination Reaction

H H │ │ H - C - C – H │ │ H OH

Dil aqueous OH - a

t 60C

Substitution Reaction with OH- Nucleophile

Hydroxide OH - – acts as a nucleophile• electron pair donor to electron deficient carbon

Substitution Reaction

H H │ │ H - C - C – H │ │ H Br


H H │ │ H - C - C – H │ │ H OH

H H │ │ C = C │ │H H

Dil aqueous OH - a

t 60C Conc alcoholic OH - at 100C

Substitution Reaction with OH- Nucleophile Elimination Reaction with Conc alcoholic OH- at 100C

OH - + C2H5OH ↔ H2O + C2H5O-

C2H5O- (ethoxide ion) is a stronger base due to

• Inductive effect of alkyl group pushing electron to O• Acts as a base not nucleophile by removing H+/proton acceptor


Substitution Reaction Elimination Reaction

H H │ │ H - C - C – H │ │ H Br


H H │ │ H - C - C – H │ │ H OH

H H │ │ C = C │ │H H

Dil aqueous OH - a

t 60C Conc alcoholic OH - at 100C

Substitution Reaction with OH- Nucleophile Elimination Reaction with Conc alcoholic OH- at 100C

OH - + C2H5OH ↔ H2O + C2H5O-

C2H5O- (ethoxide ion) is a stronger base due to

• Inductive effect of alkyl group pushing electron to O• Acts as a base not nucleophile by removing H+/proton acceptor

Elimination Mechanism


CH3-CH2-Br + C2H5O- → CH2=CH2 + Br- CH3-CH2-Br + C2H5O

- → CH2=CH2 + Br-


Inverted, Bond breaking and forming together Carbocation formation

E1, 1 stepE2, 2 steps

H H H │ │ │ H - C - C - C – H │ │ │ H H Br

1- bromopropane

H H H H │ │ │ │ H - C - C - C – C -H │ │ │ │ H H Br H

H H H H H │ │ │ │ │ H - C - C - C – C –C -H │ │ │ │ │ H H Br H H


Dilute aqueous OH - at 60C

Conc alcoholic OH - at 100C

2-bromobutane

3-bromopentane

Substitutio

n Dilute aqueous OH - at 60C




Elimination Substitutio

n

Substitutio

nElimination

Elimination

H H H │ │ │ H - C - C - C – H │ │ │ H H Br

1- bromopropane

H H H │ │ │ H - C - C - C – H │ │ │ H H OH

H H H │ │ │ H - C - C = C – H │ H

H H H H │ │ │ │ H - C - C - C – C -H │ │ │ │ H H Br H

H H H H │ │ │ │ H - C - C - C – C -H │ │ │ │ H H OH H

H H H H │ │ │ │ H - C - C - C = C -H │ │ H H

H H H H │ │ │ │ H - C - C = C - C -H │ │ H H

H H H H H │ │ │ │ │ H - C - C - C – C –C -H │ │ │ │ │ H H Br H H

H H H H H │ │ │ │ │ H - C - C - C – C –C -H │ │ │ │ │ H H OH H H

H H H H H │ │ │ │ │ H - C - C = C – C –C -H │ │ │ H H H

H H H H H │ │ │ │ │ H - C - C - C = C –C -H │ │ │ H H H




propan-1-ol propene

2-bromobutane

butan-1-ol But-1-ene

3-bromopentane

But-2-ene

pentan-3-ol pent-2-ene pent-2-ene

Substitutio

n Dilute aqueous OH - at 60C


Different products



Same products

Elimination Substitutio

n

Substitutio

nElimination

Elimination

│ │C- C - CI │ │

│ │C - C -OH │ │

│ │C- C – NH2 │ │

│ │C- C – CN │ │

│ │ │C- C – C-NH2

│ │ │

│ │C- C – COOH │ │

Reaction Pathways


Nucleophilic substitution

SN2/ NH 3

SN2/ C

N-

SN2 / Warm OH-

EliminationHigh Temp 100C /Conc alcoholic OH-

│ │C = C │ │

Amine

Nitrile

Alcohol

Aci

d H

ydro

lysi

s

Hydrogenation/Reduction

H2 /Nickel catalyst

Carboxylic Acid Amine

│ │C- C │ │H H

│ │C - C │ │H CI

│ │C - C │ │Br Br

│ │C - C │ │CI CI

Addition H 2

Add

ition

HC

I

Ad

dit

ion

Br 2 Addition CI

2

Addition Reaction

AlkeneHalogenoalkane

C –C–C-Br → C – C – C – C-NH2 C = C → C – C – C-NH2

Convert 1 bromopropane to butan-1-amine Convert ethene to propan 1-amine

Convert ethene to ethanamine

C = C → C – C-NH2

Reaction Pathways

C – C → C – C │ │ │ │H OH CI CI

Convert ethanol to 1,2 dichloroethane

C –C–C-Br → C – C – C – C-NH2 C = C → C – C – C-NH2

Convert 1 bromopropane to butan-1-amine Convert ethene to propan 1-amine

Two steps1st Nucleophilic Substitution with CN2nd Reduction of CN with H2

Three steps1st Addition with HCI2nd Nucleophilic Substitution with CN3rd Reduction of CN with H2

Convert ethene to ethanamine

C = C → C – C-NH2

Two steps1st Addition with HCI2nd Nucleophilic Substitution with NH3

Reaction Pathways

C – C → C – C │ │ │ │H OH CI CI

Convert ethanol to 1,2 dichloroethane

Two steps1st Elimination of water2nd Addition of CI2

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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