ib chemistry on nucleophilic substitution and elimination reaction
DESCRIPTION
IB Chemistry on Nucleophilic Substitution and Elimination Reaction.TRANSCRIPT
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 SN2 OH-
OH-
H H │ │ CH3 CH2- C- Br + CN- CH3 CH2-C –CN + Br - │ │
H H
SN2 SN2 CN-
CN
H H │ │ CH3 CH2- C- Br + NH3 CH3 CH2-C –NH2 + Br - │ │
H H
SN2 SN2 NH3 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 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
HO:- + CH3CH2Br [HO---CH2(CH3)---Br] - CH3CH2OH + Br-
CH3CH2Br + OH- → CH3CH2OH + Br-
SN2
Nucleophilic Substitution with Hydroxide OH -
Mechanism for Nucleophilic Substitution
HO: -
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 through transition state
1o Halogenoalkane by SN2
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CH3CH2Br + OH- → CH3CH2OH + Br-
Nucleophilic Substitution with Hydroxide OH -
Single Step Nucleophilic Substitution SN2 • Undergo SN2 mechanism, Bimolecular Nucleophilic Substitution
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
Bond making and bond breaking
Nucleophile CN- attack
Br2 leaving group
Single step
CN- + CH3CH2Br → [NC---CH2(CH3)---Br]- → CH3CH2CN + Br-
CH3CH2Br + CN- → CH3CH2CN + Br-
SN2
Nucleophilic Substitution with Cyanide ion CN-
Mechanism for Nucleophilic Substitution
NC: - NC NC
Use to increase the length of carbon • propanenitrile reduced using hydrogen with nickel catalyst to amine • propanenitrile hydrolysed using acid to carboxylic acid
CH3CH2CH2NH2
CH3CH2COOH
CH3CH2Br + CN- → Br- + CH3CH2CN
Propanenitrile
H
│
:CN- + CH3 – C –Br
│
H
Bond Breaking and Making at transition state Bromine substituted with CN group
H H
│ │ CH3 - C – Br + :CN- CH3 – C –CN + 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 + :CN- → CH3CH2CN + Br- Single step through transition state
1o Halogenoalkane by SN2
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CH3CH2Br + CN- → CH3CH2CN + Br-
Nucleophilic Substitution with Cyanide ion CN-
Single Step Nucleophilic Substitution SN2 • Undergo SN2 mechanism, Bimolecular Nucleophilic Substitution
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
Bond making and bond breaking
Nucleophile NH3 attack
Br2 leaving group
Single step
:NH3 + CH3CH2Br → [H3N---CH2(CH3)---Br] → CH3CH2NH2 + H+ + Br-
CH3CH2Br + NH3 → CH3CH2NH2 + Br-
SN2
Nucleophilic Substitution with Ammonia NH3
Mechanism for Nucleophilic Substitution
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
Bond Breaking and Making at transition state Bromine substituted with NH3 group
H H
│ │ CH3 - C – Br + :NH3 CH3 – C –NH2 + 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 + :NH3 → CH3CH2NH2 + Br- Single step through transition state
1o Halogenoalkane by SN2
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CH3CH2Br + NH3 → CH3CH2NH2 + Br-
Nucleophilic Substitution with Ammonia NH3
H H │ │ H - C - C – H
│ │
H Br
Substitution vs Elimination Reaction
H H
│ │ H - C - C – H
│ │ H OH
H H │ │ C = C │ │ H H
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
Hydroxide OH - – acts as a nucleophile • electron pair donor to electron deficient carbon
CH3-CH2-Br + C2H5O- → CH2=CH2 + Br- CH3-CH2-Br + C2H5O
- → CH2=CH2 + Br-
Substitution Reaction Elimination Reaction
Inverted, Bond breaking and forming together Carbocation formation
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
Substitution vs Elimination Reaction
Dilute aqueous
OH - at 60C
Conc alcoholic
OH - at 100C
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
Dilute aqueous
OH - at 60C
Conc alcoholic
OH - at 100C
Different products
Dilute aqueous
OH - at 60C Conc alcoholic
OH - at 100C
Same products
│ │ C- C - CI │ │
│ │ C - C -OH │ │
│ │ C- C – NH2 │ │
│ │ C- C – CN │ │
│ │ │ C- C – C-NH2
│ │ │
│ │ C- C – COOH │ │
Reaction Pathways
Substitution Reaction Elimination Reaction
Nucleophilic substitution
SN2 / Warm OH-
Elimination
High Temp 100C /Conc alcoholic OH- │ │ C = C │ │
Amine
Nitrile
Alcohol
Carboxylic Acid Amine
│ │ C- C │ │ H H
│ │ C - C │ │ H CI
│ │ C - C │ │ Br Br
│ │ C - C │ │ CI CI
Ad
dit
ion
Br 2
Addition Reaction
Alkene Halogenoalkane
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 steps 1st Nucleophilic Substitution with CN 2nd Reduction of CN with H2
Three steps 1st Addition with HCI 2nd Nucleophilic Substitution with CN 3rd Reduction of CN with H2
Convert ethene to ethanamine
C = C → C – C-NH2
Two steps 1st Addition with HCI 2nd Nucleophilic Substitution with NH3
Reaction Pathways
C – C → C – C │ │ │ │ H OH CI CI
Convert ethanol to 1,2 dichloroethane
Two steps 1st Elimination of water 2nd Addition of CI2