ib chemistry on nucleophilic substitution, sn1, sn2 and protic solvent

http://lawrencekok.blogspot.com

Prepared by Lawrence Kok

Tutorial on Nucleophilic Substitution, SN1 and SN2 reactions

http://lawrencekok.blogspot.com/

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


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


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

https://www.youtube.com/watch?v=bJMUKNbAsTY

https://www.youtube.com/watch?v=bJMUKNbAsTY

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


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

https://app.box.com/s/rt2288tslan0dspia59xs7pe1ye9pjlh

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


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



halogen gp• High reactivity – due to polarity

of C+- CI -C - Brᵟ+ ᵟ-

electron


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


E


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


H


C - Br OH ..ᵟ-ᵟ+

NucleophileElectrophile

ᵟ+C = Cᵟ-


Organic Rxn

Addition rxn Substitution rxn

Nucleophilic Substitution

Free Radical Substitution

Electrophilic Substitution


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




H ᵟ+C = Cᵟ-


Free radicle

CI CICI CI. .

:


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



halogen • High reactivity – polarity of C+-

Br -

Nucleophile– Lone pair electron – Donate electron pair - (Lewis base)


C - Brᵟ+ ᵟ-

electron


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


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

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

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

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


Click here view SN2

SN2Substitution

Nucleophilic


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


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

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 -


C - Brᵟ+ ᵟ-

electron


O–H ....

ᵟ-C

ᵟ+

+ OH- ᵟ+ ᵟ-


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

https://www.youtube.com/watch?v=JmcVgE2WKBE


https://www.youtube.com/watch?v=JmcVgE2WKBE

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


Click here to view

- 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


2 step mechanism

3ₒ SN1

http://www.youtube.com/watch?v=JmcVgE2WKBE&feature=plcp

H Br H ׀ ׀ ׀ H - C – C – C – H ׀ ׀ ׀ ׀׀ H H H

+ :OH- ᵟ+


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


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)


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

https://www.youtube.com/watch?v=9-j1HTGDo00&index=50&list=WL

https://www.youtube.com/watch?v=cyZUgH4yoCo


https://www.youtube.com/watch?v=cyZUgH4yoCo




H ᵟ+C = Cᵟ-


Free radicle

CI CICI CI. .

:


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


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


• 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


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

https://www.youtube.com/watch?v=ZRMSHG18aMY&index=51&list=WL

https://www.youtube.com/watch?v=StzknRJPY_E

https://app.box.com/s/c27ofmuyxmp9ikwid4zx9echwtvaohcf

https://www.youtube.com/watch?v=StzknRJPY_E

https://www.youtube.com/watch?v=ZRMSHG18aMY&index=51&list=WL

https://app.box.com/s/c27ofmuyxmp9ikwid4zx9echwtvaohcf

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

http://creativecommons.org/licenses/

http://lawrencekok.blogspot.com/

ib chemistry on nucleophilic substitution, sn1, sn2 and protic solvent

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