IB Chemistry on Free Radical Substitution, Nucleophilic Substitution and Addition Reaction

Reaction of Alkanes

Reactivity for Alkanes

• Low reactivity - Strong stable bond between C - C, C - H

• Low reactivity - Low polarity of C - H bonds • Saturated hydrocarbons – Non polar bonds

Reactions for Alkanes • Combustion reaction • Free Radical Substitution reaction

Complete combustion – produces CO2 + H2O •C2H6 + 7/2O2 → 2CO2 + 3H2O •Incomplete combustion – produces C, CO, CO2, H2O • 2C3H8 + 7O2 → 2C + 2CO + 8H2O + 2CO2

Free radical Substitution •CH3CH3 + CI2 → CH3CH2CI + HCI

Initiation

• Homolytic fission- bond breaking by radical formation. Covalent bond split and each atom obtain an electron

(unpaired electrons)

• UV radiation needed

CI – CI → CI● + CI●

Combustion reaction Substitution reaction

Free Radical Substitution Mechanism

• Initiation, Propagation and Termination

CH4 + CI2 → CH3CI + HCI

Propagation

• Radical reacting with molecule

CI● + H-CH3 → CI-H + CH3●

CH3● + CI - CI → CH3CI + CI●

Termination

• Radical reacting with radical forming molecule

CI● + CI● → CI-CI CI● + CH3● → CH3CI

CH3● + CH3● → CH3-CH3

Reactions for Alkanes

UV

CH4 + CI2 → CH3CI + HCI

If excess CH4 used - CI● radical form react with CH4

- Chloromethane formed

Free radical Substitution Reaction Mechanism

Initiation, Propagation and Termination Reaction Mechanism

UV

If limited CH4 used – CI● radical react with product chloromethane

- Dichloromethane formed

CH4 + CI● -> CH3CI CH3CI + CI● -> CH2CI2

Addition reaction CH2=CH2 + Br2 → CH2Br–CH2Br CH2=CH2 + CI2 → CH2CI–CH2CI CH2=CH2 + HCI → CH3–CH2CI CH2=CH2 + H2O → CH3–CH2OH catalyst nickel, H3PO4 at 300C

H H │ │ C = C │ │

H H

H H

│ │

H- C - C - H

│ │

Br Br

H H

│ │

H- C - C -H

│ │

CI CI

H H

│ │

H - C - C -H

│ │

H CI

H H │ │ H - C - C - H

│ │

H OH

Reaction of Alkenes

Reactivity for Alkenes

• High reactivity - Unstable bond between C = C • High reactivity – Weak pi bond overlap between p orbitals • Unsaturated hydrocarbons – pi bond, weak p orbital overlap Reactions for Alkenes • Combustion reaction • Addition reaction

Complete combustion – produces CO2 + H2O C2H4 + 3O2 → 2CO2 + 2H2O •Incomplete combustion – produces C, CO, CO2, H2O 2C2H4 + 7/2O2 → 2C + CO + 4H2O + CO2

Combustion reaction Addition reaction

Addition CI2 Addition Br2

Addition HCI Addition H2O catalyst nickel, H3PO4 at 300C

H │ CH3 - C – OH │ H

Types of alcohol Primary alcohol 1 0 – One alkyl gp on C attached to OH group

Secondary alcohol 2 o – Two alkyl gp on C attached to OH group

Tertiary alcohol 3 o – Three alkyl gp on C attached to OH group

CH3 │ CH3 - C – OH │

H

CH3 │ CH3 - C – OH │ CH3

Primary alcohol 10

Secondary alcohol 20

Tertiary alcohol 30

Reactions of Alcohols • Functional group hydroxyl (OH) Production of ethanol by

• Yeast sugar fermentation C6H12O6 → 2C2H5OH + 2CO2

• Hydration of ethene with steam C2H4 + H2O → C2H5OH Reaction for alcohol • Combustion reaction • Oxidation reaction

Reaction of Alcohol

•Complete combustion excess oxygen – produces CO2 + H2O C2H6OH + 3O2 → 2CO2 + 3H2O •Incomplete combustion – produces C, CO, CO2, + H2O 2C2H5OH + 4O2 → C + 2CO + 6H2O + CO2

Combustion reaction

Primary alcohol (1o) – Oxidised to aldehyde to carboxylic acid Secondary alcohol (2o) – Oxidised to ketone Tertiary alcohol (3o) – Cannot be oxidised

Oxidation reaction

H H │ │ CH3-C-O-H + [O] CH3- C=O + H2O │

H K2Cr2O7/H

+

MnO4- /H

+

Reaction of Alcohol

CH3 CH3

│ │ CH3-C –O H + [O] CH3- C= O + H2O │

H

H OH │ │ CH3- C= O + [O] CH3-C=O

MnO4- /H

+

K2Cr2O7/H+

CH3

│ CH3-C – OH + [O] │

CH3

X

Oxidation of alcohol

Primary alcohol (1o) – Oxidised to aldehyde to carboxylic acid Secondary alcohol (2o) – Oxidised to ketone Tertiary alcohol (3o) – Cannot be oxidised

Types of alcohol

Primary alcohol 10 – Two Oxidisable Hydrogen

Secondary alcohol 2o – One Oxidisable Hydrogen

Tertiary alcohol 3o – No Oxidisable Hydrogen

Primary alcohol 10 - Oxidised to Aldehyde

K2Cr2O7/H+

MnO4- /H

+

Secondary alcohol 20 - Oxidised to Ketone

Tertiary alcohol 30 - Cannot be Oxidise

Aldehyde oxidised to Ethanoic acid

Oxidising agent permanganate(VII) / dichromate(VI) • Ethanol oxidised to Aldehyde • MnO4

- reduces from purple (Mn7+) to pink (Mn2+) • Cr2O7

2- reduces from orange (Cr6+) to green (Cr3+)

Reaction of Alcohol

Primary alcohol (1o) – Oxidised to aldehyde to carboxylic acid Secondary alcohol (2o) – Oxidised to ketone Tertiary alcohol (3o) – Cannot be oxidised

Oxidation of alcohol

Ethanol to Ethanal (Distillation) 1. Ethanol + acidified dichromate(VI)/permanganate(VII) and warm it 2. Collect ethanal/distillate (Distillation)

Oxidation of alcohol using oxidising agent

Ethanol to Ethanoic acid (Reflux) 1. Ethanol + acidified dichromate(VI)/permanganate(VII) and warm it 2. Collect ethanoic acid/distillate using reflux

Distillation

Reflux

CH3CH2OH + MnO4-/Cr2O7

2- → CH3CHO + Mn2+/Cr3+

Oxidising agent permanganate(VII) / dichromate(VI) • Ethanol oxidised to Etanoic acid • MnO4

- reduces from purple (Mn7+) to pink (Mn2+) • Cr2O7

2- reduces from orange (Cr6+) to green (Cr3+)

distillation

reflux

CH3CH2OH + MnO4-/Cr2O7

2- → CH3COOH + Mn2+/Cr3+

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

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

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

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

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

H H

CH3 CH3 │ │ CH3 C- Br + OH- CH3 C- OH + Br- │ │

CH3 CH3

CH3 CH3 │ │ CH3 C- Br + OH- CH3 C- OH + Br- │ │

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

H H O │ │ ║ CH3 CH- C-O-H + [O] CH3 CH-C=O CH3CH – C OH │ │ │ │

CH3 H CH3 CH3

Oxidation of 2-Methylpropan-1-ol to 2-Methylpropanal to 2-Methylpropanoic acid

Oxidation of 2-Methylpropan-2-ol

CH3 CH-CH2CH3 + [O] CH3 CH CH2CH3 │ ║

OH O

Questions on Oxidation Reaction

CH3

│ CH3-C – OH + [O] │

CH3

MnO4- /H

+

K2Cr2O7/H+

K2Cr2O7/H+

MnO4- /H

+

Oxidation of Butan-2-ol to Butanone

K2Cr2O7/H+

MnO4- /H

+

Primary alcohol 10

Tertiary alcohol 30

Secondary alcohol 20

X

Aldehyde

Ketone

Carboxylic acid

X