Download - 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