ib chemistry on free radical substitution, nucleophilic substitution and addition reaction
DESCRIPTION
IB Chemistry on Free Radical Substitution, Nucleophilic Substitution, Addition Reaction and Oxidation of AlcoholTRANSCRIPT
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