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

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IB Chemistry on Free Radical Substitution, Nucleophilic Substitution, Addition Reaction and Oxidation of Alcohol

IB Chemistry on Free Radical Substitution, Nucleophilic Substitution, Addition Reaction and Oxidation of Alcohol

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  • 1. Reaction of AlkanesReactivity for Alkanes• Low reactivity - Strong stable bond between C - C, C - H• Low reactivity - Low polarity of C - H bonds• Saturated hydrocarbons – Non polar bondsReactions for Alkanes• Combustion reaction• Free Radical Substitution reaction Reactions for Alkanes Combustion reaction Substitution reaction Complete combustion – produces CO2 + H2O Free radical Substitution •C2H6 + 7/2O2 → 2CO2 + 3H2O •CH3CH3 + CI2 → CH3CH2CI + HCI •Incomplete combustion – produces C, CO, CO2, H2O • 2C3H8 + 7O2 → 2C + 2CO + 8H2O + 2CO2 Free Radical Substitution Mechanism • Initiation, Propagation and Termination Initiation CH4 + CI2 → CH3CI + HCI • Homolytic fission- bond breaking by radical formation. Covalent bond split and each atom obtain an electron (unpaired electrons) • UV radiation needed UV CI – CI → CI● + CI● 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
  • 2. Free radical Substitution Reaction Mechanism Initiation, Propagation and Termination Reaction Mechanism UV CH4 + CI2 → CH3CI + HCIIf excess CH4 used - CI● radical form react with CH4 If limited CH4 used – CI● radical react with product chloromethane - Chloromethane formed - Dichloromethane formed CH3CI + CI● -> CH2CI2 CH4 + CI● -> CH3CI
  • 3. 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 Combustion reaction Addition reactionComplete combustion – produces CO2 + H2O Addition reactionC2H4 + 3O2 → 2CO2 + 2H2O CH2=CH2 + Br2 → CH2Br–CH2Br•Incomplete combustion – produces C, CO, CO2, H2O CH2=CH2 + CI2 → CH2CI–CH2CI2C2H4 + 7/2O2 → 2C + CO + 4H2O + CO2 CH2=CH2 + HCI → CH3–CH2CI CH2=CH2 + H2O → CH3–CH2OH catalyst nickel, H3PO4 at 300C H H H H │ │ │ │ H- C - C -H H- C - C-H │ │ │ │ CI CI Br Br Addition CI2 Addition Br2 H H │ │ C = C │ │ H H Addition HCI Addition H2O catalyst nickel, H3PO4 at 300C H H H H │ │ │ │ H - C - C -H H- C - C-H │ │ │ │ H CI H OH
  • 4. Reaction of Alcohol 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 Types of alcohol H Primary alcohol 1 0 – One alkyl gp on C attached to OH group │ CH3 - C – OH Primary alcohol 10 │ H Secondary alcohol 2 o – Two alkyl gp on C attached to OH group CH3 │ CH3 - C – OH Secondary alcohol 20 │ H Tertiary alcohol 3 o – Three alkyl gp on C attached to OH group CH3 │ CH3 - C – OH Tertiary alcohol 30 │ CH3 Combustion reaction Oxidation reaction•Complete combustion excess oxygen – produces CO2 + H2O Primary alcohol (1o) – Oxidised to aldehyde to carboxylic acidC2H6OH + 3O2 → 2CO2 + 3H2O Secondary alcohol (2o) – Oxidised to ketone•Incomplete combustion – produces C, CO, CO2, + H2O Tertiary alcohol (3o) – Cannot be oxidised2C2H5OH + 4O2 → C + 2CO + 6H2O + CO2
  • 5. Reaction of Alcohol 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 2 – One Oxidisable Hydrogen o Tertiary alcohol 3o – No Oxidisable Hydrogen Primary alcohol 10 - Oxidised to Aldehyde H H │ MnO4- /H+ │ CH3-C-O-H + [O] CH3- C=O + H2O │ K2Cr2O7/H+ H Aldehyde oxidised to Ethanoic acid H OH │ │ MnO4- /H+ CH3- C= O + [O] CH3-C=O K2Cr2O7/H+Secondary alcohol 20 - Oxidised to Ketone CH3 CH3 │ │ MnO4- /H+ CH3-C –O H + [O] CH3- C= O + H2O │ K2Cr2O7/H+ HTertiary alcohol 30 - Cannot be Oxidise CH3 │ CH3-C – OH + [O] │ X CH3
  • 6. Reaction of Alcohol Distillation Oxidation of alcohol Primary alcohol (1o) – Oxidised to aldehyde to carboxylic acid Secondary alcohol (2o) – Oxidised to ketone Tertiary alcohol (3o) – Cannot be oxidisedOxidation of alcohol using oxidising agent Ethanol to Ethanal (Distillation) 1. Ethanol + acidified dichromate(VI)/permanganate(VII) and warm it 2. Collect ethanal/distillate (Distillation) CH3CH2OH + MnO4-/Cr2O72- → CH3CHO + Mn2+/Cr3+ Oxidising agent permanganate(VII) / dichromate(VI) • Ethanol oxidised to Aldehyde distillation • MnO4- reduces from purple (Mn7+) to pink (Mn2+) • Cr2O72- reduces from orange (Cr6+) to green (Cr3+) Reflux Ethanol to Ethanoic acid (Reflux) 1. Ethanol + acidified dichromate(VI)/permanganate(VII) and warm it 2. Collect ethanoic acid/distillate using reflux CH3CH2OH + MnO4-/Cr2O72- → CH3COOH + Mn2+/Cr3+ Oxidising agent permanganate(VII) / dichromate(VI) • Ethanol oxidised to Etanoic acid • MnO4- reduces from purple (Mn7+) to pink (Mn2+) reflux • Cr2O72- reduces from orange (Cr6+) to green (Cr3+)
  • 7. Reaction of Halogenoalkanes Reactivity for halogenoalkanes • Carbon bonded to halogen group – F, CI, Br, I • High electronegativity on halogen group H H • High reactivity – due to polarity of C+- CI -, C+- Br - │ │ CH3 - C – Br H - C – Br • Nucleophile – species with lone pair electron – donate to carbon center │ │ •Reaction for Halogenoalkanes H H • 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 Primary halogenoalkane 10 - SN2 CH3CH2Br + OH- → CH3CH2OH + Br -Nucleophilic Substitution SN2• Undergo SN2 mechanism, Bimolecular Nucleophilic Substitution SN2CH3CH2Br + OH- → CH3CH2OH + Br-• Single step mechanism – Bond breaking and Bond making in transition state• Involve collision between 2 molecules Single Step• Rate is dependent on concentration of CH3CH2Br and OH-• Molecularity = 2• Experimentally rate expression = k [CH3CH2Br][OH-] Bond making and bond breaking Br2 leaving group Transition state Single step Nucleophile OH attack OH- + CH3CH2Br → [ HO---CH2(CH3)---Br ] → CH3CH2OH + Br-
  • 8. 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 CH3 Types of halogenoalkane │ Primary 10 – One or NO alkyl gp on C attach to halogen gp CH3 - C – Br Secondary 2o –Two alkyl gp on C attach to halogen gp │ CH3 Tertiary 3o – Three alkyl gp on C attach to halogen gp Tertiary halogenoalkane 30 – SN1 (CH3)3CBr + OH- → (CH3)3COH + Br - 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 SN1 (CH3)3C+ + OH- → (CH3)3COH • Rate is dependent on concentration of (CH3)3CBr • Molecularity = 1 • Experimentally rate expression = k [(CH3)3CBr](CH3)3CBr → (CH3)3C+ + Br- 1st step (slow)(CH3)3C+ + OH- → (CH3)3COH 2nd step (fast)
  • 9. 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 CH3 │ Types of halogenoalkane CH3 - C – Br Primary 10 – One or NO alkyl gp on C attach to halogen gp │ Secondary 2o –Two alkyl gp on C attach to halogen gp H Tertiary 3o – Three alkyl gp on C attach to halogen gpSecondary halogenoalkane 20 -SN1 and SN2 CH3CH(CH3)Br + OH- -> CH3CH(CH3)OH + Br-Nucleophilic Substitution SN2• Undergo SN2 mechanism, Bimolecular Nucleophilic Substitution SN2CH3CH(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-] ANDNucleophilic Substitution SN1• Undergo SN1 mechanism, Unimolecular Nucleophilic Substitution SN1CH3CH(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 productCH3CH(CH3)+ + OH- → CH3CH(CH3)OH• Rate is dependent on concentration of CH3CH(CH3)Br• Molecularity = 1• Experimentally rate expression = k [CH3CH(CH3)Br]
  • 10. Questions on Nucleophilic Substitution Primary halogenoalkane 10 - SN2 H H │ │ SN2 OH SN2 OH CH3 CH2- C- Br + OH- CH3 CH2-C –OH + Br - │ │ H H Single step mechanism - Bond breaking + Bond making in transition state Tertiary halogenoalkane 30 - SN1 CH3 CH3 │ │ SN1 SN1CH3 C- Br + OH- CH3 C- OH + Br- │ │ OH OH CH3 CH3 Two step mechanism – Formation of carbocation Single step mechanism - Bond breaking + Bond making in transition state Secondary halogenoalkane 20 - SN2 and SN1 OH OH SN2 CH3 CH3 │ │ CH3 C- Br + OH- CH3 C- OH + Br- SN1 │ │ H H OH OH Two step mechanism - Formation of carbocation
  • 11. Questions on Oxidation ReactionOxidation of 2-Methylpropan-1-ol to 2-Methylpropanal to 2-Methylpropanoic acidPrimary alcohol 10 Aldehyde Carboxylic acid H H O │ MnO4- /H+ │ MnO4- /H+ ║CH3 CH- C-O-H + [O] K2Cr2O7/H+ CH3 CH-C=O K2Cr2O7/H+ CH3CH – C OH │ │ │ │ CH3 H CH 3 CH3Oxidation of Butan-2-ol to Butanone Secondary alcohol 20 Ketone MnO4- /H+ CH3 CH-CH2CH3 + [O] CH3 CH CH2CH3 │ K2Cr2O7/H+ ║ OH OOxidation of 2-Methylpropan-2-ol Tertiary alcohol 30 X CH3 │ CH3-C – OH + [O] X │ CH3

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