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Free Radical Chain Reaction: Alkane + Halogen

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Presentation on theme: "Free Radical Chain Reaction: Alkane + Halogen"— Presentation transcript:

1 Free Radical Chain Reaction: Alkane + Halogen
1) Initiation: photochemical homolysis RDS 2) Propagation Bond stability depends on size of atom X H C Methyl free radical fast slow X X 2 X UV light Halogen free radicals Homolytic fission 3) Termination Reaction comes to a stop if 2 free radicals pair up X fast Additional halogens can add on during propagation… H C X Further reaction will result in the substitution of all hydrogens with halogens C H fast Ethane (bi-product) C H X fast

2 + + – + – + – Electrophilic Addition: Alkene + Halogen C C X X X
Dipole induced Bond lengthens and weakens Halogen approaches alkene: δ – δ+ X Heterolytic Fission (electrophile) slow C + X Carbocation Anion (nucleophile) will attack carbocation through backside attack Anion depends on solution Inert solution Dissolved in NaCl Dissolved in water Cl C + X fast X C + fast Anions Cl– X-- Anion X– (halide anion) OH C + X fast Anions OH– X– Example - Bromine and Ethene (X = Br2) Inert Solution: ,2 – dibromoethane Bromine Dissolved in Water: 2-bromo ethan-1-ol Bromine Dissolved in NaCl: bromo, 2-chloro ethane

3 Electrophilic Addition: Alkene + Hydrogen Halide
Hydrogen Halide approaches alkene C + H Carbocation X Halide ion (Nucleophile) Backside attack fast Halo alkane C slow δ – δ+ X H (electrophile) Heterolytic Fission Stability of Carbocations Existing dipole strengthened Bond lengthens and weakens Tertiary (3°) Secondary (2°) Primary (1°) R C + R C + H H C + R > > Note: for asymmetric alkenes, Markovnikov’s rule is used Hydrogen will bond to the carbon that is richest in hydrogens already -This allows for the most stable carbocation to form Hydrogen will be abstracted from the Carbon atom that has the least Hydrogens already *Positive inductive effect : alkyl groups donate electron density to centre carbon -Carbon with the most electron density is the most stable Example – Hydrogen iodide and 2-methyl pent-2-ene(X = I2) C C I C + H I slow fast C δ – δ+ I H Heterolytic Fission (electrophile) C Iodine ion (nucleophile) Tertiary carbocation 2-iodo 2-methyl pentane

4 Electrophilic Addition: Alkene + Dilute Sulphuric Acid (with silver catalyst)
Sulphuric Acid approaches alkene C + H Carbocation OSO3H Hydrogen sulphate ion Backside attack fast Ethyl hydrogen sulphate O SO3H C slow δ – δ+ HSO3O H (electrophile) Heterolytic Fission Existing dipole strengthened Bond lengthens and weakens C O SO3H H Water (warm) fast Hydrolysis Ethanol H2SO4 Net Reaction: Hydration of an Alkene C O H H+ / H2SO4 Alkene Alcohol This reaction is industrially important in the preparation of alcohols

5 Ethane 1,2-diol (antifreeze)
Oxidation of Alkenes: Alkene + Oxidizing Agent KMnO4 MnO4-- Cold, dilute Potassium Permanganate Oxidizing Agent Permanganate ion Purple Oxid # Mn = +7 Oxidizing Agent: Mn O __ Mn O C O Mn C OH C Alkene Ethane 1,2-diol (antifreeze) Manganese (IV) Oxide Oxid # Mn = +4 Permanganate Ion Oxidizing Agent Purple Transition state Alkene + Hydrogen : Reduction of Alkenes/ Hydrogenation C H This method is used in the food industry to solidify oils into edible fats (ex: margarine) High Temperature C H (g) Alkene Catalyst: Transition Metal Pt, Ni, Pd Heterogeneous Catalysis -Absorption, reaction, desorption Alkane

6 Addition Polymerization: Alkene + Free Radical
Alkyl free radical Free radical is reproduced to continue the polymerization R Free Radical C Alkene High pressure High temperature Catalyst C R Alkyl free radical C Free radical R C Alkene Termination occurs when 2 free radicals combine Polyethene C Product: n Note: The degree of polymerization can be altered by changing temperatures, pressures, catalysts. This will result in many polyethenes with a variety of different characteristics

7 Effect of Steric Hindrance on Secondary Haloalkane:
SN2: Haloalkane Nucleophilic Substitution Rate Determining Step = k [R-X]1 [Nucleophile]1 H C X R Nu C H X R C H Nu R Nu _ slow fast H Nucleophile Only strong bases can displace the halide ion (OH--, CN--) H Primary Haloalkane Transition State Trigonal bipyramidal Bonds must flatten themselves for a planar arrangement High Potential Energy, unstable compound Note: Inversion of configuration Possibility of optical isomers Secondary Haloalkanes will undergo SN2 when steric hindrance does not occur (when alkyl groups are not too large): Effect of Steric Hindrance on Secondary Haloalkane: R’ C H X R Nu _ Big and bulky alkyl groups Nucleophile does not have space to attack carbon long chains/ branching Nucleophile’s path is blocked

8 SN1: Haloalkane Nucleophilic Substitution
Note: Polar solvents will hydrate the carbocation and stabilize it This also provides energy for Step I R’ C R’’ + R O H δ – Rate Determining Step = k [R-X]1 R’ C R’’ X R R’ C R’’ + R X _ Step I : slow Heterolytic Fission Tertiary Carbocation Intermediate Most stable due to positive inductive effect of alkyl groups Transient existence Halide Ion Tertiary Haloalkane Secondary Alkanes will undergo SN1 if alkyl groups are too large hence, steric hindrance is present Nucleophilic Substitution Nu _ R’ C R’’ Nu R R’ C R’’ + R fast Nucleophile Does not require a strong base Step II: Note: possibility of optical isomers Racemic Mixture

9 E2: Haloalkane Elimination Reaction
Elimination occurs instead of substitution when pathway of nucleophile is blocked, hence the Lewis base will remove an electrophile from the haloalkane instead OH _ C H X R O H slow H C δ + δ – Heterolytic Fission Alkene Water Primary Haloalkane Secondary Haloalkanes will undergo E2when steric hindrance does not occur (when alkyl groups are not too large) Note: Elimination requires more energy than substitution The elimination reaction is favoured if: a) the base is strong b) the temperature is high

10 Step I : Step II : E1: Haloalkane Elimination Reaction _ X _ OH O C
Heterolytic Fission Tertiary Haloalkane R’ C R’’ + R X _ slow Step I : Tertiary Carbocation Stable due to positive inductive effect of alkyl groups Halogen Ion (product) Nucleophilic Elimination OH _ Hydroxide Ion Nucleophile fast C R’’ + R H O Alkene Water Step II : Note: Markovnikov’s rule still applies -When adding Hydrogen, adds to carbon that is richest in Hydrogens already -When eliminating Hydrogen, removes from carbon that has least Hydrogens Example – 2-bromo 2-methyl butane OH _ Br _ C2H5 C CH3 C CH3 H slow O H Br fast C CH3 + H Heterolytic Fission Water 2-methyl but-2-ene

11 Excess Acid+ High Temperature:
Dehydration of Alcohols: Elimination Reaction H C O C H O H OPO3H2 + OPO3H2 __ δ + δ – δ + δ – Dihydrogen Phosphate ion Phosphoric Acid (dehydrating agent) Protonated Alcohol Alcohol OPO3H2 __ H + H3 PO4 Acid regenerated Excess Acid+ High Temperature: C H + Alkene C H + Heterolytic Fission O H Carbocation Water Excess Alcohol + Low Temperature: C H + O Ether Alcohol Heterolytic Fission +

12 K2 Cr2O7 Cr2O72-- [O] Oxidation of Alcohols: Alcohol + Oxidizing Agent
Potassium Dichromate Oxidizing Agent Dichromate ion Orange Oxid # Cr= +6 Source of Oxygen Primary Alcohol Secondary Alcohol Tertiary Alcohol C R H O C R R’ H O [O] C R R’ R’’ O H [O] C R H O Aldehyde C R R’ H O Ketone No Oxidation Occurs Central carbon does not have any Hydrogens to be oxidized O H O H Excess Oxidizing Agent [O] Reflux/Bone C R H OH O Carboxylic Acid

13 Esterification (condensation reaction): Alcohol + Carboxylic Acid
Water Catalyst: concentrated sulphuric acid Homogeneous catalysis Dehydrating Agent (loss of water) -shifts equilibrium right, produces more product Note: Water is a product, not a solvent So it is included in the equilibrium calculation 𝐾 𝑐 = 𝐻 2 𝑂 [𝐸𝑠𝑡𝑒𝑟] 𝐴𝑙𝑐𝑜ℎ𝑜𝑙 [𝐴𝑐𝑖𝑑] This is an example of condensation polymerization:  A reaction in which two molecules join together to form a larger molecule with the loss of a smaller molecule Another example of condensation polymerization is the formation of peptide linkages in proteins Condensation Polymerization N C O R’ H Carboxylic Acid C R’ H O Amine N R H Peptide Linkage

14 Alcohol + Concentrated Halide Acid
Heterolytic Fission C R R’ R’’ O H H C R R’ R’’ O + H X X Halide ion (nucleophile) δ + δ – Strong Acid HCl HBr HI Protonated Alcohol (intermediate) Tertiary Alcohol Reacts most readily Alcohols are soluble in the concentrated halide acid Therefore, the reactant solution is colourless C R R’ R’’ + Carbocation Tertiary carbocations are the most stable O H Water C R R’ R’’ X Halogenoalkane Useful method to prepare alkyl halides C R R’ R’’ + X Note: This is an important test for alcohols, tertiary alcohols will react rapidly whereas primary and secondary alcohols will react more slowly or not at all Therefore, reaction can be used to distinguish between 1°, 2°, and 3° alcohols Halogenoalkanes are not soluble in the concentrated halide acid Therefore, the product solution is cloudy

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