PowerPoint Presentation - Organic Chemistry Fifth Edition

Free Radical ReactionsHalogenation of AlkanesRH + X2 RX + HX1Orbital hybridization in carbocations and carbanions:Reactive Intermediates

Free radicals can be thought of as sp2 hybridized or quickly interconverting sp3 hybridized.Reactive Intermediates:Free Radicals

RH + X2 RX + HXexplosive for F2exothermic for Cl2 and Br2endothermic for I2Energetics2Halogenation Thermodynamics

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Chlorination of Methane5carried out at high temperature (400 C)CH4 + Cl2 CH3Cl + HClCH3Cl + Cl2 CH2Cl2 + HClCH2Cl2 + Cl2 CHCl3 + HClCHCl3 + Cl2 CCl4 + HClChlorination of Methane6

Mechanism of Chlorination of Methane15The initiation step "gets the reaction going"by producing free radicalschlorine atomsfrom chlorine molecules in this case.Initiation step is followed by propagationsteps. Each propagation step consumes onefree radical but generates another one.:..Cl..:Cl..:....Cl..:.:Cl.....+Free-radical chain mechanism.Initiation step: (Light or Heat is Necessary)Mechanism of Chlorination of Methane16Which of the above initiates reactions most readily? Question

A)B)C)10Cl:.....H3C : HH : Cl:....H3C.++First propagation step:Second propagation step:++Cl:.....H3C.:..Cl..:Cl..:..H3C....:Cl:Mechanism of Chlorination of Methane18Cl:.....H3C : HH : Cl:....H3C.++Second propagation step:++++First propagation step:Cl:.....H3C.:..Cl..:Cl..:..H3C : HH : Cl:....:..Cl..:Cl..:..H3C....:Cl:H3C....:Cl:Mechanism of Chlorination of Methane18Almost all of the product is formed by repetitivecycles of the two propagation steps.Cl:.....H3C : HH : Cl:....H3C.++Second propagation step:++++First propagation step:Cl:.....H3C.:..Cl..:Cl..:..H3C : HH : Cl:....:..Cl..:Cl..:..H3C....:Cl:H3C....:Cl:18stop chain reaction by consuming free radicals

hardly any product is formed by termination stepbecause concentration of free radicals at anyinstant is extremely low

+H3C.Cl:.....H3C....:Cl:Termination Steps20QuestionThe step shown below is a _____________ step of the free-radical chlorination ofchloromethane.

A)initiationB)propagationC)chain-terminatingD)bond cleavage

QuestionFor the free-radical reaction below, light is involved in which of the following reactionsteps?

A)Initiation onlyB)Propagation onlyC)Termination onlyD)Initiation and propagation

Halogenation of Higher Alkanes21CH3CH3 + Cl2CH3CH2Cl + HCl420C(78%)Cl + Cl2 + HClh(73%)can be used to prepare alkyl chlorides from alkanes in which all of the hydrogens are equivalent to one another

Chlorination of Alkanes22Major limitation:

Chlorination gives every possible monochloride derived from original carbonskeleton.

Not much difference in reactivity ofdifferent hydrogens in molecule.Chlorination of Alkanes23CH3CH2CH2CH3 Cl2hChlorination of butane gives a mixture of1-chlorobutane and 2-chlorobutane.CH3CH2CH2CH2ClCH3CHCH2CH3 Cl(28%)(72%)Example23

Percentage of Product that Results from Substitution of Indicated Hydrogen if Every Collision with Chlorine Atoms is Productive10%10%10%10%10%10%10%10%10%10%24

Percentage of Product that Actually Results from Replacement of Indicated Hydrogen4.6%4.6%4.6%18%18%18%4.6%4.6%4.6%18%24divide by 4.64.64.6= 1184.6= 3.9A secondary hydrogen is abstracted 3.9 times faster than a primary hydrogen by a chlorine atom.

4.6%18%Relative Rates of Hydrogen Atom Abstraction26Similarly, chlorination of 2-methylbutane gives a mixture of isobutyl chloride and tert-butyl chlorideHCH3CCH3 CH3CH3CCH2Cl CH3HClCH3CCH3 CH3hCl2(63%)(37%)27QuestionHow many monochlorination products do you expect to obtain from the chlorination of2-methylbutane?A)twoB)threeC)fourD)five

Percentage of Product that Results from Replacement of Indicated Hydrogen7.0%37%28

377.0divide by 77= 17= 5.3A tertiary hydrogen is abstracted 5.3 times faster than a primary hydrogen by a chlorine atom.

Relative Rates of Hydrogen Atom Abstraction29Radicals - Resonance

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Hyperconjugation contributes more to thermodynamic stability than resonance.QuestionTrue (A) / False (B)

Vinyl free radicals are more thermodynamically stable than benzylic and allylic free radicals.QuestionTrue (A) / False (B)

R3CH>R2CH2>RCH3chlorination: 5 41bromination:1640821Chlorination of an alkane gives a mixture of every possible isomer having the same skeletonas the starting alkane. Useful for synthesis only when all hydrogens in a molecule are equivalent.

Bromination is highly regioselective for substitution of tertiary hydrogens. Major synthetic application is in synthesis of tertiary alkyl bromides.Selectivity of Free-radical Halogenation31Radicals - Bromination

How many mono-bromination products are expected from the following reaction?

A. 5B. 4 C. 3D. 2E. 1Question

33Chlorination is useful for synthesis only when all of the hydrogens in a molecule are equivalent. (64%)Cl2hSynthetic Application of Chlorination of an AlkaneCl32QuestionAn alkane with a molecular formula of C8H18 reacts with Cl2 in the presence of light andheat to give a single monochloride C8H17Cl. What is the most reasonable structure for the starting alkane?A)CH3CH2CH2CH2CH2CH2CH2CH3B)(CH3CH2)2CHCH2CH2CH3C)(CH3)2CHCH2CH2CH(CH3)2D)(CH3)3CC(CH3)3Bromination is highly selective for substitution of tertiary hydrogens. Major synthetic application is in synthesis of tertiary alkyl bromides. (76%)Br2hHCH3CCH2CH2CH3CH3BrCH3CCH2CH2CH3CH3Synthetic Application of Bromination of an Alkane33QuestionWhich of the following best describes a mechanistic feature of the free-radical bromination (Br2, light) of 2-methylpropane?A)The initiation step involves cleavage of a C-H bond.B)The free-radical (CH3)3C is produced in one propagation step and reacts with Br2 in another.C)The reaction is characterized by the homolytic cleavage of the C-Br bond.D)The reaction is concerted; i.e., it occurs in a single step.Which reaction has a faster rate? Which product is kinetically favored?

A. I) and I) B. II) and II) C. I) and II)D. II) and I)

QuestionI)II)38Which reaction has a propagation step that is endothermic? Which reaction is more regioselective?

A. I) and I) B. II) and II) C. I) and II)D. II) and I)

QuestionI)II)

39Three mono-substituted isomers form in the halogenation of butane. The products are optically inactive.

Stereochemistry

40Bromination of optically active 3-methylhexane produces only (S)-3-bromo-3-methylhexane.

QuestionTrue (A) / False (B)41How many products (including stereoisomers) are expected in the following halogenation?

A. 5B. 4 C. 3D. 2E. 1

Question42Resonance and regioselectivity:Allylic/Benzylic Bromination

A mixture is obtained that includes the di-brominated product.Br-Di-bromination can be avoided using a specialized reagent:N-bromosuccinimide (NBS)

43N-bromosuccinimide (NBS)

Allylic/Benzylic Bromination

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What is the major product of the following reaction?Question

(A) Barbamide, a cyanobacterial peptide containing a trichloromethyl group and (B) dysidenin, a barbamide-related compound isolated from a sponge-cyanobacterial associationHalogenated Marine Natural Products

Polymers21Free radical conditions are frequently used to form polymers.Recall that a polymerization process joins together many small units called monomers in a long chain.Radical Polymerization

48Radical Polymerization

Radical polymerizations commonly proceed through a chain reaction mechanism.

49Radical Polymerization

50Radical Polymerization

51Radical polymerizations generally produce chains of monomers with a wide distribution of lengths.

Because the mechanism proceeds through 1 carbon free radical intermediates, it is usually difficult.

Radical Polymerization52Branching is common in radical polymerizations.

Branching makes polymer materials more flexible, such as a polyethylene used for squeeze bottles.When catalysts are used to minimize branching, more rigid materials are produced, such as the material used for squeeze bottle caps.

Radical Polymerization

53Many derivatives of ethylene are polymerized.Radical Polymerization

54Radical Polymerization

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Stereoregular Polymersatacticisotacticsyndiotactic21Atactic PolypropyleneRandom stereochemistry of methyl groups attached to main chain (stereorandom)Properties not very useful for fibers etc.Formed by free-radical polymerization

13Isotactic PolypropyleneStereoregular polymer; all methyl groups onsame side of main chainUseful propertiesPrepared by coordination polymerization under Ziegler-Natta conditions

13Syndiotactic Polypropylene

Stereoregular polymer; methyl groups alternate side-to-side on main chainUseful propertiesPrepared by coordination polymerization under Ziegler-Natta conditions13Properties of PolymersIsotactic and syndiotactic polymers are stronger and stiffer due to their regular packing arrangement.

Anionic polymerization usually gives isotactic or syndiotactic polymers.

Free radical polymerization is nearly random, giving branched atactic polymers. 60Free Radical Polymerization

61Polystyrene & Others

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