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Chlorination of Higher Alkanes

3 o. 2 o. 1 o. 1 o. 1 o. Chlorination of Higher Alkanes. Chlorination of an alkane involves the substitution of a chlorine atom for a hydrogen atom on the alkane. All H’s are equivalent in CH 4 and CH 3 CH 3 so only one product forms. In many other alkanes, more

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Chlorination of Higher Alkanes

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  1. 3o 2o 1o 1o 1o Chlorination of Higher Alkanes • Chlorination of an alkane involves the substitution of a chlorine atom for a hydrogen atom on the alkane. • All H’s are equivalent in CH4 and CH3CH3 so only one product forms. • In many other alkanes, more than one type of hydrogen atom is present and more than one product can form.

  2. Chlorination of Higher Alkanes • The presence of more than one type of hydrogen atom in an alkane often leads to a mixture of products.

  3. Chlorination of Higher Alkanes • The predicted (statistical) distribution and experimental results are different because 1-chloropropane forms from a 1o free radical

  4. Chlorination of Higher Alkanes • while 2-chloropropane forms from a 2o free radical

  5. Chlorination of Higher Alkanes Example: Draw the mechanism for the chlorination of propane:

  6. Chlorination of Higher Alkanes • As a rule of thumb, the major product is often the one that is formed from the most stable free radical. • There are EXCEPTIONS! • Very large ratio of 1o H to 2o or 3o H may lead to 1o alkyl halide as the major product hu 62% 38%

  7. Reactivity of Halogens • All halogens react with methane via the same mechanism, but they have different relative reactivities. • Relative reactivityrefers to relative rates of reaction • high reactivity: very fast rate • low reactivity or unreactive: slow or zero rate F2 > Cl2 >Br2 > I2

  8. Reactivity of Halogens • The differences in reactivity of the halogens toward alkanes can be explained by differences in: • Activation energy for the propagation steps • DHrxn • As Ea decreases, more collisions will have enough energy for the reaction to occur • higher reactivity

  9. Reactivity of Halogens • Since DG ~ DHrxn for most organic reactions, as the reaction becomes more exothermic, the reaction becomes more spontaneous • higher reactivity Ea Overall Relative Halogen (kcal/mol) DHrxn Reactivity F2 1.2 -102 high Cl2 3.8 -24.5 medium Br2 18.6 -7.5 low I2 34 +11 unreactive

  10. Reactivity of Halogens • F2 is the most reactive halogen • low activation energy • strongly exothermic reaction • I2 is very unreactive. • High activation energy • endothermic reaction

  11. Reactivity of Halogens • The chlorination of propane leads to a broad mixture of products: • 40% 1-chloropropane • substitution of a 1o H • 60% 2-chloropropane • substitution of a 2o H • Bromination of propane is more selective: • 3% 1-bromopropane • 97% 2-bromopropane

  12. Reactivity of Halogens • The RDS for chlorination of propane is exothermic. • According to the Hammond Postulate, thetransition state will more closely resemble the reactants. • Little bond breakage has occurred. • Small difference in Ea for formation of 1o vs. 2o radical. • Stability of the free radical is less important in determining the structure of the final product.

  13. Reactivity of Halogens

  14. Reactivity of Halogens • The RDS for the bromination of propane is endothermic. • According to the Hammond Postulate, the transition state will resemble the products. • More bond cleavage and new bond formation has occurred. • Large difference in Ea for formation of 1o vs. 2o radical • Stability of the free radical is much more important in determining structure of product.

  15. Reactivity of Halogens

  16. Bromination of Alkanes Example: Predict the major product of the following reactions.

  17. Reactive Intermediates • Reactive intermediates are highly reactive, short-lived species that are never present in high concentration. • Four important reactive intermediates: • carbonium ion (carbocation) • free radical • carbanion • carbene

  18. Reactive Intermediates • Carbonium ion: • a chemical species in which: • carbon atom is bonded to 3 other atoms • carbon atom has a positive charge • planar, sp2 hybridized • electrophile: • “electron lover” • an electron pair acceptor

  19. d+ d+ d+ Reactive Intermediates • The stability of a carbonium ion increases • as the number of alkyl groups attached to the positively charged carbon increases • inductive effect • donation of electron density through the sigma bonds • partial overlap of filled orbitals with empty ones • through resonance stabilization

  20. 3o substituted allylic 2o allyl methyl vinyl 1o Reactive Intermediates • Stability of carbonium ion: > > > Least stable Most stable

  21. Reactive Intermediates • Free radical: • carbon atom with an unpaired electron • The stability of a free radical increases • as the number of alkyl groups attached to the positively charged carbon increases • through resonance stabilization • Stability: allyl > 3o > 2o > 1o > methyl > vinyl .

  22. Reactive Intermediates • Carbanion: • a trivalent carbon atom that bears a negative charge • a nucleophile • electron pair donor • Carbene: • an uncharged, divalent carbon atom that has a nonbonding pair of electrons

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