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Alkenes and Alkynes I. Addition Reactions

Alkenes and Alkynes I. Addition Reactions. Chapter Eight. Types of Additions. =>. Sect 8.1: Addition Reactions. Addition of H-X. Addition of Water. Sect. 8.2: Introduction to Mechanisms. mechanism two step rate determining step (i.e. slow step) energy diagram transition states

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Alkenes and Alkynes I. Addition Reactions

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  1. Alkenes and Alkynes I.Addition Reactions Chapter Eight WWU -- Chemistry

  2. Types of Additions =>

  3. Sect 8.1: Addition Reactions WWU -- Chemistry

  4. Addition of H-X WWU -- Chemistry

  5. Addition of Water WWU -- Chemistry

  6. Sect. 8.2: Introduction to Mechanisms • mechanism • two step • rate determining step (i.e. slow step) • energy diagram • transition states • intermediates WWU -- Chemistry

  7. MECHANISM STEP-BY-STEP ACCOUNT OF WHAT HAPPENS :X- X step 1 step 2 C C C C C C + E E + E intermediates are formed during a reaction but are not products Intermediate WWU -- Chemistry

  8. ENERGY PROFILE two step reaction intermediate TS1 E N E R G Y TS2 - X C C + E step 1 step 2 DH C C X product C C + E E WWU -- Chemistry

  9. ACTIVATED COMPLEXES correspond to transition states for each step - X X C C C C C C + E E intermediate + E - X + C C C C + E + E show bonds in the process of breaking or forming ACTIVATED COMPLEXES (bonds are half formed or half broken) WWU -- Chemistry

  10. Sect 8.3: Electrophilic Addition to a Double Bond WWU -- Chemistry

  11. Hyperconjugation WWU -- Chemistry

  12. CARBOCATION STABILITY HYPERCONJUGATION H .. electrons in an adjacent C-H s bond help to stabilize the positive charge of the carbocation by proximity (overlap) R + C C H R H R C R C H R R C H R R Most stable Least stable 2 > > + + + tertiary secondary primary WWU -- Chemistry

  13. Can you recognize the following carbocations? 1o, 2o, 3o 2 o 2 o 1 o 2 o 2 o 3 o WWU -- Chemistry

  14. Sect 8.4: Addition of Hydrogen Halides: Markovnikov’s rule WWU -- Chemistry

  15. Markovnikov’s Rule In the ionic addition of an acid to the carbon-carbon double bond of an alkene, the hydrogen of the acid attaches itself to the carbon atom which already holds the greaternumber of hydrogens. • “Them that has, gets!” • “The richer get richer!” (V. W. Markovnikov -- 1838 - 1904) WWU -- Chemistry

  16. Markovnikov WWU -- Chemistry

  17. MARKOVNIKOV RULE When adding HX to a double bond the hydrogen of HX goes to the carbon which already has the most hydrogens C H C H 3 2 C l + HCl ..... conversely, the anion X adds to the most highly substituted carbon ( the carbon with most alkyl groups attached). WWU -- Chemistry

  18. REGIOSELECTIVE REACTION C H C H C H 3 3 3 HCl C H C C H C H C C H + C H C H2 C H 3 2 3 3 3 C l C l major minor one of the possible products is formed in larger amounts than the other one Compare REGIOSPECIFIC only one of the possible products is formed (100%). WWU -- Chemistry

  19. Mechanism (Markovnikov) Secondary C+ Major product WWU -- Chemistry

  20. Mechanism (anti-Markovnikov) Primary carbocation Minor! WWU -- Chemistry

  21. COMPETING PATHWAYS rate-determining step higher energy intermediate slower faster lower energy intermediate 1 o 2 o rate-determininng (slow) step WWU -- Chemistry

  22. Markovnikov Addition to an Alkene WWU -- Chemistry

  23. Mechanism 3 o C+ WWU -- Chemistry

  24. SOME ADDITIONAL EXAMPLES only major product is shown C H C H 3 3 C l + HCl C H C H 3 2 C l + HCl C H C H C H C H 3 2 + HCl C l WWU -- Chemistry

  25. Sect 8.5 Addition of Sulfuric Acid to an Alkene WWU -- Chemistry

  26. ALKYL HYDROGEN SULFATES O - O S O H O S O H O 3 SLOW C C C C C C + O H H H O S O H alkyl hydrogen sulfate O cold water room temp FOLLOWS MARKOVNIKOFF RULE WWU -- Chemistry

  27. Addition of Water to an Alkene WWU -- Chemistry

  28. Mechanism of Hydration WWU -- Chemistry

  29. Sect 8.6 Addition of Bromine to an Alkene WWU -- Chemistry

  30. ADDITION OF BROMINE B r B r C C l 4 C C C C C C SLOW + B r B r B r B r d- d+ : B r B r alkene polarizes bromine WWU -- Chemistry

  31. THE REACTION IS STEREOSPECIFIC ANTI ADDITION - anti B r H H “open“ carbocation would give both cis and trans + B r H syn B r H B r syn anti H H H B r B r B r B r H cis compound trans compound NOT OBSERVED ACTUAL PRODUCT WWU -- Chemistry

  32. WHAT WOULD EXPLAIN FORMATION OF ONLY THE trans PRODUCT ? ..... A BRIDGED OR CYCLIC INTERMEDIATE WWU -- Chemistry

  33. CYCLIC BROMONIUM ION note size of bromine + WWU -- Chemistry

  34. BRIDGED BROMONIUM ION Br + Br Br bridging blocks approach from this side WWU -- Chemistry

  35. FORMATION OF ENANTIOMERS symmetric intermediate Addition could also start from the top with bromide attacking the bottom. + ENANTIOMERS WWU -- Chemistry

  36. ADDITION OF BROMINE TO 2-BUTENE * * C H C H C H C H C H C H C H C H 3 3 3 3 + B r B r B r 2 2n possible stereoisomers C H H C H C H 3 3 3 H C H H H 3 cis-2-butene trans-2-butene WILL THESE STEREOISOMERS GIVE THE SAME PRODUCTS? WWU -- Chemistry

  37. NO ! THEY GIVE DIFFERENT PRODUCTS cis trans enantiomers meso These results can only be explained by stereospecific anti addition. WWU -- Chemistry

  38. - B r B r H H H H C H 3 C H 3 H C 3 H C B r+ B r 3 B r B r H H B r C H 3 H C H 3 H C H B r 3 B r H C 3 B r B r B r B r C H H 3 3 H H C H H H C 3 3 - cis ADD TO LEFT ADD TO RIGHT ROTATE ROTATE C H ENANTIOMERS WWU -- Chemistry

  39. _ _ trans + ADD TO LEFT ADD TO RIGHT ROTATE ROTATE MESO IDENTICAL (also meso) WWU -- Chemistry

  40. Bromination of an Unsymmetrical Alkene WWU -- Chemistry

  41. Stereochemistry of Bromination of Alkenes • Simple alkenes: Addition of bromine or chlorine goes exclusively anti, with the formation of a bridged ion • If a resonance-stabilized open-chain carbocation is possible, there may be a mixture of mechanisms, with some molecules reacting via a bridged ion and some molecules reacting via an open-chain carbocation WWU -- Chemistry

  42. Stereochemistry of Bromination of Alkenes--Part Two • In cases where a resonance-stabilized carbocation is possible, if the solvent is made more polar (acetic acid or nitromethane), the proportion of molecules reacting via an open-chain carbocation increases. • For simple alkenes, changing solvents has little or no effect on stereochemistry. WWU -- Chemistry

  43. Sect 8.7 Halohydrin Formation Br2 + H2O HO-Br + HBr WWU -- Chemistry

  44. Mechanism WWU -- Chemistry

  45. Sect. 8.8: Carbocation Rearrangements WWU -- Chemistry

  46. Sect 8.10 Free Radical Addition of HBr to Alkenes (anti-Markovnikov!) WWU -- Chemistry

  47. ADDITION OF HBr Markovnikov Addition Oxygen Anti-Markovnikov Addition WWU -- Chemistry

  48. WWU -- Chemistry

  49. STABILITY OF CARBON RADICAL INTERMEDIATES Radicals are electron-deficient just like carbocations and have the same stability order. lowest energy highest energy tertiary secondary primary methyl and they are stabilized by resonance and / or hyperconjugation. ( ) ( ) ( ) etc. C H 2 ( ) WWU -- Chemistry

  50. Sect. 8.11 and 8.12: Hydrogenation of Alkenes and alkynes WWU -- Chemistry

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