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Understanding Electrophilic Reactions and Carbocation Stability in Organic Chemistry

This section delves into electrophilic reactions, emphasizing the importance of carbocation stability. The electrophilic step yields carbocation intermediates, which dictate the reaction's kinetics and pathways. Key concepts such as Markovnikov's rule, regioselectivity in electrophilic additions, and the principles of electrophilic aromatic substitution (SEAr) are discussed. The impact of substituents on carbocation stabilization and reaction rates is explained, along with examples of enzymatic reactions and mechanisms like Friedel-Crafts acylation. A comprehensive review of these concepts will enhance your understanding of organic electrophilic reactions.

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Understanding Electrophilic Reactions and Carbocation Stability in Organic Chemistry

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  1. Electrophilic reactions The electrophilic step: leads to carbocation intermediate thus, carbocation stability is key! intro

  2. electrophilic C-C bond-forming steps 15.1

  3. next, we can get nucleophilic addition: 15.1

  4. or, carbocation intermediate can be deprotonated: an important variant - the electrophilic aromatic substitution 15.1

  5. Thinking about the electrophilic attack: 15.2A

  6. the thermodynamic picture: 15.2A

  7. review from fall semester! Regiochemistry of electrophilic addition – two possible outcomes: it depends on C+ stability! 15.2B

  8. two possible addition products 15.2B

  9. reaction A has more stable intermediate – goes faster 15.2B

  10. Markovnikov’s rule: when acid is added to double bond, the more substituted carbon gets the heteroatom there can be exceptions! 15.2B

  11. enzymatic electrophilic additions (SAM) 15.2C

  12. another example: can you rationalize the regiochemistry? 15.2C

  13. regiochemistry: 15.2C

  14. 15.2C

  15. 15.2C

  16. (we’ve already seen the hydroboration-oxidation rxn)

  17. Good review – draw out mechanism for these! 15.2D

  18. a way to ‘protect’ alcohols (we saw one way to protect ketones in 11.4B) more stable C+ acetal – stable to bases, nucleophiles 15.2D

  19. protection example: 15.2D

  20. (skip TBDMS ethers – go to section 15.3)

  21. Electrophilic addition followed by elimination: 15.3

  22. isomerization example: 15.3A

  23. substitution example: 15.3B

  24. 15.3B

  25. next reaction in the pathways extends the chain: 15.3B

  26. Is this SN1 or SN2? Compare to protein prenylation reaction from Chapter 9: 15.3B

  27. again, F-labeling experiment was done: F substitution slows down reaction dramatically! 15.3B

  28. A slightly different kind of electrophilic addition-elimination: (step 1) steps 1&2 are the targets of Roundup we saw steps 2 & 3 in Chapter 14 15.4

  29. Question: is this a Michael or electrophilic addition?

  30. It cannot be a Michael! 15.4

  31. 15.4

  32. experiment: reaction run in D2O with deoxy analog 15.4

  33. 15.4

  34. skip to section 2.1C supplement (aromaticity)

  35. Electrophilic aromatic substitution: the MCAT-writer’s best friend (abbreviated SEAr) 15.5A

  36. pi-bonds are very stable: high energy barrier for electrophilic attack! 1: electrophile needs to be extra reactive 2: carbocation intermediate needs to be stabilized 15.5A

  37. C+ stabilized by EDG 15.5A

  38. enzymatic example intermediate is stabilized by N! 15.5B

  39. 15.5B

  40. (read next two examples on your own)

  41. SEAr / decarboxylation (see p. 602, top) 15.5B

  42. SEAr in the lab: the Friedel-Crafts reactions alkylation – plain vanilla alkyl halide won’t cut it! We need a carbocation 15.6A

  43. or, you can generate the C+ electrophile using alkene + strong acid . . .or, alcohol plus strong acid! 15.6A

  44. Friedel-Crafts acylation 15.6A

  45. 15.6A

  46. Ring-activating/directing effects ortho-para director meta-director 15.6B

  47. ring-activating, ortho-para directing substitutuents: 15.6B

  48. 15.6B

  49. 15.6B

  50. ring-deactivating, m-directing substituents: the exception: halogens are o/p directors! 15.6B

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