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Understanding the Basics of Organic Chemistry Mechanisms

This article explains the fundamentals of organic chemistry mechanisms, focusing on the understanding of electron flow. Learn the steps involved in dehydration of alcohols and acid-catalyzed hydration reactions.

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Understanding the Basics of Organic Chemistry Mechanisms

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  1. Mechanisms Some Basic Orgo I Reactions

  2. Understanding the basics… • Mechanisms are the most mind-boggling part of organic chemistry. • Students, generally speaking, have spent their time memorizing their way through science courses. • Mechanisms require a student to UNDERSTAND the fundamentals of electron flow…

  3. Everyone knows that electrons are negatively charged. • Everyone knows that electrons are attracted to things with positive charges. • Yet, the understanding of a “mechanism” remains elusive to many students… • Let’s review the basics…

  4. Electron flow is always from the electron-rich to the electron-poor species. • The electron-rich species is a Lewis Base (must have a lone pair) and is called the “nucleophile”. • The electron-poor species is a Lewis Acid (must have empty orbital) and is called the “electrophile”.

  5. Examples of Nucleophiles: • Examples of Electrophiles:

  6. SO – electron flow is always from nucleophile to electrophile, electron-rich to electron-poor… • Watch the direction of your arrows • from lone pairs to carbocation…

  7. from anion to cation… • from anion to partial positive charge…

  8. from alkene pi bond to cation or partial positive charge…

  9. When working through a mechanism, the goal is NOT to memorize the steps of a mechanism of a SPECIFIC molecule. When you do that, typically you become too focused on the structures provided in a single example. • If that happens, you will get confused when the next mechanism problem has a DIFFERENT structure.

  10. What you want to do is make a game plan - break down the steps of the mechanism, into little parts or steps. • The basic little steps are easier to remember. • By knowing the steps, you know how the mechanism progresses, regardless of the structure you are given to work with. • SO – break them down…

  11. Dehydration of Alcohols Identify this mechanism – Starts with alcohol, ends with alkene… losing water… Dehydration, Acid-catalyzed… H+ from sulfuric or phosphoric acid…

  12. Dehydration of Alcohols Steps Involved: • Convert –OH to H2O (use that acid!) • Loss of H2O and carbocation formation • Removal of H+, resulting in formation of pi bond to complete the conversion to alkene • E1 mechanism – think Zaitsev and Trans!

  13. Dehydration of Alcohols Step 1: Convert –OH to H2O

  14. Dehydration of Alcohols Step 2: Loss of H2O (“spontaneous dissociation”) to form carbocation

  15. Dehydration of Alcohols Step 3: Removal of H+, resulting in formation of pi bond to complete conversion to alkene

  16. Dehydration of Alcohols - Again Try another example: Alcohol to alkene (using acid) = dehydration (make water, lose water, form alkene)

  17. Dehydration of Alcohols - Again Step 1: Convert –OH to H2O Remember the soul purpose for the acid is to turn the –OH into a water molecule. Now it wants to leave…

  18. Dehydration of Alcohols - Again Step 2: Make the leaving group leave – “spontaneous dissociation” occurs:

  19. Dehydration of Alcohols - Again Step 3: Form the pi bond, make an alkene Find the beta-H on the more substituted side (Zaitsev!)… use your water as a Lewis Base and pull that H+ off, forming a pi bond… Done!

  20. Dehydration of Alcohols – And Again Try one more example: Do the steps… and check that your arrows and intermediates look like those you’re about to see…

  21. Dehydration of Alcohols – And Again Step 1: Convert –OH to H2O Always draw the arrow from electron rich (lone pairs!) to electron poor (positive charge!)

  22. Dehydration of Alcohols – And Again Step 2: Spontaneous Dissociation… Next step? Form the pi bond…

  23. Dehydration of Alcohols – And Again Step 3: Form the pi bond, make an alkene Find the beta-H on the more substituted side that has an H… use your water and pull it off, forming a pi bond… Done!

  24. Acid-Catalyzed Hydration Identify this mechanism – Starts with alkene, ends with alcohol…

  25. Acid-Catalyzed Hydration Steps Involved: • Reaction of pi bond with H+ (acid catalyst!) resulting in Markovnikov carbocation formation • Addition of H2O (this is where the OH is coming from) • Removal of extra proton (H+) to finish formation of –OH.

  26. Acid-Catalyzed Hydration Step 1: Reaction of pi bond with H+(acid cat.) resulting in Carbocation formation

  27. Acid-Catalyzed Hydration Step 2: Addition of H2O

  28. Acid-Catalyzed Hydration Step 3: Removal of extra proton (H+) to finish formation of –OH.

  29. Acid-Catalyzed Hydration - Again Try Again… Identify the mechanism – alkene to alcohol, using acid and water…

  30. Acid-Catalyzed Hydration - Again Step 1: React the pi bond with H+and form that carbocation: We don’t have to show this new H but make sure you are drawing the Markovnikov carbocation!

  31. Acid-Catalyzed Hydration - Again Step 2: Add the H2O (the green H is still there, just didn’t show it in the second structure) Now - Finish it off… pull the extra proton…

  32. Acid-Catalyzed Hydration - Again Step 3: Removal of extra proton (H+) to finish formation of –OH.

  33. Acid-Catalyzed Hydration – And Again ‘Third time’s the charm”… Try one more example: Do the steps… and check that your arrows and intermediates look like those you’re about to see…

  34. Acid-Catalyzed Hydration – And Again Step 1: React that nucleophilic pi bond with the proton H+: We don’t have to show this new H but make sure you are drawing the Markovnikov carbocation!

  35. Acid-Catalyzed Hydration – And Again Step 2: Add the H2O Finish it off… pull the extra proton…

  36. Acid-Catalyzed Hydration – And Again Step 3: Removal the extra proton (H+) to finish formation of –OH.

  37. Addition of H-X Identify this mechanism – Starts with alkene, ends with single halide…

  38. Addition of H-X Steps Involved: • Reaction of pi bond with H+ (of H-X), concurrent separation of X- , and formation of Markovnikov carbocation intermediate. • Attack on carbocation by X- to finish formation of product

  39. Addition of H-X Step 1: Reaction of pi bond with H+ (of H-X), concurrent separation of X-, and formation of carbocation intermediate.

  40. Addition of H-X Step 2: Attack of X- to finish formation of product.

  41. Addition of H-X - Again Try it again… Identify the mechanism… Adding an Br and an H to an alkene…

  42. Addition of H-X - Again Step 1: Reaction of pi bond with H+ (of H-X), concurrent separation of X-, and formation of carbocation intermediate.

  43. Addition of H-X - Again Step 2: Attack of X- to finish formation of product.

  44. Addition of HX – Once Again Try it again… Identify the mechanism… Adding a chloride (and an H, not drawn in) to an alkene…

  45. Addition of HX – Once Again Step 1: React the pi bond with H+ (of H-X), separate off the X-, form the more substituted carbocation intermediate.

  46. Addition of HX – Once Again Step 2: Attack of X- to finish formation of product.

  47. Addition of X2 Identify this mechanism – Starts with alkene, ends with two halides…

  48. Addition of X2 Steps Involved: • Attack by pi bond on polarized X-X with Halonium Ion formation • Attack of X- to pop open three-membered ring and finish formation of product.

  49. Addition of X2 Step 1: Attack by pi bond on polarized X-X with Halonium Ion formation (and loss of X-)

  50. Addition of X2 Step 2: Attack of X- to pop open three-membered ring and finish formation of product.

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