6. An Overview of Organic Reactions

1. 6. An Overview of Organic Reactions Why this chapter? To understand organic and/or biochemistry, it is necessary to know: -What occurs -Why and how chemical reactions take place We will see how a reaction can be described

2. 6.1 Kinds of Organic Reactions • In general, we look at what occurs and try to learn how it happens • Common patterns describe the changes • Addition reactions – two molecules combine • Elimination reactions – one molecule splits into two

3. Substitution – parts from two molecules exchange • Rearrangement reactions – a molecule undergoes changes in the way its atoms are connected

4. What kind of reaction is the transformation shown below? Learning Check: • an elimination reaction • a rearrangement reaction • a substitution reaction • an addition reaction • none of these

5. What kind of reaction is the transformation shown below? Solution: • an elimination reaction • a rearrangement reaction • a substitution reaction • an addition reaction • none of these

6. 6.2 How Organic Reactions Occur: Mechanisms • In a clock the hands move but the mechanism behind the face is what causes the movement • In an organic reaction, we see the transformation that has occurred. Themechanism describes the steps behind the changes that we can observe • Reactions occur in defined steps that lead from reactant to product

7. Steps in Mechanisms • We classify the types of steps in a sequence • A step involves either the formation or breaking of a covalent bond • Steps can occur in individually or in combination with other steps • When several steps occur at the same time they are said to be concerted

8. Types of Steps in Reaction Mechanisms • Bond formation or breakage can be symmetrical or unsymetrical • Symmetrical- homolytic • Unsymmetrical- heterolytic Bond Breaking Bond Making

9. Indicating Steps in Mechanisms • Curved arrows indicate breaking and forming of bonds • Arrowheads with a “half” head (“fish-hook”) indicate homolytic and homogenic steps (called ‘radical processes’) • Arrowheads with a complete head indicate heterolytic and heterogenic steps (called ‘polar processes’)

10. 6.3 Radical Reactions • Not as common as polar reactions • Radicals react to complete electron octet of valence shell • A radical can break a bond in another molecule and abstract a partner with an electron, giving substitution in the original molecule • A radical can add to an alkene to give a new radical, causing an addition reaction

11. Steps in Radical Substitution • Three types of steps • Initiation – homolytic formation of two reactive species with unpaired electrons • Example – formation of Cl atoms form Cl2 and light • Propagation – reaction with molecule to generate radical • Example - reaction of chlorine atom with methane to give HCl and CH3. • Termination – combination of two radicals to form a stable product: CH3. + CH3. CH3CH3

12. Steps in Radical Substitution: Monochlorination of Methane Initiation Propagation

13. Steps in Radical Substitution Termination With excess concentration of Cl2 present continued reaction is probable with formation of dichloro, trichloro, and tetrachloromethanes.

14. Radical Substitution Example: Biosynthesis of Prostaglandins p. 141

15. Radical Example: p. 142

16. Learning Check: Propose a mechanism. Add curved arrows to indicate the flow of electrons: p. 142

17. Solution: Propose a mechanism. Add curved arrows to indicate the flow of electrons: p. 142

18. In a radical chain reaction, what would be the best description of the following reaction?H3C• + •Cl → CH3Cl Learning Check: • propagation • elimination • initiation • termination • substitution

19. In a radical chain reaction, what would be the best description of the following reaction?H3C• + •Cl → CH3Cl Solution: • propagation • elimination • initiation • termination • substitution

20. Radical Substitution: With >1 kind of H • When there is • >1 type of H then there is • >1 option for radical formation and therefore • >1 option for a monohalogenation product.

21. In the reaction of Cl2 with 2-methylbutane, how many monochlorinated isomers are produced? Learning Check: • 2 • 3 • 4 • 5 • 6

22. In the reaction of Cl2 with 2-methylbutane, how many monochlorinated isomers are produced? Solution: • 2 • 3 • 4 • 5 • 6

23. 6.4 Polar Reactions • Molecules can contain local unsymmetrical electron distributions due to differences in electronegativities • This causes a partial negative charge on an atom and a compensating partial positive charge on an adjacent atom • The more electronegative atom has the greater electron density • Elements such as O, F, N, Cl more electronegative than carbon

24. p. 143

26. Polarizability • Polarization is a change in electron distribution as a response to change in electronic nature of the surroundings • Polarizability is the tendency to undergo polarization • Polar reactions occur between regions of high electron density and regions of low electron density

27. Polarizability Bonds inherently polar already can be made more polar by reactions with acids or bases. p. 144

28. Polarizability Bonds not inherently polar can be polarizable as interactions with solvent or other polar molecules effect the electron distribution. 2.66 2.58 2.55 2.55 Large atoms with loosely held electrons are more polarizable than small atoms with few tightly held electrons. So: S is more polarizable than O I is more polarizable than Cl p. 144

29. Generalized Polar Reactions • An electrophile, an electron-poor species, combines with a nucleophile, an electron-rich species • An electrophile is a Lewis acid • A nucleophile is a Lewis base • The combination is indicate with a curved arrow from nucleophile to electrophile

30. Fig. 5-1a, p. 145

31. Learning Check: Which of the following is likely to be a nucleophile and which an electrophile? p. 146

32. Solution: Which of the following is likely to be a nucleophile and which an electrophile? E E N N p. 146

33. Learning Check: Is BF3 is likely to be a nucleophile or an electrophile? p. 146

34. Solution: Is BF3 is likely to be a nucleophile or an electrophile? E p. 146

35. Which of the following is expected to be the worst nucleophile? Learning Check: • NH3 • H2O • BH3 • ethylene • (CH3) 3P

36. Which of the following is expected to be the worst nucleophile? Solution: • NH3 • H2O • BH3 • ethylene • (CH3) 3P

37. 6.5 An Example of a Polar Reaction: Addition of HBr to Ethylene • HBr adds to the  part of C-C double bond • The  bond is e- rich, allowing it to function as a nucleophile • H-Br is electron deficient at the H since Br is much more electronegative, making HBr an electrophile

38. Mechanism of Addition of HBr to Ethylene • HBr electrophile is attacked by  electrons of ethylene (nucleophile) to form a carbocation intermediate and bromide ion • Bromide adds to the positive center of the carbocation, which is an electrophile, forming a C-Br  bond • The result is that ethylene and HBr combine to form bromoethane • All polar reactions occur by combination of an electron-rich site of a nucleophile and an electron-deficient site of an electrophile

39. Fig. 5-2a, p. 148

40. Learning Check: What product would you expect? p. 142

41. Solution: What product would you expect? p. 142

42. 6.6 Using Curved Arrows in Polar Reaction Mechanisms • Curved arrows are a way to keep track of changes in bonding in polar reaction • The arrows track “electron movement” • Electrons always move in pairs • Charges change during the reaction • One curved arrow corresponds to one step in a reaction mechanism

43. Rules for Using Curved Arrows • The arrow (electrons) goes from the nucleophilic reaction site (Nu: or Nu:-) to the electrophilic reaction site (sink, E or E+) • The nucleophilic site can be neutral or negative

44. Rules for Using Curved Arrows The nucleophilic site can be negative or neutral

45. The electrophilic site can be positive or neutral

46. The octet rule must be followed The hydrogen already has two e-s so when another pair moves in the 2 already owned have to leave.

47. What is the role of the alkene in the reaction above? Learning Check: • electrophile • nucleophile • free radical • catalyst • Lewis acid

48. What is the role of the alkene in the reaction above? Solution: • electrophile • nucleophile • free radical • catalyst • Lewis acid

49. Learning Check: Add curved arrows to indicate the flow of electrons: p. 151

50. Solution: Add curved arrows to indicate the flow of electrons: p. 151