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CHE 240 Unit IV Stereochemistry, Substitution and Elimination Reactions CHAPTER SIX Terrence P. Sherlock Burlington County College 2004.

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Presentation on theme: "CHE 240 Unit IV Stereochemistry, Substitution and Elimination Reactions CHAPTER SIX Terrence P. Sherlock Burlington County College 2004."— Presentation transcript:

1 CHE 240 Unit IV Stereochemistry, Substitution and Elimination Reactions CHAPTER SIX Terrence P. Sherlock Burlington County College 2004

2 Chapter 62 Polarity and Reactivity Halogens are more electronegative than C. Carbon-halogen bond is polar, so carbon has partial positive charge. Carbon can be attacked by a nucleophile. Halogen can leave with the electron pair. =>

3 Chapter 63 Classes of Alkyl Halides Methyl halides: only one C, CH 3 X Primary: C to which X is bonded has only one C-C bond. Secondary: C to which X is bonded has two C-C bonds. Tertiary: C to which X is bonded has three C-C bonds. =>

4 Chapter 64 Classify These: =>

5 Chapter 65 Dihalides Geminal dihalide: two halogen atoms are bonded to the same carbon Vicinal dihalide: two halogen atoms are bonded to adjacent carbons. =>

6 Chapter 66 IUPAC Nomenclature Name as haloalkane. Choose the longest carbon chain, even if the halogen is not bonded to any of those C’s. Use lowest possible numbers for position. =>

7 Chapter 67 Systematic Common Names Name as alkyl halide. Useful only for small alkyl groups. Name these: =>

8 Chapter 68 “Trivial” Names CH 2 X 2 called methylene halide. CHX 3 is a haloform. CX 4 is carbon tetrahalide. Examples:  CH 2 Cl 2 is methylene chloride  CHCl 3 is chloroform  CCl 4 is carbon tetrachloride. =>

9 Chapter 69 Uses of Alkyl Halides Solvents - degreasers and dry cleaning fluid Reagents for synthesis of other compounds Anesthetic: Halothane is CF 3 CHClBr  CHCl 3 used originally (toxic and carcinogenic) Freons, chlorofluorocarbons or CFC’s  Freon 12, CF 2 Cl 2, now replaced with Freon 22, CF 2 CHCl, not as harmful to ozone layer. Pesticides - DDT banned in U.S. =>

10 Chapter 610 Dipole Moments  = 4.8 x  x d, where  is the charge (proportional to  EN) and d is the distance (bond length) in Angstroms. Electronegativities: F > Cl > Br > I Bond lengths: C-F < C-Cl < C-Br < C-I Bond dipoles: C-Cl > C-F > C-Br > C-I 1.56 D 1.51 D 1.48 D 1.29 D Molecular dipoles depend on shape, too! =>

11 Chapter 611 Boiling Points Greater intermolecular forces, higher b.p.  dipole-dipole attractions not significantly different for different halides  London forces greater for larger atoms Greater mass, higher b.p. Spherical shape decreases b.p. (CH 3 ) 3 CBr CH 3 (CH 2 ) 3 Br 73  C 102  C =>

12 Chapter 612 Densities Alkyl fluorides and chlorides less dense than water. Alkyl dichlorides, bromides, and iodides more dense than water. =>

13 Chapter 613 Halogenation of Alkanes All H’s equivalent. Restrict amount of halogen to prevent di- or trihalide formation Highly selective: bromination of 3  C =>

14 Chapter 614 Allylic Halogenation Allylic radical is resonance stabilized. Bromination occurs with good yield at the allylic position (sp 3 C next to C=C). Avoid a large excess of Br 2 by using N-bromosuccinimide (NBS) to generate Br 2 as product HBr is formed. =>

15 Chapter 615 Reaction Mechanism Free radical chain reaction  initiation, propagation, termination. =>

16 Chapter 616 Substitution Reactions The halogen atom on the alkyl halide is replaced with another group. Since the halogen is more electronegative than carbon, the C-X bond breaks heterolytically and X - leaves. The group replacing X - is a nucleophile. =>

17 Chapter 617 Elimination Reactions The alkyl halide loses halogen as a halide ion, and also loses H + on the adjacent carbon to a base. A pi bond is formed. Product is alkene. Also called dehydrohalogenation (-HX). =>

18 Chapter 618 S N 2 Mechanism Bimolecular nucleophilic substitution. Concerted reaction: new bond forming and old bond breaking at same time. Rate is first order in each reactant. Walden inversion. =>

19 Chapter 619 S N 2 Energy Diagram One-step reaction. Transition state is highest in energy. =>

20 Chapter 620 Uses for S N 2 Reactions Synthesis of other classes of compounds. Halogen exchange reaction. =>

21 Chapter 621 S N 2: Nucleophilic Strength Stronger nucleophiles react faster. Strong bases are strong nucleophiles, but not all strong nucleophiles are basic. =>

22 Chapter 622 Trends in Nuc. Strength Of a conjugate acid-base pair, the base is stronger: OH - > H 2 O, NH 2 - > NH 3 Decreases left to right on Periodic Table. More electronegative atoms less likely to form new bond: OH - > F -, NH 3 > H 2 O Increases down Periodic Table, as size and polarizability increase: I - > Br - > Cl - =>

23 Chapter 623 Polarizability Effect =>

24 Chapter 624 Bulky Nucleophiles Sterically hindered for attack on carbon, so weaker nucleophiles. =>

25 Chapter 625 Solvent Effects (1) Polar protic solvents (O-H or N-H) reduce the strength of the nucleophile. Hydrogen bonds must be broken before nucleophile can attack the carbon. =>

26 Chapter 626 Solvent Effects (2) Polar aprotic solvents (no O-H or N-H) do not form hydrogen bonds with nucleophile Examples: =>

27 Chapter 627 Crown Ethers Solvate the cation, so nucleophilic strength of the anion increases. Fluoride becomes a good nucleophile. =>

28 Chapter 628 S N 2: Reactivity of Substrate Carbon must be partially positive. Must have a good leaving group Carbon must not be sterically hindered. =>

29 Chapter 629 Leaving Group Ability Electron-withdrawing Stable once it has left (not a strong base) Polarizable to stabilize the transition state. =>

30 Chapter 630 Structure of Substrate Relative rates for S N 2: CH 3 X > 1° > 2° >> 3° Tertiary halides do not react via the S N 2 mechanism, due to steric hindrance. =>

31 Chapter 631 Stereochemistry of S N 2 Walden inversion =>

32 Chapter 632 S N 1 Reaction Unimolecular nucleophilic substitution. Two step reaction with carbocation intermediate. Rate is first order in the alkyl halide, zero order in the nucleophile. Racemization occurs. =>

33 Chapter 633 S N 1 Mechanism (1) Formation of carbocation (slow) =>

34 Chapter 634 S N 1 Mechanism (2) Nucleophilic attack => Loss of H + (if needed)

35 Chapter 635 S N 1 Energy Diagram Forming the carbocation is endothermic Carbocation intermediate is in an energy well. =>

36 Chapter 636 Rates of S N 1 Reactions 3° > 2° > 1° >> CH 3 X  Order follows stability of carbocations (opposite to S N 2)  More stable ion requires less energy to form Better leaving group, faster reaction (like S N 2) Polar protic solvent best: It solvates ions strongly with hydrogen bonding. =>

37 Chapter 637 Stereochemistry of S N 1 Racemization: inversion and retention =>

38 Chapter 638 Rearrangements Carbocations can rearrange to form a more stable carbocation. Hydride shift: H - on adjacent carbon bonds with C +. Methyl shift: CH 3 - moves from adjacent carbon if no H’s are available. =>

39 Chapter 639 Hydride Shift =>

40 Chapter 640 Methyl Shift =>

41 Chapter 641 S N 2 or S N 1? Primary or methyl Strong nucleophile Polar aprotic solvent Rate = k[halide][Nuc] Inversion at chiral carbon No rearrangements Tertiary Weak nucleophile (may also be solvent) Polar protic solvent, silver salts Rate = k[halide] Racemization of optically active compound Rearranged products =>

42 Chapter 642 E1 Reaction Unimolecular elimination Two groups lost (usually X - and H + ) Nucleophile acts as base Also have S N 1 products (mixture) =>

43 Chapter 643 E1 Mechanism Halide ion leaves, forming carbocation. Base removes H + from adjacent carbon. Pi bond forms. =>

44 Chapter 644 A Closer Look =>

45 Chapter 645 E1 Energy Diagram Note: first step is same as S N 1 =>

46 Chapter 646 E2 Reaction Bimolecular elimination Requires a strong base Halide leaving and proton abstraction happens simultaneously - no intermediate. =>

47 Chapter 647 E2 Mechanism =>

48 Chapter 648 Saytzeff’s Rule If more than one elimination product is possible, the most-substituted alkene is the major product (most stable). R 2 C=CR 2 >R 2 C=CHR>RHC=CHR>H 2 C=CHR tetra > tri > di > mono => minor major

49 Chapter 649 E2 Stereochemistry =>

50 Chapter 650 E1 or E2? Tertiary > Secondary Weak base Good ionizing solvent Rate = k[halide] Saytzeff product No required geometry Rearranged products Tertiary > Secondary Strong base required Solvent polarity not important Rate = k[halide][base] Saytzeff product Coplanar leaving groups (usually anti) No rearrangements =>

51 Chapter 651 Substitution or Elimination? Strength of the nucleophile determines order: Strong nuc. will go S N 2 or E2. Primary halide usually S N 2. Tertiary halide mixture of S N 1, E1 or E2 High temperature favors elimination. Bulky bases favor elimination. Good nucleophiles, but weak bases, favor substitution. =>

52 Chapter 652 Secondary Halides? Mixtures of products are common. =>

53 Chapter 653


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