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ORGANOHALIDES + Nucleophilic Reactions (S N 1/2, E1/E2/E1cB) CH21 PS CLASS.

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Presentation on theme: "ORGANOHALIDES + Nucleophilic Reactions (S N 1/2, E1/E2/E1cB) CH21 PS CLASS."— Presentation transcript:

1 ORGANOHALIDES + Nucleophilic Reactions (S N 1/2, E1/E2/E1cB) CH21 PS CLASS

2 Preparation of Organohalides From ALKENES C=C [just review old lessons] FOR TERTIARY ALCOHOLS, we can simply use H-X (gas) X=Cl,Br in ether, 0°C

3 Preparation of Organohalides FOR TERTIARY ALCOHOLS, we can simply use H-X (gas) X=Cl,Br in ether, 0°C – Follows S N 1 so a carbocation is formed, – be careful with rearrangements!

4 Preparation of Organohalides FOR PRIMARY/SECONDARY ALCOHOLS: SOCl 2 / PBr 3

5 Practice

6 Alkyl Fluorides Also from ALCOHOLS + HF / Pryidine (CH 3 CH 2 ) 2 NSF 3

7 Grignard Reagents Reaction of R-X with Mg over ether/THF to form R-Mg-X organometallic compound.

8 Grignard Reagents: reduction of R-X

9 More samples:

10 Nucleophilic Reactions R-X, alkyl halides are ELECTROPHILES (positive or electron-poor) They react with NUCLEOPHILES/BASES (negative or electron-rich) Either substitution – C-C-X becomes C-C-blah + X- or elimination reactions – C-C-X becomes C=C + X-

11 SUBSTITUTION REACTIONS S – substitution: R-X + Nu  R-Nu + X- N – Nucleophilic 1 or 2  unimolecular or bimolecular rates INVERSION (change of stereochemistry) CAN HAPPEN!

12 Try this first…

13 S N 2  BIMOLECULAR Bimolecular simply refers to the rate depending on BOTH reactants because of the nature of the mechanism Rate = k[RX][Nu] Rate depends on both because there is ONE SINGLE COLLISION BETWEEN RX and Nu to form a Nu-R-X transition state

14 S N 2  BIMOLECULAR 100% INVERSION OF STEREOCHEMISTRY OCCURS! SUBSTRATE LEAVING GROUP

15 Factors that affect S N 2 RXNS: STERIC EFFECTS TO INCOMING Nu: – C=C-X (vinylic) and Ar-X (aryl) TOTALL UNREACTIVE

16 Factors that affect S N 2 RXNS: THE NUCLEOPHILE

17 Factors that affect S N 2 RXNS: THE LEAVING GROUP should be stable on its own as a free anion Comparing halides, we go down the column

18 Factors that affect S N 2 RXNS: Alcohols and fluorides usually do not undergo SN2 because OH- and F- aren’t good leaving groups This is why we use SOCl 2 and PBr 3 … THEY CONVERT THE –OH INTO A BETTER LEAVING GROUP

19 Factors that affect S N 2 RXNS: Reaction SOLVENT can also affect the reaction. We prefer POLAR APROTIC SOLVENTS – POLAR but no –OH or –NH in the molecule (no H 2 O, NH 3, etc…) Polar protic solvents form a CAGE around Nu

20 Practice

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24 S N 1  UNIMOLECULAR Unimolecular: rate depends only on the substrate (mechanism), almost opposite of SN2 Rate = k[RX] Rate is only dependent on the slowest step which is the spontaneous dissociation of your leaving group. (molecules just don’t easily dissociate!)

25 S N 1  UNIMOLECULAR

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27 STEREOCHEM IS LOST, A RACEMATE FORM IS MADE, but usually not 50:50

28 S N 1  UNIMOLECULAR STEREOCHEM IS LOST, A RACEMATE FORM IS MADE, but usually not 50:50 An ION PAIR BLOCKS THE OTHER SIDE!

29 Factors that affect S N 1 RXNS: SUBSTRATE:

30 Factors that affect S N 1 RXNS: LEAVING GROUP: An –OH in acidic medium can become –OH 2 + and leave as H 2 O which is very favorable

31 Factors that affect S N 1 RXNS: NUCLEOPHILE: no effect, almost at all.

32 Factors that affect S N 1 RXNS: SOLVENT: rates increase if you stabilize carbocation transition state. POLAR PROTIC!

33 Factors that affect S N 1 RXNS: SOLVENT: rates increase if you stabilize carbocation transition state. POLAR PROTIC!

34 PRACTICE

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41 Elimination Reactions More compliated (different mechanisms) The loss of H-X can lead to a MIXTURE of alkene products (C-C-X  C=C + HX) But we can predict the most stable/major poduct ZAITZEV’S RULE: base-induced eliminations will form more stable alkene

42 E2 elimination Again, bimolecular so a single collision between your Base B: and the alkyl halide.

43 E2 elimination Anti-periplanar is favored for transition state

44 E2 elimination Anti-periplanar is favored for transition state

45 Practice

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49 E1 reaction Unimolecular, ALSO spontaneously forms carbocation, but then followed by loss of H+ (taken by a base B: and not an attack by Nu:) COMPETES WITH SN1 reactions!

50 E1 reaction

51 E1 reactions No need for anti periplanar geometry

52 PRACTICE

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54 E1cB Unimolecular, but this time CARBANION formed because a proton H+ is first removed by a base. cB stands for “conjugate base” because you deprotonate your carbon C-H into a C- and H+ Usually favored for poor leaving groups (e.g. – OH) Carbanion can be stabilized with C=O groups nearby

55 E1cB

56 PRESENCE OF C=O NEARBY CAN GIVE RESONANCE STABILIZATION!

57 PREDICTING WHAT PREDOMINATES:

58 Slight Clarifications: BASE vs. NUCLEOPHILE BASE Affinity for a PROTON Strong base like R-O - or OH- NUCLEOPHILE Usually a LEWIS BASE In this context, how attracted to a CARBON

59 PRACTICE

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