SN2 Reactions of Benzylic Halides
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1 SN2 Reactions of Benzylic Halides SN2 of benzylic halides is faster than allylic halides. How can this be explained if there is no carbocation intermediate? Slide 1 1
2 Allylic SN2 Faster: Two Arguments 1. steric hinderance (VWF) 2. molecular orbital interactions LUM of C-X bond can adopt a coplanar arrangement with p- orbitals of p-bond = electron delocalization over three orbitals = lower energy LUM = lower activation energy = faster reaction Why does lower energy LUM result in lower activation energy? Slide 2 2
3 Benzylic SN2 Faster: Two Arguments 1. steric hinderance (VWF) 2. molecular orbital interactions Cl LUM of C-X bond can adopt a coplanar arrangement with p- orbitals of p-bond = more p-orbitals in benzene than alkene = more electron donation into LUM of C-Cl bond = weaker C-X bond = faster reaction Slide 3 3
4 Preparation of Alkenylbenzenes Same methods used to prepare alkenes can be used to prepare alkenylbenzenes Dehydrogenation Dehydration Dehydrohalogenation Slide 4 4
5 Elimination of Benzylic Hydrogens Addition reactions to alkenes can also be applied to addition reactions to alkenylbenzenes Hydrogenation omination Slide 5 5
6 Regioselective Addition of Electrophiles to Alkenylbenzenes Addition to alkenylbenzenes favors (faster) formation of the more stable benzylic carbocation Slide 6 6
7 Self Test Question What is the major regioisomeric halohydrin formed by the reaciton below? 2, H 2 no light 0º C A B C D H H E University of Illinois at Chicago CHEM 232, Spring 2010 Slide 7 Lecture 25: April 15 7
8 Self Test Question What is the major regioisomeric halohydrin formed by the reaciton below? 2, H 2 no light 0º C δ + δ + A B C D H H E University of Illinois at Chicago CHEM 232, Spring 2010 Slide 7 Lecture 25: April 15 7
9 Self Test Question What is the major regioisomeric halohydrin formed by the reaciton below? 2, H 2 no light 0º C δ + δ + A B C D H H E University of Illinois at Chicago CHEM 232, Spring 2010 Slide 7 Lecture 25: April 15 7
10 CHEM 232 rganic Chemistry I at Chicago Lecture 26 rganic Chemistry 1 Professor Duncan Wardrop April 15,
11 Self Test Question With respect to cyclononatetraene, which process below will form an aromatic species? A. addition of one π-electron to give C9H10 B. addition of two π-electrons to give C9H10 2 C. loss of H + from the sp 3 -hybridized carbon to give C9H9 D. loss of H+ from an sp2-hybridized carbon to give C9H9 E. loss of :H from the sp3-hybridized carbon to give C9H9 + University of Illinois at Chicago CHEM 232, Spring 2010 Slide 9 Lecture 25: April 15 9
12 Self Test Question With respect to cyclononatetraene, which process below will form an aromatic species? A. addition of one π-electron to give C9H10 B. addition of two π-electrons to give C9H10 2 C. loss of H + from the sp 3 -hybridized carbon to give C9H9 D. loss of H+ from an sp2-hybridized carbon to give C9H9 E. loss of :H from the sp3-hybridized carbon to give C9H9 + University of Illinois at Chicago CHEM 232, Spring 2010 Slide 10 Lecture 25: April 15 10
13 Self Test Question With respect to cyclononatetraene, which process below will form an aromatic species? 2 A. addition of one π-electron to give C9H10 B. addition of two π-electrons to give C9H10 2 C. loss of H + from the sp 3 -hybridized carbon to give C9H9 D. loss of H+ from an sp2-hybridized carbon to give C9H9 E. loss of :H from the sp3-hybridized carbon to give C9H9 + University of Illinois at Chicago CHEM 232, Spring 2010 Slide 11 Lecture 25: April 15 11
14 Self Test Question With respect to cyclononatetraene, which process below will form an aromatic species? A. addition of one π-electron to give C9H10 B. addition of two π-electrons to give C9H10 2 C. loss of H + from the sp 3 -hybridized carbon to give C9H9 D. loss of H+ from an sp2-hybridized carbon to give C9H9 E. loss of :H from the sp3-hybridized carbon to give C9H9 + University of Illinois at Chicago CHEM 232, Spring 2010 Slide 12 Lecture 25: April 15 12
15 Self Test Question With respect to cyclononatetraene, which process below will form an aromatic species? A. addition of one π-electron to give C9H10 B. addition of two π-electrons to give C9H10 2 C. loss of H + from the sp 3 -hybridized carbon to give C9H9 D. loss of H+ from an sp2-hybridized carbon to give C9H9 E. loss of :H from the sp3-hybridized carbon to give C9H9 + University of Illinois at Chicago CHEM 232, Spring 2010 Slide 12 Lecture 25: April 15 12
16 Self Test Question With respect to cyclononatetraene, which process below will form an aromatic species? A. addition of one π-electron to give C9H10 B. addition of two π-electrons to give C9H10 2 C. loss of H + from the sp 3 -hybridized carbon to give C9H9 D. loss of H+ from an sp2-hybridized carbon to give C9H9 E. loss of :H from the sp3-hybridized carbon to give C9H9 + University of Illinois at Chicago CHEM 232, Spring 2010 Slide 13 Lecture 25: April 15 13
17 Self Test Question With respect to cyclononatetraene, which process below will form an aromatic species? A. addition of one π-electron to give C9H10 B. addition of two π-electrons to give C9H10 2 C. loss of H + from the sp 3 -hybridized carbon to give C9H9 D. loss of H+ from an sp2-hybridized carbon to give C9H9 E. loss of :H from the sp3-hybridized carbon to give C9H9 + University of Illinois at Chicago CHEM 232, Spring 2010 Slide 14 Lecture 25: April 15 14
18 Self Test Question Predict the product. A NH 2 B 1. 2, CH 2 Cl 2 2. NaNH 2 (excess) 3. H 2 C H 4. B 2 H 6, diglyme 5. H 2 2, NaH D H University of Illinois at Chicago CHEM 232, Spring 2010 E H Slide 15 Lecture 25: April 15 15
19 Self Test Question Predict the product. A NH 2 B 1. 2, CH 2 Cl 2 2. NaNH 2 (excess) 3. H 2 H C H B 2 H 6, diglyme 5. H 2 2, NaH 5. H D H University of H H Illinois at Chicago CHEM 232, Spring 2010 BH 2 E H Slide 15 Lecture 25: April 15 15
20 Self Test Question Predict the product. A NH 2 B 1. 2, CH 2 Cl 2 2. NaNH 2 (excess) 3. H 2 H C H B 2 H 6, diglyme 5. H 2 2, NaH 5. H D H University of H H Illinois at Chicago CHEM 232, Spring 2010 BH 2 E H Slide 15 Lecture 25: April 15 15
21 Review: Addition Reactions to Alkenes δ + C C + E Y δ E Y Electrophilic Addition (AdE) In all addition reactions, the alkene is the nucleophile (Lewis base: weakly held π-electrons are used/donated to form stronger s-bonds). Reagent being added to the alkene is an electrophile (Lewis acid: accepting electrons to form s-bonds). Slide 16 16
22 Review: Addition Reactions to Alkenes δ + C C + E Y δ E Y E C C Y Electrophilic Addition (AdE) In all addition reactions, the alkene is the nucleophile (Lewis base: weakly held π-electrons are used/donated to form stronger s-bonds). Reagent being added to the alkene is an electrophile (Lewis acid: accepting electrons to form s-bonds). Slide 16 16
23 Review: Addition Reactions to Alkenes δ + C C + E Y δ E Y variations of this mechanism apply to different electrophiles E C C Y Electrophilic Addition (AdE) In all addition reactions, the alkene is the nucleophile (Lewis base: weakly held π-electrons are used/donated to form stronger s-bonds). Reagent being added to the alkene is an electrophile (Lewis acid: accepting electrons to form s-bonds). Slide 16 16
24 Review: Addition Reactions to Alkenes C C + δ + δ H Cl H Cl H C C Cl Electrophilic Addition (AdE) In all addition reactions, the alkene is the nucleophile (Lewis base: weakly held π-electrons are used/donated to form stronger s-bonds). Reagent being added to the alkene is an electrophile (Lewis acid: accepting electrons to form s-bonds). Slide 17 17
25 Review: Addition Reactions to Alkenes C C + C C Electrophilic Addition (AdE) In all addition reactions, the alkene is the nucleophile (Lewis base: weakly held π-electrons are used/donated to form stronger s-bonds). Reagent being added to the alkene is an electrophile (Lewis acid: accepting electrons to form s-bonds). Slide 18 18
26 Review: Addition Reactions to Alkenes C C + H BH 2 H 2 B H H 2 B H Electrophilic Addition (AdE) In all addition reactions, the alkene is the nucleophile (Lewis base: weakly held π-electrons are used/donated to form stronger s-bonds). Reagent being added to the alkene is an electrophile (Lewis acid: accepting electrons to form s-bonds). Slide 19 19
27 Review: Addition Reactions to Alkenes C C + H H H H H Pd H Electrophilic Addition (AdE) In all addition reactions, the alkene is the nucleophile (Lewis base: weakly held π-electrons are used/donated to form stronger s-bonds). Reagent being added to the alkene is an electrophile (Lewis acid: accepting electrons to form s-bonds). Slide 20 20
28 Self Test Question Assuming that benzene reacts similarly to alkenes, which structure could not be an intermediate for AdE with the general electrophile depicted? δ + δ E Y E E E E E A B C D E University of Illinois at Chicago CHEM 232, Spring 2010 Slide 21 Lecture 25: April 15 21
29 Self Test Question Assuming that benzene reacts similarly to alkenes, which structure could not be an intermediate for AdE with the general electrophile depicted? δ + δ E Y E E E E E A B C D E University of Illinois at Chicago CHEM 232, Spring 2010 Slide 21 Lecture 25: April 15 21
30 Self Test Question Assuming that benzene reacts similarly to alkenes, which structure could not be an intermediate for AdE with the general electrophile depicted? δ + δ E Y E E E E E A B C D E University of Illinois at Chicago CHEM 232, Spring 2010 Slide 21 Lecture 25: April 15 21
31 Formation of Arenium Ion arenium ion: carbocation formed from an aromatic ring formation of arenium ions is slow since the ground state is so stable (loss of aromaticity) requires very reactive (high energy) electrophiles Slide 22 22
32 Arenium Ions arenium ion: also known as a s-complex arenium ions are allylic carbocations stabilized by resonance no aromatic Slide 23 23
33 Fate of Arenium Ion Electrophilic aromatic substitution (SE2Ar) Electrophilic addition (AdE2Ar) Slide 24 24
34 Fate of Arenium Ion Electrophilic aromatic substitution (SE2Ar) rearomatization Electrophilic addition (AdE2Ar) Slide 24 24
35 Potential Energy Diagram Slide 25 25
36 Preparation of Electrophiles Nitronium Ion N HN 3 + H 2 S 4 N + H 2 + HS 4 H N H nitric acid H H 2 + N H H N nitronium ion Slide 26 26
37 Nitration Slide 27 27
38 Preparation of Electrophiles Sulfur trioxide S H 2 S 4 S 3 + H 2 + HS 4 H H S nitric acid + H H H S H S H S sulfur trioxide Slide 28 28
39 Sulfonation Slide 29 29
40 Preparation of Electrophiles omine complex Slide 30 30
41 omination Slide 31 31
42 Preparation of Electrophiles Carbocations R X + AlCl 3 (R = 3º or 2º) R + + AlCl 4 Slide 32 32
43 Friedel-Crafts Alkylation Slide 33 33
44 Preparation of Electrophiles Acylium Ions R C R C R Cl + AlCl 3 R C + AlCl 4 Slide 34 34
45 Friedel-Crafts Acylation Slide 35 35
46 Summary of Some Representative Electrophilic Aromatic Substitutions nitration H HN 3, H 2 S N 4 2 sulfonation H H 2 S 4 S 2 H bromination H 2, Fe 3 alkylation acylation H H RCl, AlCl 3 R Cl AlCl 3 R R Slide 36 36
47 Self Test Question If anhydrides also react with AlCl3 to give acylium ions, what is the product for the acylation reaction below? A B CH 3 CH 3 + H 3 C CH 3 acetic anhydride AlCl 3 C D CH 3 H E CH 3 University of Illinois at Chicago CHEM 232, Spring 2010 Slide 37 Lecture 25: April 15 37
48 Self Test Question If anhydrides also react with AlCl3 to give acylium ions, what is the product for the acylation reaction below? A B CH 3 CH 3 + H 3 C CH 3 acetic anhydride AlCl 3 C D H CH 3 H 3 C CH 3 H3C C + acylium ion AlCl 3 CH 3 E CH 3 University of Illinois at Chicago CHEM 232, Spring 2010 Slide 37 Lecture 25: April 15 37
49 Self Test Question If anhydrides also react with AlCl3 to give acylium ions, what is the product for the acylation reaction below? A B CH 3 CH 3 + H 3 C CH 3 acetic anhydride AlCl 3 C D H CH 3 H 3 C CH 3 H3C C + acylium ion AlCl 3 CH 3 E CH 3 University of Illinois at Chicago CHEM 232, Spring 2010 Slide 37 Lecture 25: April 15 37
50 Complication of Friedel-Crafts Alkylations rearrangement to more stable carbocations always predominates 1º alkyl chlorides (except ethyl chloride) cannot be employed in FC alkylations Slide 38 38
51 Reduction of Aryl Ketones Clemmensen Reduction Zn(Hg) HCl Wolf-Kishner Reduction H 2 NNH 2, KH Slide 39 39
52 Alternative Strategy to Alkybenzenes R R + Cl R Slide 40 40
53 Alternative Strategy to Alkybenzenes methylene group (CH2) R R + Cl R Slide 40 40
54 Self Test Question CN Predict the product. A N N Cl 1. benzene, AlCl 3 2. Zn(Hg), HCl 3. NBS, CHCl 3 4. NaCN B C N CN CN N D CN N University of Illinois at Chicago CHEM 232, Spring 2010 Slide 41 Lecture 25: April 15 41
55 Self Test Question CN Predict the product. A N 1. N N Cl 1. benzene, AlCl 3 2. Zn(Hg), HCl 3. NBS, CHCl 3 4. NaCN 2. N 3. N 4. B C D N CN CN N CN N University of Illinois at Chicago CHEM 232, Spring 2010 Slide 41 Lecture 25: April 15 41
56 Self Test Question CN Predict the product. A N 1. N N Cl 1. benzene, AlCl 3 2. Zn(Hg), HCl 3. NBS, CHCl 3 4. NaCN 2. N 3. N 4. B C D N CN CN N CN N University of Illinois at Chicago CHEM 232, Spring 2010 Slide 41 Lecture 25: April 15 41
57 CHEM 232 rganic Chemistry I at Chicago Next Lecture... Chapter 12: Sections Quiz Next Week... Synthesis Problem Chapters 11 & 12 42
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