CHEM 232 rganic Chemistry I at Chicago Lecture 27 rganic Chemistry 1 Professor Duncan Wardrop April 20, 2010 1
Self Test Question Nitrosonium (not nitronium) cations can be generated by treating sodium nitrite (NaN2) with a strong acid. This relatively weak electrophile can undergo SEAr with phenol. What is the major product when phenol reacts with a nitrosonium ion? H Na N H + x2 H 2 N A B N 2 NNa D E H N NH2 N nitrosonium ion C H NNa University of Illinois at Chicago CHEM 232, Spring 2010 Slide 2 2
Self Test Question Nitrosonium (not nitronium) cations can be generated by treating sodium nitrite (NaN2) with a strong acid. This relatively weak electrophile can undergo SEAr with phenol. What is the major product when phenol reacts with a nitrosonium ion? H + N A B N 2 NNa D E H H N NH2 phenol nitrosonium ion C H NNa University of Illinois at Chicago CHEM 232, Spring 2010 Slide 3 3
Self Test Question Which substituent would not activate an aromatic ring toward SEAr (i.e. is not an electron donating group)? N A B C D E CH3 C N 2 NH 2 University of Illinois at Chicago CHEM 232, Spring 2010 Slide 4 4
Strongly Activating Substituents HN 3 H 2 S 4 N 2 k rel = 1.0 HN N 3 2 + H 2 S 4 k rel = 25 N 2 H H H HN N 3 2 + H 2 S 4 k rel = 1,000 N 2 Slide 5 5
Review: Alkyl Groups are Activating and rtho/para Directing N ortho nitration N 2 N 2 N 2 2º 2º 3º N para nitration N 2 N 2 N 2 2º 3º 2º Both ortho nitration and para nitration provide an arenium ion with a 3º carbocation contributor. Slide 6 6
Strongly Activating Substituents H N ortho nitration H N 2 H N 2 H N 2 H N para nitration H H H N 2 N 2 N 2 Why is a hydroxyl group (-H) more electron donating and thus more activating than a methyl group? Slide 7 7
Strongly Activating Substituents H N ortho nitration H N 2 H N 2 H N 2 H N 2 H N para nitration H H H H N 2 N 2 N 2 N 2 Additional resonance structure for ortho & para SEAr when substituent has a lone pair of electrons = more stable arenium ion = faster SEAr (more activated) Slide 8 8
Summary of Reactivity & Directing Effects Classification of Substituents in Electrophilic Aromatic Substitution (SEAr) Very strongly activating Strongly activating Activating Standard = H Deactivating Strongly Deactivating Very strongly deactivating Activating: greater electron donation = more stable arenium ion = faster SEAr Slide 9 9
Summary of Reactivity & Directing Effects Classification of Substituents in Electrophilic Aromatic Substitution (SEAr) Very strongly activating Strongly activating Activating Standard = H Deactivating Strongly Deactivating Very strongly deactivating NH 2 1º amine (amino) H N R 2º amine (alkylamino) H alcohol (hydroxyl) R N R 3º amine (dialkylamino) Slide 10 10
Summary of Reactivity & Directing Effects Classification of Substituents in Electrophilic Aromatic Substitution (SEAr) Very strongly activating Strongly activating Activating Standard = H Deactivating Strongly Deactivating Very strongly deactivating H N R amide (acylamino) R ether (alkoxy) R ester (acyloxy) Slide 11 11
Degree of Electron Donation The acyl group on nitrogen increases the delocalization of the lone pair of electrons, thus decreasing its ability to be dontated to the aromatic ring. H NH 2 N R vs. H N R Slide 12 12
Summary of Reactivity & Directing Effects Classification of Substituents in Electrophilic Aromatic Substitution (SEAr) Very strongly activating Strongly activating Activating Standard = H Deactivating Strongly Deactivating Very strongly deactivating R alkane (alkyl) C C alkene (alkenyl) Ar aromatic (aryl) Slide 13 13
Summary of Reactivity & Directing Effects Classification of Substituents in Electrophilic Aromatic Substitution (SEAr) Very strongly activating Strongly activating Activating Standard = H Deactivating Strongly Deactivating Very strongly deactivating All are also ortho/ para directors Slide 14 14
Summary of Reactivity & Directing Effects Classification of Substituents in Electrophilic Aromatic Substitution (SEAr) Very strongly activating Strongly activating Activating Standard = H Deactivating Strongly Deactivating Very strongly deactivating Deactivating greater electron withdrawal = less stable arenium ion = slower SEAr Slide 15 15
Summary of Reactivity & Directing Effects Classification of Substituents in Electrophilic Aromatic Substitution (SEAr) Very strongly activating Strongly activating Activating Standard = H Deactivating Strongly Deactivating Very strongly deactivating ortho/para directors X halogen (halo) X halomethyl Slide 16 16
Summary of Reactivity & Directing Effects X N ortho nitration X N 2 X N 2 X N 2 X N 2 X N para nitration X X X X N 2 N 2 N 2 N 2 Although halogens deactivate aromatic rings through inductive effect, they are still ortho/ para directors since they have lone pairs. Slide 17 17
Summary of Reactivity & Directing Effects Classification of Substituents in Electrophilic Aromatic Substitution (SEAr) Very strongly activating Strongly activating Activating Standard = H Deactivating Strongly Deactivating Very strongly deactivating C H aldehyde (formyl) C Cl acid chloride (acyl chloride) C R ketone (acyl) C N nitrile (cyano) C H carboylic acid (carboxyl) C R ester (alkyl carboxylate) H S sulfonic acid (sulfonyl) Slide 18 18
Summary of Reactivity & Directing Effects Classification of Substituents in Electrophilic Aromatic Substitution (SEAr) Very strongly activating Strongly activating Activating Standard = H Deactivating Strongly Deactivating Very strongly deactivating C C Slide 19 19
Summary of Reactivity & Directing Effects Classification of Substituents in Electrophilic Aromatic Substitution (SEAr) Very strongly activating Strongly activating Activating Standard = H Deactivating Strongly Deactivating Very strongly deactivating CF 3 N 2 trifluoromethyl nitro Slide 20 20
Summary of Reactivity & Directing Effects Classification of Substituents in Electrophilic Aromatic Substitution (SEAr) Very strongly activating Strongly activating Activating Standard = H Deactivating Strongly Deactivating Very strongly deactivating All are also meta directors Slide 21 21
Self Test Question List the aromatic compounds below in order of increasing rate of SEAr. Br a b c d e University of Illinois at Chicago CHEM 232, Spring 2010 A. a,b,c,d,e B. e,d,c,b,a C. b,e,d,c,a D. d,e,c,b,a E. c,a,b,d,e Slide 22 22
Multiple Substituent Effects H 3 C AlCl 3 All positions on the aromatic ring are equivalent. Slide 23 23
Multiple Substituent Effects Br 2 Br Fe N 2 N 2 Directing effects reinforce each other; substitution here takes place ortho to the methyl group and meta to the nitro group. Slide 24 24
Multiple Substituent Effects NH Cl Br 2 acetic acid NH Br Cl Regioselectivity is controlled by most activating substituent. Slide 25 25
Multiple Substituent Effects HN 3 N 2 H 2 S 4 C( ) 3 C( ) 3 When activating effects are similar, substitution occurs ortho to smallest substituent (least sterics) Slide 26 26
Multiple Substituent Effects HN 3 H 2 S 4 N 2 When electronic effects are similar/same, steric effects control regioselectivity; here substitution occurs at least sterically hindered ortho position. Slide 27 27
Self Test Question Predict the major product. A H 3 C S 3 H Br B H 3 C S 3 H Br H 3 C S 3 H Br 2, FeBr 3 C H 3 C S 3 H D H 3 C Br Br S 3 H University of Illinois at Chicago CHEM 232, Spring 2010 Slide 28 28
SEAr in Synthesis Br Br X Br + Cl Br + Cl Slide 29 29
Self Test Question Which retrosynthesis is not feasable? A. S 3 H B. N 2 C. 2 N D. H H University of Illinois at Chicago CHEM 232, Spring 2010 Slide 30 30
CHEM 232 rganic Chemistry I at Chicago Quiz This Week... Synthesis Problem Chapters 11 & 12 31