Bowman Chem 345 Lecture Notes by Topic Electrophilic Aromatic Substitution (EAS): Aromatic rings have a tendency to be unreactive due to their inherent stability. However, aromatic rings can react given the right incentives. One way, they can react is with strong electrophiles. In a typical EAS reaction, a hydrogen is replaced by an electrophile. General Scheme: H E E+ The reaction mechanism for EAS reactions follows three parts. 1.) Formation of the electrophile. This is dependent on the electrophile. 2.) Reaction of a pi bond from the benzene ring to form the sigma intermediate 3.) Deprotonation of the sigma intermediate to reform the aromatic ring. This step is irreversible. General Mechanism: The placement of the electrophile on the aromatic ring and the rate of the reaction is dependent on the substituents on the aromatic ring. The following list of groups on aromatic rings is a good list to know. Activating Groups Deactivating Groups O OR O R NR2 OR R H X SO3H CN NO2 NR3 > > > > > ~ > > > > R=H or sp3 C X=Halogen meta directors ortho/para directors Bowman Chem 345 Lecture Notes by Topic Y The terms ortho, meta, and para are used to show the relative ortho ortho positions between two groups on a benzene ring. Groups ortho to each other have a 1,2 relationship, meta 1,3, and para 1,4. meta meta para You can often predict whether a group is going to be an ortho/para or a meta director from resonance structures. The incoming electrophile wants to react with the carbons with the highest electron density. Me Me Me Me O O O O The sites ortho/para to the OMe bare a negative charge, so those sites have the highest electron density. OMe is an ortho/para director. O O O O O O O O N N N N The sites ortho/para to the NO2 bare a positive charge, so those sites have the lowest electron density. NO is a meta director. 2 A more acceptable way of explaining the activating/directing effects of various substituents is to look at the energy of the reaction. Energy Diagram The rate determining step is the addition of the electrophile to the benzene ring. By the Hammond postulate, the transition state most closely resembles the structure closest in energy to it. In this case, the sigma intermediate is closest in energy to the transition state, therefore the more stable the carbocation intermediate, the more stable the transition state, and the faster the reaction. This can be used to explain why some groups direct ortho/para and some meta. Bowman Chem 345 Lecture Notes by Topic Alkyl Groups: ortho E+ E E E major meta E+ E E E para E+ E E E major In the case of alkyl groups, the resonance structures of the sigma intermediate have a tertiary carbocation when the electrophile adds ortho or para to the alkyl group. If an electrophile adds meta, then only resonance structures with secondary carbocations can be formed. The presence of an alkyl group adjacent to a carbocation lowers the energy by hyperconjugation (the carbocation feeds on some of the electron density from the bonds attached to adjacent sp3 C’s). This lowers the energy of the intermediates (and the subsequent transition states) leading to ortho and para products but not to the meta products. Bowman Chem 345 Lecture Notes by Topic The energies are a bit exaggerated. Bowman Chem 345 Lecture Notes by Topic Electron Withdrawing Groups: Carbonyls, NO2, Nitriles (meta directors) O O O O ortho E+ E E E destabilizing O O O O meta E+ E E E O O O O para E+ E E E destabilizing You can draw the same intermediates for electron withdrawing groups. In this case, the more substituted carbocations in the ortho and para cases are actually destabilizing structures. Electron withdrawing groups raise the energy of the carbocations by pulling electron density away from them. The meta intermediates do not have any stabilizing interactions, but at least they do not have destabilizing interactions. Bowman Chem 345 Lecture Notes by Topic Strong Electron Donating Groups: OR, NR2 (ortho-para directors) OMe E major ortho OMe OMe OMe OMe E+ E E E meta OMe OMe OMe OMe E+ E E E para OMe OMe OMe OMe E+ E E E OMe major E In the case of strong electron donating groups, the resonance structures of the sigma intermediate have an extra resonance structure (that has a complete octet) when the electrophile adds ortho or para to the electron donating transition states leading to ortho and para products. This drastically lowers the barrier to the reaction. Bowman Chem 345 Lecture Notes by Topic Weak Electron Donating Groups: Halogens (ortho-para directors) Cl E ortho Cl Cl Cl Cl E+ E E E meta Cl Cl Cl Cl E+ E E E para Cl Cl Cl Cl E+ E E E Cl E In the case of weak electron donating groups, the resonance structures of the sigma intermediate still have an extra resonance structure which stabilized addition to the ortho and para positions, but the halogens are weak pi donors. They are also very electronegative which destabilizes all of the structures via the inductive effect. While the electron density donated (through the pi system) is enough to direct to ortho and para positions, the electron density withdrawn through the sigma system (inductive effect) makes the reaction proceed at a slower rate. (The inductive effect applies to nitrogen and oxygen as well. But N and O are much stronger pi donors and that makes up for any electron density being pulled away through the sigma system). Bowman Chem 345 Lecture Notes by Topic Predicting the product(s). Typically ortho and para directors give mixtures of ortho and para products. The para product is favored by sterics and the ortho product is favored by statistics (two ortho positions as compared to one para position). In most cases, the para product is the major product. I am okay with you defaulting to para being the major product. If you have more than one directing group, then see if there is overlap between the directing group and that will be the most likely product. If there is not overlap between the two, then focus on the most activating group. The major product will likely be the one from that group. If there is a tie, use sterics to break the tie. E+ para beats ortho E E+ both direct to the same spot O N 2 O2N E + E E OMe is a stronger activating group than Me MeO MeO E+ There is a tie, so electrophile goes to the spot directed by both groups E but is less sterically hindered. Bowman Chem 345 Lecture Notes by Topic We have been using the E+ as a generic electrophile. The main groups we are going to be using in EAS reactions are Br, Cl, NO2, and carbonyls. The generic reaction conditions for each of these reactions are as follows: Br 2 Br FeBr3 Can work on aromatic Cl 2 Cl rings that are strongly AlCl 3 deactivated (ie: meta director) O H SO 2 4 N HNO3 O O O Cl AlCl3 O 1.) POCl3, DMF 2.) H2O H There are limited examples of Friedel-Crafts/Vilsmeier-Haack reactions working on deactivated rings (with a meta director). For the purposes of this course, a Friedel-Crafts reaction or Vilsmaier- Haack reaction will need a strong electron donating group (ie: O, N) when a meta director is on the aromatic ring. Bowman Chem 345 Lecture Notes by Topic The actual conditions/reagents in the lab that you use may vary depending on the starting material. These will be the standard conditions that you should use in this course. At the end of these notes, there are a couple of variants that use the same principles, but have been optimized to give better yields. Do not memorize the variants. They are only there to serve as examples of what is possible. Halogen Electrophiles: General Conditions: Br 2 Br FeBr3 Can work on aromatic rings that are strongly Cl 2 Cl deactivated (ie: meta director) AlCl3 Bromination: Chlorination: Bowman Chem 345 Lecture Notes by Topic Literature examples:1,2,3,4 O O O O Cl2 Br Cl 2 Br AlCl3 AlCl3 Br Br TBCA O N O OMe O no reaction OMe NBS N after 1 week O CH CO H N N MeCN 3 2 Br Br Br (63%) NBS O TBCA OMe NBS OMe TBCA Br Br Br MeCN + + Br CF CO H (97%) 3 2 Br Br 15 min Br (73%) (7%) (1%) NH NH NH2 NBS 2 2 + O N TBCA 2 O2N Br MeCN Br OMe OMe (48%) (11%) CF3CO2H 24 h (80%) Cl Cl O OMe NCS OMe N O ZrCl4 CH Cl NCS 2 2 Br OMe OMe OMe NBS OMe NCS ZrCl4 ZrCl4 CH2Cl2 CH2Cl2 OMe OMe Cl NCS Cl NBS ZrCl ZrCl4 4 O CH Cl CH2Cl2 O 2 2 Br Br OMe Br2/HBr OMe Br2/HBr NH2 NH2 AcOH AcOH Br Br Br Br H H N Br2/HBr N O AcOH O Br 1 Zaczek, N. M.; Tyszkiewicz, R. B. J. Chem. Ed. 1986, 63, 484. 2 Zysman-Colman, E.; et. al. Can. J. Chem. 2009, 87, 440-447. 3 De Almeida, L. S.; De Mattos, M. C. S.; Esteves, P. M. Synlett 2013, 24, 603-606. 4 Zhang, Y.; Shibatomi, K.; Yamamoto, H. Synlett 2005, 18, 2837-2842. Bowman Chem 345 Lecture Notes by Topic Nitration: Bowman Chem 345 Lecture Notes by Topic Friedel-Crafts Acylation: Bowman Chem 345 Lecture Notes by Topic Vilsmeier-Haack Reaction: There are a couple of examples in the literature of placing a carbonyl on a deactivated aromatic ring by using the Vilsmeier-Haack conditions5 or Friedel- Crafts6 but these examples are exceedingly rare.