Aromatic compounds are less reactive than many other molecules because of their stability

Transcription

1 rganic Chemistry otes by Jim Maxka C 16: Reactions of Aromatic Compounds Topics: Aromatic compounds are less reactive than many other molecules because of their stability Formation of very T electrophiles can lead to AS lectrophilic Aromatic Substitution WG and DG can influence the reactivity and regiochemistry of benzene reactions Benzene can react through a benzyne intermediate with strong base and a good LG Benzene can react towards a nucleophile as long as there is an WG in the o or p position Benzene has special reactivity at the benzyl position Connections: - 2 S C 3 C 3 C3 2 u u =, 2, 2,, I, F C16-1

2 rganic Chemistry otes by Jim Maxka Reactions of Aromatics There are 3 main ways that Aromatic Compounds react: (1) Addition onto Benzene: lectrophilic Aromatic Substitution (AS) (2) Benzene with LG: 2 (Benzyne Intermediate) or ucleophilic Aromatic Substitution (AS) (3) Reactions at the benzyl C: oxidation, reduction and radical bromination. lectrophilic Aromatic Substitution (AS) Benzene is stablized by energy and so benzene is less reactive than an alkene. For example, an alkene reacts with a variety of electrophiles. What are the products of the following reactions? C 3 C 2 3 C C 3 C 2 3 C C C 3 C 2 3 C C 3 owever, the reactions with benzene are different. Benzene 2 Benzene Benzene 2 ( ) 2 - C - C3 2 Benzene can react, but we need a very strong electrophile: A super-electrophile! When benzene does react, we call it lectrophilic Aromatic Substitution (AS). lectrophilic Aromatic Substitution is really divided in parts: (1) Creation of the super -electrophile,. (2) Addition of the to the Aromatic ring (3) 1 limination of to return to aromaticity C 3 C3 Draw arrows to accomplish the following steps in the reaction. S S Addition limination Final product = substitution All of the reactions proceed the same except the nature of the super-electrophile ( ) can vary. Formation of the super-electrophile: nly certain electrophiles are strong enough to react with benzene. They are always formed by reaction with a strong protic acid or Lewis Acid. The types of the reactions can be seen below. These reactions are so useful synthetically, that we need to look at followup reactions as well. Reaction Reagents Product Follow-up Rxn itration 3 / 2 S 4 2 itrobenzene Reduction omination 2 /Fe 3 omobenzene Benzyne or AS Sulfonation fuming sulfuric acid S 3 Phenylsulfonic acid Sodium Fusion Friedel-Crafts Alkylation R/Al 3 R Alkylbenzene xidation of BS Friedel-Crafts Acylation RC=/Al 3 RC= Acylbenzene Reduction C16-2

3 rganic Chemistry otes by Jim Maxka itration itration takes place via the formation of 2 (nitronium ion). 2 is strong enough to react with benzene. Draw arrows for the following steps to make 2. The official reagents are 3 2 S 4 : Draw the necessary arrows. 2 nitric acid verall, the reaction is 2 S 4 3 The followup reaction for nitration is the reduction of itrobenzene to form aniline. Since reduction is loss of bonds to and gain of bonds to. The reaction involves the exchange of the oxygens on with hydrogens. This reaction is accomplished by oxidizing a metal, either Sn or Zn, or Sn 2 (aq) and adding to provide the hydrogens. Since aniline is a base, to get aniline, we need basic workup. Formally, the reaction goes like this: 2 Sn 3 Sn2 a workup 2 2 omination The formation of the super-electrophile is from the reaction of 2 with the Lewis Acid Fe 3. Draw arrows: (bromonium) ion is strong enough to react with benzene. Fe 3 Fe 3 Fe 4 verall reaction is written: Fe 3 The followup reaction for bromination involves the reaction of benzene with a LG. Benzene can react to eliminate the LG to give a very reactive benzyne intermediate that usually reacts with the solvent or a substitution reaction when there is a strong WG in the ortho or para position. More later. Sulfonation The formation of the super-electrophile S 3 is from fuming sulfuric acid. Fuming sulfuric acid is more acidic than sulfuric acid. Below is the acid rain reaction. The inverse can dehydrate sulfuric acid. When ulfuric acid reacts with itself to protonate an group on another sulfuric aicd and then eliminate it. S 3 is a good enough to react with benzene.. S (g) 2 S - S S S 2 C16-3

4 rganic Chemistry otes by Jim Maxka verall Benzene 2 S 4 (fuming) The followup reaction for sulfonation is a fusion. a fusion involves the reaction with a(g) at C. The a must be dry or other reactions occur. Phenoxide (phenol anion) is produced. The reaction must be acidified to make phenol. The reaction can be written: S 3 a (dry) - dilute aq acid heat workup Friedel-Crafts alkylation: The Friedel-Crafts reaction is the Lewis acid catalyzed ionization of the C- bond (like in -). The catalyst is Al 3. The super-electrophile is. A poor example of this is below. Arrows: 3 C Al 3 3 C Al 3 C 3 Al 4 Since there is a C, watch out for rearrangements. Consider the problem of reacting propyl chloride with benzene in Al 3. You don t get much propyl benzene from this reaction. We ll learn how to make propyl benzene soon. 3 C 3 C C 2 C2 Al 3 3 C C 2 C C C 2 1,2-hydride shift Two products are produced. Which is the major? Which is the minor? There are other issues with this reaction that will be discussed in the next section. The followup reaction of alkylation is reactions at the newly formed benzyl Carbon. You already learned the BS can selectively replace an in the allyl or benzyl positons with a. We will learn some oxidation chemistry soon. Friedel-Crafts acylation Acylation reaction does not rearrange because of resonance stabilization of the acylonium ion. 3 C C Al 3 3 C C Al C 3 C Al 4 C 3 C 3 verall with an acyl chloride reagent: 3 C C Al 3 The followup reaction for acylation is reduction of the the benzyl carbonyl. This position is easy to reduce with either Zn, (as with nitro) or with 2, Pt. ote that benzene is not reduced with 2 and Pt. Why? C 3 Zn, r Pt, 2 C 2 C3 C16-4

5 rganic Chemistry otes by Jim Maxka Practice: Predict the products of the following reactoins: Benzene 2 (Fe 3 ) Benzene 3 ( 2 S 4 ) Benzene 2 S 4 (fuming) Benzene 2 ( 2 S 4 ) Benzene (C 3 ) 3 C- (Al 3 ) The Second Substitution What is the effect of a previously substituted benzene? It depends if we have an WG or DG. Since the reaction is the pi bond in the benzene ring reaching out for the, then adding electron density to ring will make the reaction go better; withdrawing electron density from the ring will make reaction go worse. There are two ways to justify of this effect, Reactivity and Directing ffects The case for DG: Anisole and aniline have a strong DG because the lone pair electrons on the or can interact with the aromatic ring by resonance. Draw the 4 other resonance contributors for anisole: R R R R? Would anisole react better than benzene with? In what positions would the most like go? Anisole is so reactive that it reacts with 2 without iron catalyst. Be sure to review the DG s. ow for the case of WG: As we saw before, reacting with benzene and WG is more difficult than benzene by itself, because the ring has less electron density and will be less able to react with. Let s look at itrobenzene. Draw the 4 additional resonance structures:? Would nitrobenzene be less reactive than benzene? Why? Remember you are reacting. What positions would be less likely to be substituted? Why? Remember you are reacting. Which position(s) would be more likely to be substituted? itrobenzene reacts very slowly with a second mole of 2 (from 3 // 2 S 4 ). The reaction requires reflux and long reaction times. Why is this fact a good thing for the synthesis of nitrobenzene? The second nitro group goes in the meta position. C16-5

6 rganic Chemistry otes by Jim Maxka Reactivity of Aromatic Compounds is related to the pka of the p-substituted benzoic acids ne way to understand the activation/deactivation effects is to look at a chart that plots the pka s of substituted benzoic acids. What we see is that the electronic effects that influence the electron density between the - of the acids is an indication of the electronics that influence activation or deactivation. The chart on the next page plots pka from 3 on the right (very unreactive) to 5 on the left (very reactive) with benzene in the middle at We use this chart to compare reactivities of different substituted aromatic compounds. less acidic more reactive towards Me 4 S 2 C 3 Me (C 3 ) 2 C 3 C 2 more acidic less reactive towards xample of activation/deactivation Below are the results of a student s nitration of toluene: The student had previously nitrated benzene. The nitration of benzene took about 30 minutes. The student noted that toluene reacted in just about one minute. C 3 C 3 C 3 C S % 3% 40% Justify the difference in reaction time: Justify the ortho/para to meta ratio. C16-6

7 rganic Chemistry otes by Jim Maxka Resonance vs Inductive ffects: In the chart above, it is not hard to see that the on the far right, benzene is substituted by _G and on the far left by _G. The hard one to explain is the effect of the halogens, F,,, I = X. X is very electronegative (WD) but has LP that can be DG. We say that X is resonance donating but inductively electron withdrawing. The inductive effect is another way of saying through sigma bonds, and resonance is through pi bonds. Therefore, X directs to the o,p positions but is a little less reactive than benzene. In general, which effect is strongest? The following chart might help explain inductive vs resonance effects. Type Reaction Favored next xample ote Influence position DG Resonance Activating o,p anisole LP electrons DG Inductive Activating o,p toluene o LP electrons WG Resonance* Deactivating M nitrobenzene, Resonance benzaldehyde structures WG Inductive* Deactivating M R 3 -C 6 5 o resonance structures alides DG Res-/ Slightly o,p bromobenzene ffects cancel WG Induct- deactivating each other out. * Friedel Crafts reaction does not occur. Practice: assify the following as inductive or resonance WG/DG or both. 3 C 3 - C 3 Practice rder the following in terms of reactivity: Most Least Medium Methoxybenzene (C 3 C 6 5 ) benzene benzaldehyde (C 6 5 C(=)),-dimethylaniline Toluene itrobenzene omobenzene is only slightly less reactive than benzene. Why? itrobenzene is much more unreactive than benzene. Why? Methoxybenzene (anisole) is much more reactive than benzene. Why? Why can isopropyl chloride react with Al 3 and bromobenzene but not nitrobenzene? C16-7

8 rganic Chemistry otes by Jim Maxka More Resonance Structures to explain directing effects: rtho, para directors Aniline para like ortho otice there are resonance structures for o,p and for meta. Meta directors para like ortho otice there are charges next to each other for o,p and for meta. C16-8

9 rganic Chemistry otes by Jim Maxka Back to the Friedel-Crafts reaction: The reaction of benzene with (C 3 ) 2 C (Al 3 ) goes to isopropylbenzene very easily. But there is a problem. What about the reactivity of isopropylbenzene: more or less reactive than benzene?. A practical consequence is multi-substitution: Al 3 Al 3 R R R more or less reactive than Benzene? Most chemists prefer to perform the Friedel-Crafts acylation. Why? Consider the reaction for the synthesis of acetophenone. 3 C C Al 3 C C 3 more or less reactive than Benzene? If we react with 2 (Pt) we can reduce the carbonyl in the benzyl position. Draw the complete reaction to make propyl benzene. It s not the same products that we would get if we reacted benzene with C 3 C 2 C 2 (Al 3 ) Draw the major product of this reaction: Summary of AS: 1. Benzene needs super or cannot react. 2. DG-Substituted Benzenes (ACTIVATD) react faster than benzene towards super and even reacts with normal. 3. WG-Substituted Benzenes (DACTIVATD) react slower than benzene towards ; regular, are you kidding? 4. Activated aromatics direct o,p. Deactivated aromatics direct m. alogens are deactivated o,p directors. 5. alogens are resonance DG, inductive WG. You need to know the difference between DG/WG resonance/inductive Benzene with Leaving Groups: Benzyne and ucleophilic Substituion (AS) Benzyne Benzyne intermediates. Benzene with a LG could conceivably go through elimination with a strong base, creating an alkyne intermediate. Why is the benzyne so unstable? What is this mechanism called? Base takes a proton on the C next to C with LG 2 C16-9

10 rganic Chemistry otes by Jim Maxka Because benzyne is so hard to form, we need an extremely strong base or we need harsh conditions. a/ 2 can be used with high temperatures and pressure. Why is benzyne so hard to form? C 3 C 3 i T/P C 3 C 3 Draw possible structures Consider the results of the following reaction a/ 2 must react >300 0 C and 150 atm (ortho substituted LG): ow consider the results of meta-chlorotoluene reacting with a/ 2 >300 0 C and 150 atm: C 3 i T/P C 3 C 3 C 3 Draw possible structures C 3 C 3 i T/P You can also consider the results of the para-chlorotoluene reaction with strong base. Two products can form. C 3 C 3 i T/P C16-10

11 rganic Chemistry otes by Jim Maxka Finally benzyne can react with a diene to form a Diels Alder adduct. Which Diene was used to make this molecule? Write out the reaction conditions. Mg Mg -MgF 2 C F F 2 C ucleophilic Aromatic Substitution: We just studied the reaction, in which a strong base reacts with benzene substituted with a group. ow, let s look at another reaction in which benzene has a Leaving Group and reacts by nucleophilic aromatic substitution. We already studied aromatic substitution. W RACTI: Start with a nucleophile. Remember a good nucleophile has strong charge like or it has a reactive of electrons like: 3 or an amine. A good nucleophile can also be classified as a good. What do you expect for the result of the reaction of bromobenzene with - at low pressures? u: u??????? Well, doesn t look too good. Can you list some problems with this intermediate???? owever, if there is an WG in the ortho or para position, then we can stabilize the negative charge from the intermediate. Try this! This is called the Meisenheimer Intermediate. u: This is how we can get away with nucleophilic aromatic substitution. ot exactly like S n 2, but similar. So now, you have to watch for two reactions with benzenelg: if there is strong base or - with high pressures and heat the reaction goes by benzyne; if there is an WG in the ortho or para position, then the reaction will go by ucleophilic Aromatic Substitution at normal temperatures and pressure, with moderate nucleophiles. This is hard because all benzenes in this situation have LGs. Practice: Can you justify why 2 in the meta position does not react through the Meisenheimer intermediate? u u u - C16-11

12 rganic Chemistry otes by Jim Maxka More Practice: Predict the main products for the reactions below. Indicate the mechanism. 2 / 3 - reflux 2-350o C C psi What is the main difference between the two mechanisms? What would happen if we reacted meta-bromonitrobenzene with aqueous a solution at normal temperatures? Benzylic C Chemistry: omination (BS), xidation, and Reduction Finally, we have chemistry at the benzylic position. ow, we always need practice. Finish the resonance structures for the benzyl cation (like the allyl cation). C 2 C 2 C 2 C2 C 2 C2 C 2 C2 The benzylic position is reactive to three types of reagents: radicals (review allylic bromination), oxidation and 2 reduction. Benzylic omination Toluene reacts with BS, light or FRI to make benzylbromide. C 3 BS, FRI or 2 light hv Predict the product of the reaction of ethylbenzene BS (FRI) What is the difference between the following reaction conditions: (a) toluene 2 (Fe 3 ) (DARK) o-bromotoluene p-bromotoluene (b) toluene 2 (Fe 3 ) (LIGT) o-bromotoluene p-bromotoluene benzyl bromide. C16-12

13 rganic Chemistry otes by Jim Maxka Benzylic xidation. The benzyl position is oxidized by Mn - 4 (permanganate) or Cr 3 in sulfuric acid (Jones). The result is a carboxylic acid in the benzyl posiiton no matter how many carbons are hanging off benzene, as long as there are s to lose. xidation/reduction in organic chemistry: Simply put xidation is gain of bonds to. Loss of bonds to. Reduction is gain of bonds to. Loss of bonds ot. Predict the following oxidation products. Toluene permanganate Propylbenzene Jones reagent tert-butylbenzene Jones reagent Benzylic Reduction: We saw that a C= in the benzyl position can be easily reduced by catalytic hydrogenation ( 2 /Pt). What is the result of the following sequence? C3 C 2 Al 3 2 /Pt Synthetic Strategy Aromatic Synthetic strategy. See book Synthesis of p-bromobenzoic acid vs. m-bromobenzoic acid from benzene. Targets You must start with benzene and react with any reagent that will give the exact aromatic compound. Since some groups are o,p-directors and others are m-directors, it is important which order you add the groups and transform them. Retrosynthetic Analysis Friedel Crafts Alkylation Benzylic oxidation C 3 o,p-director C 3 Al 3 2 Fe 3 o,p-director para KMn 4 aromatic bromination 3 C 2 Fe 3 C 3 Al 3 C16-13

14 rganic Chemistry otes by Jim Maxka o,p-director aromatic bromination m-director meta ow make p-bromonitrobenzene Benzylic oxidation 2 Fe 3 Friedel Crafts Alkylation C 3 KMn C 3 Al To make m-bromonitrobenzene: 2 Case Study: Synthesis of Sulfanilamide, a Sulfa drug 2 C 3 C 3 C steps 2 S 2 2 S 2 2 S sulfanilamide Summaries 1. Understand how aromatics react differently than alkenes. 2. Know the reagents and products of these AS reactions a. Aromatic nitration b. Aromatic bromination c. Aromatic sulfonation d. Friedel-Crafts alkylation e. Friedel-Crafts acylation 3. Know the reagents and products of the followup reactions a. Aromatic nitration aniline b. Aromatic bromination reactions with LG c. Aromatic sulfonation sodium fusion d. Friedel-Crafts alkylation e. Friedel-Crafts acylation reduction at the benzyl position. 4. Know the reagents and conditions of the benzyne reaction. 5. Know the reagents and conditions of the AS reaction. 6. Understand how the benzylic position is selectively brominated, oxidized or reduced. 7. Understand how the 2 nd substituent AS (o,m or p) is influenced by WG/DG. 8. Understand how the reactivity of the benzene ring is influenced by WG/DG. 9. Understand the principles of retrosynthetic design and how to decide what steps are required. 10. Use retrosynthetic-thinking to design 3 step syntheses of aromatics. C16-14