Chapter 16- Chemistry of Benzene: Electrophilic Aromatic Substitution

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1 Chapter 16- Chemistry of Benzene: Electrophilic Aromatic Substitution Ashley Piekarski, Ph.D. Substitution Reactions of Benzene and Its Derivatives Benzene is aroma%c What does aromatic mean? Reac9ons of benzene lead to the reten9on of the aroma9c core 1

2 Why do I care, Dr. P? In the last chapter, we learned about the structure and stability of aroma9c compounds We established a structure- property rela%onship! This valuable information will help us understand the reactions these molecules undergo and the products that are made Rela9onship cri9cal to understanding of how biological molecules/pharmaceu9cal agents are synthesizedà real- world applica%ons! Electrophilic Aromatic Substitution Reactions: Bromination Benzene s π electrons par9cipate as a Lewis base in reac9ons with Lewis acids The product is formed by loss of a proton, which is replaced by bromine FeBr 3 is added as a catalyst to polarize the bromine reagent 2

3 Bromination of alkenes What was the product of the bromina9on of propene? This doesn t happen with benzene Why do you think that is? Addition Intermediate in Bromination The addi9on of bromine occurs in two steps In the first step the π electrons act as a nucleophile toward Br 2 (in a complex with FeBr 3 ) This forms a ca9onic addi9on intermediate from benzene and a bromine ca9on The intermediate is not aroma(c and therefore high in energy 3

4 Formation of Product from Intermediate The ca9onic addi9on intermediate transfers a proton to FeBr 4 - (from Br - and FeBr 3 ) This restores aroma%city! (in contrast with addi9on in alkenes) Other Aromatic Substitutions Chlorine and iodine (but not fluorine, which is too reac9ve) can produce aroma9c subs9tu9on with the addi9on of other reagents to promote the reac9on Chlorina9on requires FeCl 3 (acts as a catalyst to make chlorine a bexer electrophile) AKA Valium (an9- anxiety drug) 4

5 Other Aromatic Substitutions Chlorine and iodine (but not fluorine, which is too reac9ve) can produce aroma9c subs9tu9on with the addi9on of other reagents to promote the reac9on Chlorina9on requires FeCl 3 (acts as a catalyst) Iodine must be oxidized to form a more powerful I + species (with Cu + or peroxide) Aromatic Nitration The combina9on of nitric acid and sulfuric acid produces NO 2 + (nitronium ion) The reac9on with benzene produces nitrobenzene 5

6 Aromatic Sulfonation Subs9tu9on of H by SO 3 (sulfona9on) Reac9on with a mixture of sulfuric acid and SO 3 Reac9ve species is sulfur trioxide or its conjugate acid Aromatic Hydroxylation Not performed in the laboratory Does occur in biological pathways! 6

7 Alkylation of Aromatic Rings: The Friedel Crafts Reaction Alkyla9on among most useful electrophilic aroma9c subs9tu9on reac9ons! Very powerful rxn. to put in your organic toolbox! Aroma9c subs9tu9on of R + for H + Aluminum chloride promotes the forma9on of the carboca9on Alkylation of Aromatic Rings: The Friedel Crafts Reaction 7

8 Limitations of the Friedel-Crafts Alkylation Only alkyl halides can be used (F, Cl, I, Br) Aryl halides and vinylic halides do not react (their carboca9ons are not stable enough to form) Will not work with rings containing an amino group subs9tuent or a strongly electron- withdrawing group Control Problems Mul9ple alkyla9ons can occur because the first alkyla9on is ac9va9ng 8

9 Carbocation Rearrangements During Alkylation Similar to those that occur during electrophilic addi9ons to alkenes What are the two types of carboca9on rearrangements and why do they occur? Carbocation Rearrangements During Alkylation 9

10 Learning check Predict the product and write the complete stepwise mechanism for the reac9on below: CH 3 CH 2 Cl AlCl 3 Learning check Predict the product and write the complete stepwise mechanism for the reac9on below: Cl AlCl 3 10

11 Acylation of Aromatic Rings Reac9on of an acid chloride (RCOCl) and an aroma9c ring in the presence of AlCl 3 introduces acyl group, COR Benzene with acetyl chloride yields acetophenone Mechanism of Friedel-Crafts Acylation Similar to alkyla9on Reac9ve electrophile: resonance- stabilized acyl ca9on An acyl ca9on does not rearrange 11

12 Learning check Predict the product and write the complete stepwise mechanism for the reac9on below: O Cl AlCl 3 Substituent Effects in Aromatic Rings Subs9tuents can cause a compound to be more or less reac9ve than benzene Subs9tuents affect the orienta9on of the reac9on the posi9onal rela9onship is controlled ortho- and para-directing activators, ortho- and paradirecting deactivators, and meta-directing deactivators 12

13 Origins of Substituent Effects An interplay of induc(ve effects and resonance effects Induc%ve effect - withdrawal or dona9on of electrons through a σ bond Resonance effect - withdrawal or dona9on of electrons through a π bond due to the overlap of a p orbital on the subs9tuent with a p orbital on the aroma9c ring It is cri9cal you know the difference between these two defini9ons! 13

14 Inductive Effects Controlled by electronega9vity and the polarity of bonds in func9onal groups Halogens, C=O, CN, and NO 2 withdraw electrons through σ bond connected to ring Alkyl groups donate electrons Resonance Effects Electron Withdrawal C=O, CN, NO 2 subs9tuents withdraw electrons from the aroma9c ring by resonance π electrons flow from the rings to the subs9tuents 14

15 Resonance Effects Electron Donation Halogen, OH, alkoxyl (OR), and amino subs9tuents donate electrons π electrons flow from the subs9tuents to the ring Effect is greatest at ortho and para An Explanation of Substituent Effects Ac%va%ng groups donate electrons to the ring, stabilizing the carboca9on intermediate Deac%va%ng groups withdraw electrons from the ring, destabilizing the carboca9on intermediate 15

16 Ortho- and Para-Directing Activators: Alkyl Groups Alkyl groups ac9vate: direct further subs9tu9on to posi9ons ortho and para to themselves Alkyl group is most effec9ve in the ortho and para posi9ons because they form the most stable intermediates! Ortho- and Para-Directing Activators: OH and NH 2 Alkoxyl, and amino groups have a strong, electron- dona9ng resonance effect Most pronounced at the ortho and para posi9ons 16

17 Ortho- and Para-Directing Deactivators: Halogens Electron- withdrawing induc9ve effect outweighs weaker electron- dona9ng resonance effect Resonance effect is only at the ortho and para posi9ons, stabilizing carboca9on intermediate Meta-Directing Deactivators Induc9ve and resonance effects reinforce each other Ortho and para intermediates destabilized by deac9va9on of carboca9on intermediate Resonance cannot produce stabiliza9on 17

18 Summary Table: Effect of Substituents in Aromatic Substitution Trisubstituted Benzenes If the direc9ng effects of the two groups are the same, the result is addi9ve 18

19 Substituents with Opposite Effects If the direc9ng effects of two groups oppose each other, the more powerful ac9va9ng group decides the principal outcome Usually gives mixtures of products Meta-Disubstituted Compounds The reac9on site is too hindered To make aroma9c rings with three adjacent subs9tuents, it is best to start with an ortho- disubs9tuted compound 19

20 Learning check Predict the products of the reac9ons below: a) CH 3 SO 3 H 2 SO 4 b) OCH 3 Br 2 FeBr 3 NO 2 O c) CH 3 1. HNO 3, H 2 SO 4 2. H 2 /Pd Nucleophilic Aromatic Substitution Aryl halides with electron- withdrawing subs9tuents ortho and para react with nucleophiles Form addi9on intermediate that is stabilized by electron- withdrawal Halide ion is lost to give aroma9c ring 20

21 Benzyne Phenol is prepared on an industrial scale by treatment of chlorobenzene with dilute aqueous NaOH at 340 C under high pressure The reac9on involves an elimina9on reac9on that gives a triple bond The intermediate is called benzyne Evidence for Benzyne as an Intermediate Bromobenzene with 14 C only at C1 gives subs9tu9on product with label scrambled between C1 and C2 Reac9on proceeds through a symmetrical intermediate in which C1 and C2 are equivalent must be benzyne 21

22 Structure of Benzyne Benzyne is a highly distorted alkyne The triple bond uses sp 2 - hybridized carbons, not the usual sp The triple bond has one π bond formed by p p overlap and another by weak sp 2 sp 2 overlap Oxidation of Aromatic Compounds Alkyl side chains can be oxidized to CO 2 H by strong reagents such as KMnO 4 and Na 2 Cr 2 O 7 if they have a C- H next to the ring Converts an alkylbenzene into a benzoic acid, Ar R Ar CO 2 H Benzene ring cannot be oxidized! 22

23 Bromination of Alkylbenzene Side Chains Reac9on of an alkylbenzene with N- bromo- succinimide (NBS) and benzoyl peroxide (radical ini9ator) introduces Br into the side chain Mechanism of NBS (Radical) Reaction Abstrac9on of a benzylic hydrogen atom generates an intermediate benzylic radical Reacts with Br 2 to yield product Br radical cycles back into reac9on to carry chain Br 2 produced from reac9on of HBr with NBS 23

24 Reduction of Aromatic Compounds Aroma9c rings are inert to cataly9c hydrogena9on under condi9ons that reduce alkene double bonds Can selec9vely reduce an alkene double bond in the presence of an aroma9c ring Reduc9on of an aroma9c ring requires more powerful reducing condi9ons (high pressure or rhodium catalysts) Reduction of Aryl Alkyl Ketones Aroma9c ring ac9vates neighboring carbonyl group toward reduc9on Ketone is converted into an alkylbenzene by cataly9c hydrogena9on over Pd catalyst 24

25 Synthesis of Trisubstituted Benzenes These syntheses require planning and considera9on of alterna9ve routes Ability to plan a sequence of reac9ons in right order is valuable to synthesis of subs9tuted aroma9c rings Learning check Propose a synthesis for 3- bromo- 2methylbenzenesulfonic acid. 25

26 Learning check Propose a synthesis for 4- chloro- 1- nitro- 2- propylbenzene. 26

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