Learning Guide for Chapter 18 - Aromatic Compounds II
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1 Learning Guide for Chapter 18 Aromatic Compounds. lectrophilic aromatic substitution ntroduction Mechanism Reagents and Products lectrophiles ffects of stituents FriedelCrafts alkylation and acylation ntroduction. lectrophilic Aromatic stitution Which of the following reactions will give product? 2 addition atoms added to the molecule without taking anything away 2 no reaction addition would destroy aromaticity Fe 3 2 substitution is replaced by product is still aromatic What can you conclude? Mechanism aromatic rings undergo substitution instead of addition to preserve aromaticity different reagents are needed n addition reactions of alkenes, how does the C=C react? weak u or base or acid u weak u strong u
2 When an aromatic ring reacts, it is a very weak nucleophile. What will be formed? LG Ch 18 p 2 very weak u more reactive sp 3 4! e nonaromatic strong acid base weak sometimes called a sigma complex ow would the activation energy of the first steps compare? C formed in both, has resonance in the second but aromaticity destroyed in the second much higher ow would the two electrophiles compare? second one is much stronger, more reactive Why does the C react as an acid rather than an electrophile at the end? to restore aromaticity What would you predict about the strength the acid and the base in the second step? weak summing up: Why is the reaction call "electrophilic aromatic substitution"? reagent is an starting material must be aromatic is replaced by What are the two steps involved? 1 ring attacks, forms nonaromatic C w/ resonance structures 2 weak base pulls off on same C as, restoring aromaticity ow does this reaction compare to the reactions we studied in the last section? don't have to start with aryl halides benzene attacks, rather than being attacked
3 Reagents and Products LG Ch 18 p 3 What are the names and products in the following reactions? chlorination 2 Al 3 bromination 2 Fe 3 or Fe 2 ow can that be the same as that? Fe and 2 react to form Fe 3 iodination 2 3 nitration 2 S sulfonation S 3 2 S 4 S 3 S = sulfonic acid group FreidelCrafts alkylation Al 3 R R FreidelCrafts acylation Al 3 R R We will focus on the first 5, then come back to the last 2. You will need to memorize these reagents and the products that they form; however, you should also be able to use some reasoning based on their mechanisms to help you.
4 LG Ch 18 p 4 lectrophiles Most of the reagents require a preliminary reaction in order to form a highly reactive nucleophile. The base required for the second step will be different for each reaction. chlorination: reactive complex forms between 2 and Al 3 Al Al where would this complex be attacked? What reagents are being used up, and which are catalytic? benzene, 2 used up to make chlorobenzene Al 3 catalytic Al bromination: reactive complex forms between 2 and Fe 3. Fe 3 Fe 3 Fe 3 is ionic; is ne Fe bond truly covalent? same kind of mechanism as above pair of e in benzene ring attacks on the end, forming C w/ 3 resonance structures bromine takes a to give bromobenzene and iodination: redox reaction between iodine, nitric acid, and some additional acid 1/ what will be the base?
5 S 3 is a gas; ir is also the anhydride of sulfuric acid the mixture is called "fuming sulfuric acid" because it reacts w/ water vapor to form clouds of sulfuric acid when opened sulfonation: S 3 is the electrophile; sulfuric acid is the solvent LG Ch 18 p 5 S S S 3 S 3 what will be the base? another molecule of the C S 3 nitration: sulfuric and nitric acid react to form nitronium ion S Where would this be attacked? 2 S 4 3 nitric acid reacts as a base!! S S nitronium ion what will be the base? 2 What are the reactive electrophiles and bases in each reaction? chlorination 2 Al 3 complex; Al 4 bromination 2 Fe 3 complex; Fe 4 iodination ion; 2 sulfonation sulfur trioxide; PhS 3 nitration nitronium ion; 2
6 ffects of stituents First of all, more than one isomer is possible. 40% 3% 57% LG Ch 18 p 6 n order for electrophilic aromatic substitution reactions to be useful, they must work on a variety of aromatic compounds. owever, when we do the reaction on a ring which already contains some other substituent, some complications come up. 25 times faster 2 S mostly ortho, para ow many products are possible? 2 When you isolate these products, you will find that they are present in different amounts. Secondly, the reaction is about 25 times faster with toluene than with benzene. To understand why these ratios of products are formed, and why the reaction is faster, we need to get some more data. What other reactions could we try? 25 times faster different reagents try aromatic rings with different substituents 2 Fe 3 very similar results not the reagents, the mechanism 45% 2% 53% mostly ortho, para 10 times slower 2 S % 1% 64% mostly ortho, para 10,000 times faster 2 S % 0.01% 55% mostly ortho, para 100,000 times slower 2 2 S % 93% all kinds of results substituent is affecting the ratedetermining step in different ways for different isomers 2 0.7% mostly meta
7 LG Ch 18 p 7 After doing hundreds of these reactions, you can sumarize the results as follows: 1 All of the reagents give the same results for a given starting material. 2 stituents on the aromatic ring can be divided into three groups: Group 1 Activating, rtho/paradirecting Groups R R R 2 R mildly activating strongly activating alkyl groups or atoms with electron pairs on an atom connected to the ring all give mostly ortho and para products alkyl groups moderately speed up the reaction groups with electron pairs dramatically speed up the reaction Group 2 Deactivating, Metadirecting Groups 2 S 3 strongly deactivating all have atoms with full or partial charges connected to the ring all give mostly meta products all slow down the reaction Group 3 Mildly deactivating, rtho/paradirecting Groups mildly deactivating halogens moderately slow the reaction give ortho/para products
8 LG Ch 18 p 8 To answer the question why, we must look at how the presence of a substituent affects the ratelimiting step. Which step will be rate limiting? rtho: formation of C with resonance structures preliminary steps can't be, results don't change final step should be fast restores aromaticity can't go to the ispo C must have an to finish ortho positive charge has density on C with substituent ortho Meta: meta meta positive charge does not have density on C with substituent Para: para positive charge has density on C with substituent para f the substituent can stabilize the positive charge, what will happen to the energy of the intermediate? it will go down, the reaction will be faster f the substituent destabilizes the negative charge, what will happen to the energy of the intermediate? it will go up, the reaction will be slower
9 LG Ch 18 p 9 Group 1 alkyl substituents and substituents with electron pairs on the atom next to the ring alkyl ortho para energy diagram benzene meta ortho/para electrondonating more stable 3 o C ortho reaction takes lower pathway goes faster, gives o,p products is electroneg less stable new resonance structure! energy diagram para meta benzene ortho/para Group 2: substituents with fully or partially positively charged atoms on the benzene ring ortho para energy diagram CF3 ortho/para meta benzene Group 3: halogen substituents ortho energy diagram meta ortho/para benzene
10 LG Ch 18 p 10 What products will predominate in each of the following cases, and why? Will they be slower or faster than the same reaction on a benzene ring? 2 S all reactions are slower, but o/p are slowest meta wins deactivating, meta directing nitration adds 2 S 3 S 3 activating, o/p directing 2 S 4 sulfonation adds S 3 3 S o/p reactions are faster, meta are slower C 2 C all reactions are slower, but o/p are slowest meta wins Al 3 deactivating, meta directing chlorination adds deactivating, o/p directing 2 3 iodination adds all reactions are slower, but o/p are a little faster activating, o/p directing 2 Fe 3 bromination adds all reactions a little faster, o/p more than meta
11 LG Ch 18 p 11 When combining reactions to form syntheses, the directing effect of substituents must be taken into account. 2 2 S 4 2 Fe Fe 3 2 Fe 3 o, p When more than one substituent is present, they may reinforce the directing effect, or they may conflict. S 3 reinforce S 3 2 S 4 3 S S 3 conflict 2 Al 3 When conflicts occur, what takes priority? same priority mixtures 1. strong o/p directing groups 2. mild o/p directing groups 3. all meta directing groups 2 S Fe
12 FriedelCrafts Alkylation and Acylation LG Ch 18 p 12 What reagents are needed for FreidelCrafts alkylation and acylation reactions? What substituent is added? alkylation acylation Al 3 C 3 Al 3 C 3 reagents: Al 3, alkyl chloride product: alkyl group added reagents: Al 3, acid chloride product: ketone added ow do the reactive electrophiles form? alkylation 3 C Al 3 C Al R forms a complex with Al 3 (just like 2 does) acylation Al Al C C 3 C C 3 acid chloride forms a complex, then dissociates Al 4 ow does the aromatic ring react with these electrophiles? benzene ring attacks, base pulls off a alkylation Al 4 acylation Al 4
13 LG Ch 18 p 13 What are the three major limitations of the FreidelCrafts alkylation reaction? 1) rearrangements 2) multiple substitutions 3) won't work on strongly deactivated rings 1) primary alkyl halides always give rearranged products Al 3 2) the product is more reactive than the staring material, so multiple products form Al 3 3) if a ring contains a strong deactivating group, the reaction is too slow (electrophile isn't strong enough) strongly deactivating 2 Al 3 C 3 C 2 no reaction mildly deactivating Al 3 C 3 C 2 activating Al 3 C 3 C 2 Which of these limitations does acylation overcome? 1) no rearrangements 2) no multiple substitutions 3) still won't work on strongly deactivated rings
14 1) acylium ions have resonance, so they don't rearrange Al 3 2) the product is less reactive than the starting material (use reaction above) LG Ch 18 p 14 less reactive no more products 3) if the ring has a strong deactivating group, it is still too slow Al 3 no reaction ow are acylation product distributions different from other electrophilic aromatic substitutions? because of steric hindrance, little para product forms Al 3 ow can the emmenson reaction be used with FreidelCrafts acylation to give products that could not be made with Friedel Crafts alkylation? Al 3 alkylation wrong product Al 3 Zn/g use acylation, then remove C= with emmensen reduction
15 LG Ch 18 p 15 ow could 1isobutyl4nitrobenzene be synthesized from benzene? 2 Al 3 Zn/g 2 S 4 3 What about 1isobutyl3nitrobenzene? 2 2 Al 3 2 S 4 3 Synthesize 1ethoxy2,4dinitrobenzene from benzene. 2 Zn/g you don't have to do the reduction right away leave it on as a meta directing group if that is what you need! Al 3 2 S 4 2 S para
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