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Chapter 19

CHEM 267: Chapter 19 Textbook Notes

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CHEM 267
Monica Barra

Chem 267 Chapter 19 Notes Winter 2012 19.1 Introduction to Electrophilic Aromatic Substitution (pg. 859)  When benzene undergoes an addition reaction with Br , no 2eaction occurs (too stable) o However, when iron (Fe) is introduced into the mixture, a reaction takes place  Instead of an addition reaction, an electrophilic aromatic substitution occurs o One aromatic proton is replaced by an electrophile 19.2 Halogenation  Fe enhances the electrophilicity of the bromine (Br) atom  Steps: 1. Aromatic ring functions as a nucleophile and attacks the electrophile (Br), forming a positively charged intermediate called a sigma complex (arenium ion) which resonates 2. Sigma complex is then deprotonated (restoring aromaticity and the Lewis acid) 3. AlBr 3an also serve as a suitable alternative to FeBr 3  NOTE: no additional products would occur! (it is already stable/aromatic)  General mechanism for electrophilic aromatic substitution: 19.3 Sulfonation  When benzene is treated with fuming sulphuring acid (fuming H SO ), sulfonic acid is obtained 2 4 o Fuming sulphuric acid is a mixture of H SO and SO gas (SO = very strong electrophile!) 2 4 3 3  Reaction mechanism is the same as all electrophilic aromatic substitution reactions (but with an extra proton transfer at the end to protonate the electronegative oxygen atom)  NOTE: the reaction between benzene and fuming H SO is 2eve4sible! 19.4 Nitration  When benzene is treated with a mixture of nitric acid and sulfuric acid, a nitrobenzene is formed  Reaction proceeds via an electrophilic aromatic substitution (EAS) with a nitronium ion (NO ) a2+ the electrophile (which is formed from the acid-base reaction between HNO and H SO ) 3 2 4  Once the nitro group is on the aromatic ring, it can be reduced to give an amino group (NH ) 2 19.5 Friedel-Crafts Alkylation  Now we deal with electrophiles in which the electrophilic center is a carbon atom  The Friedel-Crafts alkylation reaction puts an alkyl group on an aromatic ring  The alkyl halide is not electrophilic enough to react with benzene, but in the presence of a Lewis acid, the alkyl halide is converted into a more electronegative carbocation (that can now react)  benzene can now react with the electrophile (carbocation) the same way as a normal EAS rxn  Many different alkyl halides can be used in a Friedel-Crafts alkylation o 2° and 3° halides = readily converted into carbocations in the presence of AlCl 3 o 1° alkyl halides = NOT converted into carbocations (too high in energy!)  Rxn still occurs, but electrophile is a complex between the 1° halide and A3Cl  HOWEVER: most 1° alkyl halides (containing >2 carbons) cannot be used effectively because their complexes with AlCl 3eadily undergo carbocation rearrangement (hydride shift/methyl shift), resulting in a more stable 2° or 3° carbocation o NOTE: a mixture of products will be obtained (w/ and w/o carbocation rearrangement) Rules for the Friedel-Crafts alkylation reaction: 3 1. The carbon atom connected to the halogen MUST be sp hybridized (vinyl and aryl carbocations are not stable enough to react) 2. Polyalkylation can occur (alkylation at multiple sites on the benzene ring) 3. There are certain groups (ex. nitro group, NO ) that are incompatible with the reaction 2 19.6 Friedel-Crafts Acylation  Similar to installing an alkyl group to benzene, an acyl group can also be installed on benzene o This results in the production of an aryl ketone  Acylation = a reaction that installs an acyl group  Friedel-Crafts acylation = acyl chloride is treated with a Lewis acid to form a cation (acylium ion)  The acylium ion is an excellent electrophile, producing another EAS reaction  The aryl ketone that is produced can further be reduced using a Clemmensen reduction o the net result is the installation of an alkyl group  In the presence of zinc amalgam and HCl, the carbonyl group is completely reduced (the double- bonded O is replaced by 2 single-bonded H’s) 2  NOTE: this is a useful method for installing alkyl groups that cannot be installed with the direct alkylation process (due to carbocation rearrangement and resulting in a mixed product)  RULE: polyacylation is not observed (acyl group deactivates ring toward further acylation) 19.7 Activating Groups Nitration of Toluene  Toluene (benzene with a methyl substituent) undergoes nitration ~25 times faster than benzene o ∴ the methyl group is said to “activate” the aromatic ring (due to the methyl group’s ability to donate electron density due to hyperconjucation; stabilizing the sigma complex)  Regiochemistry: the nitro group could be installed ortho, meta, or para to the methyl group, but ortho and para products predominate (very little meta product is produced) o This is because the ortho and para products contain resonance structures where the positive charge is directly adjacent to the electron-donating alkyl group o The sigma complex from the meta attack doesn’t have this stability (∴ higher in energy) o As well, the ortho attack involves a slightly hiaher E than the para attack because the nitro group is closer to the methyl group in ortho (steric hindrance) compared to para o HOWEVER: there is still more ortho product than para (even though ortho attack involves a higher E ), simply because there are 2 ortho attack sites compared to para (only 1 site) a  Methyl group is an ortho-para director (directs incoming nitro group to ortho and para positions) Nitration of Anisole  Methoxybenzene (anisole) undergoes nitration 400 times faster than toluene! o ∴ Methoxy group is a more powerful activator than a methyl group  Induction suggests that the methoxy group is electron withdrawing (since O is more EN than C)  Resonance suggests that the methoxy group is electron donating (donates lone pair & resonates)  RULE: whenever resonance and induction compete with each other, resonance is dominant  Anisole is so reactive, that treatment of bromine without a Lewis acid gives a trisubstituted product  Methoxy group is also an ortho-para director o b/c the sigma complex for the ortho and para attack have 1 additional resonance structure o para is favoured over ortho because the ortho product has more steric hindrance RULE: all activators are ortho-para directors! 3 19.8 Deactivating Groups  unlike activating groups (which increase the rate of EAS), deactivating groups are electron- withdrawing substituents that decrease the rate of EAS  the nitro group (NO2) is a deactivator because: o Induction suggests that it is electron withdrawing (b/c N is very electronegative) o Resonance suggests that it is also electron withdrawing as it needs to withdraw electron density from the ring in order to resonate the positive charge around the ring  ∴ since there is no competition between resonance and induction, NO2is electron withdrawing  NOTE: nitrobenzene is 100,000 times less reactive than benzene towards nitration and can only be accomplished at an elevated temperature  NOTE: the meta product predominates the ortho and para product! (opposite of activators) o The Ortho and para products are unstable b/c one of the resonance structures in their sigma complex has a +ve charge directly adjacent to the electronegative, positively charged nitrogen atom (unstable b/c N is pulling electron density from an already positive C atom) RULE: most deactivators are meta directors! 19.9 Halogens: The Exception  NOTE: Halogens are an exception to the rule that deactivators are meta directors
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