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Chapter 19

CHEM267 Chapter Notes - Chapter 19: Electrophilic Aromatic Substitution, Nucleophilic Aromatic Substitution, Nitro Compound


Department
Chemistry
Course Code
CHEM267
Professor
Monica Barra
Chapter
19

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Chem 267 Chapter 19 Notes Winter 2012
19.1 Introduction to Electrophilic Aromatic Substitution (pg. 859)
When benzene undergoes an addition reaction with Br2, no reaction 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. AlBr3 can also serve as a suitable alternative to FeBr3
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 H2SO4), sulfonic acid is obtained
o Fuming sulphuric acid is a mixture of H2SO4 and SO3 gas (SO3 = very strong electrophile!)
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 H2SO4 is reversible!
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 (NO2
+) as
the electrophile (which is formed from the acid-base reaction between HNO3 and H2SO4)
Once the nitro group is on the aromatic ring, it can be reduced to give an amino group (NH2)

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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 AlCl3
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 AlCl3
HOWEVER: most 1° alkyl halides (containing >2 carbons) cannot be used effectively because their
complexes with AlCl3 readily 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:
1. The carbon atom connected to the halogen MUST be sp3 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, NO2) that are incompatible with the reaction
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)
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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 higher Ea 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 Ea), simply because there are 2 ortho attack sites compared to para (only 1 site)
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!
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