CHM 247 CH.16
Chemistry of Benzene: ElectrophilicAromatic Substitution
• Electrophilic aromatic substitution reaction: A reaction in which an electrophile (E )
reacts with an aromatic ring and substitutes for one of the ring hydrogens.
• Ability to undergo electrophilic substitution is a good test of aromaticity.
• Aromatic rings can have halogens, nitro groups (-NO ),2sulfonic acid groups (-SO H)3
hydroxyl groups, alkyl groups, or acyl groups (-C=OR) added via substitution.
16.1 ElectrophilicAromatic Substitution Reactions: Bromination
• Aromatic rings are less reactive towards electrophiles than alkenes.
• Bromination of benzene takes place with a catalyst like FeBr .3The catalyst makes the Br 2
molecule more electrophilic by polarizing it.
• This reaction leads to a less stable carbocation, making it endergonic and slow.
• If addition occurred, the ring would lose its stability gained from being an aromatic
compound, therefore, only substitution occurs.
16.2 OtherAromatic Substitutions
Aromatic Fluorination, Chlorination, and Iodination
• Chlorine and iodine can be introduced with electrophilic substitution reactions, but
fluorine is too reactive and gives poor yield. Therefore other agents of Fluorine are used.
2 is also not reactive enough under standard conditions and therefore not used.
Aromatic Nitration • Aromatic rings are nitrated by a reaction with a mixture of concentrated nitric and
sulfuric acids. Electrophile is the nitronium ion.
• Nitro groups can be reduced to amines.
• Reaction uses H 2O a4d SO . N3te reversible reaction.
• FORWARD in concentrated acid, BACKWARDS in dilute acid.
16.3 Alkylation and Acylation of Aromatic Rings: The Friedel-Crafts Reaction
• Alkylation: Introduction of an alkyl group onto a molecule. For example, aromatic rings
can be alkylated to yield arenes, and enolare anions can be alkylated to yield α-
substituted carbonyl compounds.
• Friedel-Crafts reaction: An electrophile aromatic substitution reaction to alkylate or
acylate an aromatic ring.
• Treat aromatics with an alkyl chloride in the presence ofAlCl to generate a carbocation.
Loss of H completes the reaction. •
• LIMITATION: Only alkyl halides can be used.Aromatic halides and vinylic halides are
not reactive under these conditions.
• LIMITATION: Only work with weak electron withdrawing groups on the aromatic
compound. Strong groups (C=O, SO , N3 NO, NH , NHR2 NRR, ester, carboxylic acid,
sulfuric acid, and NO2) will not work.
• LIMITATION: Difficult to stop at one alkylation, so the reaction will often continue.
• LIMITATION: The carbocation formed can undergo re-arrangement, leading to a
product yield that is low.
• Acylation: The introduction of an acyl group, -COR, onto a molecule. For example,
acylation of an alcohol yields an ester, acylation of an amine yields an amide, and
acylation of an aromatic ring yields an alkyl aryl ketone.
• Acyl group: A –COR group.
• Aromatic ring is acylated in presence ofAlCl 3eacting with a carboxylic acid chloride.
• NO carbocation rearrangement due to polarity.
• NO multiple reactions occur.
• LIMITATION: Functional group compatibility.
16.4 Substituent Effects in Substituted Aromatic Rings • Only one substituent can form when an electrophilic substitution occurs on benzene. But
a substitution carried out on a ring that already has a substituent has 2 effects:
- Substituents affect the reactivity of the aromatic rings. Some substituents activate the
ring making it more reactive, others deactivate it, making it less reactive. Electron
withdrawing groups are activators, making it more reactive.
- Substituents affect the orientation of the reaction. The nature of the substituent
present determines whether ortho, meta, or para products form.
• Substituents can be classed as 3 groups, ortho and para directing activators, ortho and
para directing deactivators, and meta directing deactivators.All meta directing groups are
16.5 An Explanation of Substituent Effects
Activation and Deactivation ofAromatic Rings
• Activating groups donate electron density to the ring. Deactivating groups with