Chemistry 243 - Lecture 13 1 C 2004 Barry Linkletter, UPEI Lecture 13 Chemistry of Aromatic Compounds Chapter 5 Chemistry 243 - Lecture 13 2 C 2004 Barry Linkletter, UPEI Substitution Reactions of Benzene and Its Derivatives Benzene is aromatic: a cyclic conjugated compound with 6 ! electrons Reactions of benzene lead to the retention of the aromatic core Electrophilic aromatic substitution replaces a proton on benzene with another electrophile Chemistry 243 - Lecture 13 3 C 2004 Barry Linkletter, UPEI Bromination of Aromatic Rings Benzenes ! electrons participate as a Lewis base in reactions 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 Chemistry 243 - Lecture 13 4 C 2004 Barry Linkletter, UPEI Addition Intermediate in Bromination The addition of bromine occurs in two steps In the first step the ! electrons act as a nucleophile toward Br 2 (in a complex with FeBr 3 ) This forms a cationic addition intermediate from benzene and a bromine cation The intermediate is not aromatic and therefore high in energy (see Figure 16.2)
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Chemistry 243 - Lecture 13 1
C 2004 Barry Linkletter, UPEI
Lecture 13
Chemistry of Aromatic Compounds
Chapter 5
Chemistry 243 - Lecture 13 2
C 2004 Barry Linkletter, UPEI
Substitution Reactions of Benzene and
Its Derivatives
� Benzene is aromatic: a cyclic conjugatedcompound with 6 ! electrons
� Reactions of benzene lead to the retention ofthe aromatic core
� Electrophilic aromatic substitution replaces aproton on benzene with another electrophile
Chemistry 243 - Lecture 13 3
C 2004 Barry Linkletter, UPEI
Bromination of Aromatic Rings
� Benzene�s ! electrons participate as a Lewis basein reactions with Lewis acids
� The product is formed by loss of a proton, whichis replaced by bromine
� FeBr3 is added as a catalyst to polarize thebromine reagent
Chemistry 243 - Lecture 13 4
C 2004 Barry Linkletter, UPEI
Addition Intermediate in Bromination
� The addition of bromine occurs in two steps
� In the first step the ! electrons act as anucleophile toward Br2 (in a complex with FeBr3)
� This forms a cationic addition intermediate frombenzene and a bromine cation
� The intermediate is not aromatic and thereforehigh in energy (see Figure 16.2)
Chemistry 243 - Lecture 13 5
C 2004 Barry Linkletter, UPEI
Formation of Product from
Intermediate
� The cationic additionintermediate transfersa proton to FeBr4
-
(from Br- and FeBr3)
� This restoresaromaticity (in contrastwith addition inalkenes)
Chemistry 243 - Lecture 13 6
C 2004 Barry Linkletter, UPEI
Other Aromatic Substitutions
� The reaction with bromine involves a mechanismthat is similar to many other reactions of benzenewith electrophiles
� The cationic intermediate was first proposed by G.W. Wheland of the University of Chicago and isoften called the Wheland intermediate
Chemistry 243 - Lecture 13 7
C 2004 Barry Linkletter, UPEI
Aromatic Chlorination and Iodination
� Chlorine and iodine (but not fluorine, which is too reactive)
can produce aromatic substitution with the addition of other
reagents to promote the reaction
� Chlorination requires FeCl3
� Iodine must be oxidized to form a more powerful I+ species
(with Cu+ or peroxide)
Chemistry 243 - Lecture 13 8
C 2004 Barry Linkletter, UPEI
Aromatic Nitration
� The combination of nitric acid and sulfuric acid producesNO2
+ (nitronium ion)
� The reaction with benzene produces nitrobenzene
Chemistry 243 - Lecture 13 9
C 2004 Barry Linkletter, UPEI
Aromatic Sulfonation
� Substitution of H by SO3 (sulfonation)
� Reaction with a mixture of sulfuric acid and SO3
� Reactive species is sulfur trioxide or its conjugate acid
� Reaction occurs via Wheland intermediate and is reversible
Chemistry 243 - Lecture 13 10
C 2004 Barry Linkletter, UPEI
Alkylation of Aromatic Rings: TheFriedel�Crafts Reaction
� Aromatic substitution ofa R+ for H
� Aluminum chloridepromotes the formationof the carbocation
� Wheland intermediateforms
Chemistry 243 - Lecture 13 11
C 2004 Barry Linkletter, UPEI
Limitations of the Friedel-Crafts
Alkylation
� Only alkyl halides can be used (F, Cl, I, Br)
� Aryl halides and vinylic halides do not react (theircarbocations are too hard to form)
� Will not work with rings containing an amino groupsubstituent or a strongly electron-withdrawing group
Chemistry 243 - Lecture 13 12
C 2004 Barry Linkletter, UPEI
Control Problems
� Multiple alkylations can occur because the first
alkylation is activating
Chemistry 243 - Lecture 13 13
C 2004 Barry Linkletter, UPEI
Carbocation Rearrangements During
Alkylation� Similar to those that occur during electrophilic additions to
alkenes
� Can involve H or alkyl shifts
Chemistry 243 - Lecture 13 14
C 2004 Barry Linkletter, UPEI
Acylation of Aromatic Rings
� Reaction of an acid chloride (RCOCl) and an aromatic ring inthe presence of AlCl3 introduces acyl group, !COR
� Benzene with acetyl chloride yields acetophenone