2444-111111

2444-111111 - EXAM AVG. 76.4 / 100 Stnd. Dev. 40.1 FINAL...

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EXAM 3 AVG. 76.4 / 100 Stnd. Dev. 40.1 FINAL EXAM Wed. 12/14 8-10 am A120 + CLAS 110
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Electrophilic Aromatic Substitution 2 H Br H Br Br– Br H Br + Al Cl 3 H Br 40C 40A Br + AlCl 3 Br H Br Br H Br 1 39 H + 18 ! ! + 40B AlCl 3 Br AlCl 3 Br The reaction of diatomic bromine with aluminum chloride (AlCl 3 ) gives complex Br + AlCl 3 Br as the product before there is any reaction with benzene . • This cationic bromine atom is so reactive, however, that even a weak base like benzene will react to form a carbocation, 40 . • This high energy intermediate is called a Wheland intermediate or an arenium ions 40 is reasonable stabilized as shown in 40A, which leads to sufFcient stability that the reaction generates this intermediate. 40 is resonance stabilized but not aromatic.
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Electrophilic Aromatic Substitution 3 H Br H Br Br– Br H Br + Al Cl 3 H Br 40C 40A Br + AlCl 3 Br H Br Br H Br 1 39 H + 18 ! ! + 40B AlCl 3 Br AlCl 3 Br • Arenium ion 40 is a resonance stabilized intermediate with the three resonance contributors shown in 40A , and it is sometimes written in an abbreviated form ( 40C ) to represent the resonance. • It is known that the Fnal product is 18 , which requires loss of hydrogen atom from 40 .
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Electrophilic Aromatic Substitution 4 The reaction of benzene and bromine in the presence of aluminum chloride to give bromobenzene is an electrophilic aromatic substitution reaction . • The mechanism of this reaction is formally a two-step process: ( 1 ) Lewis-base-Lewis acid reaction between benzene and Br + , followed by ( 2 ) an E1 reaction that leads to loss of a proton to regenerate the aromatic benzene ring. Br H + 18 Br– Br H Br + Al Cl 3 H Br 40A Br + AlCl 3 Br H Br H Br 1 ! ! + AlCl 3 Br
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Nitrobenzene 5 • The nitronium ion, NO 2 + , is formed when nitric acid is mixed with the stronger sulfuric acid, nitric acid functions as a base. • Benzene reacts as a Lewis base with the nitronium ion to give resonance stabilized arenium ion intermediate, 41 . • The second step is the E1 reaction and loss of a proton to give nitrobenzene, 42 . N O O O H S O O H O O H N O O O H H S O O O O H H 2 SO 4 N O O S O O O O H + + H 3 O + nitronium ion base acid H NO 2 H NO 2 H NO 2 H NO 2 NO 2 - H 1 41 42
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Friedel-Crafts Alkylation 6 If benzene reacts with a carbocation, a new carbon-carbon bond is formed, and electrophilic aromatic substitution will give an arene. • The reaction of benzene and its derivatives with carbocations is generically called the
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2444-111111 - EXAM AVG. 76.4 / 100 Stnd. Dev. 40.1 FINAL...

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