16 chemistry of benzene electrophilic aromatic substitution
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16. Chemistry of Benzene: Electrophilic Aromatic Substitution - PowerPoint PPT Presentation

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16. Chemistry of Benzene: Electrophilic Aromatic Substitution. Based on McMurry’s Organic Chemistry , Chapter 16. Benzene is aromatic: a cyclic conjugated compound with 6  electrons Reactions of benzene lead to the retention of the aromatic core

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16 chemistry of benzene electrophilic aromatic substitution l.jpg

16. Chemistry of Benzene: Electrophilic Aromatic Substitution

Based on

McMurry’s Organic Chemistry, Chapter 16

Substitution reactions of benzene and its derivatives l.jpg

Benzene is aromatic: a cyclic conjugated compound with 6 Substitution electrons

Reactions of benzene lead to the retention of the aromatic core

Electrophilic aromatic substitution replaces a proton on benzene with another electrophile

Substitution Reactions of Benzene and Its Derivatives

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16.1 Bromination of Aromatic Rings Substitution

  • Benzene’s  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

  • FeBr3 is added as a catalyst to polarize the bromine reagent

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Addition Intermediate in Bromination Substitution

  • The addition of bromine occurs in two steps

  • In the first step the  electrons act as a nucleophile toward Br2 (in a complex with FeBr3)

  • 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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Formation of Product from Intermediate Substitution

  • The cationic addition intermediate transfers a proton to FeBr4- (from Br- and FeBr3)

  • This restores aromaticity (in contrast with addition in alkenes)

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Aromatic Chlorination and Iodination Substitution

  • 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)

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Aromatic Nitration Substitution

  • The combination of nitric acid and sulfuric acid produces NO2+ (nitronium ion)

  • The reaction with benzene produces nitrobenzene

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Aromatic Sulfonation Substitution

  • 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

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Alkali Fusion of Aromatic Sulfonic Acids Substitution

  • Sulfonic acids are useful as intermediates

  • Heating with NaOH at 300 ºC followed by neutralization with acid replaces the SO3H group with an OH

  • Example is the synthesis of p-cresol

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16.3 Alkylation of Aromatic Rings: The Friedel–Crafts Reaction

  • Aromatic substitution of a R+ for H

  • Aluminum chloride promotes the formation of the carbocation

  • Wheland intermediate forms

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Limitations of the Friedel-Crafts Alkylation Reaction

  • Only alkyl halides can be used (F, Cl, I, Br)

  • Aryl halides and vinylic halides do not react (their carbocations are too hard to form)

  • Will not work with rings containing an amino group substituent or a strongly electron-withdrawing group

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Control Problems Reaction

  • Multiple alkylations can occur because the first alkylation is activating

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Carbocation Rearrangements During Alkylation Reaction

  • Similar to those that occur during electrophilic additions to alkenes

  • Can involve H or alkyl shifts

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How many the substituted benzenes below can be subjected to Friedel-Crafts alkylation?

  • Only one

  • Two of them

  • All of them

  • None of them

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16.4 Acylation of Aromatic Rings Friedel-Crafts alkylation?

  • Reaction of an acid chloride (RCOCl) and an aromatic ring in the presence of AlCl3 introduces acyl group, COR

    • Benzene with acetyl chloride yields acetophenone

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Mechanism of Friedel-Crafts Acylation Friedel-Crafts alkylation?

  • Similar to alkylation

  • Reactive electrophile: resonance-stabilized acyl cation

  • An acyl cation does not rearrange

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16.5 Substituent Effects in Aromatic Rings Friedel-Crafts alkylation?

  • Substituents can cause a compound to be (much) more or (much) less reactive than benzene

  • Substituents affect the orientation of the reaction – the positional relationship is controlled

    • ortho- and para-directing activators, ortho- and para-directing deactivators, and meta-directing deactivators

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Origins of Substituent Effects Friedel-Crafts alkylation?

  • An interplay of inductive effects and resonance effects

  • Inductive effect - withdrawal or donation of electrons through a s bond

  • Resonance effect - withdrawal or donation of electrons through a  bond due to the overlap of a p orbital on the substituent with a p orbital on the aromatic ring

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Inductive Effects Friedel-Crafts alkylation?

  • Controlled by electronegativity and the polarity of bonds in functional groups

  • Halogens, C=O, CN, and NO2withdraw electrons through s bond connected to ring

  • Alkyl groups donate electrons

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Resonance Effects – Electron Withdrawal Friedel-Crafts alkylation?

  • C=O, CN, NO2 substituents withdraw electrons from the aromatic ring by resonance

  •  electrons flow from the rings to the substituents

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Resonance Effects – Electron Donation Friedel-Crafts alkylation?

  • Halogen, OH, alkoxyl (OR), and amino substituents donate electrons

  •  electrons flow from the substituents to the ring

  • Effect is greatest at ortho and para

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16.6 An Explanation of Substituent Effects Friedel-Crafts alkylation?

  • Activating groups donate electrons to the ring, stabilizing the Wheland intermediate (carbocation)

  • Deactivating groups withdraw electrons from the ring, destabilizing the Wheland intermediate

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Ortho- and Para-Directing Activators: Alkyl Groups Friedel-Crafts alkylation?

  • Alkyl groups activate: direct further substitution to positions ortho and para to themselves

  • Alkyl group is most effective in the ortho and para positions

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Ortho- and Para-Directing Activators: OH and NH Friedel-Crafts alkylation?2

  • Alkoxyl, and amino groups have a strong, electron-donating resonance effect

  • Most pronounced at the ortho and para positions

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Ortho- and Para-Directing Deactivators: Halogens Friedel-Crafts alkylation?

  • Electron-withdrawing inductive effect outweighs weaker electron-donating resonance effect

  • Resonance effect is only at the ortho and para positions, stabilizing carbocation intermediate

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Meta-Directing Deactivators Friedel-Crafts alkylation?

  • Inductive and resonance effects reinforce each other

  • Ortho and para intermediates destabilized by deactivation from carbocation intermediate

  • Resonance cannot produce stabilization

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16.7 Trisubstituted Benzenes: Additivity of Effects Substitution

  • If the directing effects of the two groups are the same, the result is additive

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Substituents with Opposite Effects Substitution

  • If the directing effects of two groups oppose each other, the more powerful activating group decides the principal outcome

  • Often gives mixtures of products

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Meta-Disubstituted Positions Are Unreactive Substitution

  • The reaction site is too hindered

  • To make aromatic rings with three adjacent substituents, it is best to start with an ortho-disubstituted compound

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Bromination of phenol with excess bromine yields tribromophenol. Which isomer?

  • 2,3,4-Tribromophenol

  • 2,4,5-Tribromophenol

  • 2,4,6-Tribromophenol

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16.8 Nucleophilic Aromatic Substitution tribromophenol. Which isomer?

  • Exactly the opposite electron demand as before!

  • While protons attached to electron-rich aromatics can be displaced by electrophiles, halogens attached to electron-depleted aromatics can be displaced by nucleophiles

  • Nucleophilic aromatic substitution is similar to SN1/SN2 in its outcome, but follows a completely different mechanism

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Addition-Elimination Reactions tribromophenol. Which isomer?

  • Typical for aryl halides with nitro groups in ortho or para position

  • Addition to form intermediates (Meisenheimer complexes) by electron-withdrawal

  • Halide ion is then lost by elimination

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…but brute force will do the job! conditions…

However, this can’t be the same mechanism

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Elimination-Addition: Benzynes conditions…

  • The reaction involves an elimination reaction that gives a triple bond, followed by rapid addition of water

  • The intermediate is called benzyne

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Evidence for Benzyne as an Intermediate conditions…

  • Bromobenzene with 14C only at C1 gives substitution product with label scrambled between C1 and C2

  • Reaction proceeds through a symmetrical intermediate in which C1 and C2 are equivalent— must be benzyne

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Structure of Benzyne conditions…

  • Benzyne is a highly distorted alkyne

  • The triple bond uses sp2-hybridized carbons, not the usual sp

  • The triple bond has one  bond formed by p–p overlap and by weak sp2–sp2 overlap

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What type of reaction is this? conditions…

  • Nucleophilic substitution with benzyne intermediate

  • Diels-Alder cycloaddition

  • Friedel-Crafts alkylation

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16.10 Oxidation of Aromatic Compounds conditions…

  • Alkyl side chains can be oxidized to CO2H by strong reagents such as KMnO4 and Na2Cr2O7 if they have a C-H next to the ring

  • Converts an alkylbenzene into a benzoic acid, ArR  ArCO2H

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Bromination of Alkylbenzene Side Chains conditions…

  • Reaction of an alkylbenzene with N-bromo-succinimide (NBS) and benzoyl peroxide (radical initiator) introduces Br into the side chain

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Mechanism of NBS (Radical) Reaction conditions…

  • Abstraction of a benzylic hydrogen atom generates an intermediate benzylic radical

  • Reacts with Br2 to yield product

  • Br· radical cycles back into reaction to carry chain

  • Br2 produced from reaction of HBr with NBS

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16.11 Reduction of Aromatic Compounds conditions…

  • Aromatic rings are inert to catalytic hydrogenation under conditions that reduce alkene double bonds

  • Can selectively reduce an alkene double bond in the presence of an aromatic ring

  • Reduction of an aromatic ring requires more powerful reducing conditions (high pressure or rhodium catalysts)

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Reduction of Aryl Alkyl Ketones conditions…

  • Aromatic ring activates neighboring carbonyl group toward reduction

  • Ketone is converted into an alkylbenzene by catalytic hydrogenation over Pd catalyst

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Summary of reactions conditions…not involving aromatic substitution

  • Reduction of nitrobenzenes to anilines

  • Conversion of benzenesulfonic acids to phenols

  • Side chain oxidation of alkylbenzens to benzoic acids

  • Side chain halogenation of alkylbenzenes

  • Reduction of benzenes to cyclohexanes

  • Reduction of aryl ketones to alkyl benzenes