Chem Benzene

8/4/2019 Chem Benzene

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Chemistry of Benzene

2302261

Anawat Ajavakom


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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 thearomatic core.

Electrophilic aromatic substitution replaces a

proton on benzene with another electrophile.

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Bromination of Aromatic Rings

Benzenes electrons participate as a Lewis base inreactions 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

The addition of bromine occurs in two steps.

In the first step the electrons act as a nucleophiletoward Br2 (in a complex with FeBr3).

This forms a cationic addition intermediate frombenzene and a bromine cation.

The intermediate is higher in energy (not aromatic).

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Formation of Product from Intermediate

The cationic additionintermediate transfers a

proton to FeBr4- (from Br-and FeBr3).

This restores aromaticity

(in contrast with additionin alkenes).

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This cation intermediate (Whelandintermediate) was proposed byGeorge Willard Wheland (1907-1974).


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Aromatic Chlorination and Iodination

Chlorine and iodine (but not fluorine, which is tooreactive) can produce aromatic substitution with theaddition 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

The combination of nitric acid and sulfuric acidproduces NO2

+ (nitronium ion).

The reaction with benzene produces nitrobenzene.

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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.

Viareversible Wheland intermediate

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Alkali Fusion of Aromatic Sulfonic Acids

Sulfonic acids are useful as intermediates.

Heating with NaOH at 300 oC followed by neutralizationwith acid replaces the SO3H group with an OH.

Example is the synthesis of p-cresol.

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Alkylation of Aromatic Rings: The Friedel

Crafts Reaction Aromatic substitution of

a R+ for H.

Aluminum chloridepromotes the formationof the carbocation.

Wheland intermediate isformed.

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Limitations of the Friedel-Crafts Alkylation

Only alkylhalides can be used (F, Cl, I, Br).

Arylhalides and vinylichalides do not react (theircarbocations are too hard to form).

Will not work with rings containing an amino groupsubstituent or a strongly electron-withdrawing group.

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Control Problems

Multiple alkylations can occur because the firstalkylation is activating.

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Carbocation Rearrangements During

Alkylation Similar to those that occur during electrophilic

additions to alkenes

Can involve H or alkyl shifts

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Acylation of Aromatic Rings

Reaction of an acid chloride (RCOCl) and anaromatic 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

Similar to alkylation

Reactive electrophile: resonance-stabilized acylcation

An acyl cation does not rearrange

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Substituent Effects in Aromatic Rings

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-

directingdeactivators, and meta-directing deactivators.

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Inductive Effects

Controlled by electronegativity and the polarity ofbonds in functional groups.

Halogens, C=O, CN, and NO2 withdrawelectronsthrough bond connected to ring.

These are called Electron Withdrawing Group.

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Resonance Effect


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Resonance Effects Electron Donation

Halogen, OH, alkoxyl (OR), and amino substituentsdonateelectrons.

electrons flow from the substituents to the ring.

Effect is greatest at orthoand para.

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X, OH, OR inductively withdraw e-s and deactivate ring

Resonance interactions are weaker, affecting orientation


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Deactivating groupswithdraw electrons fromthe ring, destabilizing the

Wheland intermediate.

Activating groups donateelectrons to the ring,stabilizing the Wheland

intermediate.


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Ortho-and Para-Directing Activators: Alkyl Groups

Alkyl groups activate: direct further substitution to positionsorthoand para to themselves.

Alkyl group is most effective in the orthoand parapositions.

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Ortho-and Para-Directing Activators: OH and NH2

Alkoxyl, and amino groups have a strong, electron-donatingresonance effect. Most pronounced at the orthoand parapositions.

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Ortho-and Para-Directing Deactivators: Halogens

Electron-withdrawing inductive effect has weaker electron-donating resonance effect.

Resonance effect is only at the orthoand parapositions,stabilizing carbocation intermediate.

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Meta-Directing Deactivators

Inductive and resonance effects reinforce each other. Orthoand para intermediates destabilized by deactivation

from carbocation intermediate.

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Summary Table: Effect of Substituents in

Aromatic Substitution

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Trisubstituted Benzenes: Additivity of Effects

If the directing effects of the two groups are thesame, the result is additive.

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Substituents with Opposite Effects

In case of two groups with different directing effects,the more powerful one decides the principal outcome.

Usually gives mixtures of products.

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Meta-Disubstituted Compounds Are Unreactive

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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Nucleophilic Aromatic Substitution

Aryl halides withEWGs orthoandparareact with Nu.

Form intermediatethat is stabilized by

electron-withdrawal.

Halide ion is lost togive aromatic ring.

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Benzyne Industrial preparation of phenol; treatment of PhCl

with dil. aq. NaOH at 340 oC under high pressure.

The reaction involves an elimination reaction that

gives a triple bond. The intermediate is called benzyne.

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Evidence for Benzyne as an Intermediate

Bromobenzene with 14C only at C1 gives substitutionproduct with label scrambled between C1 and C2.

Reaction proceeds through a symmetrical

intermediate in which C1 and C2 are equivalentmust be benzyne.

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Structure of Benzyne

Benzyne is a highly distorted alkyne.

The triple bond uses sp2-hybridized carbons, not theusual sp.

The triple bond has one bond formed by ppoverlap and by weak sp2sp2 overlap.

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Oxidation of Aromatic Compounds

Alkyl side chains can be oxidized to CO2H bystrong reagents such as KMnO4 and Na2Cr2O7 if theyhave a C-H next to the ring.

Converts an alkylbenzene into a benzoic acid, ArR ArCO2H.

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Bromination of Alkylbenzene Side Chains

Reaction of an alkylbenzene with N-bromo-succinimide (NBS) and benzoyl peroxide

(radical initiator) introduces Br into the sidechain.

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Mechanism of NBS (Radical) Reaction

Abstraction of a benzylic hydrogen atom generatesan 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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Reduction of Aromatic Compounds

Aromatic rings are inert to catalytic hydrogenationunder 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

Aromatic ring activates neighboring carbonyl grouptoward reduction.

Ketone is converted into an alkylbenzene by catalytic

hydrogenation over Pd catalyst.

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Chem Benzene

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