WWU-Chemistry Aromatic Substitution Chapter 22

WWU-Chemistry Sections to skip!! Skip sections 22.1 (most), 22.3, 22.4, and 2.13 through Keep (review from c. 351, for some of you) Keep 22.17

WWU-Chemistry Problems In text: End of Chapter:

WWU-Chemistry Sect Nomenclature

WWU-Chemistry Catechols

Nomenclature-- examples

WWU-Chemistry Chapter 22: Aromatic Substitutions, Monosubstitution reactions on benzene Sect Electrophilic aromatic substitution mechanism Sect Nitration Sect Halogenation Sect Friedel-Crafts Reactions Sect Sulfonation Reactions (skip fall 2006)

WWU-Chemistry Sect Electrophilic aromatic substitution.

WWU-Chemistry Sect. 22.5: Nitration conc. HNO 3 and H 2 SO 4 react to make electrophile, NO 2 + nitro aromatics are important intermediates reduction of nitro groups give anilines

WWU-Chemistry Mechanism of Aromatic Nitration Step 1: Where does the electrophile come from? Nitronium ion (NO 2 + ) is the electrophile that reacts with the benzene ring.

WWU-Chemistry Mechanism of Aromatic Nitration (Step 2)

WWU-Chemistry Mechanism of Aromatic Nitration (Step 3)

WWU-Chemistry Sect. 22.6: Halogenation active electrophile is a bromonium or chloronium ion need Lewis acid catalyst ( FeX 3 ) to activate X 2

WWU-Chemistry Sect. 22.7: Friedel-Crafts Alkylation alkyl halide + AlCl 3 -->carbocation + AlCl 3 X - watch out for carbocation rearrangements! more than one alkylation can occur --> mixtures!

WWU-Chemistry Friedel-Crafts Acylation acid chloride + AlCl 3 --> acylium ion + AlCl 4 - cation rearrangements are NOT observed! acylation will only occur ONCE... reaction VERY sensitive to substituents-- an acyl group prevents further reaction

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Aromatic substitution on Benzene Sect. 22.8:Sulfonation (skip, fall 06) Sect. 22.9:Summary

WWU-Chemistry Sect and 22.11: Directing effects methoxy group releases electrons by resonance effect: ortho and para director nitro group withdraws electrons by inductive and resonance effect: meta director

WWU-Chemistry These are ortho and para directors! All are electron releasing!!

WWU-Chemistry All ortho/para directing groups have pairs of electrons next to the benzene ring! The only exception are alkyl groups. They are also ortho/para directors.

WWU-Chemistry Why do ortho/para groups direct as they do? Resonance!!

WWU-Chemistry These are meta directors! All are electron withdrawing!!

WWU-Chemistry Now let’s look at a meta directing group This is an example of Electrophilic Aromatic Substitution (EAS).

WWU-Chemistry Why does the nitration reaction take place preferentially at the meta position? Let’s ask a “what if” question.

WWU-Chemistry ortho meta para BAD!

WWU-Chemistry meta substitution preferred because the + charge is never next to the CO 2 R group

WWU-Chemistry Activation during substitution All ortho and para directing groups are activating relative to benzene, except halogen substituents. Halogens are weakly deactivating but are still o, p- directors.

WWU-Chemistry Deactivation during substitution All meta directors are strongly deactivating relative to benzene.

WWU-Chemistry Sect and 22.17: Some synthetic examples involving aromatic substitution

WWU-Chemistry ortho/para directors can work together with meta directors. They reinforce each other.

WWU-Chemistry Strong o/ p directors win over weak o, p and meta directors.

WWU-Chemistry Substitution RARELY occurs in-between two substituents--too hindered!

WWU-Chemistry Some groups can be modified to change their directing effects.

WWU-Chemistry Good stuff! Order of reaction is critical!

WWU-Chemistry Some more good stuff!

WWU-Chemistry An explosive!

WWU-Chemistry 2,4,6-Trinitrotoluene = TNT

WWU-Chemistry Some miscellaneous examples Nitration of 3-nitrobenzoic acid Acylation of 1,3-dimethylbenzene Acylation of 1,4-dimethylbenzene Make 2-methyl-1-phenylpropane

WWU-Chemistry Sect Aromaticity and Huckel’s Rule Aromatic compounds 4n + 2 pi electrons n = 1 6 pi electrons systems :

WWU-Chemistry

Other n = 1 aromatics ::: :: 6 electrons 6 electrons, one pair not involved! 6 electrons, one pair not involved! All are aromatic!

WWU-Chemistry

n = 0 aromatic: 2 pi electrons

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Some Antiaromatic compounds : Not aromatic!

WWU-Chemistry Diazonium ions, Azo Dyes and the Sandmeyer Reaction - from Chapter 23 (not covered 06) Sect 23.16: Diazonium ion formation Sect 23.17: Sandmeyer reaction Sect 23.19: Azo dyes