Part 7. The chemistry of Aromatic Compounds

Doc Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study Notes for UK KS5 A/AS GCE IB advanced level organic chemistry students US K12 grade 11 grade 12 organic chemistry the physical and chemical properties of aromatic compounds chemistry of benzene and methylbenzene

7.0 INDEX of all My Aromatic Chemistry Notes

7.1 Molecular structure and nomenclature of aromatic compounds

(7.1 includes an extensive introduction on how to write and display aromatic formulae including isomers - lots of examples)

7.2 Proof of the structure of benzene, aromaticity and an introduction to electrophilic substitution in arenes

7.3 Sources & synthesis of including alkylation (electrophilic substitution mechanism) and the physical properties of arenes (aromatic hydrocarbons) and use of arenes like benzene in fuels

7.4 Free radical addition reactions of benzene and methylbenzene with hydrogen (hydrogenation) and side-chain chlorination substitution of the methyl group of methylbenzene

7.5 Electrophilic substitution - ring nitration of benzene and methylbenzene, properties and uses of nitro-aromatics

7.6 Electrophilic substitution - ring halogenation of benzene & methylbenzene, properties & uses of aryl halides

7.7 Electrophilic substitution - ring sulfonation of arenes, properties & uses of alkylbenzenesulfonic acids

7.8 Electrophilic substitution - ring acylation of arenes, properties & uses of aromatic ketones & aromatic aldehydes

7.9 The physical and chemical properties of phenol and some of its derivatives and their uses

7.10 Physical & chemical properties of phenylamine, selected derivatives including diazonium ions and azo dyes

7.11 The physical and chemical properties of benzoic acid and selected derivatives

7.12 The structure, properties and uses of polyesters and polyamides involving aromatic monomers

7.13 Examples of aromatic compounds from the pharmaceutical industry and those found in natural products

7.14 Explaining the orientation of products of putting a 2nd substituent into a monosubstituted benzene derivative

(See short guide below. Perhaps 7.14 is beyond what is normally required in pre-university chemistry courses?)

See also Part 15 Organic Chemistry Spectroscopy - examples of IR, mass, H-1 NMR and C-13 NMR spectra

and specific spectra of aromatic compounds

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A short guide to the orientation of products with a 2nd electrophilic substitution into a benzene ring

The theoretical 2nd substituent positions for an initial substituent atom/group G in a monosubstituted benzene derivative.

The diagram also shows the theoretical yields for a 2nd substituent position from an electrophilic substitution reaction.

Notes (i) and (ii) below give a brief description as to why the actual % yields of the three possible isomeric disubstituted benzene derivatives rarely correspond with the theoretical % quoted above.

This wrongly assumes that all five hydrogen atoms are equally susceptible to electrophilic attack and substitution, BUT PLEASE be aware, the text below is a summarising simplification, electrophilic substitution in benzene derivatives is quite a complicated situation, so this is just a rough guide for pre-university level chemistry students.

(i) 2, 4 and 6 directing groups

If the atom of group G is directly bonded to the benzene ring does not have any π bonding the ring is usually activated compared to benzene itself.

The atom/group G tends to increase the electron density of the ring and more so at the 2, 4 and 6 positions, compared to the 3 and 5 positions.

Therefore the 2 and 4 positions become the preferred substitution points in the benzene ring e.g. -CH₃, -CH₂CH₃, -OH, -NH₂, OCH₃, groups usually promote 2- and 4- position substitution.

The small electron density shift is sometimes described as a plus inductive shift (+I effect), but this does not necessarily coincide with an atom of electronegativity higher than carbon e.g. Cl, N and O.

The reason being the lone pairs of Cl, N and O interact with the ring to increase the electron density and this electron pair donation often overrides the difference in electronegativity effect (this is all about conjugation and possible resonance hybrid structures - see section 7.14 for more details).

(ii) 3 and 5 directing groups

If the atom of group G is directly bonded to the benzene ring does have any π bonding the ring is usually deactivated compared to benzene itself.

In this case the atom/group G tends to decrease the electron density of the ring and more so at the 2, 4 and 6 positions, compared to the 3 and 5 positions.

Therefore the 3 position become the preferred substitution point in the benzene ring e.g. -NO₂, -COOH, -COOR, -C≡N, COCH₃, -SO₂OH, groups usually promote 3- position substitution.

The small electron density shift is sometimes described as a minus inductive shift (-I effect), and often coincides with an atom of electronegativity higher than carbon e.g. N and O.

BUT, π bonding is involved too, and can facilitate electron pair withdrawal from the benzene ring (again this is all about conjugation and possible resonance hybrid structures - see section 7.14 for more details).

The following notes below were written before the expanded notes listed above, but they do have basic reaction conditions, reagents and general equations as well as detailed notes on the mechanisms involved.

Nitration to give nitro-aromatics like nitrobenzene

Chlorination to chloro-aromatics like chlorobenzene

Alkylation to give alkyl-aromatics like methylbenzene [Friedel-Crafts reaction]

Acylation to give aromatic ketones [Friedel-Crafts reaction]

Sulfonation to give a sulfonic acid like benzenesulfonic acid

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* THE SPECTRA OF ORGANIC COMPOUNDS *
Infrared spectra of AROMATIC HYDROCARBONS The infrared spectrum of Benzene The infrared spectrum of Methylbenzene The infrared spectrum of 1,2-dimethylbenzene The infrared spectrum of 1,3-dimethylbenzene The infrared spectrum of 1,4-dimethylbenzene The infrared spectrum of Ethylbenzene The infrared spectrum of Propylbenzene The infrared spectrum of (1-methylethyl)benzene  /  2-phenylpropane (Cumene) The infrared spectrum of 1,3,5-trimethylbenzene The infrared spectrum of Chlorobenzene The infrared spectrum of phenol The infrared spectrum of benzaldehyde The infrared spectrum of benzoic acid The infrared spectrum of methyl 2-hydroxybenzoate (methyl salicylate)	Mass spectra of AROMATIC HYDROCARBONS The mass spectrum of Benzene The mass spectrum of Methylbenzene The mass spectrum of 1,2-dimethylbenzene The mass spectrum of 1,3-dimethylbenzene The mass spectrum of 1,4-dimethylbenzene The mass spectrum of Ethylbenzene The mass spectrum of Propylbenzene The mass spectrum of (1-methylethyl)benzene  /  2-phenylpropane (Cumene) The mass spectrum of 1,3,5-trimethylbenzene The mass spectrum of Chlorobenzene The mass spectrum of phenol The mass spectrum of benzaldehyde The mass spectrum of benzoic acid The mass spectrum of methyl 2-hydroxybenzoate (methyl salicylate)
H-1 NMR spectra of AROMATIC HYDROCARBONS The H-1 NMR spectrum of Benzene The H-1 NMR spectrum of Methylbenzene The H-1 NMR spectrum of 1,2-dimethylbenzene The H-1 NMR spectrum of 1,3-dimethylbenzene The H-1 NMR spectrum of 1,4-dimethylbenzene The H-1 NMR spectrum of Ethylbenzene The H-1 NMR spectrum of Propylbenzene The H-1 NMR spectrum of (1-methylethyl)benzene  /  2-phenylpropane (Cumene) The H-1 NMR spectrum of 1,3,5-trimethylbenzene The H-1 NMR spectrum of Chlorobenzene The H-1 NMR spectrum of phenol The H-1 NMR spectrum of Benzaldehyde The H-1 NMR spectrum of benzoic acid The H-1 NMR spectrum of methyl 2-hydroxybenzoate (methyl salicylate)	C-13 NMR spectra of AROMATIC HYDROCARBONS The C-13 NMR spectrum of Benzene The C-13 NMR spectrum of Methylbenzene The C-13 NMR spectrum of 1,2-dimethylbenzene The C-13 NMR spectrum of 1,3-dimethylbenzene The C-13 NMR spectrum of 1,4-dimethylbenzene The C-13 NMR spectrum of Ethylbenzene TheC-13 NMR spectrum of Propylbenzene The C-13 NMR spectrum of (1-methylethyl)benzene  /  2-phenylpropane (Cumene) The C-13 NMR spectrum of 1,3,5-trimethylbenzene The C-13 NMR spectrum of Chlorobenzene The C-13 NMR spectrum of phenol The C-13 NMR spectrum of Benzaldehyde The C-13 NMR spectrum of benzoic acid The C-13 NMR spectrum of methyl 2-hydroxybenzoate (methyl salicylate)

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All Advanced A Level Organic Chemistry Notes

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 All Advanced Organic Chemistry Notes

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