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Notes on the Structure and Nomenclature-naming of Aromatic Compounds

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KEYWORDS: fast track to nomenclature examples of ... Hydrocarbons (Arenes) ... Halo-Aromatics ... Phenols ... Aldehydes and Ketones ... Carboxylic Acids and Derivatives ... Amines (prim/sec/tert) ... Amides ... Diazonium Salts and azo dyes ... Nitro-Aromatics ... Sulphonic Acids

* TYPE IN NAME AROMATIC QUIZ * organic chemical id tests * Summary of FUNCTIONAL GROUPS

and please note that some 'old' names are quoted in () though their use should be avoided if possible, however, many are still used - just put one into GOGGLE!

See ALKANES page for comparison with aliphatic and alicyclic compounds

Aromatic Hydrocarbons (Arenes)

benzene C₆H₆,

methylbenzene (toluene) C₇H₈, C₆H₅CH₃ (c) doc b

ethylbenzene and propylbenzene

phenylethene ('styrene'), is named as a derivative of ethene,

the C₆H₅- aromatic ring grouping is called a phenyl group when quoted as a substituent prefix.

So phenylethene is named as a derivative of ethene.

1,2-dimethylbenzene , 1,3-dimethylbenzene (c) doc b , 1,4-dimethylbenzene

(once called xylenes, ortho, meta and para-xylene)

1-ethyl-2-methylbenzene

1-ethyl-3-methylbenzene

1-ethyl-4-methylbenzene,

1,2-diethylbenzene , 1,3-dipropylbenzene

1-methyl-2-propylbenzene

1-methyl-3-propylbenzene

1-methyl-4-propylbenzene

1-ethyl-4-propylbenzene

4-ethyl-1,2-dimethylbenzene: (uses lower numbers than 1-...-3,4-...)

Halo-arenes

or aryl halide, aromatic halogen compounds, the halogen is directly attached to the benzene ring.

chlorobenzene and 1,2 or 1,3 or 1,4-dichlorobenzene ,,

chloromethylbenzene, or (chloromethyl)benzene (benzyl chloride)

It is not a true aryl halogen compound, the halogen atom is in a non-aromatic side chain, so it is a primary aliphatic halogenoalkane.

are true aryl halides with the halogen attached directly to the benzene ring.

These can also be named as 2/3/4-chloro-1-methylbenzene or 2/3/4-chloromethylbenzene

They are isomeric with (chloromethyl)benzene above.

2,3-dichloromethylbenzene and 1-chloro-2,4-dimethylbenzene

Phenols

If the OH group (hydroxy) is directly attached to a benzene ring, the molecule is classified as a 'phenol'.

If not, the molecule is classified as an aliphatic alcohol.

This difference is illustrated below with molecules containing a benzene ring (Phenols ROH, R=aryl only)

phenol,

2-chlorophenol (o-chlorophenol),3-methylphenol (m-methylphenol, meta-cresol)

2-aminophenol, 4-aminophenol and 2-nitrophenol

3-hydroxybenzoic acid and 2-hydroxybenzoic acid

2,5-dichloro-4-methylphenol

BUT if the OH is not attached to a benzene ring you get an aliphatic alcohol which is isomeric with an phenols and an ether.

phenylmethanol (old name 'benzyl alcohol') is a primary aliphatic alcohol

which is isomeric with methylphenols and the ether, methoxybenzene (anisole).

1-phenylethanol, which is isomeric with the ether

ethoxybenzene (phenetole) and the ethylphenols or dimethylphenols.

Aromatic aldehydes and ketones

True aromatic aldehydes, R-CHO, have the aldehyde group -CHO directly attached to the ring e.g.

orbenzaldehyde, and 2-hydroxybenzaldehyde

Ketones, R₂C=O, are often 'mixed' aliphatic-aromatic in the sense that one R group is alkyl and the other is aryl e.g.

1-phenylethanone (acetophenone, methyl phenyl ketone) , and

Diphenylmethanone (diphenyl ketone), (c) doc b is a completely aromatic ketone.

Aromatic acids and their derivatives

Note: quite a few of the 'old' names are still used and accepted

, , , , benenecarboxylic acid, benzoic acid

, , , , benzenecarbonyl chloride, benzoyl chloride

, , , , benzenecarboxamide, benzamide

, , , , methyl benzenecarboxylate, methyl benzoate

methyl 2-hydroxybenzoate

, , , ethyl benzenecarboxylate, ethyl benzoate

, , , propyl benzenecarboxylate, propyl benzoate

2 3 or 4-hydroxybenzenecarboxylic acid, 2 3 or 4-hydroxybenzoic acid

2-chlorobenzoic acid

benzene-1,2-dicarboxylic acid (or 1,3 or 1,4)

Primary AROMATIC amines

(primary, 2 H's and only one R group attached to the N of the amine group, R-NH₂ where R = alkyl or aryl)

The amino (prefix) or amine (suffix) group is directly attached to the aromatic benzene ring

e.g. the simplest is C₆H₅NH₂, phenylamine

2- or 3- or 4-chlorophenylamine

1,3-diaminobenzene, , 3-aminobenzoic acid,

2-methylphenylamine, methyl-2-phenylamine, 1-amino-2-methylbenzene or 2-aminomethylbenzene

This is isomeric with benzylamine (c) doc b which is a primary aliphatic amine because the amine group is not directly attached to the ring.

Secondary AROMATIC amines

(secondary, one H and two R groups attached to the N of the amine group, R₂NH where R = alkyl or aryl)

N-methylphenylamine, N-ethylphenylamine

and diphenylamine

TERTIARY AMINES

(tertiary, no H and three R groups attached to the N of the amine group, R₃N where R = alkyl or aryl)

N,N-dimethylphenylamine , N,N-diethylphenylamine

and triphenylamine

PRIMARY ACID AMIDES

(primary, no alkyl/aryl R group and 2H's on the N of amide group)

The simplest primary aromatic acid amide is

benzamide or benzenecarboxamide,, ,

SECONDARY ACID AMIDES

(secondary, 1 alkyl/aryl R group and 1H on the N of amide group)

N-phenylethanamide,

N-methylbenzamide, , N-phenylbenzamide,

POLYAMIDES

Are secondary amides formed in a condensation reaction between a carboxylic acid and an amine.

Water is eliminated between the two 'monomers' to give the secondary, polyamide (polymer) linkage ...

-COOH + H₂N- ==> -CO-NH- + H₂O

KEVLAR is an aromatic polyamide formed from benzene-1,4-dicarboxylic acid and 1,4-diaminobenzene

TERTIARY ACID AMIDES

Tertiary amides would have no H and 2 aryl/alkyl groups on N of amide group.

N,N-dimethylbenzamide,

DIAZONIUM SALTS and AZO DYES:

Diazonium salts are formed when primary aromatic amines reaction with nitrous acid

e.g. C₆H₅NH₂(aq) + HNO_2(aq) + H⁺_(aq) ==> C₆H₅N₂⁺_(aq) + 2H₂O_(l)

The diazonium cation has a nitrogen-nitrogen triple bond system directly attached to the benzene ring e.g.

(1) from phenylamine

(2) from 4-methylphenylamine

(3) from 2-aminobenzoic acid.

In alkaline solution these diazonium salts couple with phenols and aromatic amines to form azo dyes.

These dyes have benzene rings linked with an azo -N=N- bond system e.g.

reacting (1) with phenol gives

reacting (1) with phenylamine gives

reacting (2) with phenol gives

reacting (2) with phenylamine gives

NITRO-AROMATICS

These have the nitro -NO₂ group directly attached to the ring. On reduction they form primary aromatic amines.

nitrobenzene, ; 1,3-dinitrobenzene,

1-chloro-2-nitrobenzene , 1-chloro-3-nitrobenzene , 1-chloro-4-nitrobenzene

1-methyl-2-nitrobenzene, ; 3-nitrobenzoic acid,

Sulphonic Acids

These molecules have a strongly mono-basic acidic group -SO₂OH directly attached to the benzene ring e.g.

benzenesulphonic acid, ,(or benzenesulfonic acid)

2-, 3- or 4-methylbenzenesulphonic acid,, , (or ....sulfonic acid)

GENERAL REVISION NOTES

Revision notes for studying revising tutoring teaching Advanced Level GCE AS A2 CHEMISTRY courses in unofficial support the Chemistry in any advanced-subsidiary AQA, Edexcel, OCR, CIE, WJEC, SQA and CCEA (NI) UK or Cambridge/London/Edexcel International and OCR/CIE and International Baccalaureate (IB) examinations.