Monocyclic Aromatic Hydrocarbons - Advances in Chemistry (ACS

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Monocyclic Aromatic Hydrocarbons

Unsaturated hydrocarbon ring systems containing one or more six-mem­ bered rings in each of which the unsaturation may be represented formally by three alternating (conjugated) double bonds have long been known as aromatic hydrocarbons. The class name for these compounds is arene. This chapter deals only with monocyclic arenes—i.e., with benzene and substituted benzenes. Polycyclic arenes are the subject of Chapters 4 and 5, and unsaturated alicyclic (nonaromatic) hydrocarbons are treated in Chapter 2. Recommended Nomenclature Practice Simple monocyclic arenes are best named systematically as derivatives of the parent compound benzene, although trivial names for a few substituted benzenes are recognized in the 1957 I U P A C Rules.

20 In Nomenclature of Organic Compounds; Fletcher, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1974.

3.

Monocyclic Aromatics CH=CH

2

1,3,5-Trimethylbenzene Mesitylene

Etheny lbenzene Styrene

7

CH

3

1-1 sopropyl-4-methylbenzene p-Cymene (fixed numbering shown)

2 3

CH CHCH

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3

9

3

8 10

All other simple monocyclic aromatic hydrocarbons are best named systematically as derivatives o f benzene, particularly when an added substituent is identical with one already present.

CH

3

1,2,3,5 -Tetramethy lbenzene not 2-Methylmesitylene

Positions o f substituents are indicated by Arabic numerals except that o(ortho), m- (meta), and p- (para) may be used instead of 1,2-, 1,3-, and 1,4-, respectively, for disubstituted benzene derivatives only. Lowest possible numbers are given to the carbon atoms carrying substituents, choice between alternatives being made as with the aliphatic hydrocarbons (Chapter 2); however, when a name is based on a recognized trivial name other than benzene, priority for lowest numbers is given to substituents implied by the parent trivial name. The isomeric cymenes have fixed numbering, as i n the example (p-cymene) shown above.

CH CH 2

3

1-E thy 1- 3 -propy lbenzene ra-E thy lpropy lbenzene

CH CHCH 3

3

1,4-Diisopropylbenzene p-Diisopropylbenzene not p-Isopropylcumene CH CHCH 3

3

In Nomenclature of Organic Compounds; Fletcher, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1974.

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NOMENCLATURE OF ORGANIC COMPOUNDS

l-fm-Butyl-4-me thy lbenzene 4-tert-Butyltoluene p-terf-Butyltoluene

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CH

3

Complex monocyclic arenes composed of benzene rings and aliphatic chains are named either as substituted aromatic hydrocarbons or as substituted aliphatic hydrocarbons (see also Chapter 2). Choice between these methods is made (1) to produce the maximum number of substitutions in the parent compound, and (2) so that smaller structural units are named as substituents on a larger parent compound. Thus, a hydrocarbon containing several aliphatic groups attached to benzene as the nucleus is named as a derivative o f benzene; similarly, a hydrocarbon containing two or more benzene rings attached to one straight chain is named as a derivative of the straight-chain hydrocarbon.

CH

3

CH CH CHCH CH 2

2

2

3

3 -Methyl-1 -phenylpentane

Triphenylmethane

In Nomenclature of Organic Compounds; Fletcher, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1974.

3.

Monocyclic Aromatics

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Finally, a hydrocarbon composed of one aliphatic chain and one benzene ring is usually named as a derivative of the larger structural unit. If the aliphatic chain is unsaturated, the compound may be named as a derivative of that chain, regardless of chain size; however, when the unsaturated chain contains no more than three carbon atoms, the compound is usually named as a derivative of benzene.

^

^

CH2CH2CH2CH2CH3

^

^CH CH CH CH CH CH CH 2

2

Or «

^-C=CHCH

2

3

2

2

Pentylbenzene

2

3

1-Phenylheptane

2-Phenyl-2-butene

CH,

=CH