Para- prefix

This method is useful for portraying chemical formulae in typescripts and manuscripts, as a lot of information is conveyed in a condensed manner. However, it suffers from the problem that it does not give a pictorial representation of the stereochemistry of the molecule. In order to overcome this deficiency, it is possible to include the appropriate Cahn-Ingold-Prelog prefix, namely R or S, to indicate diastereo details, or one of the cis, trans, ortho, meta or para prefixes to indicate geometrical details. ... 

Benzene molecules bearing alkyl substituents are called alkylbenzenes. The simplest of these is methylbenzene (common name, toluene), shown in Figure 27-1 Ic. The dimethyl-benzenes are called xylenes. Three different compounds (Table 27-6) have the formula CgH4(CH3)2 (see margin). These three xylenes are constitutional isomers. In naming these (as well as other disubstituted benzenes), we use prefixes ortho- (abbreviated o-), meta- m-), or para- (p-) to refer to relative positions of substituents on the benzene ring. The ortho- prefix refers to two substituents located on adjacent carbon atoms for example, 1,2-dimethylbenzene is o-xylene. The meta- prefix identifies substituents on C atoms 1 and 3, so 1,3-dimethylbenzene is 7w-xylene. The para- prefix refers to substituents on C atoms 1 and 4, so 1,4-dimethylbenzene is p-xylene. 

In Summary Simple monosubstituted benzenes are named by placing the substituent name before the word benzene. For more highly substituted systems, 1,2-, 1,3-, and 1,4- (or ortho-, meta-, and para-) prefixes indicate the positions of disubstitution. Alternatively, the ring is numbered, and substituents labeled with these numbers are named in alphabetical order. Many simple substituted benzenes have common names. 

If one now considers disubstituted Pc derivatives, particular descriptors are employed to designate the relation between the two positions. For substituents on the same phenyl ring, ortho, meta, and para prefixes are still valid, while inter-ring disubstitution gives rise to pseudo-gem, pseudo-ortho, pseudo-para, and pseudo-meta prefixes (Figure 8.3). Finally, a last remark about the configurational stability of Pc derivatives chiral [2.2]paracyclophanes can indeed racemize above 200°C by a process that was proven to involve benzylic-bond homolytic scission, rotation, and recombination of the diradical. 

OH groups are in the para or 1,4 position to each other. This use of the prefix is confined to disubstituted benzene derivatives in such cases as para-hydrogen and paraldehyde the prefix has no uniform structural significance and is always written in full. 

The prefix ortho signifies a 1 2 disubstituted benzene ring meta signifies 1 3 disubstitu tion and para signifies 1 4 disubstitution The prefixes o m and p can be used when a substance is named as a benzene derivative or when a specific base name (such as ace tophenone) is used For example... 

Structural prefixes should be italicized and connected with the chemical formula by a hyphen cis-, trans-, anti-, syn-, cyclo-, catena-, o- or ortho-, m- or meta-, p- or para-, sec- (secondary), tert-(tertiary), v- (vicinal), meso-, as- for asymmetrical, and 3- for symmetrical. 

The prefixes e- and n- refer to equilibrium and normal states. For is the low temperature ortho-para composition existing at 20.39 K, composition above 200 K. See text. 

If several groups are attached to the benzene ring, their names as well as their relative positions should be indicated. For example, dimethylbenzene or xylene, CgH (CH,)2, has three geometric isomers, with prefixes ortho-, meta-, and para-, indicating the relative positions of the two methyl groups. 

When two hydrogens in the ring are substituted by the same reagent, three isomers are possible. The prefixes ortho, meta, and para are used to indicate the location of the substituents in 1,2- 1,3- or 1,4-positions. For... 

Disubstituted benzenes are named using one of the prefixes ortho- (o), meta- (in), or para- (p). An ortho-disubstituted benzene has its two substituents in a 1,2 relationship on the ring, a meta-disubstituted benzene has its two substituents in a 1,3 relationship, and a para-disubstituted benzene has its substituents in a 1,4 relationship. 

Para- (Section 15.1) A naming prefix used for 1,4-disub-stituted benzenes. 

Chemists do not always use position numbers to describe the branches that are attached to a benzene ring. When a benzene ring has only two branches, the prefixes ortho-, meta-, and para- are sometimes used instead of numbers. 

Simple phenolics are substituted phenols. The ortho, meta and para nomenclature refers to a 1,2-, 1,3- and 1,4-substitution pattern of the benzene ring, respectively, where in this case one of the functional groups is the hydroxyl group. With three functional groups, the substitution pattern can be 1,3,5, which, when all three substituents are identical, is designated as a mt /fl-tri-substitution pattern, whereas the 1,2,6, substitution pattern is indicated by the prefix v/c (Figure 1-1). 

The compounds are listed in alphabetical order, whereby numbers, prefixes such as para or O- have been ignored. For example, 7 -coumaric acid is listed under C , and 1,2-dihydroxybenzene is listed under D When applicable, the compound may be listed multiple times. In the example above, 1,2-dihydroxybenzene is also listed under B , as benzene, 1,2-dihydroxy . ... 

Disubstituted benzenes are named using the prefixes ortho-, meta-, and para- to specify the substitution patterns. These terms are abbreviated o-, m-, andp-. Numbers can also be used to specify the substitution in disubstituted benzenes. 

Aromatic acids of the form Ar—COOH are named as derivatives of benzoic acid, Ph—COOH. As with other aromatic compounds, the prefixes ortho-, meta-, and para-may be used to give the positions of additional substituents. Numbers are used if there are more than two substituents on the aromatic ring. Many aromatic acids have historical names that are unrelated to their structures. 

There are three different ways that two groups can be attached to a benzene ring, so a prefix— ortho, meta, or para—can be used to designate the relative position of the two substituents. Ortho, meta, and para are also abbreviated as o, m, and p, respectively. 

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