Aromatic compounds, naphthenic rings

Difficulties arise in using such a classification in that in the fractions boiling above 200°C (390°F) the molecules can no longer be placed in one group because most of them are of a typically mixed nature. Purely naphthenic or aromatic molecules occur very seldom cyclic compounds generally contain paraffinic side chains and often even aromatic and naphthenic rings side by side. More direct chemical information is often desirable and can be supplied by means of the correlation index (Cl). 

Hydrogenation of the aromatic ring to form naphthenic compounds has been proposed as a route to faciUtate the separation of the Cg aromatic isomers (31). The spread in boiling points of the naphthenic compounds is 12°C vs a spread of 8°C for the aromatic compounds. However, the cycloparaffinic products obtained from OX and EB boil only 3°C apart, impeding the separation. 

Aiicyclic. Closed ring structures that fall into one of three different subgroups (1) saturated cycloparaffms—also called naphthenes—such as cyclohexane or cyclopentane, and (2) cycloolefins such as cyclopen tadiene—but not to be confused with aromatic compounds with the benzene ring. 

Naphthenes may undergo ring cleavage or side chain removal when thermally cracked. Longer side chain naphthenes are the most susceptible to side chain removal by thermal cracking. Aromatic compounds are the most resistant to thermal cracking conditions. The ease by which various hydrocarbons are cracked thermally can be ranked as follows ... 

Naphthenic-aromatic compounds are formed by the condensation of an aromatic ring with a cycloparaffin. Examples of naphthenic-aromatics include indane and tetralin and their derivatives. These compounds are common constituents of distillate fuel and light gas oil fractions. 

Along these lines a more or less complete separation is possible of paraffins, naphthenes with 1 ring, 2 rings, etc., benzene derivatives, mono-aromatics with 1 ring, 2 rings, etc., as shown in Table VI. The physical data for each fraction enable the corresponding amounts of structural compounds present to be calculated, as will be discussed below. 

Thus the contributions of these compounds to the specific refraction r of the fractions cancel out [cf. eqn. (71)] this is the reason that diagrams for the ring analysis of saturated mineral oil fractions based on specific refraction and molecular weight are also valid for the estimation of naphthenic rings in aromatic fractions cf. p. 24). 

The di- + triaromatic subfraction analyzes as 54% monoaromatics and 47% diaromatics by proton NMR. The average molecule also contains many short alkyl substituents, more than one aromatic ring, and one naphthene ring. The average molecular weight calculated from NMR is lower than that obtained by VPO. These data indicate that non-condensed di- and triaromatics are present in this subfraction. Compounds such as ... 

The number of naphthenic rings in aromatic compounds is determined by the assignment of Z values, where every naphthenic ring indicates a deficiency of two hydrogens. The distribution of compound types indicates that monoaromatic compounds consist largely of mono-. 

In addition to aromatics with benzene ring structures, modern refinery processes tend to increase the number of hydrocarbons with simpler types of carbon ring structures. Typical chemicals include cyclopentane, where the straight-chain pentane has been wrapped into a five-carbon ring. Other transformations of aliphatic hydrocarbons include cylcohexane and cyclopentane. These ring compounds are usually called naphthenes. 

This method gives the distribution by family (paraffins, isoparaffins, naphthenes with one or more rings, etc., aromatics) and by number of carbon atoms (up to a maximum of C33 for saturated compounds and up to C22 for aromatic compounds). 

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