Paraffin homologous series

A similar concept was suggested by Kaiser,23 who calls his parameter a separation number (SN), or Trennzahl (TZ) in German. It is the number of analytes that can be resolved between two consecutive members of the paraffin homologous series x and x + 1. It can be calculated by Eq. (27). 

To find the Kovats index for a given solute on a given stationary phase, a few members of the paraffin homologous series are chromatographed and plotted. Then the solute is run under the same conditions and its Index value is determined from the graph. It is best if the paraffins chosen bracket the retention volume of the analyte. If the flow rate is kept constant during the gathering of these data, then adjusted retention times can be plotted. Alternatively, the index can be calculated from equation 4,... 

The saturated open-chain hydrocarbons form a homologous series called the paraffin series or the alkane series. The composition of each of the members of the series corresponds to the formula CnH2n + 2> where n is the number of carbon atoms in the molecule. All the members of the series are unreactive. They do not react readily at ordinary temperatures with such reagents as acids, alkalies, or oxidizers. 

It is also not practical to expect that a single equation will correlate simultaneously the properties of NaCl with hexane, since there are too many differences between them. When we study the properties within a homologous series, such as the normal paraffins that differ only in the number of —CH2—units, we isolate only one parameter to study. In many cases, the molecular properties change smoothly with the number of units, which can be used as the predictor. The one-parameter populations set out in table 5.4 are frequently used. 

Reactivity of Hydrocarbons. - Each homologous series in a liquid fuel can exhibit different kinetics upon reforming under similar reaction conditions. For example, aromatic compounds are the most difficult to reform and require higher temperatures and lower space velocities. Aromatics also contribute significantly to carbon formation, compared to paraffins and naphthenes. At the same reaction conditions, the H2 production rates are typically in the order aromatics naphthenes. ° The relative reactivities of various higher hydrocarbons are summarized in Table 12. 

The lower members of the homologous series of 1. Alcohols 2. Aldehydes 3. Ketones 4. Acids 5. Esters 6. Phenols 7. Anhydrides 8. Amines 9. Nitriles 10. Polyhydroxy phenols 1. Polybasic acids and hydro-oxy acids. 2. Glycols, poly-hydric alcohols, polyhydroxy aldehydes and ketones (sugars) 3. Some amides, ammo acids, di-and polyamino compounds, amino alcohols 4. Sulphonic acids 5. Sulphinic acids 6. Salts 1. Acids 2. Phenols 3. Imides 4. Some primary and secondary nitro compounds oximes 5. Mercaptans and thiophenols 6. Sulphonic acids, sulphinic acids, sulphuric acids, and sul-phonamides 7. Some diketones and (3-keto esters 1. Primary amines 2. Secondary aliphatic and aryl-alkyl amines 3. Aliphatic and some aryl-alkyl tertiary amines 4. Hydrazines 1. Unsaturated hydrocarbons 2. Some poly-alkylated aromatic hydrocarbons 3. Alcohols 4. Aldehydes 5. Ketones 6. Esters 7. Anhydrides 8. Ethers and acetals 9. Lactones 10. Acyl halides 1. Saturated aliphatic hydrocarbons Cyclic paraffin hydrocarbons 3. Aromatic hydrocarbons 4. Halogen derivatives of 1, 2 and 3 5. Diaryl ethers 1. Nitro compounds (tertiary) 2. Amides and derivatives of aldehydes and ketones 3. Nitriles 4. Negatively substituted amines 5. Nitroso, azo, hy-drazo, and other intermediate reduction products of nitro com-pounds 6. Sulphones, sul-phonamides of secondary amines, sulphides, sulphates and other Sulphur compounds... 

The viscosity of a liquid is related directly to the type and size of the molecules which make up the liquid. The variation of liquid viscosity with molecular structure is not known with exactness however, the viscosities of liquids which are members of a homologous series are known to vary in a regular manner, as do most other physical properties. For example, pure paraffin hydrocarbons exhibit a regular increase in viscosity as the size and complexity of the hydrocarbon molecules increase. 

The usefulness of retention data from gas chromatography can be enhanced by reporting standardized times or retention indices (RI), which involves expressing retention in terms of a ratio of the retention time (RT) of an analyte to the RT of a standard. Retention scaling based on the Kovats (1965) method requires the chromatographic separation of a homologous series of normal paraffins, esters, and others, producing an index that is the ratio of the RT of an analyte minus the RT of a less retentive standard to the RT difference between... 

Table 1.1 shows that much of the work has been done on hydrocarbons and, in particular, homologous series. As a result, several very accurate methods are available for estimating the melting points of normal alkanes. For example, Broadhurst (1962) reports errors of less than 0.5°C for paraffins with chain lengths between 44 and 100 carbons. Similarly, Hanson... 

Many of the published estimation methods have been derived for specific homologous series, i.e., particular chemical classes such as n-paraffins, alcohols, substituted benzenes, etc. Table 2.1 provides a list of references for such methods. Within such a class, boiling point estimation can be fairly accurate (e.g., having average absolute errors under 10°C) ... 

In homologous series such as the n-paraffins heats of adsorption increase regularly with carbon number (Fig. 3). 

FIGURE 3 Variation of limiting heat of sorption (-AH0) with chain length nfor homologous series of linear paraffins. 

The main problem with a is that there is no single standard to which data have been ratioed, and consequently there are no tabulations of relative retention data in the literature. Kovats suggested that a series of standards be used, and he proposed the n-paraffins. Before discussing his proposal further, we need to examine the relationship between retention volume and the members of a homologous series such as the paraffins. 

For LC a similar relationship should apply if the retention mechanism shows the expected theoretical dependence on carbon number. The situation is more complex since the partition coefficient is a function of many intermolecular forces. Several papers have been published showing a homologous series retention like that described for GC.5 In principle then, the retention index concept should also apply in those cases. However, little interest has been shown in developing an index for LC, probably because the paraffins are not usually run by LC and the modes of analysis by LC are much more variable and complex, so that the data are not as widely usable. 

Reference Eq. (6-20) for an infinite chain of covalently bonded methylene groups can be considered to be an asymptotic limit for the homologous series of n-alkanes. By substitution of w into the exponent of Eq. (6-20) by the corresponding term, wI (., which represents a matrix composed of a paraffin with i carbon atoms, an equation for the diffusion coefficient Dski for trace amounts of a paraffin with k carbon atoms results ... 

With the homologous series of n-alkanes, such an upper limit, Mrmaxk, for the relative molecular mass of a trace paraffin kina solution of paraffin i is obtained for each member of the series. Consequently, an analogous equation to 6-32 can be written for the series ... 

Investigating the viscosity of a homological series of liquid normal paraffins, Doolittle [84] pointed out that the direct relationship between viscosity ( resistance to flow ) and free volume ( relative volume of molecules per unit free space ) is an intuitive hypothesis and the experimental dependence is described better by a logarithmic equation... 

Although aniline does possess a dipole moment the cohesion energy is predominantly of the London type. The critical solubility point—technically actually the i i demixing point—with hydrocarbon mixtures (aniline point) is an important characteristic quantity. The aniline point rises with the molecular weight in a homologous series. For aromatic hydrocarbons the critical solubility point is the lowest, for paraffins the highest, olefins and cyclic hydrocarbons are in between, in agreement with the variation of the specific cohesion (Table 33). 

However, the concept of retention index has been shown to be more universal in its application. This system uses a homologous series of -paraffins to provide the reference points on the scale (E. Kovats,... 

Fig. 4. The determination of the retention index of phenobarbitone from a plot of an homologous series of -paraffins.

Homologous Series.—K series of compounds, the members of which differ in composition by a constant amount, and whose physical constants change uniformly, constitute what has been termed an homologous series. This particular series which we are discussing is known as the homologous series of the saturated or paraffin hydrocarbons which are also open chain or a-cyclic compounds, the general formula of which is CnH2 +2. 

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