INDEX homologous series

Location of the compound within a class (or homologous series) of compounds. Reference to the literature or to tables of the physical properties of the class (or classes) of organic compounds to which the substance has been assigned, will generally locate a number of compounds which boil or melt within 6° of the value observed for the unknown. If other physical properties e.g., refractive index and density for a hquid) are available, these will assist in deciding whether the unknown is identical with one of the known compounds. In general, however, it is more convenient in practice to prepare one, but preferably two, crystalhne derivatives of the substance. 

Similarly, the increase in the number of isomers in other homologous series (e.g., in the series starting with naphthalene and anthazene) is asymptotically proportional to the number of isomers of the alcohol series. The proportionality factor can easily be derived from the cycle index of the permutation group of the replaceable bonds of the basic compound. 

Detectability may be a significant problem with homologous series of unsaturated compounds, particularly //-alkanes. For these compounds, refractive index detection or evaporative light-scattering, both of which are described elsewhere in the book, may be of use. Indirect photometry is a useful detection scheme for compounds that do not absorb in the UV. Acetone, methylethyl ketone, methyl propyl ketone, methyl isopropyl ketone, methyl isobutyl ketone, and acetophenone are added to an acetonitrile/water mobile phase, generating a negative vacancy peak when the nonchro-mophoric analyte emerges and a positive peak if the ketone is adsorbed and displaced.70 Dodecyl, tetradecyl, cetyl, and stearyl alcohols also have been derivatized with 2-(4-carboxyphenyl)-5,6-dimethylbenzimidazole and the derivatives separated on Zorbax ODS in a mobile phase of methanol and 2-propanol.71... 

Since members of a homologous series have incremental boiling point differences and if the amount of any homolog in the moving gas phase is related to vapor pressure at the temperature of the experiment, plots of log k vs. carbon number should also be a straight line. (The enthalpy of vaporization increases monotonically with carbon number.) This in fact is observed in gas-liquid equilibrium separation systems. It is the basis of retention index systems pioneered by Kovats for qualitative identification. 

An isolated cs plane or a random array of cs planes, known as the Wadsley defect, still gives rise to nonstoichiometry. Regularly recurring cs planes result in a homologous series of stoichiometric intermediate phases. Occurrence of such equidistant cs planes in a crystal indicates a cooperative mechanism. The formula of a cs phase depends on the cs plane index as well as the width of the parent slab between cs planes. If MO is the formula of the parent line phase, the formula of the homologous series of oxides resulting from cs may be represented as where n is the width (number of... 

The molecular weight and the Kovats retention index can then be combined to aid in the identification of the component. A linear relationship exists between the molecular weight and retention index for a homologous series of compounds. The relationship varies for each class of compound thus, a clue can be obtained regarding the type of compound present which can be verified by some other technique. 

In Figure 3, a straight-line relationship between a retention index and molecular weight is illustrated for homologous series of n-alcohols, n-aldehydes, n-acetates, and n-hydrocarbons. The compound having molecular weight 130 and retention index 1180 corresponds to n-pentyl acetate. Since the relationship in Figure 3 holds for all types of related compounds, it is obvious that this technique could be widely used in identification of GC peaks. 

Correlations of viscosity with density and refractive index have been evaluated for various homologous series [7] and correlations between viscosity and boiling point and between viscosity and vapor pressure have been reported, for example, for n-alkyl /3-ethoxypropionates [8]. Viscosity correlations with vapor pressure are represented by the Porter equation [9] ... 

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... 

The properties of Ge(OR)4 allow them to be considered more likely to be the esters of an inorganic acid than metal alkoxides these are colorless volatile liquids, containing monomeric tetrahedral molecules. The solid crystalline form is known only for R = Bu, OC6Hnc, and also 2,6-substituted phenoxides. All the members of the Ge(OR)4 homologous series are characterized by thoroughly determined physical characteristics — density, refraction index, surface tension, viscosity (and calculated parachor values), dipole moments in different solvents [222, 857, 1537] (Table 12.9). The results of the investigation of vapor pressure, density, viscosity polytherms, and so on. permitted rectification for the preparation of samples of high purity for sol-gel and MOCVD applications [682, 884]. 

The best results are attained within the template molecules approach. In order to arrive at approximations to the scaling functions we select a homologous series of molecules, G,. For this series, the reduced topological invariants are functions of only the index i. We formally invert the functions k(i) to obtain direct relations between the reduced invariants. One can say that the homologous series forms a template for the scaling relations. 

Fig. 7. Correlation of the UV spectra (in In units), In X(B), with connectivity index, x(T), (Eq. 11) for three homologous series of hydrocarbons 1 = benzene 5 = pentacene 6 = chrysene, 9 = L1A2(L2)jA2L1 10 = phenanthrene 13 = L1(L2)3A2L1. Circles correspond to beta absorption bands while triangles correspond to para bands...

At this point it should be remarked that multivariate regression with latent variables is a useful tool for describing the relationship between complex processes and/or features in the environment. A specific example is the prediction of the relationship between the hydrocarbon profile in samples of airborne particulate matter and other variables, e.g. extractable organic material, carbon preference index of the n-alkane homologous series, and particularly mutagenicity. The predictive power was between 68% and 81% [ARMANINO et al., 1993]. VONG [1993] describes a similar example in which the method of PLS regression was used to compare rainwater data with different emission source profiles. 

Qualitative analysis is enhanced if data are acquired on more than one system. For example, in GC it is fairly common and easy to run a sample on each of two columns that are chosen to be widely different in their polarities. The results can be plotted as net retention volumes or as Kovats index values on either linear or log scales as shown in Figure 6.5. In either case, straight lines result for homologous series, thus aiding qualitative identifications. The principle is simple the more data, the more reliable the analysis. 

The retention index has become the standard method for reporting GC data. By definition, the members of any homologous series should differ from each other by 100 units just as the standards do. This relationship is not always exact,2 and the index is somewhat temperature dependent.3 Nevertheless, it is very popular, and McReynolds4 has published a self-consistent book of indices for 350 compounds on 77 stationary phases and at 2 temperatures. Other homologous series have also been used as standards in specific industries. 

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. 

When a molecule takes part in a reaction, it is properties at the molecular level which determine its chemical behaviour. Such intrinsic properties cannot be measured directly, however. What can be measured are macroscopic molecular properties which are likely to be manifestations of the intrinsic properties. It is therefore reasonable to assume that we can use macroscopic properties as probes on intrinsic properties. Through physical chemical models it is sometimes possible to relate macroscopic properties to intrinsic properties. For instance 13C NMR shifts can be used to estimate electron densities on different carbon atoms in a molecule. It is reasonable to expect that macroscopic observable properties which depend on the same intrinsic property will be more or less correlated to each other. It is also likely that observed properties which depend on different intrinsic properties will not be strongly correlated. A few examples illustrate this In a homologous series of compounds, the melting points and the boiling points are correlated. They depend on the strengths of intermolecular forces. To some extent such forces are due to van der Waals interactions, and hence, it is reasonable to assume a correlation also to the molar mass. Another example is furnished by the rather fuzzy concept nucleophilicity . What is usually meant by this term is the ability to donate electron density to an electron-deficient site. A number of measurable properties are related to this intrinsic property, e.g. refractive index, basicity as measured by pK, ionization potential, HOMO-LUMO energies, n — n ... 

Fig- 17 A plot of tnolecular connectivity index, jg(B) s of a homologous series rf polyhex graphs against the corresponding quantities, %(A) s, of the corresponding Clar graphs-... 

It has been shown that gas-Hquid chromatographic methods are particularly suitable for a quantitative characterization of the polarity of solvents. In gas-liquid chromatography it is possible to determine the solvent power of the stationary liquid phase very accurately for a large number of substances [98, 99, 259, 260]. Many groups of substances exhibit a certain dependence of their relative retention parameters on the solvation characteristics of the stationary phase or of the separable components. In determining universal gas-chromatographic characteristics, the so-called retention index, I, introduced by Kovats [100], is frequently used. The elution maxima of individual members of the homologous series of n-alkanes (C H2 +2) form the fixed points of the system of retention indices. The retention index is defined by means of Eq. (7-41),... 

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.

In a homologous series of OP compounds, increasing potency for AChE inhibition and cholinergic toxicity correlates with decreasing potency for NTE inhibition and OPIDN. The relative inhibitory potency (RIP) of an OP compound or its active metabolite for NTE versus AChE in vitro can be used as a convenient index of the probable neuropathic potential of the compound. A commonly used measure of inhibitory potency is the IC50, the concentration required to inhibit 50% of the enzyme activity under a standardized set of reaction conditions and time of incubation of the inhibitor with the enzyme preparation. A better measure of inhibitory potency is the bimolecular rate constant of inhibition, ki. When... 

Retention index There are two types of retention index, namely the isothermal retention and the hnear temperature programming index. Both indices express the retention characteristics of a chemical compound analysed by GC to the retention of the homologous series of normal ahphatic hydrocarbons analysed under identical conditions. Under both retention index systems, a chemical compound is bracketed by two ahphatic hydrocarbons that are assigned a retention index value corresponding to the number of carbon atoms in the hydrocarbon molecule multiphed by 100. The isothermal retention index, RL, is defined as a logarithmic interpolation between two successive ahphatic hydrocarbons eluted just prior to and just after the compound A under the isothermal GC conditions, and is calculated for compoxmd A as follows ... 

The retention index related to a given stationary phase is determined as follows. When a mixture containing compounds belonging to a homologous series of... 

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