Series, homologous

The second reason for studying homologous series is more fundamental. Most structure-property studies involving changes in polarity of groups, polarizability, and molecular geometry, require substantial 

We briefly discussed in an earlier volume the behavior of the isomeric and homologous organomagnesium bromides compared to the organolithium compounds as a means of furthering our understanding of the thermochemistry of the latter species . Here, we will discuss only the magnesium compounds. Discussion of the cycloalkylmagnesium bromides is deferred to a later section in this chapter. 

The linear correlation of enthalpies of formation with the number of carbon atoms is a useful and well-known feature of homologous series of functionalized organic compounds. The slope of the regression line for the gaseous n-alkanes (CH3—(CH2)j —H), —20.6 kJmol-, and the similar values of the slopes for other CH3—(CHi) —Z series is often called the universal methylene increment . In the liquid phase, the increment for the n-alkanes is —25.6 0.1 klmoD. The most accurate determination of the increment 

TABLE 4. Enthalpies of reaction and enthalpies of formation of organomagnesium bromides in ether solution (kJ mol ) 

Joel F. Liebman, Torkil Holm and Suzanne W. Slayden 

Saturated hy drocarbons, OuHgn + g Alcohols, OaH jO. Aldehydes, CnH nO. Acids, CrtHjnOja Ketones, C H,nO. 

The terms of any homologous series must all have the same constitution, i.e., their constituent atoms must be similarly arranged within the molecule. 

Isomerism—Metamerism—Polymerism.—Two substances are said to be isomeric, or to be isomeres of each other, when they have the same centesimal composition. If, for instance, we analyze acetic acid and methyl formiate, we find that each body consists of C, O and H, in the following proportions  

This similarity of centesimal composition may occur in two ways. The two substances may each contain in a molecule the same numbers of each kind of atom or one maty contain in each molecule the same kind of atoms as the other, but in a higher multiple. In the above Instance, for example, each substance may have the composition CaHiOs or one may have that formula and the other, C.HiaO , or CaH.OaXS. In the former case the substances are said to be metameric, in the latter polymeric. Whether two substances are metameric or polymeric can only be determined by ascertaining the weights of their molecules, which is usually accomplished by determining the sp. gr. of their vapors (see p. 37). 

The sp. gr. of the vapor of acetic acid is the same as that of methyl foriniate, and, consequently, each substance is made up of molecules, each containing CsH.Oa. But the two substances differ from each other greatly in their properties, and their differences are at once indicated by their typical or graphic formulae  

The observation that the CH2 group contributes a constant amount to the property of a compound has been made for many properties and also applies to Vm- Van Krevelen [11] has listed the CH2 contribution to VM from 12 different GCMs ranging between 16.1 and 16.6 cm3 mol-1. Jannelli et al. [12] have reported a simple linear relationship between Vm at 25°C and NCh2 for n-alkanenitriles (C2-Cg). Further, they review analogous relationships for alkanes, alcohols, diols, ethers, and amines. 

For 1-substituted n-alkanes with the general formula CmH2m+iX, Huggins [15] evaluated the following relationship  

Huggins [18] also applied eq. 3.3.2 to n-alkyl n-alkanoates with the general formula C9H2 +iC(=0)0C/)H2/)+i and m = q + p. The corresponding constants A and B are listed in Table 3.3.2. 

Smittenberg and Mulder [16,17] introduced the concept of the limit of a property,  

into structure-density relationships. P , represents the property of a hypothetical compound with an infinite number of carbon atoms. This concept is especially useful to estimate properties of polymers. For the density at 20°C, P becomes d . Smittenberg and Mulder derived the following equation  

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The surface tensions for solutions of organic compounds belonging to a homologous series, for example, R(CH2)nX, show certain regularities. Roughly, Traube [145] found that for each additional CH2 group, the concentration required to give a certain surface tension was reduced by a factor of 3. This rule is manifest in Fig. lll-15b the successive curves are displaced by nearly equal intervals of 0.5 on the log C scale. 

Spreading velocities v are on the order of 15-30 cm/sec on water [39], and v for a homologous series tends to vary linearly with the equilibrium film pressure, it", although in the case of alcohols a minimum seemed to be required for v to be appreciable. Also, as illustrated in Fig. IV-3, substrate water is entrained to some depth (0.5 mm in the case of oleic acid), a compensating counterflow being present at greater depths [40]. Related to this is the observation that v tends to vary inversely with substrate viscosity [41-43]. An analysis of the stress-strain situation led to the equation... 

Fig. X-9. Zisman plots of the contact angles of various homologous series on Teflon O, RX , alkylbenzenes (f), n-alkanes , dialkyl ethers , siloxanes A, miscellaneous polar liquids. (Data from Ref. 78.)...

A major contribution to the rational organization of contact angle data was made by Zisman and co-workers. They observed that cos 6 (advancing angle) is usually a monotonic function of 7l for a homologous series of liquids. The proposed function was... 

As discussed in Chapter III, the progression in adsoiptivities along a homologous series can be understood in terms of a constant increment of work of adsorption with each additional CH2 group. This is seen in self-assembling monolayers discussed in Section XI-IB. The film pressure r may be calculated from the adsorption isotherm by means of Eq. XI-7 as modified for adsorption from dilute solution ... 

Systematic experimental investigations of these transport effects on reaction rates can either be done by varying solvents in a homologous series to change viscosity without affecting other physicochemical or chemical properties... 

Nitromethane, CH3NO2, the first member of the homologous series, can, however, be readily prepared by a special reaction. When equimolecular amounts of sodium nitrite and sodium monochloroacetate are heated together in aqueous solution, the chlorine in the monochloroacetate is replaced by the nitro group, and the sodium nitroacetate thus formed undergoes hydrolysis follow ed by decarboxylation ... 

I his reaction is of great value for ascending the homologous series of alcohols ... 

Two methods may conveniently be used to ascend the homologous series of aromatic hydrocarbons ... 

In ascending a homologous series, the solubilities of the members tend to become more and more like that of the hydrocarbon from which they may be regarded as being derived. 

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. 

The lower members of other homologous series of oxygen compounds— the acids, aldehydes, ketones, anhydrides, ethers and esters—have approximately the same limits of solubility as the alcohols and substitution and branching of the carbon chain has a similar influence. For the amines (primary, secondary and tertiary), the limit of solubility is about C whilst for the amides and nitriles it is about C4. 

Group I. This includes the lower members of the various homologous series (4-5 atoms in a normal chain) that contain oxygen and/or nitrogen in their structures they are soluble iu water because of their low carbon content. If the compound is soluble in both water and ether, it would also be soluble in other solvents so that further solubility tests are generally unnecessary the test with sodium bicarbonate solution should, however, be performed (see Section XI,6). 

On nonpolar columns, the compounds of a homologous series separate as a function of their boiling points, and linear relationships have been established between the logarithms of the retention volumes and the number of carbon atoms in the 2-, 4-, and 5-positions (see Fig. III-l). 

Unbranched alkanes of the type CH3(CH2) cCH3 are often referred to as n alkanes and are said to belong to a homologous series... 

Homologous series (Section 2 9) Group of structurally related substances in which successive members differ by a CH2 group... 

Within a homologous series such as the -butyl-, -hexyl-, and -octyl-substituted methacrylates there is a trend for those polymers with the bulkier substituents to show smaller values of f. Note that there is also a correlation between substituent size and Tg within this series. 

In the methacrylate homologous series, the effect of side-chain bulkiness is just the opposite. In this case, however, the pendant groups are flexible and offer less of an obstacle to free rotation than the phenyl group in polystyrene. As chain bulk increases, molecules are wedged apart by these substituents, free volume increases, and Tg decreases. 

One of the most sensitive tests of the dependence of chemical reactivity on the size of the reacting molecules is the comparison of the rates of reaction for compounds which are members of a homologous series with different chain lengths. Studies by Flory and others on the rates of esterification and saponification of esters were the first investigations conducted to clarify the dependence of reactivity on molecular size. The rate constants for these reactions are observed to converge quite rapidly to a constant value which is independent of molecular size, after an initial dependence on molecular size for small molecules. The effect is reminiscent of the discussion on the uniqueness of end groups in connection with Example 1.1. In the esterification of carboxylic acids, for example, the rate constants are different for acetic, propionic, and butyric acids, but constant for carboxyUc acids with 4-18 carbon atoms. This observation on nonpolymeric compounds has been generalized to apply to polymerization reactions as well. The latter are subject to several complications which are not involved in the study of simple model compounds, but when these complications are properly considered, the independence of reactivity on molecular size has been repeatedly verified. 

The polarizabihties of molecules ia a homologous series iacrease steadily with increa sing aumbers of atoms. Therefore, the relative streagths of... 

Characteristics of common inorganic oxidizers are Hsted in Table 5. In any homologous series, potassium perchlorate-containing propellants bum fastest ammonium nitrate propellants bum slowest (79,80). 

Formic acid [64-18-6] (methanoic acid) is the first member of the homologous series of alkyl carboxyHc acids. It occurs naturally ia the defensive secretions of a number of insects, particularly of ants. Although the acid nature of the vapors above ants nests had been known since at least 1488, the pure acid was not isolated until 1671, when the British chemist John Ray described the isolation of the pure acid by distillation of ants (1). This remained the main preparative method for more than a century until a convenient laboratory method was discovered by Gay-Lussac (2). The preparation of formates using carbon monoxide was described by Berthelot in 1856. 

Formic acid exhibits many of the typical chemical properties of the aHphatic carboxyHc acids, eg, esterification and amidation, but, as is common for the first member of an homologous series, there are distinctive differences in properties between formic acid and its higher homologues. The smaller inductive effect of hydrogen in comparison to an alkyl group leads, for example, to formic acid = 3.74) being a considerably stronger acid than acetic acid... 

Naphthenic acids are lepiesented by a general formula where n indicates the carbon number and specifies a homologous series. The ... 

Some substances are odorous, others are not. Humans can smell at a distance if one smells the roses in a garden, it is not ordinarUy considered that part of the rose is in contact with the nose. Substances of different chemical constitution may have similar odors. Substances of similar constitution usuaUy have similar odors, eg, in a homologous series nevertheless, even stereoisomers may have different odors. Substances of high molecular weight are usuaUy inodorous and often nonvolatile and insoluble. The quaUty as weU as the strength of odor may change on dUution. 

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