Homologous series alkenes

Alkenes also form a homologous series as the carbon number increases, the number of possible isomeric structures for each member increases more rapidly than in the case of the alkane series. 

The alkenes make up a homologous series of hydrocarbons with the general formula C H2 . Alkenes show two types of structural isomerism, position isomerism and chain isomerism. Geometrical isomerism also exists because of the lack of free rotation about the C=C double bond. 

If a sample contains one or more members of a homologous series, identifications can be made using a plot of log tR against the number of carbon atoms, previously prepared from standards. The plot, which is valid for one temperature only, is linear and can be used for alkanes, alkenes, alcohols, aldehydes, ketones, esters and ethers. 

Alkenes (olefins) are unsaturated hydrocarbons that contain carbon-carbon double bonds. A double bond consists of a cr bond and a tt bond. A tt bond is weaker than a a bond, and this makes tt bonds more reactive than cr bonds. Thus, it bond is considered to be a functional group. Alkenes form a homologous series with general molecular formula C H2 . The simplest members of the series are ethene (C2H4), propene (CsHg), butene (C4Hg) and pentene (CsHjo)-... 

The homologous series known as alkenes is also called unsaturated hydrocarbons or olefins. The general formula for the alkene family is CnH2 . The distinguishing feature of the alkene structure is the carbon-carbon double bond which, as we have discussed previously, is a four-electron bond formed by the sharing of two electrons from each of two carbon atoms. 

Skubla [9] has designed a group contribution scheme which applies for various homologous series. His method relies on eq. 6.2.1 where both coefficients a0 and a have to be derived from group contributions and with respect to Nc. The model applies for n-alkanes, 1-alkenes, n-alkylcyclopentanes and n-alkylcyclohexanes, alkylbenzenes, 1-bromoalkanes, 1-alkanols, di-n-alkyl ethers, carboxylic acids and esters, 1-alkanethiols, 1-aminoalkanes, dialkylamines, alkaneamides, and some... 

Solubility-Molar Volume Relationships The correlation between aqueous solubility at room temperature and the molar volume has been studied by McAuliffe [5] for different hydrocarbon classes. He discusses linear relationships, presented as graphs, describing the decrease in solubility with increasing molar volume for the homologous series of alkanes, alkenes, alkandienes, alkynes, and cycloalkanes. 

Bond cleavage would be expected to lead to a homologous series of alkanes, alkenes, and esters to Cio if the fatty acid were oleate, but it would give higher homologs if the fatty acid were stearate. The principal products are, in fact, alkanes, alkenes, and a homologous series of methyl esters. The highest member of the alkane series found in irradiated methyl oleate... 

In the remainder of this section we identify members of several homologous series of alkenes and apply equation 4 to their enthalpies of formation. In this way we can assess... 

TABLE 2. Regression analyses of equations 4a and 7 for homologous series of alkenes and corresponding alkanes (kJ mol 1 )a... 

We deduced from equations 4 and 7 that only when the slope, m, is approximately equal to 1 will the enthalpies of hydrogenation for a homologous series of alkenes be reasonably constant. Inspecting the slope and intercept data from equation 7 in Table 2, together with the actual experimental enthalpies of hydrogenation, shows our conclusion to be valid. We define for each alkene homologous series... 

Because the enthalpies of formation of all homologous series of alkenes correlate with the number of carbons according to equation 4, any two series must correlate with each other. We can recast equation 10 in linear form as equation 11. 

Just as for the enthalpy of hydrogenation in equation 7, the enthalpies of isomerization between alkene members of two homologous series will be constant and equal to the y-intercept ( A//jsom ) only when m = 1. For homologous series with non-parallel slopes... 

Comparing the effect of the double-bond migration within the identical cr-frameworks of isomeric isoalkyl-1-alkenes and the 2-methyl-1-alkenes, we find that the individual enthalpies of isomerization for Cg-Cs are —10.4, —10.9 and —11.3 kJmol-1. These mono- to disubstituted isomerization enthalpies are very similar to those for isomerization of the 1-n-alkenes to the tram -2-n-alkenes, homologous series which also possess a common structural skeleton. 

All the enthalpies of formation for acyclic aliphatic aldehydes and ketones which are discussed in this section are tabulated by Pedley and coworkers16. The number of these carbonyl compounds whose enthalpies of formation have been measured are few compared to the abundant data available for alkenes and so there are fewer decisions to make on which data to include. However, we then lack the means to make the comprehensive analyses we would prefer. The demonstrated linearity34 of the enthalpies of formation of the members of a homologous series vs the number of carbon atoms, nc, in the molecules provides an excellent visual method for determining the quality of the data any enthalpy of formation which deviates significantly from linearity is of questionable reliability. The linear relationship can be expressed as in equation 4 and the results of the regression analyses of this equation for aldehydes and ketones appear in Table 4. 

We have often wondered if there is a large enough nc such that equation 4 exhibits a break or curvature and the bond additivity changes within a homologous series to become essentially the same as for the n alkanes. If such a change were to occur in the 2- and 3-n-alkenes at nc ca 7-9, it might avoid the current conundrum of reversed stability order. 

Alkenes form another homologous series of hydrocarbons of the general formula C H2 where n is the number of carbon atoms. The alkenes are more reactive than the alkanes because they each contain a double covalent bond between the carbon atoms (Figure 14.9). Molecules that possess a double covalent bond of this kind are said to be... 

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