Alkanes having more than five carbon

Cycloalkanes that have more than five carbon atoms undergo substitution reactions. In the substitution reactions of alkanes there can be more than one product, but in cycloalkanes, there can only be a single product. 

With alkanes having more than four carbons, the names of the straight-chain isomers are systematic and derive from Greek roots pentane for five C atoms, hexane for six, and so on. There are... 

Alkenes that contain fewet than five carbon atoms are gases at room temperature. The boiling points of the alkenes, like those of alkanes, increase with an increasing number of carbon atoms because the London forces increase (Table 5.3). And, like alkanes, alkenes with branched alkyl groups have lower boihng points. Branched alkenes have more compact structures than the unbranched isomers, and thus less intermolecular contact, which diminishes the intermolecular London forces. 

Compounds like the two different C4H10 molecules and the three different C5H12 molecules, which have the same molecular formula but different structures, are called isomers (Section 20.8). The number of possible alkane isomers grows rapidly as the number of carbon atoms increases, from five isomers for C6H14 to more than 6 x 1013 isomers for C4oH82 ... 

Why are five-membered and six-membered rings more common than the other sizes Adolf von Baeyer first attempted to explain the relative stabilities of cyclic molecules in the late nineteenth century, and he was awarded a Nobel Prize for this work in 1905. Baeyer reasoned that the carbon atoms in acyclic alkanes have bond angles of 109.5°. (We now explain this bond angle by the tetrahedral geometry of the sp hybridized carbon atoms.)... 

For nonpolar substances with the same molar mass, the strength of the dispersion forces is influenced by molecular shape. Shapes that allow more points of contact have more area over which electron clouds can be distorted, so stronger attractions result. Consider the two five-carbon alkanes, pentane (also called n-pentane) and 2,2-dimethylpropane (also called neopentane). These isomers have the same molecular formula (C5H,2) but different shapes. n-Pentane is shaped like a cylinder, whereas neopentane has a more compact, spherical shape, as shown in Figure 12.16. Thus, two n-pentane molecules make more contact than do two neopentane molecules. Greater contact allows the dispersion forces to act at more points, so n-pentane has a higher boiling point. Figure 12.17 summarizes how to analyze the intermolecular forces in a sample. 

The estimates of the RSE for small cyclic alkanes, cyclo-olefins, and cyclic ethers using all three reactions are listed in Table 3.21. The failure of the isodesmotic reaction is readily apparent by examining the RSE of the cyclic alkanes. For the rings having five or more carbon atoms, the isodesmic method predicts that there is no strain energy. Rather, it predicts that these cyclic species are more stable than their acyclic counterparts. The homodesmotic and GE reactions, which are identical for the cyclic alkanes, provide RSEs in excellent agreement with CRSE estimates. 

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