Melting points of alkanes

The boiling points and melting points of alkanes increase with increasing molecular weight. 

Source K. J. Burch and E. G. Whitehead, Melting Points of Alkanes, Journal of Chemical and Engineering Data 49 858-863 (2004). 

The boiling points of alkanes are lower than those of almost any other type of compound with the same molecular weight. In general, both boiling and melting points of alkanes increase with increasing molecular weight (Table 3.4). 

The melting points of alkanes increase with increasing molecular weight. The increase, however, is not as regular as that observed for boiling points, because the ability of molecules to pack into ordered patterns of solids changes as the molecular size and shape change. 

The effect of chain branching on the melting points of alkanes is more difficult to predict. Generally, however, branching that produces highly symmetrical structures results in... 

Fig. 3.7 Melting points of alkanes (a) and perfluoroalkanes (b). (From Ref. 19. Reproduced by permission of The Macmillan Co.)...

Paraffin wax is macrocrystalline, britde, and is composed of 40—90 wt % normal alkanes, with the remainder C g—isoalkanes and cycloalkanes. Paraffin wax has Httle affinity for oil content fully refined paraffin has less than 1 wt % cmde scale, 1—2 wt %, and slack [64742-61-6] above 2 wt %. Within these classes, the melting point of the wax determines the actual grade, with a range of about 46—71°C. Typical properties of petroleum waxes are listed in Table 3. 

Pentaerythritol with its four primary hydroxyl groups is used for the preparation of tetraesters and presents Httie difficulty except for its high melting point of 263°C, when pure. Pentaerythritol tetraesters are used in aircraft lubes, synthetic drying oils, and alkyds. Esters derived from trimethylo1 alkanes and dipentaerythritol are also used in alkyd resins (qv). Esterification may take place in situ during preparation of the alkyd. 

The C=C group and all four atoms attached to it lie in the same plane and are locked into that arrangement by the resistance to twisting of the TT-bond (Fig. 18.7). Because alkene molecules cannot roll up into a ball as compactly as alkanes or rotate into favorable positions, they cannot pack together as closely as alkanes so alkenes have lower melting points than alkanes of similar molar mass. 

The complexes are solids but are not useful as derivatives, since they melt, with decomposition of the complex, at the melting point of urea. They are useful, however, in separating isomers that would be quite difficult to separate otherwise. Thiourea also forms inclusion compounds though with channels of larger diameter, so that n-alkanes cannot be guests but, for example, 2-bromooctane, cyclohexane, and chloroform readily fit. 

As with alkanes, the boiling points and melting points of alkenes decrease with increasing molecular weight, but show some variations that depend on the shape of the molecule. Alkenes with the same molecular formula are isomers of one another if the position and the stereochemistry of the double bond differ. For example, there are four different acyclic structures that can be drawn for butene (C4H8). They have different b.p. and m.p. as follows. 

The 1-fluoroquinuclidinium fluoride (NFQNF, 2) precipitates during the reaction and, after treatment with hot, dry acetone to remove quinuclidinium fluoride, is obtained as an extremely hygroscopic white solid which can be assayed iodometrically (reaction with aqueous acetonic KI occurs instantaneously at rt). l-Fluoroquinuclidinium fluoride (2) is readily soluble in water, methanol, ethanol, trifluoroacetic acid, and ethyl acetate, and reasonably so in acetonitrile, but appears to be insoluble in alkanes, arenes, chloromethanes, acetone, diethyl ether, tetrahy-drofuran, dimethylformamide and dimethyl sulfoxide. The fluoride 2 has a melting point of 126-128°C (dec.) and is less flammable than quinuclidine in air, igniting only on direct contact with a flame or after prolonged heating on a metal plate. It does not explode when struck with a hammer.73... 

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... 

Just as shape is a common and essential element in the construction of a sculpture, the shape of a molecule is extremely important in determining the physical and chemical properties of a substance. For the alkanes, as the number of carbon atoms in the molecule increases, thereby increasing the molecular weight, the boiling point and the melting point of the substances increase. This would indicate that intermolecular forces, the attractive forces between molecules, increase as the number of carbon atoms and the molecular weight of the alkane molecules increase. 

Much of the early discussion of micelle structure centered around the problem of whether the hydrocarbon part should be considered as solid-like or liquid-like, the latter referring to conditions similar to those in liquid alkanes. Up to high alkyl chain lengths, the melting points of the alkanes lie below ambient temperature thus providing an indication that there is a liquid-like interior. However, the constraint offered by the micelle surface may, of course, substantially change the conditions. 

Figure 6.3 Effects of hydrocarbon chain modifications on melting points of similar-sized cuticular lipids. When lipids melt, the absorption frequency of C-H symmetric stretching vibrations increases from -2849 cm1 to -2854 cm4. From right to left, compounds are (chemical change relative to n-alkane, molecular mass in daltons) filled circles, n-dotnacontane (no change, 450) open circles, palmitic acid myristyl ester (wax ester, 452) filled triangles, 13-methylhentriacontane (methyl-branched alkane, 450) open triangles, (Z)-13-tritriacontene (double bond, 462) filled squares, 9,13-dimethylhentriacontane (2 methyl branches, 464) open squares, oleic acid oleyl ester (2 double bonds and an ester link, 532). Data from Gibbs and Pomonis (1995) and Patel el al. (2001).

Fig. 4 Paths to integer folded forms in long alkanes. While solution crystallization gives integer forms (E, F2,...) directly, for melt crystallization this is true only for the E form. Below the melting point of F2, the transient non-integer form (NIF) appears and subsequently transforms isothermally either to the once-folded F2 form or the mixed-integer folded-extended (FE) form, based on layer triplets. Deuterium- labelled chain ends, prepared for neutron scattering experiments, are indicated with circles...

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