Alkanes between molecules

In the case of alkanes, the distance between the molecules in the solid phase is ca. 5 A, while it is 5-6 A in the case of the liquid phase. The distance between molecules in the gas phase, in general, is ca. I ()()() /3 = 10 times larger than in the liquid phase (water volume of 1 mol water = 18 cc volume of 1 mol gas = 22.4 L). In fact, mono-molecular film studies are the only direct method of obtaining such information at the interfaces of lipids. Considering that, only microgram quantities are enough for such information, the importance of such studies becomes clearly evident. 

As shown by Tagawa et al. [74], the alkane excited molecules have a broad absorption band in the visible region with maxima increasing from —430 to —680 nm between C5 and C20 for the -alkanes. This spectrum strongly overlaps with the absorption spectrum of the radical cations with low carbon atom number alkanes the two maxima practically coincide. With increasing carbon atom number, the red shift in the radical cation absorbance is stronger than in the Si molecule absorbance [47,49,84-86]. The decay of the excited molecule absorbance was composed of two components with 0.1- and 1.0-nsec decay times in cyclohexane, and 0.17 and 2.7 nsec in perdeuterocyclohexane [47]. The nature of the faster-decaying component is as yet unclear. 

The solubility of molecules can be explained on the basis of the polarity of molecules. Polar, e.g. water, and nonpolar, e.g. benzene, solvents do not mix. In general, like dissolves like i.e., materials with similar polarity are soluble in each other. A polar solvent, e.g. water, has partial charges that can interact with the partial charges on a polar compound, e.g. sodium chloride (NaCl). As nonpolar compounds have no net charge, polar solvents are not attracted to them. Alkanes are nonpolar molecules, and are insoluble in polar solvent, e.g. water, and soluble in nonpolar solvent, e.g. petroleum ether. The hydrogen bonding and other nonbonding interactions between molecules are described in Chapter 2. 

Alkanes have similar chemical properties, hut their physical properties vary with molecular weight and the shape of the molecule. The low polarity of all the bonds in alkanes means that the only intermolecular forces between molecules of alkanes are the weak dipole-dipole forces (see 2.5.1), which are easily overcome. As a result, compared with other functional groups, alkanes have low melting and boihng points, and low solubility in polar solvents, e.g. water, but high solubility in nonpolar solvents, e.g. hexane and dichloromethane. Most cycloalkanes also have low polarity. 

As noted above, London dispersive interactions occur even between molecules of apolar compounds like alkanes, that on average over time exhibit a rather smooth distribution of electrons throughout the whole molecular structure. This interaction occurs in all chemicals because there are momentary (order of femtosecond timescales) displacements of the electrons within the structure such that short-lived electron-rich and electron-poor regions temporarily develop. This continuous movement of electrons implies the continuous presence of short-lived dipoles in the structure. This fleeting dipole is felt by neighboring molecules whose electrons respond in a complementary fashion. Consequently, there is an intermolecular attraction between these molecular regions. In the next moment, these attractive interactions shift elsewhere in the molecule. 

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. 

Dispersion forces cannot be explained by the magnetic analogy nor by conventional electrostatics. They are weak forces that exist even in mon-oatomic gases that are symmetrical and nonpolar. It is believed that at any particular instant this symmetry is somewhat distorted due to the motion (and position) of the electrons of a given atom, which produces a momentary polarity. This momentary polarity can attract and be attracted by a similar polarity in a neighboring atom or molecule in such a way as to produce a net attraction. As we have already seen, such inductions depend on the polarizability of the molecule or atom. Dispersion forces will always be possible between molecules, but they are the only forces between nonpolar hydrocarbons such as the alkanes. For this reason alkanes are often chosen as the ideal molecules for study or for use as standards. An example of a chromatographic separation in which the only forces are dispersion forces would be the GC separation of alkanes on squalane, a branched paraffin. 

Unsaturated compounds have physical and chemical properties that differ from those of saturated compounds. For example, the boiling points of alkenes are usually slightly less than the boiling points of similar-sized alkanes (alkanes with the same number of carbon atoms). This difference reflects the fact that the forces between molecules are slightly less for alkenes than for alkanes. For example, the boiling point of ethane is -89°C, whereas the boiling point of ethene is -104°C. On the other hand, both alkenes and alkanes have a low solubility in water. Alkenes, like all aliphatic compounds, are non-polar. 

These materials mainly include graphites, carbon blacks and graphitized carbon blacks, and are frequently used as standards in adsorption at high and low surface coverage due to the lack of porosity and their homogeneous surfaces [61-64]. For these reasons it is easy to find a relationship, for the adsorption of non-polar molecules (n-alkanes), between the specific retention volumes and a molecular property of the adsorbate such as the polarizability or the molecular volume, and the amount adsorbed, V, which is nor-... 

Melting Point. Solid alkanes are soft, generally low-melting materials. The forces responsible for holding the crystal together are the same indnced-dipole/indnced-dipole interactions that operate between molecules in the liqnid, bnt the degree of organization... 

The small differences in stability between branched and unbranched alkanes result from an interplay between attractive and repulsive forces within a molecule (intramolecular forces). These forces are nucleus-nucleus repulsions, electron-electron repulsions, and nucleus-electron attractions, the same set of fundamental forces we met when talking about chemical bonding (see Section 1.12) and van der Waals forces between molecules (see Section 2.14). When the energy associated with these interactions is calculated for all of the nuclei and electrons within a molecule, it is found that the attractive forces increase more than the repulsive forces as the structure becomes more compact. Sometimes, though, two atoms in a molecule are held too closely together. WeTl explore the consequences of that in Chapter 3. 

The most straightforward cause of shape selectivity is the discrimination between molecules on the basis of their diffusion rates through the channels or cage windows. Microporous solids act as true molecular sieves, because the well-defined pores are able to select molecules on the basis of differences in dimensions of 0.1 A or less. Examples of strong molecular sieving effects include the selection of normal alkanes over branched ones by small-pore solids and the selection of para-substituted over ortho- and meta-substituted aromatics over medium-pore zeolites. This type of selectivity according to molecular diffusion rate may act on both reactant and product molecules. The much faster dehydration of n-butanol compared to isobutanol over Ca-A demonstrated by Frilette and Weisz is the classic example of reactant diffusion... 

The attractive forces between molecules possessing such dipoles are not as strong as hydrogen bonds, but they do cause aldehydes and ketones to boil at higher tanperatures than the nonpolar alkanes. The graph in I Figure 4.2 summarizes the boiling point comparison for several families of compounds. 

An important point in the use of surface energy components is the realization that for non-polar liquids, such as alkanes, only dispersion forces act between molecules, so... 

Alkenes and alkynes are classified as unsaturated hydrocarbons. They are said to be unsaturated because, unlike alkanes, their molecules do not contain the maximum possible number of hydrogen atoms. Alkenes have two fewer hydrogen atoms, and alk3mes have four fewer hydrogen atoms than alkanes with a comparable number of carbon atoms. Alkenes contain at least one double bond between adjacent carbon atoms, while alkynes contain at least one triple bond between adjacent carbon atoms. 

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