Unbranched hydrocarbon molecules

Aromatization, however, may also be envisaged as taking place via stepwise dehydrogenation of an unbranched hydrocarbon molecule followed by ring closure of the polyunsaturated intermediates. In fact, the formation of dienes was proved during the aromatization of C6 and C7 alkanes to the corresponding aromatics over monofunctional metal oxides and metal black, and bifunctional catalysts.307 308 Radiotracer studies even allowed the detection in very low concentration of hexatriene during the aromatization of n-hexane over Pt black.309 It was also proposed that aromatics are formed from the cis isomers, whereas trans isomers may be coke precursors.213 Direct experimental evidence has recently been... 

Likewise it is possible to differentiate between substituted and unsubstituted alicycles using inclusion formation with 47 and 48 only the unbranched hydrocarbons are accommodated into the crystal lattices of 47 and 48 (e.g. separation of cyclohexane from methylcyclohexane, or of cyclopentane from methylcyclopentane). This holds also for cycloalkenes (cf. cyclohexene/methylcyclohexene), but not for benzene and its derivatives. Yet, in the latter case no arbitrary number of substituents (methyl groups) and nor any position of the attached substituents at the aromatic nucleus is tolerated on inclusion formation with 46, 47, and 48, dependent on the host molecule (Tables 7 and 8). This opens interesting separation procedures for analytical purposes, for instance the distinction between benzene and toluene or in the field of the isomeric xylenes. 

The use of the old terminology n- (normal) for unbranched hydrocarbon chains, with i- (iso), s-(secondary), t- (tertiary) for branched chains is still quite common with small molecules, and can be acceptable in lUPAC names. 

Our discussion has centered around hydrocarbon molecules that consist of carbon atoms bonded to each other in long chains. These chains are called normal, straight, or unbranched. The chains may have only single bonds (alkanes), double bonds (alkenes), or triple bonds (alkynes). Hydrocarbons with only single bonds are called saturated hydrocarbons with double or triple bonds are called unsaturated. Not all hydrocarbons want to form straight chains. After all, who among us wants to be straight and serious all the time Some of us will always be comedians. In the world of hydrocarbon molecules, the comedians are the branched and cyclic molecules. 

Activity 5.3 will acquaint students with positive and negative space in macrosculptures (two- and three-dimensional works of art) and in microsculptures (unbranched, branched, and cyclic hydrocarbon molecules). 

Hydrocarbon molecules are used to illustrate the definitions of structure and structural stability introduced above and also the manner in which the properties of the electronic charge density at a bond critical point can be used to summarize the important physical characteristics of a bonded interaction between atoms (Bader et al. 1983). Figure 3.7 gives the molecular graphs for saturated hydrocarbon molecules with widely varying equilibrium structures, beginning with acyclic molecules both branched and unbranched, followed by cyclic and bicyclic molecules, and ending with a number of so-called... 

This setup is then immersed in a bulk reservoir of the same fluid of which the confined film consists. Thus, at thermodjmamic equilibrium, T and are the same in both subsystems (i.e., bulk reservoir and confined fluid). By applying an external force in the direction normal to both substrate surfaces, the thickness of the film can be altered by either expelling molecules from it or imbibing them from the reservoir until thermodynamic equilibrium is reestablished, that is, until the force exerted by the film on the surfaces equals the applied external normal force at the same T and /x. Plotting this force per radius R, F/R, as a function of h yields a damped oscillatory curve in most cases. This is illustrated by plots in Fig. 5.1 where typical curves axe shown for several fluids consisting of branched and unbranched hydrocarbons [146]. As one can see, both the period and the amplitude of oscillations depend on... 

The fatty acids are comparatively simple molecules containing unbranched hydrocarbon chains about 14-24 C atoms long together with an acidic COOH group. The predominance of the long hydrocarbon over the relatively small acidic group means that the fatty acid is essentially non-polar that is, because of the symmetrical distribution of the electron shells there are no electrical charges. The fatty acids do not dissolve easily in water -they are hydrophobic. The hydrophobic nature of fatty acids has... 

In order to start with the simplest possible conditions, we shall consider an isolated chain molecule, for example, an unbranched hydrocarbon chain with n links, i.e. with (n + 1) C atoms (see Fig. 15) n should be a... 

The strength of the London forces between alkane molecules increases as the molar mass of the molecules increases hydrocarbons with unbranched chains pack together more closely than their branched isomers. Alkanes are not very reactive. but they do undergo oxidation (combustion) and substitution reactions. 

The isomerization of the butanes and of neopentane has been studied over various types of evaporated platinum films by Anderson and Baker (68) and Anderson and Avery (108,24). Table II gives some typical results. It is clear that the proportion of parent hydrocarbon reacting to isomeric rather than to hydrogenolytic product is considerably smaller for a hydrocarbon with an unbranched as opposed to a branched chain containing an isostructural unit indeed, neopentane was studied as the archetypal molecule of the latter class. 

A group of substances which are closely associated with the gas hydrates, are the compounds of urea (and thiourea) with a large number of organic substances with long-chain molecules, such as normal saturated hydrocarbons and olefins, alcohols, acids, esters, ketones, halogenated hydrocarbons, etc.3, which were discovered accidentally by Bengen in 1940. These compounds are only produced with unbranched, non-cyclic molecules4. This is even so very specific that a method of separation can be based on it for normal and iso hydrocarbons in mixtures. 

Petrolatum is a purified mixture of semisolid saturated hydrocarbons having the general formula C H2 +2, and is obtained from petroleum. The hydrocarbons consist mainly of branched and unbranched chains although some cyclic alkanes and aromatic molecules with paraffin side chains may also be present. The USP 28 and PhEur 2005 material may contain a suitable stabilizer (antioxidant) that must be stated on the label. The inclusion of a stabilizer is not discussed in the JP 2001 monograph. 

---