The n-alkanes

Information on the dispersion curves of polyethylene has been obtained indirectly by studying the normal, unbranched, alkanes. The n-alkanes form a homologous series, are oligomers of polyethylene and have the same masses (-CH2- repeated n times). As more CH2 rmits are added they are constrained by the same forces in chains that simply grow longer. 

Standing wave solutions of different wavelength, X, embody the dynamics of each type of motion and their displacements away from equilibrium positions ( 4.3.2). Each of these solutions is identified by the number of half-waves, v, that can be accommodated in the length of the 

The LAM modes involve deformation of the in-plane C-C-C angles of the alkane. There are only n-2 such angles and so, in Eq. (10.2) mode = n-2. To assign the LAMs a twofold approach was adopted. The -alkanes have either C v ( = odd) or (n = even) symmetry. The LAMs belong to the Ai 01 B representations ( = odd) or Ag or representations (n = even). For the first mode, LAM-1 (v = 1) with ki = 7t/( -1), the eigenvectors exhibit a node at the chain centre with + and anti-mode, one at each end of the chain, LAM-1 must be totally symmetric. LAM-2 has two nodes and so must be antisymmetric, LAM-3 must be totally symmetric again since it has three nodes and so on. Thus the modes must follow the sequence  

Density functional theory was used to calculate the frequencies and symmetry assignments. As a check, the available Raman data were used, since for the n = even alkanes only the odd LAM modes are allowed. 

The harmonic dynamical treatment of a model of the -alkanes as a zigzag chain of point mass beads with unconstrained ends and an infinitely strong stretching force constant (so that only CCC bending occurs) has been presented [13]. The analytic result for the infinite chain for which the frequencies of the LAMs are  

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At ambient temperature and pressure, the first four members of the n-alkane series (methane to n-butane) are gases, the next thirteen (n-pentane through n-heptadecane) are liquids, and the higher members from n = 18 on are solids. 

Table 4.8 Cost of SCP grown on various substrates in comparison to the n-alkane process.

The tables that follow give the costs of various SCP production processes in comparative rather than in actual form. To see what this means examine Table 4.9. The production cost of raw materials for yeasts grown on n-alkanes is given as 58.5. This means that tire cost of raw materials accounts for 58.5% of the total production costs of this process. The same cost for bacteria grown on methanol is 73.8. This means that in this case 73.8% of the total production cost is accounted for by raw materials. This does not mean that the actual cost of raw materials for tire methanol process is more titan that for the n-alkanes process, as the total costs of the two processes are not necessarily similar. 

A decisive step of the molecular sieves processes is the desorption of the n-alkanes from the loaded sieve. Various procedures are in use in industry [13]. 

The desorption using a nonadsorbing medium such as nitrogen equals a pressure swing process because the purge gas reduces the partial pressure of the n-alkane and works as a vacuum. This variant also is only suitable for the isolation of n-alkanes from low molecular weight hydrocarbon mixtures like gasoline fractions. 

These calculations have been conducted on the basis of RHF optimized geometries, considering the 6-31G basis set for the n-alkane compounds (11), and the 6-31G basis set for the polyacene series (12). In both cases, the basis set contention has been checked by comparison with more thorough investigations on small compounds, such as ADC[3] calculations (11a) on n-butane based on the 6-31IG, 6-31G and 6-31G basis, or the MRSDCI ionization spectrum of ethylene as obtained by Murray and Davidson (33) using a 196-CGTO basis set. 

Oils The n-alkane series Cg-C, toluene and cyclohexane were purchased from BDH, Poole, UK, each with a stated purity of 99% reagents were used as received. Crude oil samples were obtained from two North Sea fields one located in the Norwegian sector and the other from the UK sector. Stock tank oil from the Gullfaks field was supplied by Statoil, Norway and the other stock tank oil from an undisclosed source. Both crude oils are derived from sandstone formations with reservoir temperatures of 70° and 101°C respectively. 

Lageveen et al. [41] showed that the monomer composition of aliphatic saturated poly(3HAMCL) produced by P. oleovorans is depended on the type of n-alkane used. It appeared that the n-alkanes were degraded by the subsequent removal of C2-units and it was therefore proposed that the /1-oxidation pathway was involved in poly(3HAMCL) biosynthesis. Preusting et al. [42] confirmed these results but also showed that with hexane as substrate some 3-hydroxyoctanoate and 3-hydroxydecanoate were produced, indicating that additional pathways were involved in poly(3HAMCL) biosynthesis (Table 1). 

At 375°C with the ZSM-5, the main products formed are n-alkanes. Other products are observed ramified alkanes and alkenes, 1-alkenes, aromatics and cyclic saturated hydrocarbons. The majority of hydrocarbons formed have a carbon number between 3 to 6. In the case of the zeolite Y, the n-alkanes and similar secondary products are formed but their repartition is different i.e. the normal and ramified alkanes are the main products and no cyclic compound can be observed. All these products are in higher quantity with the ZSM-5 than with the zeolite Y. This is in agreement with the calculated n-dodecane conversions. With the increase of the temperature, the same products are formed but their quantities increase. The analysis of the gaseous phase shows the presence of hydrogen, light normal and ramified alkanes and 1-alkenes. 

Fig. 4 Photoreaction of benzophenone with the n-alkanes. Plot of the second-order rate constants in CC14 at 25°C against the number of CH2 groups. (Data from Winnik and Maharaj, 1979)...

In marked contrast to the n-alkanes, the cycloalkanes exhibit thermodynamic properties where such regularities are no longer present. Heats of formation (AH ) for a substantial number of cycloalkanes are available from heats of combustion. With the exception of cyclohexane, AH°f is always more positive than the quantity — 4.926n. The difference between the two quantities leads to a quantitative assessment of the important notion of ring strain. The AH -values and strain energy data listed in Table 1 were taken from Skinner and Pilcher (1963). Other references give different but usually comparable... 

The question of the choice of the appropriate symmetry number for con-formationally labile molecules is a subtle one and is at best an approximation. It is the opinion of this author that, if a = 18 for the n-alkanes is accepted as correct, then for the sake of consistency the a-values for all of the cycloalkanes have to be set at 2n, as shown by the following example. Consider the hypothetical reaction (36) for which a statistical correction is clearly... 

Thermochemical data from the compilation of Stull et at., 1969. Entropy values are based on a 1 M standard state. The asterisk denotes symmetry-corrected quantities. Symmetry numbers were chosen as follows 18 for the n-alkanes, cis-3-hexene, dibuthyl sulphide, diethyl ether, and diethyl amine 2n for the cycloalkanes and 2 for all of the remaining ring compounds 3 for the alkanols, alkanethiols and alkyl amines 9 for the methyl alkyl sulphides... 

Calculated as the heat of combustion per methylene group minus the value (659.0) for the n alkane methylene group. 

Kovats indices (/-values) are based on the retention time of an analyte compared to retention times of the series of n-alkanes. For a particular GC phase, /-values are very reproducible from one column or from one GC to another. However, they are slightly affected by GC programming conditions. n-Alkanes have most affinity for non-polar phases and tend to elute more quickly from polar phases. In contrast, a polar analyte will elute more slowly from a polar phase and thus relative to the n-alkanes, its retention time and thus its /-value will increase as the polarity of the... 

FIGURE 9.52 Mass concentrations of acidic and neutral elutable organics in fine particles ( 2.1 (im) at Hopi Point (Grand Canyon region), at San Nicholas Island in the Pacific Ocean upwind of Los Angeles, in West Los Angeles, and downwind at Rubidoux. The bare represent all of the organics that elute between the n-alkanes C and C , where n is shown under each bar. Note the different scales for concentrations (adapted from Mazurek et al., 1997). 

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