Linear and branched alkanes

1 Linear and Branched Alkanes We know from the general correlation chart (Appendix C) that alkane groups unsubstituted by heteroatoms absorb to about 60 ppm. (Methane absorbs at — 2.5 ppm). Within this range, we can predict the chemical shifts of individual 13C atoms in a straight-chain or branched-chain hy- 

This table shows the additive shift parameters (A) in hydrocarbons the a effect of +9.1, the f effect of + 9.4 ppm, the y effect of - 2.5, the 8 effect of + 0.3, the e effect of +0.1, and the corrections for branching effects. The calculated (and observed) shifts for the carbon atoms of 3-methylpentane are 

Calculations of shift are made from the formula 8 = - 2.5 + hnA, where 8 is the predicted shift for a carbon atom A is the additive shift parameter and n is the number of carbon atoms for each shift parameter (- 2.5 is the shift of the 13C of methane). Thus, for carbon atom 1, we have 1 a1 /3-, 2 y-, and 1 5-carbon atoms. 

TABLE 4.4 The I3C Shift Parameters in Some Linear and Branched Hydrocarbons 

Carbon atom 2 has 2 a-, 2 /3-, and 1 y-carbon atoms. Carbon atom 2 is a 2° carbon with a 3° carbon attached [2°(3°) = -2.5], 

The OH substituent is attached internally (rather than terminally ) to the linear alkane chain of pentane the point of attachment is labeled a, which corresponds to C-3 of pentane, for which the shift value of 34.7 is given in Table 4.6. To this value is added the increment +41, this being the increment for an OH group attached internally to the a-carbon of 3-pentanol (see line 12, 2nd column of numbers). The shift, therefore, for the point of attachment (the 

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Vlugt T J H, Krishna R and Smit B 1999 Molecular simulations of adsorption isotherms for linear and branched alkanes and their mixtures in silicalite J. Phys. Ohem. B 103 1102-18... 

Pablo J J, M Laso, and U W Suter 1992. Estimation of the Chemical Potential of Chain Molecules by Simulation. Journal of Chemical Physics 96 6157-6162. jkstra M. 1997. Confined Thin Films of Linear and Branched Alkanes. Journal of Chemical Physics 107 3277-3288. 

The upfield shift of signals of carbon atoms in 7-position to a newly introduced substituent was recognized very early. Grant and Paul (169) found a 7-parameter of —2.5 in linear and branched alkanes. Later this group studied various other classes of hydrocarbons (33,88,100,101,170-172) and developed an interpretation of the 7-effects in terms of a polarization of the bond between the carbon concerned and an adjacent, sterically perturbed hydrogen atom (33,88) that has come to be called the Grant-Cheney approach. ... 

Stoddard solvent consists (volume basis) of linear and branched alkanes (30 to 50%), cycloalkanes (30 to 40%), and aromatic hydrocarbons (10 to 20%). Alcohols, glycols, and ketones are not included in the composition, as few, if any, of these types of compounds are expected to be present in Stoddard solvent. Possible contaminants may include lead (<1 ppm) and sulfur (3.5 ppm). 

Figure 13.10 Ratio of Henry coefficients for linear and branched alkanes over various framework types [15].

Figure 11.8 Approximately linear relationship between solvation free energy and solvent-accessible surface area for linear and branched alkanes. A best fit line passing through zero has a slope of...

Table 5 Comparison of the boiling points of linear and branched alkanes and perfluoroalkanes with the same number of carbon atoms [67]...

Hitchcock, A., and Ishii, I. (1987). Carbon K-shell excitation spectra of linear and branched alkanes. J. Electron Spectrosc. Relat. Phen. 42,11-26. 

Table 4.6 shows the effects of a substituent on linear and branched alkanes. The effect on the a-carbon parallels the electronegativity of the substituent except for bromine and iodine.t The effect at the /3-carbon seems fairly constant for all the substituents except for the carbonyl, cyano, and nitro groups. The shift to the right at the y carbon results (as above) from steric compression of a gauche interaction. For Y = N, O, and F, there is also a shift to the right with Y in the anti conformation, attributed to hyperconjugation. 

As for the dehydrogenation reaction, IRC calculations [67] indicate that the protolytic cracking of linear and branched alkanes follows different mechanisms. For ethane and propane the products of the reaction are methane and the proper alkoxide. For isobutane, as one follows the reaction path towards the products, the t-butyl cation decomposes into propene and a proton which restores the acid site of the zeolite. 

The protolytic cracking involves the attack of the zeolitic proton to a carbon atom of the alkane molecule and the simultaneous rupture of one its adjacent C-C bond. The carbon atom being attacked and the C-C bond being broken will be preferentially those which produce the most stable carbenium ion. As for the dehydrogenation reaction, the protolytic cracking of linear and branched alkanes also follow different mechanisms, the latter ones producing olefins instead of alkoxides. 

Table 4.2 lists the shifts in some linear and branched alkanes. 

TIME RESOLVED MULTICOMPONENT SORPTION OF LINEAR AND BRANCHED ALKANE ISOMERS ON ZEOLITES, USING NIR SPECTROSCOPY... 

Millot, B. M6thivier, A. Jobic, H. Moueddeb, H. Dalmon, J.A., Permeation of linear and branched alkanes in ZSM-5 supported membranes. Micropor. Mesoporo. Mater. 38 (2000) 85-95. 

Similar to linear and branched alkanes, cycloalkanes also give rise to radical cations in zeolites, spontaneously or upon y-radiolysis. This brief discussion of selected examples is intended only to give a flavor of the work being done. Thus, a 13-line radical cation spectrum (a = 0.17 mT, g = 2.003) obtained upon incorporation of 1-methylcyclohexane, 43, into zeolites [71] was identified as 1,2-dimethylcyclopentene radical cation, 44 + (two sets of protons with hyperfine couphng constants in the ratio of ca 2 1 a = 1.67 mT, 2 CH3 a = 3.42 mT, 4H) [72]. The formation of 44 + was rationalized by protonation of the 3° carbon of 43, followed by loss of H2. Loss of a proton from a rearranged carbocation may generate 44, which is oxidized to 44 + by a Lewis site. 

Small pore (6- and 8-ring) zeolite-based membranes might be used in industrial processes involving hydrogen, in air separation or in separation of linear and branched alkanes,. plying small pore apertures might lead to high separation/selectivity. 

Adsorption details of calcined DD3R crystals are listed in Table 8 [36]. As can be seen from Table 8, a complete separation between linear and branched alkanes can be achieved. Isobutane is excluded from the pore structure of DD3R. The very small adsorption observed is ascribed to external surface or intercrystalline sorption. 

Stoddard solvent consists of 30-50% linear and branched alkanes, 30-40% cycloalkanes, and 10-20% aromatic hydrocarbons. Its typical hydrocarbon chain ranges from C7 through C12 in length. 

Stoddard solvent is a petroleum distillate mixture of C7-C12 hydrocarbons, approximately 80-90% aliphatics (30-50% linear and branched alkanes, and 30-40% cyclic alkanes) and 10-20% aromatics (not PAHs). It is similar to white spirits, which is also included in the toxicological profile on Stoddard solvent (ATSDR 1995b). For additional detail, see Section 3.2 and Table E-2.b. Data regarding the health effects of Stoddard solvent in either humans or animals are limited and were judged inadequate for MRL development. Upper respiratory irritant effects were seen in animals exposed by inhalation for acute and intermediate durations these appear to be the most sensitive effects by the inhalation route. Male rat nephropathy has been reported in intermediate inhalation studies, but is not considered relevant to human health. No oral studies were located. Information on the potential carcinogenicity of Stoddard solvent is inadequate. 

For human volunteers exposed by inhalation to 100 ppm white spirit for 3 hours, a mean pulmonary uptake of 392 mg white spirit was measured, based on concentrations of white spirit in inspiratory and expiratory air (Pedersen et al. 1987). Following exposure to the same concentration, 6 hours/day for 5 consecutive days, the mean pulmonary uptake was 3,464 mg white spirit. The test material was a mixture of aliphatic hydrocarbons containing 99% linear and branched alkanes (0.99% C8-C9, 15%... 

Aliphatic EC>8-EC16 Fraction. Hydrocarbons in this fraction are oxidatively metabolized to fatty acids and alcohols, apparently mediated by cytochrome P-450 isozymes (see Miller et al. 1996 for review). Studies regarding the metabolism of hydrocarbons in this fraction in humans or animals provide suggestive evidence that metabolism may be slow. In a study of humans exposed to 100 ppm white spirit 6 hours/day for 5 days (white spirit is a mixture comprised predominately of C10-C12 linear and branched alkanes), only minor differences were observed in the GC-MS spectrum of hydrocarbons in biopsied fatty tissue, than in the spectrum of hydrocarbons in the test material (Pedersen et al. 

In the previous equation, there are two unknown quantities the values of the equilibrium constants and the true values of the accessible areas. K and are dependent on the polarisabilities of linear and branched alkanes that are not identical. But, their ratio can be measured on the reference silica, as well as the ratio of the actual contact areas. Consequently, the index of nanomorphology, Im, will be defined by ... 

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