Neopentane, effect

Shinsaka, K. and Freeman, G.R., 1974, Electron mobilities and ranges in solid neopentane Effect of the liquid-solid phase change, Can. J. Chem., 52 3556. 

Direct photochemical excitation of unconjugated alkenes requires light with A < 230 nm. There have been relatively few studies of direct photolysis of alkenes in solution because of the experimental difficulties imposed by this wavelength restriction. A study of Z- and -2-butene diluted with neopentane demonstrated that Z E isomerization was competitive with the photochemically allowed [2tc + 2n] cycloaddition that occurs in pure liquid alkene. The cycloaddition reaction is completely stereospecific for each isomer, which requires that the excited intermediates involved in cycloaddition must retain a geometry which is characteristic of the reactant isomer. As the ratio of neopentane to butene is increased, the amount of cycloaddition decreases relative to that of Z E isomerization. This effect presumably is the result of the veiy short lifetime of the intermediate responsible for cycloaddition. When the alkene is diluted by inert hydrocarbon, the rate of encounter with a second alkene molecule is reduced, and the unimolecular isomerization becomes the dominant reaction. 

Another interesting effect seen in alkanes is that increased branching lowers an alkane s boiling point. Thus, pentane has no branches and boils at 36.1 3C, isopentane (2-methylbutane) has one branch and boils at 27.85 °C, and neopentane (2,2-dimethylpropane) has two branches and boils at 9.5 °C. Similarly, octane boils at 125.7 °C, whereas isooctane (2,2,4-trimethylpentane) boils at 99.3 °C. Branched-chain alkanes are lower-boiling because they are more nearly spherical than straight-chain alkanes, have smaller surface areas, and consequently have smaller dispersion forces. 

As a result, free-radical chlorination of alkanes is a nonselective process. Except when only one type of replaceable hydrogen is present (methane, ethane, neopentane, unsubstituted cycloalkanes), all possible monochlorinated isomers are usually formed. Although alkyl chlorides are somewhat less reactive than alkanes, di- and polychlorinations always occur. The presence of a chlorine atom on a carbon atom tends to hinder further substitution at that carbon. The one exception is ethane that yields more 1,1-dichloroethane than 1,2-dichloroethane. The reason for this is that chlorination of an alkyl chloride occurs extremely slowly on the carbon atom adjacent to the one bearing the chlorine atom (vicinal effect).115... 

Table I. Effect of Surface on Initial Product Distribution in the Oxidation of Neopentane...

Figure 1. Relationship between the measured adsorption volumes, Fp (measd) and calculated void volume Vp of several zeolites. The dashed line corresponds to Vp (measd) = Vp (calcd). The symbols represent the zeolites as described in Tables I-VI A, X, L, Z (mordenite Zeolon), omega (to), and offretite-type 0. Vertical shaded areas containing plotted values of Vp (measd) correspond to calculated values of Vp for the main pore systems. The narrow area, 0, corresponds to the main c-axis void of zeolite 0. The value of Vp for Zt = Vp for zeolite 0. Symbols with the subscript t (At Xt) etc.) represent values of Vp for the total void volume shown by narrow shaded areas. The neopentane (NP) volumes lie consistently below the dashed line thus showing a paeking effect. In all of these zeolites of varying structure, the H20 and N2 volumes correspond with complete filling of the total voids even though this is not possible in the case of N2 in zeolites A, X, and L.

Among isomers the knock rating increases as the number of side chains increases. This effect, too, is very large, more than spanning the full octane number scale. Adding chains in such a way as to increase compactness or centralization raises the rating most. Neopentane is an exception. It is worse than isopentane under all test conditions. 

The results presented in this paper provide a simple explanation for the compensation effect. Neopentane (C-(CH3)4) is chosen as a specific example since neopentane can only adsorb via one of the methyl groups, and therefore its adsorption can be approximated by CH3 adsorption, which has been considered in this paper. Other, more straight alkanes like n-hexane can adsorb in a variety of geometries, which complicates the picture. 

The gain in entropy from desorption of two H atoms on Pt as a H2 molecule for a basic support is approximately 85 J mol K. Since typical conditions are Pm = 0.99 bar, with Pneo = 0.01, an increase in the order in H2 has no effect on the pressure-term in equation 8. Thus the change in the pre-exponential factor for the conversion of neopentane on an acidic versus a basic support is given by... 

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