Neopentane 2,2-dimethylpropane

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. 

Which of the following liquids is expected to have the highest vapor pressure at 0°C octane, CHjfCHil CH, pentane, Cl I i(CH2)10 I, or neopentane, 2,2-dimethylpropane, C(CH3)4 Justify your answer. 

Make the following molecules with a computational chemistry software pentane, isopentane (2-methylbutane), and neopentane (2,2-dimethylpropane). Note do not forget to do geometry optimization ... 

Neopentane (2,2-dimethylpropane) [463-82-1] M 72.2, b 79.3 , d 0.6737, n 1.38273. Purified from isobutene by passage over cone H2SO4 or P2O5, and through silica gel. 

The loss of a methyl radical from neopentane (2, 2-dimethylpropane)... 

Pentane is a colorless, flammable liquid (the first liquid member of the alkanes) that is lighter than water. It has a pleasant odor that can be detected at 900 ppm, and a moderate odor intensity is observed at 5000 ppm. It occurs as two other isomers, including isopentane [(CH3)2CHCH2CH3] and neopentane [C(CH3)4]. Isopentane (2-methylbutane) apparently has physical and physiological characteristics similar to straight-chain pentane. Neopentane (2,2-dimethylpropane) is similar to butane in physical and physiological characteristics. In air, one part per million of C5 pentane is equivalent to 3 mgm. ... 

A branched alkane which has been studied by INS is neopentane (2,2-dimethylpropane, C(CH3)4) [14]. Fig. 8.4 shows a comparison of the INS spectrum and that calculated using a classic Wilson GF force field... 

Provided a branched alkane has only primary, secondary and tertiary carbon atoms, all hydrogen atoms can be exchanged by the a/S mechanism the carbon skeletons (devoid of hydrogen atoms) of some such molecules are shown in the first row of Table 6.5. The presence of a quaternary carbon atom as in neopentane (2,2-dimethylpropane) prevents the formation of an aa-diadsorbed species, so that multiple exchange, if it oecurs, must of necessity proceed through either aa- or ay-diasorbed structures, or where possible through anaS structure. Carbon skeletons of molecules of this type are shown in the lower part of Table 6.5, but... 

High atmospheric concentrations, e.g. in pooiiy ventilated spaces, can cause oxygen deficiency, with risk of unconsciousness. The measures on this card also apply to neopentane (2,2-dimethylpropane,CEHia). Use propane If ambient temperature drops below5°C. Under no circumstances warm cylinder. Turn leaking cylinder so that leak Is on top to prevent liquid Isobutane escaping. 

The investigations which have been carried out with ethane and neopentane (2, 2-dimethylpropane) and those performed with the mixtures n 4H 0-H2S have already been described in some notes and articles, published or to be published shortly. The first results of the experiments carried out with isobutane and propane have not yet been published. 

Effect of molecular shape. For a pair of nonpolar substances with the same molar mass, stronger attractions occur for a molecular shape that has more area over which the electrons can be distorted. For example, the two five-carbon alkanes, n-pentane and neopentane (2,2-dimethylpropane) are structural isomers— same molecular formula (C5H12) but different properties. n-Pentane is more cylindrical and neopentane more spherical (Figure 12.15). Thus, two -pentane molecules make more contact than do two neopentane molecules, so dispersion forces act at more points, and n-pentane has a higher boiling point. 

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