2,2-Dimethylpropane, molecular structure

All three have five carbon atoms and 12 hydrogen atoms, so they have the molecular formula C5H12. However, as you can see, these models represent three different arrangements of atoms, pentane, 2-methylbutane, and 2,2-dimethylpropane. These three compounds are isomers. Isomers are two or more compounds that have the same molecular formula but different molecular structures. Note that cyclopentane and pentane are not isomers because cyclopentane s molecular formula is C5H10. 

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. 

When the objective is analytical the products of ozonolysis are isolated and identi lied thereby allowing the structure of the alkene to be deduced In one such example an alkene having the molecular formula C Hig was obtained from a chemical reaction and was then subjected to ozonolysis giving acetone and 2 2 dimethylpropanal as the products... 

Figure 12.3 Mass spectrum of 2,2-dimethylpropane (C5Hi2 MW = 72). No molecular ion is observed when electron-impact ionization is used. (What do you think is the structure of the M+ peak at m/z = 57 )...

Both molecules have the same molecular formula, but they have different structures. The London (dispersion) forces between pentane molecules in the liquid or solid states are stronger because the linear molecules have a larger surface area for interaction. In contrast, its isomer, 2,2 dimethylpropane, is more compact (adopting a roughly spherical shape) owing to its extensive branching, and hence has a smaller surface area for interaction (Figure 4.73). 

The preference for fragmentation at a highly snbstituted center is even more pronounced in the mass spectrum of 2,2-dimethylpropane (Figure 11-27). Here, loss of a methyl radical from the molecular ion produces the 1,1-dimethylethyl (fert-butyl) cation as the base peak at m/z = 57. This fragmentation is so easy that the molecular ion is barely visible. The spectrum also reveals peaks at m/z = 41 and 29, the result of complex structural reorganizations, such as the carbocation rearrangements considered in Section 9-3. 

For hydrocarbons, there appears to be a strong correlation between the electron mobility and the molecular and solvent structure. The electron mobilities for example in ethane, n-pentane and 2,2-dimethylpropane are 2.8 x 10, 1.5 x 10, 7.0 x 10 ps respectively at room temperature [72]. This pattern suggests that for n-alkanes [54] (1) the electron mobility decreases with increasing carbon number and (2) electron mobility is slower for linear alkanes. A possible explanation is because of irregularities of the potential in the liquid, the electron randomly scatters and becomes trapped. 

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