2-Methylbutane, 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. 

Isomer Chemical compounds with the same molecular weight and atomic composition but differing molecular structure (e.g., n-pentane and 2-methylbutane). 

Another class of struaural descriptors is the topological indices, which are derived from molecular structures represented as graphs. The molecule 2-methylbutane is shown below in two representations in the usual chemical stick figure drawing and as a graph. In the graphic representation, eadi atom is a node or vertex without elemental identification and each bond is an edge. 

The limited data available for 2-methylbutanal and 3-methylpentanal photodecomposition have been discussed by Calvert et al. (2008) these are insufficient to allow reliable estimates to be made of the photolysis frequencies. The effects of molecular structure of the aldehyde on photodecomposition efficiency have been discussed briefly by Calvert et al. (2008). All of the aliphatic aldehydes studied to date follow the pattern of photodecomposition exhibited by the aldehydes discussed in this chapter. 

If you said that all of these structures represent the same compound, you are correct. The diagram simply shows four ways of drawing a structural formula for 2-methylbutane. As you can see, the structural formula for a given hydrocarbon can be written in several ways. Before you continue reading, make sure that you understand why these structural formulas are all alike. You ll soon learn, however, that three distinctly different alkanes have the molecular formula C5H12. 

Before proceeding further, it is necessary to learn different ways of drawing the structure of organic compounds. To see how atoms are connected in 2-methylbutane (C5H12), for example, we must first write the expanded molecular formula, CH3CH(CH3)CH2CH3 and draw its structural formula, shown in Figure 16.3(a). 

What is lacking in these structures, however, is the three dimensionality of the molecule, which for 2-methylbutane is shown by the ball-and-stick molecular model in Figure 16.3(d). Depending on the purpose of the discussion, any of these representations can be used to describe the properties of the molecule. 

Application of these principles is important for interpreting the mass spectrum of 2-methylbutane (15) (Fig. 8.60). Several peaks reveal information in support of its structure. For example, the molecular ion, P, is seen at w/z 72, which is the molar mass you would calculate for 15 by using atomic masses of 12.000 for carbon and 1.000 for hydrogen. The small + 1 peak at m/z73 results from the presence of and in the compound. A P + 2 peak is not observed because it is statistically unlikely that a molecule of 2-methylbutane would contain two atoms of either or or one of each of these isotopes. As you can see, the most intense peak in the spectrum is not the molecular ion, but rather is a fragmentation peak having ml2 43. This peak is defined as the base peak its relative intensity is set as 100%, and the intensities of all other peaks in the spectrum are measured relative to it. 

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