Structural Isomers and Formal Nomenclature

With methane, ethane, and propane, there is only one way the molecule can be put together that conforms to the requirements of valency. However, when we reach butane, there are two possibilities. The carbon atoms can be arranged in a linear four-carbon chain (2.16) or a branched chain (2.17). These were initially called -butane, normal butane, and iso-butane. We describe these as structural isomers—the carbon atoms are joined together in a different pattern. Notice that I am using the stick drawings if you draw out the same structures with all the atoms there, you will quickly see why these are better  

With pentane, three structures are possible (2.18, 2.19, 2.20), -pentane normal pentane), iso-pentane, and Heo-pentane, the new pentane. However, moving on to higher alkanes, clearly, a different system is needed—new prefixes cannot be endlessly invented. If we want to name 2.21, we need to use the lUPAC system. 

Molecules (and atoms) are attracted to one another—otherwise solids and liquids would not hold together. All the types of forces between molecules are properly referred to as van der Waals forces, although this term is also sometimes used to refer specifically to dispersion forces. Table 2.4 lists the main forces between molecules (although there is increasing interest in other types of interactions specific to particular types of molecules, especially in the solid state). 

TABLE 2.4 Interionic and Intermolecular Forces Typical Energy  

Hydrogen bonding 20 Mainly N, 0, F—the link is a hydrogen atom 

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