3 - Chapter 3 Building blocks for organic molecules The...

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1 Chapter 3 – Building blocks for organic molecules The previous chapter described the formal process of generating Lewis structures. Most likely, this was a review of what you learned in general chemistry. The description in the previous chapter was intended to help you understand the electron configuration model of organic structure. However, chemists use a more practical approach when they set out to write down the Lewis structure of an organic molecule. It is based on a piecewise process of joining modular components. These modular components are called building blocks. To most chemists this process is second nature, and they do not give much thought to the process. It will become second nature to you too, if you take the time to drill and practice the skill. In the section that follows, an attempt is made to describe this practical process in detail. First, we need to construct a list of building blocks. How is it that a chemist can instinctively draw a complex molecule? The reason is that the electronic structure of atoms leads to predictable patterns of chemical bonding. These patterns can be broken down to the level of the individual atom, according to atom type (e.g., C, H, N, O, F), formal charge (-1, 0, +1) and connectedness. At the level of the individual atom, patterns of chemical bonding can be described as the distribution of electron pairs within domains, or regions of space about the atom. Electron pair domains, or EPDs, provide a convenient scheme around which the patterns of bonding can be organized. Practice and experience solidifies these patterns in the chemist’s mind. Bonding patterns, in turn, lead to the notion of a functional group, a recurring set of atoms with a specific structure that instills certain reactivity. Also important is the three-dimensionality of structure, where using the EPD model, only three major atomic possibilities are shown to exist. Familiarity with the EPD model is what gives the chemist a seemingly effortless ability to construct complex molecular drawings, even for structures that they have never before encountered – it’s a powerful graphical language that opens the way through the “monstrous and boundless thicket” of organic chemistry.
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