Chapter 3 – Building blocks for organic molecules
The previous chapter described the formal process of generating Lewis structures.
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
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
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.
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.