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How (and why) electrons flow

Image courtesy of Flickr user AndyArmstrong

So what is the key “driving force”  involved in chemistry? A chemist would say that “opposite charges attract, like charges repel.” But how can we make that concept more concrete and specific?

I’ve been going through Daniel Levy’s excellent book “Arrow Pushing in Organic Chemistry” (fuller review to come). The following excerpt nicely summarizes the driving force for electron flow (and therefore, chemical reactivity). Emphasis is from the author:

As a rule, electrons will flow from atomic centers high in electron density to atomic centers low in electron density. This dependence on polarity is similar to the way that electricity flows in an electric circuit. If there is no difference in electrical potential between the ends of a wire, electricity will not flow. If we imagine a simple hydrocarbon such as ethane, we can analogously relate this system to a nonpolarized wire. Both carbon atoms possess the same density of electrons and thus ethane has no polarity. However if functionality is added to ethane through introduction of groups bearing heteroatoms, the polarity changes and electron flow can be used to induce chemical reactions. These heteroatom bearing groups are known as functional groups and serve to donate or withdraw electron density.

While functional gorups can be either electron donating  or electron withdrawing, these properties rely upon the specific heteroatoms the functional group is composed of as well as the configuration of these heteroatoms relative to each other. With respect to the specific heteroatoms, electronegativity of the heteroatoms is the driving force influencing polarity. Thus the more electronegative the atom, the greater the affinity of electrons for this atom. As a calibration for electronegativity, the periodic table of the elements serves as an excellent resource. Specifically, moving from left to right and from bottom to top, electronegativity increases…

Dr. Levy’s book is an excellent guide to understanding the main types of reactions you’ll encounter in a typical organic chemistry course. You can buy it here.

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