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The
most important question in chemistry is "Where are the electrons?"
The answer is that electrons are generally
in higher amounts around the more electronegative atoms (e.g. F,
Cl, O, N) of a molecule. The electronegative atoms pull electron
density away from the less electronegative atoms (e.g. C,
H) to which they are bonded. Thus, understanding electronegativities
provides a simple method of deciding which portions of a molecule
have a relatively high electron density, and which portions have
a relatively low electron density. Molecules with areas of high and
low electron density are referred to as polar molecules.
Molecules that have relatively uniform electron densities are referred
to as nonpolar molecules.
Electronegativities of the atoms we will
normally encounter in organic chemistry:
H = 2.1, C = 2.5, S = 2.5, Br = 2.8, N =
3.0, Cl = 3.0, O = 3.5, F = 4.0
Physical properties such as melting
points, boiling points and solubilities can be understood by analyzing
distributions of electron density. All things being equal, molecules
that are more polar will stick to each other better, increasing
both the melting points and boiling points (it takes more energy and
thus heat to break apart the interactions between sticky molecules).
This is because unequal distributions of electrons in a polar molecule
produce corresponding partial charges (remember that electrons are
negatively charged), and opposite partial charges attract each other,
making the molecules stick together. Nonpolar molecules do not have
these partial charges, so they do not stick to each other as well.
Also, polar molecules dissolve in polar solvents, again because of
the attractions between partial charges on the solvent and the polar
molecules. Nonpolar molecules dissolve in nonpolar solvents.
Relative acidities are also understood
on the basis of electron distribution. When a molecule loses a proton,
it is acting as an Bronsted acid. Because a proton has a positive charge,
losing a proton generally leaves a negative charge behind. A molecule
will be more willing to give up a proton (i.e. it will be a
stronger acid) the better the negative charge can be absorbed after
the proton leaves. A molecule can better absorb a negative charge if
it has more electronegative atoms that can accommodate the negative
charge. Later you will see how important it is to be able to predict
relative acidities of molecules in order to predict important attributes
such as relative leaving group abilities.
Finally, and most importantly, understanding
where the electrons are located in a molecule allows for the understanding/prediction
of reactions. As you will learn, the vast majority of reactions
involve the electron rich portions of one molecule or species (the
so-called nucleophile) reacting with the extremely electron
deficient portion of another molecule (the so-called electrophile).
Being able to understand and predict reactions, then, comes down to
being able to predict the locations of these reactive sites (i.e. where
are the electrons?).
The bottom line. Understanding
where electrons are in molecules (i.e. around the more electronegative
atoms) allows you to understand/predict physical properties, acidities
and reactions of molecules. It is just that simple. In short, you will
understand organic chemistry, it will make sense to you, it will be
far easier than trying to memorize a bunch of facts and your organic
chemistry classes will be enjoyable. O.K., maybe the word enjoyable was
a stretch here, but it will be less painful than you are expecting.
I guarantee it!
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