Van der Waals BondingThe distinction is betweenBond PolarityandMolecular polarity. The total polarity of
a molecule is measured asDipole Moment. The actual calculation of dipole moment isn’t
really necessary so much as an understanding of what it means. Frequently, a guesstimate
of dipole moment is pretty easy once you understand the concept and until you get into the
more advanced organic chemistry, exact values are of little value.
Basically, the molecular polarity is, essentially, the summation of the vectors of all of the
bond polarities in a molecule.
8.4 Van der Waals Bonding
Van der Waals bonding is the collective name for three types of interactions:
1.Permanent Dipole interactions: these are the electrostatic attractive forces be-
tween two dipoles, these are responsible for fluromethane’s (CH3F) high boiling point
(about -15 deg C) compared to Nitrogen (about -180 deg C).
2.Permanent dipole / induced dipole: these are the interactions between a per-
manent dipole and another molecule, causing the latter molecule’s electron cloud to
be distorted and thus have an induced dipole itself. These are much weaker than
the permanent dipole / dipole interactions. These forces occur in permanent dipole-
molecules, and in mixtures of permanent dipole and dipole free molecules.
3.Instantaneous dipole / induced dipole: At any specific moment the electron
cloud is not necesarily symetrical, this instantaneous dipole then induces a dipole in
another molecule and they are attracted, this is the weakest of all molecular interac-
tions.A Dipole is caused by an atom or molecule fragment having a higher electronegativity
(this is a measure of its effective nuclear charge, and thus the attraction of the nucleus by
electrons) than one to which it is attached. This means that it pulls electrons closer to it,
and has a higher share of the electrons in the bond. Dipoles can cancel out by symmetry,
eg: Carbon dioxide (O=C=O) is linear so there is no dipole, but the charge distribution
is asymmetric causing a quadripole moment (this acts similarly to a dipole, but is much
weaker).
8.5 Organometallic Compounds and Bonding
Organometallic chemistry combines aspects of inorganic chemistry and organic chemistry,
because organometallic compounds are chemical compounds containing bonds between car-
bon and a metal or metalloid element. Organometallic bonds are different from other bonds
in that they are not either truly covalent or truly ionic, but each type of metal has individual
bond character. Cuprate (copper) compounds, for example, behave quite differently than
Grignard reagents (magnesium), and so beginning organic chemists should concentrate on
how to use the most basic compounds mechanistically, while leaving the explanation of
exactly what occurs at the molecular level until later and more in-depth studies in the
subject.
Basic organometallic interactions are discussed fully in a later chapter.