Polar and Nonpolar Molecules
Dipole moment
** The dipole moment is a physical property that can be measured experimentally.
** It is defined as the product of the magnitude of the charge in electrostatic units (esu) and the distance that separates them in centimeters (cm):
** The charges are typically on the order of 10-10 esu and the distances are on the order of 10-8 cm. Dipole moments, therefore, are typically on the order of 10-18 esu cm.
** For convenience, this unit, 1 × 10-18 esu cm, is defined as one debye and is abbreviated (D). (The unit is named after Peter J. W. Debye, a chemist born in the Netherlands and who taught at Cornell University from 1936 to 1966. Debye won the Nobel Prize in Chemistry in 1936.)
** In SI units 1 D = 3.336 × 10-30 coulomb meter (C . m).
** If necessary, the length of the arrow can be used to indicate the magnitude of the dipole moment.
** Dipole moments are very useful quantities in accounting for physical properties of compounds.
Polar and Nonpolar Molecules
** In the discussion of dipole moments, our attention was restricted to simple diatomic molecules. Any diatomic molecule in which the two atoms are different (and thus have different electronegativities) will, of necessity, have a dipole moment.
** In general,
a molecule with a dipole moment is a polar molecule.
** If we
examine Table (1) , however, we find that a number of molecules (e.g., CCl4,
CO2) consist of more than two atoms, have polar bonds, but have no
dipole moment.With our knowledge of the shapes of molecules, we can understand how
this can occur.
 |
| Table (1) |
Carbon tetrachloride
** Consider a
molecule of carbon tetrachloride (CCl4).
** Because the
electronegativity of chlorine is greater than that of carbon, each of the
carbon chlorine bonds in CCl4 is polar.
** Each
chlorine atom has a partial negative charge, and the carbon atom is
considerably positive.
** Because a
molecule of carbon tetrachloride is tetrahedral (Fig 1), however, the center of
positive charge and the center of negative charge coincide, and the molecule
has no net dipole moment.
 |
| Fig (1) |
** This result
can be illustrated in a slightly different way: if we use arrows (+→) to
represent the direction of polarity of each bond, we get the arrangement of
bond moments shown in Fig. 2.
 |
| Figure (2) |
** Since the
bond moments are vectors of equal magnitude arranged tetrahedrally, their effects
cancel. Their vector sum is zero. The molecule has no net dipole moment.
Chloromethane
** The
chloromethane molecule (CH3Cl) has a net dipole moment of 1.87 D.
** Since carbon
and hydrogen have electronegativities (Table 1) that are nearly the same, the
contribution of
three C-H bonds to the net dipole is negligible.
** The electronegativity
difference between carbon and chlorine is large, however, and the highly polar C-Cl
bond accounts for most of the dipole moment of CH3Cl (Fig. 3).
 |
| Figure (3) |
Solved
Problem (1): Although molecules of CO2 have polar bonds (oxygen is
more electronegative than carbon), carbon dioxide (Table 1) has no dipole
moment. What can you conclude about the geometry of a carbon dioxide molecule?
Strategy and
Answer:
For a CO2
molecule to have a zero dipole moment, the bond moments of the two carbon oxygen
bonds must cancel each other. This can happen only if molecules of carbon
dioxide are linear
Water and Ammonia
** Unshared
pairs of electrons make large contributions to the dipole moments of water and
ammonia.
** Because an
unshared pair has no other atom attached to it to partially neutralize its negative
charge, an unshared electron pair contributes a large moment directed away from
the central atom (Fig. 4). (The O-H and N-H moments are also appreciable.)
 |
| Figure (4) |
Dipole Moments in Alkenes
** Cis–trans
isomers of alkenes have different physical properties. They have different
melting points and boiling points,
** often cis–trans
isomers differ markedly in the magnitude of their dipole moments.
** Table (2)
summarizes some of the physical properties of two pairs of cis–trans isomers.
 |
| Table (2) |
Solved
Problem (2): Explain why cis-1,2-dichloroethene (Table 2) has a large dipole moment
whereas trans-1,2-dichloroethene has a dipole moment equal to zero.
Strategy and
Answer:
If we examine
the net dipole moments (shown in red) for the bond moments (black), we see that
in trans-1,2-dichloroethene the bond moments cancel each other, whereas in cis-1,2
dichloroethene they augment each other.
Reference: Organic chemistry / T.W. Graham Solomons , Craig B.Fryhle , Scott A.snyder , / ( eleventh edition) / 2014.
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