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.