Polar and Nonpolar Molecules

– In this subject, we will discuss the 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 the 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 (2.1), however, we find that several molecules (e.g., CCl₄, CO₂) consist of more than two atoms, and have polar bonds, but have no dipole moment.

– With our knowledge of the shapes of molecules, we can understand how this can occur.

Carbon tetrachloride is Nonpolar

– 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.

 

– 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.

– 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 is Polar

– 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).

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.)

 

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 (2.2) summarizes some of the physical properties of two pairs of cis-trans isomers.

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.