Edited by MURIELBOY0 BISHOP Clemson University Clemson. SC 29631
Identifying Polar and Nonpolar Molecules R. J. Tykodi Southeastern Massachusetts University, North Dartmouth, MA 02747 We touch on the polar/nonpolar character of molecules a t various places in the undergraduate chemistry curriculum-lightly in general chemistry and more in detail in physical chemistry and in advanced inorganic chemistry: nonpolar molecule.The center of positive charge in the molecule coincides with the center of negative charge, and the molecule has a zero electric dioole moment. polar molecule. The centers of pmitive and negative charge fur the molecule do not coincide, and the rndecule h a a nonzero electric dipole moment. With respect t o nonzero electric dipole moments, we ask if a given molecule has one and, if so, what effect the dipole has on the behavior of the molecule. The dioole moment of a molecule is an ingredient in iw dielectricconntant (relative permittivity) and in its molar uolarization. The behavior of the dipole moment during a ;ormal-mode vibration of the molecule determines the spectroscopic activity of that particular mode-if the vibrational motion changes the dipole moment, that particular normal mode is IR-active (otherwise not): the symmetric stretching vibration in COz, for example, is IR-inactive (no change in the dipole moment) and Raman-active. We expect dipole-iipole interactions among the mole-
Murk1 Boyd B i s l w received the BA degree
In chemisby from Huntingdon College, the MS degree in biochemistry from E m University, and me PhDdegree in cheminryhom Michigan State University. She received a porflactaal
appointment at Yale University. She Is currently a profesor in the Chemistry Department at Clemson University where she has taught courses in biochemistry, organic chemisby. general Ehemishy, and medical technology. She has been director of the medical technoiogy program, coordinata of the freshman chemistry laboratories and of the freshman learning center. She developed me safety and teacher mining program for chemistry teaching assistants. She has operated a summer science camp in chemistry for high schwl students. She has worked with the Journal a number of years, refereeing manuscripts. writing book reviews, and writing p s pers.
cules of a substance t o have an influence on its physical properties; i t comes as a surprise, therefore, to find that the influence of dipole moment on normal boiling point is quite erratic: if we look a t the boiling points of sets of isomeric molecules with identical molar-masses but varying dipole moments, we find no clear trend with increasing dipole mom e n t s e e the table (the reoorted dioole moments were measured in the gas phase (1)j. Determinine the centers of oositive and neeative charae for a molecul~can be a forbidding task, so we frequeniy resort to symmetry considerations (3, 4 ) to establish the presence or absence of a nonzero electric dipole moment for a -aiven molecule: a molecule is nonuolar (has a zero electric dipole moment) if it has two or more noncoincident proper rotation axes or if it has a center of symmetry ( 3 ) .Dealing in detail with bond dipole vectors, with symmetry operations, and with group-theoretical consequences of symmetry operations is appropriate in both the physical chemistry and the advanced inorganic chemistry courses because these ideas will be used in the discussion of molar polarization and spectroscopic activity; such detailed considerations are not appropriate to the general chemistry course because they will not have any additional applications beyond the polar/ nonuolar cateaorization of a few simde molecules. ~ b much w time and effort shouldwe devote to the polad nonpolar character of molecules in general chemistry? Not much-one or two lectures should be adequate. Once we have identified the polar or nonpolar nature of a given molecule, we don't do a&hing withthat information-we don't usually say much about molar polarization or about the details of s~ectroscooicactivitv in our aeueral chemistrv courses, and we don't find an;special rilevance of dipoG character to the nhvsical orooerties of the substances we study (except for'soke q;ali&tive observations about the nature of polar and nonpolar solvents). The polarlnonpolar character of molecules is one of those "define i t and drop it" todcs that so litter the landscaue of general chemistry. For such isolated topics that are mostlyaithout ramifications for the rest of the course, i t is important not t o "overspend" in terms of time and theoretical machinery: do what you feel needs doing, a t the simplest possible conceptual level, and move on. I consider the PLAT0 module in Chemistry 1 (4) on molecular symmetry to he an example of "overexpenditure"in that it invests heavily in the machinery of symmetry operations-an investment that does not "pay off' because nowhere else in the course is i t needed or used. Volume 66
Number 12 December 1989
1007
EIectrb Dipole Moments and Bolllng Points lor Sets of Isomeric Molecules ( 42) Molecule
CdWr 1,ldichlamthane 1.2dichlw0ethane
Diode momentldebves Boillna Point/'C
2.06
57.3
(less?)
83.5
CIH+
ethylarnine dimethylamine CaHsCl
frans-l-chlaopmpene 3-chloropmpem, cis-l-chlaopropene 2-chforopropene 4HsCIa 1.ldichlwopmpane 2,2dichlwopropane 1,Michlwapropane 1,2dichlnaprapane CsHICI Z-chloropropane 1-chloropropane
tion of the reference points is not fully symmetric, the molecule is polar. For purposes of symmetry analysis, resolve a geometric fieure into a set of suhfieures that show either a center of symmetry (the line) or multiple noncoincident proper rotation axes (the trianele. the tetrahedron. the ~ e n t a e o n .the square antiprism, &d so on). I shall call such kbfi&re$ the "simple parts" of the given geometric figure. If the satellite points of each of the "simple parts" are equally occupied (occupied by identical entities), the reference figure is fully symmetric and the molecule is nonpolar. If any of the "simple parts" has its satellite points unequally occupied (occupied by nonidentical entities), the reference figure is not fully symmetric, and the molecule is polar. Let's catalog some of the possible satellite configurations. onesolellilr: reference f i ~ r the e line, h u w a t . If the two point aare equally occupied, H-H for example, the molecule is nonpolar. If the two pointa are unequally occupied. H-
