GUEST AUTHOR Wilbur N. Moulton
Southern lllinois University Carbondole, Illinois
II
Textbook Errors,
Dipole Moments of Symmetrical Organic Molecules
The use of dipole moments to assign cis or trans configurations to olefins is frequently introduced in elementary organic textbooks.' The method is a useful one in some cases, but it is not applicable to all olefins. Some of the problems and limitations involved also arise in considering the dipole moments of substituted benzenes. The example which is most frequently given is 1,2dichloroethylene, in which case the dipole moment ( p ) of the trans isomer is zero and that of the cis isomer is about 1.85 Debye (D). The dipole moment of a molecule is the resultant of the group moments treated as vectors. As shown below in cis-1,2-dichloroethylene the resultant, primarily the vector sum of the carbonchlorine moments, is perpendicular to the carbon-tocarbon bond and in the plane of the molecule. I n trans-1,2-dichloroethylenethe carbon-chlorine moments are directed in exactly opposite directions, and there-
fore the vector resultant is zero. No error is involved in this special case, but there is when it is stated or implied that the dipole moment of all symmetrical trans-olefius will he zero. The dipole moment of symmetrical trans-olehs will be zero if, and only if, the group moments are directed along the axes of the bonds from the olefin carbons to the attached groups. Groups for which this is true are said to have axial or linear moments, and include hydrogen, the halogens, nitro, nitrile, and methyl groups. A second class of groups have moments that are not directed along the bond axis. In these cases the group moments for the trans isomer are not a t an 180' angle from each other, hence the vector sum is not zero and the dipole moment is not zero. Groups of the second class include hydroxyl, akoxyl, amino, carboxylic acid, ester, aldehyde, and ketone groups. Maleic acid and fumaric acid constitute a case of cis-trans isomerism in which the groups attached to the double bond do not have axial moments. This case is complicated by hydrogen Suggestions of material suitable for this column and guest columns suitable for publication directly are eagerly solicited. They should be sent with as many details as possihle, and particularly with references to modem textbooks, to Karol J. Mysels, Department of Chemistry, University of Southern California, Los Angeles 7, California. Since the purpose of this column is to prevent the spread and continuation of errors and not to evaluate individual texts, the source of errors discussed will not be cited. To be presented, the error must occur in at least two independent standard books.
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bonding in the free acids, but the dipole moments of the diethyl esters have been measured. The observed dipole moments are 2.54 D for diethyl maleate and 2.38 D for diethyl fumarate. Although the dipole moment for the cis isomer is greater than that for the trans isomer, the small difference between two large dipole moments would not be adequate basis for assignment
of configuration. The angle (8) between the bond axis and the bond moment can be calculated from experimental data and varies from 55" for methoxyl to 142' for the amino group attached to aromatic carbon. The calculated angle between the bond axis and bond moment for a given group are not necessarily the same in aliphatic compounds as in aromatic compounds. A value of
8 greater than 90' indicates that the group is positive with respect to the rest of the molecule to which it is attached. An error more frequently made by implication than by direct statement concerns paradisubstituted benzenes. It is often pointed out that para-dinitrobenzene has a dipole moment of zero, which is true as it is of all other cases in which two like groups with axial or linear bond moments are para to each other. From the discussion above it is obvious that it cannot be correctly assumed that all pa~adisubstitutedbenzenes have zero dipole moment. Rather in all cases involving groups with non-axial moments the para-disubstituted benzenes will have dipole moments. Examples include para-diaminobenzene with a dipole moment of 1.5 D and para-dimethoxybenzene with a dipole moment of
1.7
n~ -.
It might seem that if groups are free to rotate about a single bond2 that variously oriented dipole moments of the different conformations might average out to zero. This is not true. The fact is that if a molecule has a dipole moment in any conformation it will have a net dipole moment, since the actual dipole moment is the weighted average of the dipole moment for each conformation that the molecule assumes. The experimental dipole moment falls between the values calculated for the conformations with maximum and minimum dipole moment. I n para-diiethoxybenzene two important conformations are shown. It can be
argued that resonance interaction of the oxygen with
the ring would favor such planar conformations. Of these conformers A would have zero dipole moment and B would have a large dipole moment. The experimental value (which is smaller than that calculated for B) can be explained if one assumes that para-dimethoxybenzene exists as a mixture of the two. However, there is nothing about the experimental data that requires planar conformation. Completely free rotation about the carbonaxygen bond is not ruled out by the dipole moment data. The treatment of free versus restricted rotation is not limited to aromatic systems. An examination of this problem in an aliphatic system, in addition to its own merit, can be used to correct another error. A recent textbook states that any molecule that has a center of symmetry will not have a dipole moment. A current monograph implies the same argument by stating that hexamethylenetetramine does not have a dipole moment because it has a center of symmetry. The fact is that the presence of a center of symmetry does not necessarily prevent a molecule from having a dipole moment. In the case of hexamethylenetetramine the molecule does not have a dipole moment, but neither does it have a center of symmetry. The case of 1,2-dichloroethane will illustrate the point. This compound can exist in a number of conformers of which three are shown.
gauche (skew) conformers need to be considered. The anti conformer in addition to a plane of symmetry has a point of symmetry, and since the bond moments are aligned in such a way as to have a vector sum of zero, the dipole moment would be zero for this conformer. However, the gauche conformers, the one shown and its non-superimposable mirror image, are not symmetrical and would have a dipole moment and optical activity if each were treated independently. Experimentally, a dipole moment of 1.12 D at 32'C and 1.54 D a t 271°C is observed for 1,2-dichloroethane, and the compound is optically inactive. This can be explained by assuming that the observed dipole is the weighted average of that for each of the various conformers, the anti conformer predominating a t lower temperatures. As the temperature increases the internal energy of the molecule increases and more of the molecules assume the gauche conformation. The value a t 271' has been calculated to he about that which would be expected for a random distribution between the anti and the two gauche conformations. The molecule is optically inactive because there are an equal number of each of the two non-superimposable gauche conformers. The following general rules relate conformation, optical activity, and dipole moment. If any of the conformers in which a molecule can exist has a center of symmetry the compound will he optically inactive. If any conformer has a dipole moment, the compound will have a dipole moment. A. compound will not have a dipole only if the center of gravity of negative charge coincides with the center of gravity of positive charge in all conformers. It would seem that even a minimum treatment of the topic should include these basic principles in some form. The data used in this article and excellent treatments of the topic can be found in the books listed in the bibliography. Bibliography
anfi
gnilciie
eclipsed
The eclipsed conformation shown and other eclipsed conformers in which chlorine and hydrogen are eclipsed are too unstable to make any appreciable contribution, so only the anti (also called trans or ~taggered)~ and
Goum, E. S., "Mechanism and Structure in Orgitnio Chemistry," Henry Holt and Ca. Inc., New York, 1959, ohap. 3. S ~ T HC. , P., "Dielectric Behavior and Structure," McGmwHill Book Co., Inc., New York, 1955, chap. 7-11. W ~ L A N G. D , W., ''Advanced Organic Chemistry," 3rd ed., John Wiley and Sons, Inc., New York, 1960, pp. 430431. WEELAND,G. W., "Resonance in Organic Chemistry," John Wiley and Sons, Inc., New York, 1955, chap. 5.
' For a, discussion of the principles of eonformational analysis see
ELIEL,E. L., J. CAEM.EDUC., 37, 126 (1960).
Volume 38, Number 10, October 1961
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