AN UNCONVENTIONAL REPRESENTATION OF MULTIPLE BONDS
0
RICHARD G. GILLIS and PETER F. NELSON Melbourne Technical College, Victoria, Australia
N O ~ practice A L has always been t o write a horizontal stroke between the symbols for two elements t o represent the two electrons of a single covalent bond. Following this practice, four-electron bonds are represented by two strokes and six-electron bonds by three. The identity of the strokes tacitly implies the identity of the bonds, although many occasions arise in which this is undesirable. That a double bond is not two single bonds has been appreciated for many years on chemical grounds, and we now have also a physical picture of the difference between the pairs of electrons in a multiple bond. I n order t o emphasize the difference in chemical reactivity of the u and r electrons in a double bond, the authors have found that a very useful device for teaching purposes is to represent the u electrons by a stroke as usual, but t o indicate the a electrons by a pair of dots placed above the stroke, thus:
--..-.
Students immediately appreciate from this picture that there are two electrons available to take part in all of the addition reactions which occur, but that there is stii a bond present of the same type as simple single bonds, which form the essential skeleton of molecules and which are not ruptured by most normal reagents. This method of representation has an advantage if one wishes to use the curved arrow t o indicate conjugative (tautomeric) effects. C1
C1
C1
N
I helps better than I1 t o emphasize that the curved arrow refers to the r pair of electrons only; I11 and IV show the presence of both conjugative and inductive effects with greater clarity than other methods. The ground is now prepared for a discussion of addition reactions by ionic mechanisms, both electrophilic and nucleophilic, as well as radical additions and polymerizations. To give a concrete example, the familiar question, "Where do the electrons go to?" when the student is faced with the more conventional formulation:
has its answer implicit in the representation:
All of this can be taught without discussing any physical picture of the bonds a t all. However, eveu elementary students have little trouble in appreciating that the u bond has axial symmetry, whereas the electrons in a r bond can be considered t o move in one plane. They can usually take the further step of visualizing the six-electron bond as having one u type bond and two a type electron pairs with their planes perpendicular to one another. This is represented as XGY
with the convention that placing the dots on opposite sides of the stroke indicates that the planes of the orbitals are a t right angles. Extending this, cumulated systems are written and we can introduce the idea that the XY a pair do not interact directly with the YZ T pair since neither has a vector component in the other's plane. On the other hand, conjugated systems are represented WLX-Y-..~
to show that the WX and YZ rr pairs can have their planes parallel, and thereupon interact. Indeed, our experience is that the whole idea of delocalization of electrons is best introduced and treated'in this way. The electronic interactions in molecules such as vinyl chloride are better appreciated when seen as V, where the fact that only one of the three lone pairs on the chlorine can interact with the double-bond a pair is n -.. CHz I ; CH + Cl:
v
-
-..,- C + -.. I: ..
H+C
h/"
VI
readily obvious. I n iodoacetylene (whose dipole moment is practically zero), VI shows nicely that there are two such similar interactions. From the concept of delocalization in conjugated aliphatic compounds, it is not a difficult step to a consideration of benzene and other aromatic systems. We have used the convention, with respect t o orbitals originating on ring atoms, that dots within the ring represent electrons whose orbit,als are perpendicular t o the plane of the ring, and dots outside indicate those
OCTOBER,1954
547
with orbitals parallel to its plane. Aromatic substitution can be very effectively discussed against this background, and greater clarity given to such topics as the basicity of heterocycles, and the reactivity of the phenyl radical, VII. Without graphic assistance, stu-
VII
VIII
dents sometimes have difficultyin perceiving why the reactivity of this particular radical is not affected by resonance in the ring. With it, a comparison of the
phenyl and benzyl (VIII) radicals becomes simple and instructive. Discussion of the conjugation of the ring with external unsaturated systems is also simplified, e. g., the halobenzenes, the base strengths of anilines, and additions to beuzoyl and cinnamoyl groups in ketones, esters, etc. This method of presentation has been developed and tried over several years and found very satisfactory. The examples given here have been kept to a minimum and made fairly general; the instructor who is interested in the electronic approach to teaching organic chemistry can provide many more applications for himself.
