AN AID TO TEACHING ELECTRONIC THEORY A. A. HUMFFRAY Swinburne Technical College, Melbourne, Australia
A STRONG analogy exists between triad tautomerism
hydrogen (salt formation with metals or metal hyand the various electronic processes known as the droxides), or a t least greater reactivity of the hydrogen electomen'c effect; in fact the latter is usually said to than is shown by a hydrogen not so situated. occur by the "tautomeric mechanism,"' hut the author Examples is unaware of any detailed discussion of the analogy. Thii paper outlines the method used by the author in (1) Acidic hydrogen presenting the ideas of tautomerkm and electronic theory to technical-school students. It utilizes a Nitrous acid generalization which may he described as the "X=Y-Z R mle," and which may be stated thus: If a molecule contains the grouping X=Y-Z, where O=d-0-H X, Y, and Z are elements from Groups IV, V, or VI of Carboxylic acids Phenols the periodic table (Z may also come from Group VII, and X, Y, and Z may he all alike, or different), and if attached to Z there is one of the following: (a) a hydrogen atom (b) an unshared pair of electrons (c) a single unshared electron (d) a positive charge
Barhituric acid
then it may be expected that the "suhstituent" on Z (a, b, c, or d) will have a tendency to move reversibly to X, with bond rearrangement to X-Y=Z, as shown below in detail for each case. CASE (a): X=Y-2-H
R H R O=C-N4=0 Imides (soluble in alkali)
(2) Reactive hydrogen CzH+ I
I CHrC-C-C-OEt II I II
PC&
l
l
o=c-C-C=O
e H-X-Y=Z
I
H Malanic ester
This is the normal case of triad p r o t o t r ~ p y . ~A few examples are given, the X=Y-Z--H grouping being shown in boldface type for clarity. H
7
H
I = CHrC=C-C-OEt I II
0-H O H 0 Acetoacetic ester
0
H
H NHx-C-NH 5 NH*-C=NH II I S-H S Thiourea
6
I
R Nitropt~rsffins(primary and secondary) R-&=O
R
I + R-C-C=O I
I
a-Picoline H
R
I I I I
-R-C-C4=0
I
H H OHR H Aldehydes (aldol condensation)
0
Vitamin C (ascorbic acid)
Another characteristic of this system is acidity of the 1 R ~ ~ r cA. n E., , "Eleotronic Interpretations of Organic Chemistry," John Wiley & Sons, Inc., New York, 1947, p. 62. 2 G n ~ i u r ,H.. Editor, "Organic Chemistry," 2nd ed., John Wiley k Sons. Inc., New York, 1947, p. 1937.
The student is first introduced to the X=Y-Z rule in connection with the greater acidity of acids and phenols compared to alcohols. Thereafter little difficulty is experienced in recognizing the grouping when present in a new compound, and the student readily appreciates the reactivity of the hydrogen atoms in examples (2) above. Cases (b), (c), and (d) constitute the three possible types of electronic rearrangement considered in the usual "valence bond" treatment of resonance, when writing down "canonical forms" of a structure. (The purely hypothetical nature of "canonical forms" is
JOURNAL OF CHEMICAL EDUCATION
636
stressed in this treatment as usual.) These cases are discussed only after the student has become quite familiar with the application of the X=Y-Z rule to tautomerism.
e :-X-Y=Z+
CASE ( 6 ) : X=Y-2:
It must be noted here that X always gains one negative charge and Z always loses one negative charge, while the polarity of Y is unchanged. This is first stated dogmatically, but is easily verified from a consideration of the number of electrons associated with each atom in each of the above structures. (Note: X may be positive originally, or Z may be negative.) Examples
This example shows that no negative charge is possible on the meta carbon atoms. The discussion of orientation in aromatic substitution is one of the most useful applications of the X=Y-Z rule, being more easily grasped by students than the usual "curved arrow" method of indicating electron shifts. It may be noted here that the unshared pair of electrons has a lower reactivity than usual (i. e., a smaller tendency to form coordinate bonds), as is shown by the weaker basic strength of aniline
(1) Positively charged X
and of amides
+ The guanidine ion
e CHrC=NHs
CH-CNH,
II
YE% 4 ~ The acetamidine ion
(2) Negatively charged Z
.. + ..
O=N-0:"
II
- .. + e :o-N=O I
"
9
..
compared to aliphatic amines. CASE (c): x=y-Z.
= .x-Y=Z
The commonest example is provided by the stable aromatic free radicals, e. g., triphenylmethyl
..
The nitro group
o = c o :- e :o-c=o "
I
R
"
I
R
The carboxylate ion
(3) X and Z uncharged. Examples are benzene derivatives such as: CASE ( d ) : X=Y-Z+
in which the atom attached to the ring possesses an unshared pair of electrons. Thus:
A repetition of the process, going around the ring, gives
and, similarly,
= +X-Y=Z
,
In this case X gains one positive charge, and Z loses one positive charge; again this ir, easily vedied by considering the number of electrons associated with each atom in each of the "canonical forms." This case should he preceded by a discussion of the relative electronegativities3 of the elements; this is usually done in connection with the discussion of dipole moments and the inductive effect. (For any period of the periodic table, the electronegativity of an element, i. e., its attraction for electrons, increases with atomic number.) Examples
a
REMICK, A. E., op. eit., pp. 26, 144
DECEMBER, 1993
637
Here the carbon atom attached to the ring possesses no unshared electrons, but is linked to an oxygen atom, which has a greater electronegativity; therefore the carbonyl group is polarized as shown above. If complete polarization of the carbonyl group is assumed to occur (on the demand of the attacking reagent), giving the "structure"
In all instances the application of the X=Y-Z rule is not permitted to negate the "octet rule," e. g., the anilinium ion,
0-"" possesses theX=Y-Z+ rearrange to
H-LCl
$" \
the positive charge on the carbon can then move as postulated above, producing
and, by a repetition of the process, the following:
structure, but obviously cannot
etc.
+
without giving the nitrogen ten shared electrons. It should be noted that tautomerism, as discussed in case (a), can be treated as a combination of case (b) or (d) plus movement of an ion to follow the negative or positive charge, re~pectively.~ The examples given in this paper are illustrative of a method for providing students with a simple entry into electronic theory, based directly on the analogy between it and tautomerism. The author wishes to express his indebtedness to Dr. Francis Lions of Sydney University, who first pointed out the possibilities of the X=Y-Z rule as applied to tautomerism (case (a)).
The other meta directing groups, R-C=O, NO2, CN, etc., can be treated similarly.
COOH,
'BAKER. J. W., "Trtutomerism," D. Van Nostrand Co., New York, 1934.
