Oxidation Numbers and Their Limitations A. A. WwU Bristol Polytechnic, Coldharbour Lane, Bristol, UK
+
RNH,+X-
+ HNO,
-
RN,+X-
-
+ C(-2) 2'30) C(2) The ON approach could provide a guide through the plethora of new multi-element reagents introduced in organic synthesis. However Kauffman's method is not completely foolproof! Ambiguities arise when adjacent atoms have small or zero electronegativity (EN) differences. This is shown clearly in the isoelectronic series NO2+,Con, N20, and N3-. Kauffman implies that any Lewis structure will lead to the same ON assignment. This is only true for NOzf and COZFive linear structures can be written including two mirror-image pairs 2 and 3 for the above centro-symmetric molecules. Suitable bonding patterns are 1 = = 2 - = and? - E with the remaining electrons in lone pairs. For ON0 and OCO all structures give the same set of ON'S, viz. (-2,5, -2) and (-2, 4, -21, respectively. However, for NNO, forms 1 and 2 give (-1, 3, -2) but 3 gives (-2, 4, -2) or (0, 2, -2) if the bond pattern is reversed. In the azide ion each form gives a differentpattern (-1,1, -1); (-2,2, -1) and (-2,1,0) for 1,2,and 3, respectively. (Other possible forms can be rejected mainly on electrostatic grounds, for example, those with the same charge on adjacent atoms (2) or with excessive charge separation or with positive charges on the electronegative atoms). This ambiguity may not be of much interest for NzO and N3-, but another 16e molecule, CHzNz is of synthetic use. Diazomethane is a mild, efficient methylating agent, and we need to write mechanisms with an appropriate resonance form. Pauling picked resonance structures by calculating the expected bond lengths (2). The structures so chosen are appropriate for the molecule in the ground state, but it does not follow that such a choice is appropriate for the reactive form in a mechanism. The following analysis illustrates how a choice can be made between oxidation state patterns after introducing part of the "chemical reality" mentioned above. In methylation reactions dinitrogen is eliminated and hence we require forms with the N's in a zero state, as in diazonium salts, or equidistant from zero. This restricts the choice to 2a or 2b or 3b (Table 1). The first can be rejected because the terminal N is positive with respect to the other atoms. Hence one can ~ r e d i cthat t the reactive form will be a
+ 2H20
(We have modified Kauffman's "exploded" structures to make them typographically clearer. Equal sharing of the electron pair is denoted by and the transfer of the pair to right or left by (- or -), respectively). Reaction of diazo compounds that release dinitrogen, such as those of Sandmeyer, Gattermann, and Balz-Schiemann, are comprehensible. Comparisons can be made with similar inorganic "neutralization" reactions involving nitrogen in two oxidation states.
+
NzO + 2H,O NH,NO, N(-3) + N(5) 2N(1) NO+ N,N, + N*O N(3) + N(-1) 2N(O) + 2N(1) N3- 2H+ N, N,O H,O NO,N(3) + N(-1) 2N(O) ZN(1)
+
-
+
+
-
+
This formation reaction is seen as a redox reaction of nitrogen
-+ + -
-
H(1) 2H(0)]; nitrogen trichloride or trifluoro[H(-1) methyl hypochlorite yield dichlorine with hydrogen chloride [Cl(+l) C1(-1) 2C1(0)]. In organic chemistry the Wittig reaction is also revealed as a neutralization on carbon, the phosphorus remaining as P(5).
The assignment of integral oxidation numbers (ON) to partially charged atoms in compounds, by making them formally fully charged in ionic structures, is a useful teaching device provided that i t is not treated as a purely numerical exercise, but, rather, that contact is maintained with chemical reality. Thus i t is applied almost universally to halance inorganic redox reactions because, in spite of the artificiality of the assigned charges, there is self-compensation between the opposite charges. (A similar self-compensation, in this instance between charge and radius, allows lattice energies of incompletely ionic structures to be calculated). Kauffman's simple method for determining ON in covalent compounds ( I ) enables one to extend this balancing to mixed organic-inorganic or purely organic systems and the student to view some reactions in a new light. On the numerical side i t becomes as easy to balance the permanganate-oxalic acid reaction, for example, by ON as by the classical method of oxygen halance. On the more qualitative side, polarities may be assessed, reactions interpreted and products predicted provided that the full charge separation is an exaggerated, but true, reflection of the partial charges in actual molecules. The chemistry of nitrogen is a particularly rich field for examples with its eight possible ON'S. Consider the diazotization reaction,
+
Similar "neutralizations" occur with other elements. Hydroborates or aluminates release dihydrogen with acids
Table 1. Oxldatlon Numbers for the Resonance Forms of Dlazomethanea 1
2b
2a
e e
e e LN=N>-i=!
/
0
-1
-1
-2
-1
+1
e @ e \c- N =B / -2
1
-1
e
3a
3b
e
\c-N=@ 0' -1 -1
e e )-N=N
-2
0
0
Volume 65 Number 1 January 1988
45
combination of 2b and 3b with the least-charge-separated form predominant. This is confirmed experimentally. When NMR spectra of diazomethane in fluorosulfuric acid are examined at -120 OC, C protonation is four times faster than N protonation (3).Above -120 OC these cations pass over to MeS03F. This implies that form 3b predominates in the methylation reaction. Another interesting isoelectronic series is provided by the 26e systems in the acids H,X03. These may he written in hydridic or hydroxide forms (X-H or X-OH). I t can he seen from Table 2 that the ON of sulfur in sulfurous acid, or the hydrogen sulfite anion, has the same value in hoth forms unlike the phosphorus and chlorine acids. This implies that only the sulfur acid is likely to exist in hoth forms as has been confirmed recently using OI7NMR data (4). For stability the hydridic form requires H to he more electronegative than X. Bond polarities may also he predicted incorrectly if the ON method is applied without modification. Consider a simple comparison of trifluoro and methyl iodides. Both compounds have a homolytic chemistry. However, with polar reactants the chemistry indicates positive iodine in the former and negative in the latter. The organic products of hydrolysis with alcoholic potash are CF3H and CHaOH, respectively. The Kauffman assignment does not distinguish between the two types of iodine. (Equal sharing is assumed in the C-I bonds because of the close proximity of electronegativity values. The ON of iodine would remain the same in bothcompounds even if iodine were the more electronegative.) F H(
1
~--)c+fl F
\ H(/
H(--C+II
/' 3
-3
0
-
0
A similar situation exists with the cyanogen halides. Here the solution chemistry shows positive iodine in ICN and negative chlorine in NCCl (5). Thus on alkaline hydrolysis the former gives iodate and cyanide, the latter cyanate and chloride. There is no indication of positive iodine in Kauffman's formulation Il+C(=NI) IN-)C(-c_y 0
3
3
-3
4 -1
These difficulties can he avoided if the next nearest neighbors to the bonding atoms are taken into account. I t is generally agreed that EN values of elements in compounds are variable from compound to compound hut there is no agreed method for assessing the variability. Sanderson's approach is the simplest conceptually and is suitable for elementary teaching. He treats E N as an atomic property that changes as atoms combine; the more electronegative atoms capture electron density and decrease in EN and vice versa (5). Eventually all atoms in a molecule reach a mean EN value. With a group of n atoms attached to a bond, a better
Table 2.
Oxldatlon Numlmrs In HXO. Molecules Oxldatlon Number of Xd Wride Hydroxy form form
X
n
P S CI
3 2
8
3
1
@
@
5
r3
representation of bond polarity is given by the nth root of the ~ r o d u cof t EN values rather than a single atom value. he calculated group EN'S of CF3 = 3.46, CN = 2.69, and Me = 2.35 using Sanderson's scale would give I(l), I(1) and I(-1) in the respective iodides in agreement with the experimental polarities. More complex calculations of groups' E N values seem unnecessary in view of the small differences to the ahove values. Bratsch illustrates this in a comparative table (7). Finally another limitation arises when using electronegativity values because of the nature of their derivation and use. Usually they are derived from atomic or elementalproperties, and in compounds the electron movements are considered to he only inductive ones. However, with some elements, groups, or ligands, hack-bonding and repulsion effects can predominate as can externally caused polarization with some molecules. Thus predictions from EN'S and oxidation numher can he reversed. The classic examples are in fluorine chemistry. The most electronegative element would he expected to show the most negative polarity, yet RF? is the weakest Lewis acid of the boron halides; compounds such as PhaAsF. are less ionized than the other dihalides as shown by iblutron conductivity or NQH (8); fluorobenzene shows activated elertrophilic I , and p substitution and pfluorophenol is less acidic than phenol. In oreanometallicchemistw a truncated form of Ox's is in use because of the ahove effkcts. Thus we accept Os(8) in 0 ~ 0 4 hut , we prefer Os(0) in Os(CO)b even though CO is more electronegative than 0 s and should take the metal's valency electrons if we follow the usual assimment. Then certain ligands are considered neutral and others charged in order to maintain the noble gas rule where possible. One can view this situation as a comDetition between electroneeativity and electroneutrality pknciples applied in a rat&r ad hoc manner. Literature Clted
6. ~ s n d e r a mR , . J. Ihorgenic ~h&i&'~heinhold:
7. B2atsch.G.S. J . ChemEduc. 1985,62.101. 8. Blil1,T.B. J.Chsm.Edur 1981,58,519.
Near Ynk, 1961
1988 James Flack Norris Award Nominations Sought Nominations are being received for the 1988James Flack Norris Award for OutstandingAchievement in the Teaching of Chemistry. The annual award, consisting of an appropriate scroll and honorarium, is presented by the ACS Northeastern Section. Nominees must have served with distinction as teachers of chemistry in a university, college or secondary school. Nominating materials should include a curriculum vitae and a listing of honors, awards, and publications, especially those germane to the Norris Award, a nominating letter, and as many seconding letters as are necessary to convey the nominee's qualifications for this award. These letters should outline, in narrative form, the reasons why the candidate is deserving of the Norris Award. They could include comments on the impact of the nominee's teaching at the high school or college Level, the influence pf the nominee's textbooks, articles or activities on chemical education at the national or international level, and the success of the nominee in inspiring students to spend their professional lives in chemistry. Materials should he of the usual 8% X 11size but should not include reprints of published materials. Nominating materials should he sent before April 15 to Emily P. Dudek, Department of Chemistry, Brandeis University, Waltham MA 02254. 46
Journal of Chemical Education
