letters Oxidation Number for Fluorine

T o the Editor: The Note entitled, "Fluorine with a Positive Oxida-

tion Number" [J. CHEM. EDUC., 48,625 (1971)], makes what, appears to be a misleading suggestion. The subject involves what formal oxidation numbers should be assigned to the elements in hypofluorous acid, HOF. Those familiar with the concept of oxidation numbers realize that it is nothing more significant than a scheme used principally for the purpose of balancing redox equations. Its rather arbitrary rules lead to somewhat of a dilema in the case of HOF. The two possibilities brought forth in the Note are (1) assign hydrogen +1, oxygen -2, and fluorine +1 or (2) assign H +1, oxy- gen 0, and fluorine -1. On the grounds that HOF is a powerful oxidizing agent, the conclusion reached in the Note is that (1) should be favored. Clearly neither of these assignments is likely to be an accurate description of HOF, but it should be pointed out that strong oxidizing power could be associated with either form. There are a substantial number of strong oxidiz- ing agents with fluorine in negative formal oxidation states such as C1F3 and AgF2, so the value of this crite- rion is doubtful. Consideration of the relative electro- negativities of the elements involved would lead me

to favor (2) and, at the same time, remind niy students of the artificial nature of the concept in cases like this. Such an approach would seem to be substantially more justifiable than implying in any way that fluorine might actually bein a positive oxidation state.

T o the Editor: The existence of the molecule HOF does not require

one to assign fluorine a positive oxidation state as sug- gested by Stonestreet [J. CHEII. EDUC., 48,625 (1971)l. The oxidizing power of HOF is readily understandable if OH is treated as a pseudo-halogen group isoelectronic with fluorine. There is much evidence for this view- point: for example, hydrogen peroxide, isoelectronic with the fluorine molecule, has a similar long central bond and low dissociation energy; water and liquid hydrogen fluoride have analogous properties. The problem of assigning the direction of the dipole in the HO-F bond, and hence by convention an integral oxidation number, can be treated in general chemistry teaching by consideration of electronegativity differ- ences between the groups or atoms concerned.

Consider the cyanogen halides. The electronega- tivity of the cyano group can be taken as the geometric

Volume 49, Number 4 , April 1972 / 299

mean of C and N values yielding a series of increasing electronegativities: (I, 2.50; CN, 2.69; Br, 2.96; C1, 3.28; F, 3.92) on Sanderson's scale. Thus the cyano group is negative with respect to iodine and positive with respect to other halogens. Experimentally this makes sense because in heterolytic processes cyanogen iodide behaves as a cyanide whereas cyanogen chloride acts as a chloride. (The reactions of cyanogen bromide are less clear-cut. The polarity can swing either way depending on solvent interactions.) Returning to HOF the mean electronegativity of OH (2.89) will lead to a similar sequence with respect to halogens in which fluorine and chlorine are clearly negative, iodine is positive, and bromine almost neutral. Thus the polarity of "hypofluorous acid" is likely to be

6+ 6- HO-F

comparable with chlorine and bromine monofluorides the reactions of which show that fluorine is at the nega- tive end of the dipole.

It is the hydroxy group which should be considered to change polarity along the halogen series rather than fluorine, in the same way as polarity reverses for the cyano group.

To the Editor: The note, "Fluorine with a Positive Oxidation Num-

ber" [J. CHEX. EDUC., 48, 625 (1971)], is an excellent example of fallacious reasoning by analogy. Against the cogent argument of the relative electronegativities of oxygen and fluorine, Dr. Stonestreet concludes the fluorine cannot have the oxidation number -1 and oxygen 0 because HOF has much more "oxidizing ~ower" than any other substance with F = -1 or oxygen = 0. Assuming Dr. Stonestreet means re- activity, rather than number of oxidation equivalents (which remains the same regardless of oxidation num- ber assignments), one needs only to cite the difference in oxidizing reactivity between nitric acid and nitrate ion, in both of which N = +5 and O = -2, to de- molish the argument. Oxidation number does not determine oxidizing reactivity. All that is needed for two substances of equivalent oxidation state to differ widely in reactivity is a significant difference in available reaction pathways with different activation energies. Furthermore, oxidation number in no way describes bond energies, which depend on more subtle considerations. One may well expect HOF to be be- tween HOOH and Fz in reactivity, both of which sub- stances are more reactive than O2 as oxidizers, in spite of the several dissimilarities in oxidation numbers, as the bonding orbitals of the 0-0 bond, the F-F bond, and the 0-F bond are very similar in these three sub- stances. Oxidation numbers, which in molecules are arbitrary numbers describing polarity, should make little contribution to the situation.

To the Editor: Professor Smith presents some persuasive arguments

in his criticism of my note. It was never my intent, however, to-as Professor Smith states in his second sentence--eaclude that fluorine "cannot have the oxidation number - 1 and oxygen 0 . . . ." Since oxida- tion number assignments in covalent compounds are arbitrary (as Professor Smith himself points out), I merely gave my opinion as to how and why I shall make this arbitrary assignment. For example, in spite of Professor Smith's statement that oxidation numbers describe polarity, I-along with most chemists -shall continue to assign carbon the oxidation num- her of +2 and oxygen -2 in carbon monoxide, even though the negative end of this slightly polar molecule is the carbon end.

Decimal Time Units Proposed To the Editor:

Advantages of decimal systems of units and cur- rency are clearly recognized today by the scientific community. We still, however, cling to our not so convenient system for the measurement of time. I do not know if a decimal system for the measurement of time has previously been suggested or not; but with this letter I would like to spur discussion on this sub- ject with a view to explore further advantages and dis- advantages in adopting such a system. If found ad- vantageous, the United States can be the first country to adopt it, which will add to her already proven leader- ship in science and technology.

I would like to suggest that

(1) one day consist of 10"new" hours. Thus, one "new" hour = 0.1 day or a deciday (d.d.)

(2) one "new" hour consist of 100 "new" minutes. Thus, one "new" minute = 0.01 "new" hours or a centi-hour (c.h.) = 0.001 day or a milliday (m.d.1.

(3) one"newU minute consist of 100 "new" seconds. T h n ~ , one "new" second = 0.01 "new" minutes = 0.0001 "new" hours = 0.00001 day.

(4) our clock-dials reflect 10 "new" hours (from 0 to lo), with a "new" minute hand and a "new" second hand. This will eliminate use of "A.M." and"^.^." in expressing time.

According to such a system, for example, (a) the speed of light will be expressed as 2.59 X 10lo cm/linew" second; (b ) the speed limits on our highways might be expressed as 250 km/"new" hours or 25 km/d.h. (This is approximately equal to 62 mph as we express it to- day.)

I believe that permanent advantages gained in adopt- ing such a system will outweigh initial confusion and trouble. Of course, the word "new" before the unit can be dropped when the use of the system becomes a matter of routine.

300 / Journol of Chemical Education