C)OM~IUNIC.~TIONSTo THE
im
EDITOR
1101. i ( i
comes t o equilibrium slowly occupies some surface positions which are either relatively inaccessible or are associated with an appreciable activation energy. DIVISIONOF APPLIEDSCIENCE ~JNIVERSITY HARVARD JASUARY RECEIVED
a-ketol-3-membered hemiketal system and differ only in the orientation of the hydroxyl group of the a-keto1 system as in I and 111. These experimental results (coupled with the results of bismuth oxide oxidation in the cevine seriesIc) thus establish E. VERNON BALLOU a deduction made by Dr. R. B. LVoodward in September, 1953. 11, 1951
ON THE NATURE OF THE GERMINE-ISOGERMINE PSEUDOGERMINE ISOMERIZATIONS
.Yir: Shortly beiore discontinuance of research on the ester alkaloids oi germinc a t the Syuibb Institute, otic oi us J.F. I found that germine acetonide hydroc.hloride (1 consumes one mole oi potassium periodate and affords a crystalline oxidation product which reduces Tollcns reaxent, 111.1). 274 ?;ti3 dcc., +" c l..j4, d c . . Calcd. ior C.+,H;:,03S: i, ti.j.7!); H , \.2\. F o u n d : C , ti>.; 13, \.37. Further characterization of the substance a t Har\.rircl showed it to be a n altlehydo-r-l~icti~ne II . 0 2
0
-+ HO-I TOC 13
0-
'>
6
ox-CIIO
HO- HCOH
DEPARTMEST OF CHEMISTRY S. MORRISKUPCHAV HARVARD USIVERSITY CAMBRIDGE, MASS.,ASD MARYFIESER THESQUIBB ISSTITUTE C . R. SARAYASAY LOUISF.FIESER FOR MEDICAL RESEARCH JOSEF FRIED SEW BRUNSWICK, S.J. RECEIVED FEBRUARY 4, 1951
THE SIGN OF OXIDATION-REDUCTION POTENTIALS
Sir: The choice of sign for oxidation-reduction couples is indeed arbitrary, but i t is not generally recognized where the arbitrary choice is made. If' (1) the measured voltage of a cell is taken as positive and (2) the equation for the spontaneous cell reaction is written with the products on the right; then there is no choice of signs for the "half reaction." For example, consider the spontaneous reaction Zn
I
I1
I11
Thus inirared absorption bands a t :3.(j5 and 5.7!) p are attributable to a n aldehyde group, and a band a t .j.(.j2 p is indicative oi the ylactone iunction. Furtherinore, the substance ior:ns an oxime, n1.p. -.>; % < , i b j S \ I Kupchan D. L : ~ v i e .C. I' Deliwala a n d B . Y Andoh, i b t ! , 76, .;.5l't l ' t . 3 ; c S 11 Kiipchan anit D L a v i e , t b ~ '76, 314 ( 1 9 5 4 , 2 , 1) FI R B i r r o n And C J. \\' HroCk5, C i i w t i l i ? ill! 1 , 1 / ! . j ! r ) 51, I.ICC. I s i S i . \Ye are inde:,ted 11, 1)r U . ~ r r < ~f.ir ii ~,mmii~iicrt'n; thme resiiltr t o ( 1 3 ;>Ti( r :re pii')lich:i(,n ,i 1.. i: < m i d and \V .\ J3cobs. J . GI,/Cht,i,r , 148, 57 1 , 4 2 1
+ 2H+ = Zn++ + H2
E"
=
f0.75
(A)
one must then write H?
=
+ 2e+ 2e-
2H+
and
Eo
=
0.0
(R)
Zn = Z n r f E " = +0.76 (CJ This is necessary since the hydrogen a n d zinc couples must add up t o give the equation and the potential for the completed reaction A. U'e then must also write Zn++
+ 2e-
=
Zn
Ea
=
-0.76
(C,)
because the laws of thermodynamics require that the sign of the free energy of a reaction be reversed if the equation is reversed. The sign conversion of C1and Cs is, however, the sa-ne. There is general agreement throughout the world, with regard to the arbitrary choices (1) and ( , 2 ) . G. N. Lewis in his selection of thermodynamic conventions was therefore co;npelled to choose the signs of his "standard electrode potentials" or "half reactions" consistent with C1 and C,. Unfortunately the older European conventions were Zn
=
Zn++
Zn++
+ 2e-
+ 2e=
Zn
E" E"
= =
-0.76 +0.76
The resulting confusion has been most distressitly. I t was hoped that the Commissions of the Congress of Pure and Xpplied Chemistry meeting in Stockholm ( 1 9.53) had resolved the difficultv but such does not seem to be the case. Thus, J. 0'11.B.' has criticized my use of the Lewis conventions. T o quote, "The author can be parochial in important matters. The sign convention for electrode potentials is presented dogmatically without mention that the one used is not that of the majority and t h a t an International Commission has recently recommended its withdrawal." The facts are that the Commission voted to accept the arbitrary choices ( I ) and ( 2 ) and the (1) J . O'AI. B . , T i a n s . F a v a d n y S o r . , 49, 1.503 (1
