W. J. le Noble and Yu-Sen Chang State University of New York Stony Brook, New York 11790 Reactive intermediates are freouentlv to " nostulated . occur in chemical reactions. Almost all chemists interested in the details of chemical conversions have centered their attention on these unstable ions, radicals, carhenes, excited states and so on, and indeed, until the recent advent of the Woodward-Hoffmann rules, concerted reactions were somewhat contemptuously referred out as the "no-mechanism reactions". Even now, virtually every issue of the journals catering to organic and inorganic chemists, kineticists and so forth, contains many papers describing the most complex and ingenious methods for detecting those species. It happens relatively rarely, however, that a transient intermediate can be directly, i.e., visually, observed. The reasons for the rarity of this event are several. First of all, the intermediate must have a visible spectrum different from that of any reactant or product present in the mixture; preferably, the intermediate should be colored of a t least 10 somewhere in the visible rewith an ,.me gion, with all other species involved in the reaction colorless. Secondly, the overall reaction should he at least himolecular, so that the reaction can he initiated at will by mixing the reagents. Thirdly, we will wish to restrict our candidates to those reactions that can he carried out under reasonable conditions-preferably a t room temperature and pressure, and in a non-exotic solvent. There are additional reasons perhaps best explained by means of the following scheme. Suppose that R1 and Rz are the ingredients which upon mixing give the reactant R, which then via two consecutive steps gives the product P; I is the intermediate which is produced in the first step.
state value in which i t will not he directly observable even if its extinction coefficient em., is reasonably high (see the figure). Our last condition then is that k l > 0.1 kz. The equations governing this situation (eqn. (1)-(3)) have been given by Wiherg.'
I t may furthermore be noted that when (3) reduces to '[II,,,
=
= rp = 0, eqn.
c,
In h, - In h , k,-h,
(4)
For the case that now also kz = kl '[II,.,
=
ilk,
[I],,
=
[Ride
(6)
2/k2
(7)
tinr>.=
(5)
The reaction of chloroacetylhydrazide hydrochloride with concentrated aqueous base furnishes us with an example of this kind.
Both the initial and final solutions are colorless; however, a distinctly yellow color develops as the reaction proceeds. At a concentration of about 1 M base and a t room temperature both the formation and decay of the yellow intermediate have a half-life of a few tenths of a second. Thus, the intermediate, visible for only a fleeting- instant, is truly transient. The pathway of the reaction is probably as follows.
T o deserve a description as transient, the intermediate must have a short half-life; if I is visible for a period of minutes, we would begin to regard it as CI 0 an unstable product. Let OHus somewhat arbitrarily 'cH,-c // f H,O 'N-NH, say that I should have a half-life of no more than H/ about one second; i.e., k2 must he less than 1 sec-'. On the other hand. ~0 the decomposition of I C H ~ C \ ~ ~ must not he too rapid N, + NsH, either. If its half-life is less than a milli-second or so, HN=NH we probably would not he able to see it even if it were initially generated at a fairly high concentration. We conclude that as a fourth condition, kz must be s e c l . Finally, k l must bear some relation to ka; it is intuitively obvious that if kl
