2ND DERIVATIVE UV-VIS ABSORPTION-SPECTROSCOPY - A SUCCESSFUL METHOD TO FOLLOW KINETICS OF ORGANIC-REACTIONS, WHEN 2 BANDS ARE STRONGLY OVERLAPPED IN THE FUNDAMENTAL SPECTRA

The objetc of this paper is to find through this technique an effectiv e and practical tool to follow kinetics of some special organic reacti ons. in fact, when bands of reactants and reaction products are overla pped in the fundamental UV-VIS absorption spectra along the reaction l ime kinetics cannot be followed by the classic UV method. In many case s, the 2nd. derivative UV-VIS spectrophotometry (D-2) provides ano the r unusual alternative method to solve the problem. Even-order derivati ves are suitable to follow kinetics because the maxima in the UV-VIS d erivative spectrum can be associated with the minima and a low-noise o n-line spectra is obtained which can be discarded up to the 6th order derivative and even up to the 10th order with the newly developed comp uters. On the other hand, the first derivative does not provide tha ab ove association; and other higher odd-order derivatives are less preci se, though in practice it has proved valuable to work with spectra of he 3rd an 5th order. Here we present some examples of heterocyclic rea ctions that could not be kinetically studied by the classical UV metho d or were opportunely followed by other kinetic procedures due to the strong superposition of bands at he selected wavelength, along the lim e, in the fundamental spectra. Now we monitored the same kinetics appl ying to the D-2 technique and comparatively good results were attained . The method ''peak-to-zero'' was used for calculations. Values of k(o bs) achieved by the 2nd derivative were compared with those attained by other kinetic procedures (k(obs)') and close rate constants were ob tained. It is concluded that, when the D-2 method can be applied relia ble results are achieved because they become from clear derivative cur ves arising from a high resolution of the overlapped analytical bands.