Isotopie- und Tunneleffekte bei der Keto-Enol-Tautomerie von 5,8-Dimethyl-1-tetralon in unpolaren Lösungsmitteln (English shop)

Preview

Table of Contents, Datei (25 KB)
Extract, Datei (310 KB)

Previous investigations of the kinetics of the light-induced keto–enol rearrangement of 5,8-dimethyl-1-tetralone (DMT) were carried out predominantly in polar solvents. The results show that the rate coefficients of the photo-enolisation proceeding in the triplet state become almost temperature-independent above about 150 K and exhibit a drastic isotope effect when the hydrogen atom migrating during the keto–enol rearrangement is replaced by deuterium. This transfer reaction is one of the first examples of a hydrogen transfer dominated by tunnelling processes in an electronically excited state. The reketonisation, i.e. the back-transfer of the H atom, proceeds in the singlet ground state. Here, too, the kinetics of the reaction are decisively determined by tunnelling effects. However, their investigation in protic solvents is difficult because the enol lifetimes at low temperatures rise to 10^4 s (H migration) up to 10^6 s (~ 11.6 days) (D migration).

Initial exploratory measurements had shown that reketonisation proceeds orders of magnitude faster in non-polar media than in polar ones and should therefore be more easily measurable at low temperatures in a non-polar solvent. The aim of this work was to assign the transients occurring after laser excitation of 5,8-dimethyl-1-tetralone in non-polar solvents and to determine the rate constants of the individual decays. In particular, it was to be examined whether the tunnelling effects measured in polar media also occur in non-polar solvents and to what extent the tunnelling rates change as a function of polarity.

Through a systematic comparison of the formation and decay curves as a function of the measurement wavelength and the temperature, it was possible to assign the observed transient absorption signals to particular intermediate states and to establish a consistent reaction scheme that is in agreement with the previous measurements on this system. The measurements demonstrate that both enolisation and reketonisation become temperature-independent at low temperatures and show a pronounced isotope effect. This clearly shows that in non-polar media at low temperatures, too, the photoenolisation in the excited state of 5,8-dimethyl-1-tetralone and the back-transfer of the hydrogen in the ground state are dominated by tunnelling processes. The reaction rates of the individual processes are thereby substantially greater than in polar media.

In the case of the deuterated derivative (DMTD), a viscosity- and concentration-dependent competing reaction occurred at low temperatures which, in a temperature range varying according to the solvent viscosity, was superimposed on the undisturbed reketonisation. The reason for this is the formation of a hydrogen-bonded complex between one DMTD molecule in the enol form and one in the keto form. As a result, the reketonisation reaction is considerably delayed, because the hydrogen bond of the complex must first be broken in order for the hydrogen atom to be transferred back.

Presumably similar hydrogen-bonded complexes are also responsible for the slow reketonisation in ether and EPA as solvents: experiments in 2,3-dimethylpentane and 3-methylpentane with the addition of 10^-4 to 10^-2 mol/l diethyl ether show that even at these low ether concentrations the reketonisation is slowed down in the same way as in pure ether or EPA solutions. It follows from this that the dielectric constant of the solvent can play only a subordinate role in the reduction of the reaction rate of the reketonisation.