Activation energy hydride transfer reactions

There are substantial differences between gas-phase and liquid-phase hydride transfer reactions. In the latter, the hydride transfer occurs with a low activation energy of 13-17 kJ/mol, and no carbonium ions have been detected as intermediates when secondary or tertiary carbenium ions were present (25). 

The data for the four compounds [83]—[86] show a good linear relationship (correlation coefficient r = 0.995) between the (C-)H" C( = 0) distance and the activation energy for hydride transfer reaction of the alkoxide anion (Fig. 16). Here also there is a simple and strong correlation between geometry and reactivity ground state structures closer to the presumed transition state structure give faster reactions. 

Fig. 16 Relationship between activation energy for the hydride transfer reaction of compounds [83-86] and the distance the hydrogen atom has to travel. The data are taken from Cernik et al. (1982, 1984).

Because the activation energy of the hydride transfer reaction is lower than that of carbonium ion formation, after a short initiation period the hydride transfer reaction will maintain the carbenium ion concentration at a steady-state level. However, secondary reactions occur that give rise to short catalyst lifetimes. 

The hydride transfer reaction was found to have an activation energy of 9.9 Kcal/mole. Transfer from methylcyclopentane occurred much more easily than from methylcyclohexane where an Ea of 21.3 Kcal/mole was obtained. The difference between the two hydride donors can be rationalized by noting that solvolysis of the corresponding 1-chloro-l-methylcycloalkanes favors the cyclopentyl... 

Embedded or periodical calculations are essential to compute these effects properly. A recent study of alkylation of toluene using a DFT periodical electronic structure code demonstrated the feasibility of this approach [131]. We illustrate here how Eq. (13) can be used to compute corrections to the activation energy from the cluster approach by adding the repulsive interaction that results from the formation of bulky intermediates. We use as an example the bimolecular hydride transfer reaction. The activation energy of the corresponding elementary rate constant can then be written... 

Thus, these simple QM-cluster calculations gave deep insight into Nature s design of the three families of mono-nuclear Mo oxo-transfer enzymes. We have found that the Mo ligands are not selected to make the oxo-transfer reactions as favourable as possible, but rather exothermic by a minimal amount of energy. This is important, because the active-site must be re-reduced or re-oxidized after the oxo-transfer reaction and the reaction must also be favourable. The reduction of DMSO is trivial and can be performed by all three families. The oxidation of sulfite is also rather simple, provided that the repulsion between the active site and the substrate can be overcome. However, the hydroxylation of xanthine is much more complicated and seems to require a unique MoOS active site, in which the S ligand makes the formal hydride-transfer reaction possible. 

Most of our general considerations about hydrogen isotope effects should be equally valid for the transfer of hydride ions or hydrogen atoms, and in fact these classes of reaction have provided much of the evidence for tunnel corrections. Some of it will be summarized briefly here, beginning with solution reactions. Many oxidation reactions are believed to involve hydride ion transfer, and large isotope effects have been reported in the oxidation of l-phenyI-2,2,2-trifluoroethanol by alkaline permanganate (fc //c° = 16,/c /fc = 57, both at 25 C). The activation energies for this reaction are not known accurately, but the reported values lead to — = 2.3 kcal mol A /A = 3.0. The chromic acid oxidation... 

A new kinetic equation to estimate activation energies of various hydride transfer reactions has been developed according to transition state theory by using the Morse-type free energy curves of hydride donors and acceptors to model hydride ion release and capture, respectively. " A perfect unity of the kinetic equation and thermodynamic equation for hydride transfer reactions has been achieved. 

The CH5 group, having a carbon atom in five-fold coordination, is called a carbonium ion. Carbonium-ion formation is an easier process in larger hydrocarbons, where the reaction is between the proton and the secondary or tertiary carbon atom, but is significantly more activated in the case of methane. The carbonium ion is the intermediate toward carbenium ions (3.29), where the positively charged carbon atom has a threefold coordination. Once formed, carbenium ions can be converted to hydrocarbons and new carbonium ions, in hydride transfer reactions, as in (3.31). Because of the lower activation energies of such reactions, the route via carbenium ions usually dominates over mechanisms involving carbonium ions. Reactions of the carbonium-ion type are terminated by a step in which a proton is transferred to the zeolite lattice. 

Figure 5-3. Active site and calculated PES properties for the reactions studied, with the transferring hydrogen labelled as Hp (a) hydride transfer in LADH, (b) proton transfer in MADH and (c) hydrogen atom transfer in SLO-1. (i) potential energy, (ii) vibrationally adiabatic potential energy, (iii) RTE at 300K and (iv) total reaction path curvature. Reproduced with permission from reference [81]. Copyright Elsevier 2002...

---