HD exchange

Dr. Anbar In this case one should examine the acid effects of HD exchange on the methylene. Further, in the equilibrium... 

A vital fact not considered fully thus far is that the similar rates of HD-exchange and hydrolysis found for several typical esters (p. 129) prove that the forward and back reactions of any tetrahedral intermediate must proceed at similar rates, and thus that neither its formation nor its breakdown can alone be rate-determining. Both processes are kinetically important, and both transition states must be considered in any detailed discussion of mechanism. Both presumably have the same composition, since there is no sign of any change in kinetic order as the concentration of the reactants are varied, except where there is thought to be an actual change of mechanism. The first step, the formation of the tetrahedral intermediate, has been considered briefly. Since its breakdown is of comparable kinetic importance, this second step must now be considered. 

Figure 8 A schematic energy scheme for the HD exchange of CP. The rate determining step is K3, the dissociation of molecularly adsorbed CP to CsH ), the a) c-bonded ri -cyclopentyl intermediate. From this dissociated CP the other intermediates are formed, for example b) 7t-bonded rj2-cyclopentene. This intermediate can easily rotate with a low activation energy. Eapp is the apparent (or observed) activation energy, Etrae the true activation energy.

Figure 7 A constable plot for the HD exchange. The estimated errors are approximately 2 kJ/mol for Eact,app and 0.5 for In Aapp.

Figure 8 A series of extrapolated Arrhenius plots for the HD exchange of cyclopentane, intersecting at Ti the isokinetic temperature.

Scheme J (A) General scheme of HD-exchange in structure S. The D and H in brackets refers to the deuterated and protonated species, respectively. (B) A simplified folding scheme with unfolded (U), folded (N for native) and one intermediate (I) state with H-D exchange considered. Adapted from Ref. 89.

Hydrogen isotope exchange is facile at the -position in pyridinium salts. 3-Methyl- and 3-cyano-pyridinium methio-dides undergo exchange in the order 2 > 6 >> 4, 5 in 0.01 M NaOD, D20. The relative rates of HD exchange for the -, -, and -positions in 1-methylpyridinium chloride are 3400 3 1. 

The rates of HD exchange at the -positions for a series of N-substituted pyridinium cations and pyridine 1-oxide derivatives in D20 at 75 C (Figure 2) <1970JA7547> correlate well with the Tafts inductive parameter ] ( = 15). A positively charged nitrogen in a ring is estimated to activate the -position toward deprotonation and ylide formation by a factor of 101S. 

Figure 2 Rates of HD exchange at the -positions of pyridinium and related cations.

Upmacis, Poliakoff and Turner(lS) observed HD exchange for mixtures of Cr(CO)4(H2)2 and D2 but interestingly not for mixtures ol Cr(CO)5(H2) and D2. Kubas et al (lx) observe a similar phenomenon where HD is produced from reacting W(CO)3(PR3)2(H2) with D2 either in solution or in the solid state. In addition other cases of H—D scrambling occur readily with metal hydride complexes(U) as in the case of Cp ScH or Cp2ZrH where Cp =... 

Fig. 6 in-situ measurement of the dihydrogen HD-exchange during n-octane... 

Interestingly the Atomic Energy Commission of Canada has been working on the H2O-HD exchange reaction up until recently even though they are operating enormous heavy water plants based on the H2 O-H2 S exchange. 

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