Formic acid, isotope effect

The reaction between permangante ion and neutral formic acid follows similar bimolecular kinetics with k2 = 1.1 x 10 exp(—16.4x 10 /lt7 )l.mole . sec . No primary kinetic isotope effect was found for this path either in light or heavy water. However, Mocek and Stewart have reported that in very strong sulphuric acid the oxidations of neutral substrate by both HMnO and MnOj display substantial isotope effects. 

The oxidations of formic acid by Co(III) and V(V) are straightforward, being first-order with respect to both oxidant and substrate and acid-inverse and slightly acid-catalysed respectively. The primary kinetic isotope effects are l.Sj (25°C)forCo(IU)and4.1 (61.5 C°)for V(V). The low value for Co(lII) is analogous to those for Co(IIl) oxidations of secondary alcohols, formaldehyde and m-nitrobenzaldehyde vide supra). A djo/ h20 for the Co(III) oxidation is about 1.0, which is curiously high for an acid-inverse reaction . The mechanisms clearly parallel those for oxidation of alcohols (p. 376) where Rj and R2 become doubly bonded oxygen. 

Chen YX, Heinen M, Jusys Z, Behm RJ. 2007. Kinetic isotope effects in complex reaction networks Formic acid electro-oxidation. ChemPhysChem 8 380-385. 

An interesting catalytic ruthenium system, Ru(7/5-C5Ar4OH)(CO)2H based on substituted cyclopentadienyl ligands was discovered by Shvo and coworkers [95— 98]. This operates in a similar fashion to the Noyori system of Scheme 3.12, but transfers hydride from the ruthenium and proton from the hydroxyl group on the ring in an outer-sphere hydrogenation mechanism. The source of hydrogen can be H2 or formic acid. Casey and coworkers have recently shown, on the basis of kinetic isotope effects, that the transfer of H+ and TT equivalents to the ketone for the Shvo system and the Noyori system (Scheme 3.12) is a concerted process [99, 100]. 

Methanol still proceeds through an initial C H bond scission, but reacts with water before the OH bond breaks. Alternatively, formaldehyde formation likely occurs along the same pathway as CO formation. This is true if HCO is an intermediate in the decomposition pathway. Furthermore, the lack of a kinetic isotope effect for CH3OD indicates that formaldehyde is not the product of an initial O-H scission.94 Because formaldehyde and formic acid are not the thermodynamically favored products of methanol oxidation, they must be the result of kinetic limitations preventing the full oxidation to C02, analogous to the production of H202 for the reduction of oxygen (see next section). 

A primary kinetic isotope effect (kn/ko = 6.03 at 298 K) was observed for the oxidation of formic and oxalic acids by benzyltrimethylammonium tribromide (BTMAB) to carbon dioxide. The kinetics of oxidation of pyridoxine to pyridoxal by broma-mine-T and bromamine-B ° and caffeine by bromamine-B have been investigated. 

Isotope effect studies of the mechanism of hydration of alkynes with formic acid as water donor485b,485c leading to ketones (equation 234a) have been undertaken recently4854 by observing the kinetic isotope fractionation of 13C in the course of carbon monoxide... 

Similar reaction pathways have also been found for the oxidation of dimethyl sulfide to dimethyl sulfoxide and dimethyl sulfoxide to dimethyl sulfone by [Ru(bpy)2(py)(0)]2+ with respective rate constants of 17.1 and 0.13 M l s"1 in MeCN at 298 K (48). The complex [Ru(bpy)2 (py)(0)]2+ has also been used electrocatalytically for the oxidation of alcohols, aldehydes, alkenes, and aromatics (23, 49). The kinetics of oxidation of formic acid/formate ion by [Ru(bpy)2(py)(0)]2 +, with a large kinetic isotope effect [ HC02-/ADCo2- = 19 (25°C, /r = 1.0 M)], has been reported (50). A two-electron hydride transfer has been suggested for the oxidation of HC02 by [Ru(bpy)2(py)(0)]2+. A similar mechanism has also been suggested for the oxidation of alcohols (51) and aromatics (52) by [Ru(bpy)2(py)(0)]2+ and other related Ru(IV) oxo complexes (28,... 

The hydrolysis rates of 1,3-dioxolanes are decreased by substitution of four methyl groups at the positions 4 and 5. 2,4,4,5,5-Pentamethyl-l,3-dioxolane reacts 6.5 times more slowly than 2-methy 1-1,3-dioxolane [161]. The AS value is decreased by 9 eu. A Hammett p value of—2.0 has been found for the substituent effect on the hydrolysis of 2-phenyl-4,4,5,5-tetramethyl-l,3-dioxolane in dilute aqueous hydrochloric acid [167]. The data obey the simple Hammett equation, and it is not necessary to apply o+ values. For the reaction of the unsubstituted 2-phenyl-4,4,5,5-tetramethyl compound, the solvent isotope effect is kH/kD = 0.42, and AS is —14.2 eu. General catalysis by formic acid has been observed in the hydrolysis of the p-methoxy compound. However, the rate is not significantly increased by addition of strong nucleophiles. 

Only a few publications have been devoted to the kinetic isotope effects in the decomposition of formic acid, and only the metals Ag and Au were investigated. The most extensive work was published by Block and Krai (21), who measured the rate of the decomposition of formic acid over silver wires between 190 and 250°C, using the isotopic compositions HCOOH, HCOOD, DCOOH, and DCOOD. The kinetic isotope effect a, which is defined as the number of times the reaction... 

Block and Krai studied the isotope effect in the zero-order pressure region as a function of temperature, using partially deuterated formic acid molecules. The results are summarized in Table III. 

Gel bshtein, Shcheglova, and Temkin (102) confirmed the results of Hammett, also for the decomposition in phosphoric acid. The nature of the solute has an appreciable influence, however at the same value of H0, the reaction is much faster in phosphoric acid than in sulfuric acid. They concluded that reaction (a) is in equilibrium (as already assumed by Hammett), and that one of the decomposition reactions (b), (c), or (d) is rate-determining. To decide between these alternative possibilities, Ropp el al. (104, 105) and Biegeleisen el al. (106) studied the decomposition of labeled formic acid compounds in concentrated sulfuric acid. From their observations that the 13C- and 14C-isotope effects are rather high (viz., 7 and 9% respectively), and that of H-D is low, they concluded that a carbon-oxygen bond, and not a carbon-hydrogen bond, is broken in the rate-determining step. This means that reaction (b) is the slowest. 

R.P. Bell and J.E. Crooks, Secondary hydrogen isotope effect in the dissociation constant of formic acid, Trans. Faraday Soc. 58 (1962), pp. 1409-1411. 

---