Aldehydes hydrogenation kinetics

This zinc metalloenzyme [EC 1.1.1.1 and EC 1.1.1.2] catalyzes the reversible oxidation of a broad spectrum of alcohol substrates and reduction of aldehyde substrates, usually with NAD+ as a coenzyme. The yeast and horse liver enzymes are probably the most extensively characterized oxidoreductases with respect to the reaction mechanism. Only one of two zinc ions is catalytically important, and the general mechanistic properties of the yeast and liver enzymes are similar, but not identical. Alcohol dehydrogenase can be regarded as a model enzyme system for the exploration of hydrogen kinetic isotope effects. 

The kinetics of aldehyde hydrogenation over cobalt and rhodium carbonyl have been examined by Heil and Marko (16, 19) who proposed Mechanism 2. 

Hydroxy ketones can be reduced with aldehydes in a Tishchenko-type reaction, (101) —> (102), using a zirconocene catalyst.165 The reaction provides a stereoselective route to a //-l,3-diols. A labelling experiment with CpZrE and MeCDO indicated that the aldehydic hydrogen ends up on the alcohol carbon. The kinetic isotope effect of ca 1.8 suggests that hydride transfer may be rate limiting. 

Kinetic calculations and DFT studies have been used to identify the operative mechanism in the reaction of chlorine atoms with alkyl aldehydes. In the linear series, abstraction of aldehydic hydrogen is predominant for ethanal and propanal only, with other hydrogens taking over for longer chains. Relative reactivity of l°/2°/3° sites is also identified." ... 

Kinetic investigation of the reaction of cotarnine and a few aromatic aldehydes (iV-methylcotarnine, m-nitrobenzaldehyde) with hydrogen eyanide in anhydrous tetrahydrofuran showed such differences in the kinetic and thermodynamic parameters for cotarnine compared to those for the aldehydes, and also in the effect of catalysts, so that the possibility that cotarnine was reacting in the hypothetical amino-aldehyde form could be completely eliminated. Even if the amino-aldehyde form is present in concentrations under the limit of spectroscopic detection, then it still certainly plays no pfi,rt in the chemical reactions. This is also expected by Kabachnik s conclusions for the reactions of tautomeric systems where the equilibrium is very predominantly on one side. 

The addition of water across carbon-carbon double bonds, a reaction thoroughly investigated by Lucas and Taft, requires strong activation and is catalyzed by hydrogen ions and hydroxyl ions. Addition of water across the 0= =0 bond of aldehydes has also been studied kinetically. Whereas chloral and formaldehyde are largely hydrated (at equilibrium in dilute aqueous solution), acetaldehyde and other... 

It is noteworthy that even a separate treatment of the initial data on branched reactions (1) and (2) (hydrogenation of crotonaldehyde to butyr-aldehyde and to crotyl alcohol) results in practically the same values of the adsorption coefficient of crotonaldehyde (17 and 19 atm-1)- This indicates that the adsorbed form of crotonaldehyde is the same in both reactions. From the kinetic viewpoint it means that the ratio of the initial rates of both branched reactions of crotonaldehyde is constant, as follows from Eq. (31) simplified for the initial rate, and that the selectivity of the formation of butyraldehyde and crotyl alcohol is therefore independent of the initial partial pressure of crotonaldehyde. This may be the consequence of a very similar chemical nature of both reaction branches. 

The synthesis of a-amino acids by reaction of aldehydes or ketones with ammonia and hydrogen cyanide followed by hydrolysis of the resulting a-aminonitrile is called the Strecker synthesis. Enzymatic hydrolysis has been applied to the kinetic resolution of intermediate a-aminonitriles [90,91]. The hydrolysis of (rac)-phenylglycine nitrile... 

However, the pattern is complicated by several factors. The sugar molecules to be hydrogenated mutarotate in aqueous solutions thus coexisting as acyclic aldehydes and ketoses and as cyclic pyranoses and furanoses and reaction kinetics are complicated and involve side reactions, such as isomerization, hydrolysis, and oxidative dehydrogenation reactions. Moreover, catalysts deactivate and external and internal mass transfer limitations interfere with the kinetics, particularly under industrial circumstances. 

All the oxidants convert primary and secondary alcohols to aldehydes and ketones respectively, albeit with a great range of velocities. Co(III) attacks even tertiary alcohols readily but the other oxidants generally require the presence of a hydrogen atom on the hydroxylated carbon atom. Spectroscopic evidence indicates the formation of complexes between oxidant and substrate in some instances and this is supported by the frequence occurrence of Michaelis-Menten kinetics. Carbon-carbon bond fission occurs in certain cases. 

Typical non-enolising aldehydes are formaldehyde and benzaldehyde, which are oxidised by Co(III) Ce(IV) perchlorate and sulphate , and Mn(III) . The main kinetic features and the primary kinetic isotope effects are the same as for the analogous cyclohexanol oxidations (section 4.3.5) and it is highly probable that the same general mechanism operates. kif olko20 for Co(III) oxidation of formaldehyde is 1.81 (ref. 141), a value in agreement with the observed acid-retardation, i.e. not in accordance with abstraction of a hydroxylic hydrogen atom from H2C(OH)2-The V(V) perchlorate oxidations of formaldehyde and chloral hydrate display an unusual rate expression, viz. 

In contrast to kinetic models reported previously in the literature (18,19) where MO was assumed to adsorb at a single site, our preliminary data based on DRIFT results suggest that MO exists as a diadsorbed species with both the carbonyl and olefin groups being coordinated to the catalyst. This diadsorption mode for a-p unsaturated ketones and aldehydes on palladium have been previously suggested based on quantum chemical predictions (20). A two parameter empirical model (equation 4) where - rA refers to the rate of hydrogenation of MO, CA and PH refer to the concentration of MO and the hydrogen partial pressure respectively was developed. This rate expression will be incorporated in our rate-based three-phase non-equilibrium model to predict the yield and selectivity for the production of MIBK from acetone via CD. 

The detailed composition, referring to classes of compounds, is shown for C6 in Figure 9.3 with and without precolumn hydrogenation. In addition to paraffins, there are olefins—mainly with terminal double bond—and small amounts of alcohols (and aldehydes). The low detection limit of gas chromatography (GC) analysis allows precise determination even of minor compounds and provides exhaustive composition data also for use in kinetic modeling. Because of the short sampling duration of ca. 0.1 s,8 time-resolved selectivity data are obtained. 

As normally practiced in a cobalt process, the aldehyde product contains about 10% alcohol, formed by subsequent hydrogenation. Marko (34) reported that the hydrogenation is more sensitive to carbon monoxide partial pressure than is the hydroformylation reaction and, in the region between 32 and 210 atm, is inversely proportional to the square of the partial pressure. The full kinetic expression for alcohol formation is expressed by Eq. (17). 

The kinetics of the ionic hydrogenation of isobutyraldehyde were studied using [CpMo(CO)3H] as the hydride and CF3C02H as the acid [41]. The apparent rate decreases as the reaction proceeds, since the acid is consumed. However, when the acidity is held constant by a buffered solution in the presence of excess metal hydride, the reaction is first-order in acid. The reaction is also first-order in metal hydride concentration. A mechanism consistent with these kinetics results is shown in Scheme 7.8. Pre-equilibrium protonation of the aldehyde is followed by rate-determining hydride transfer. 

A competition between stoichiometric hydrogenation of acetone and acetophenone resulted in hydrogenation of the acetone [42]. Competitions of this type could be influenced by both the basicity of the ketone, as well as by the kinetics of hydride transfer to the protonated ketone. An intramolecular competition between an aliphatic and aromatic ketone resulted in preferential hydrogenation of the aliphatic ketone, with the product shown in Eq. (24) being isolated and fully characterized by spectroscopy and crystallography. Selective ionic hydrogenation of an aldehyde over a ketone was also found with HOTf and [Cp(CO)3WH],... 

Kinetic and mechanistic studies by Casey et al. provided further insight into the mechanistic details of the hydrogenation of ketones and aldehydes, using a more soluble analogue of Shvd s catalyst (with p-tolyl groups instead of two of the Ph groups) [72]. The kinetics of hydrogenation of benzaldehyde by the Ru complex shown in Eq. (43) were first order in aldehyde and first order in the Ru complex the... 

Transfer hydrogenation is a mild and efficient means of reducing aldehydes, and can be advantageous over other reagents such as sodium borohydride. Clearly, the product is a primary alcohol and therefore not chiral, but a chiral center might be alpha to the aldehyde, in which case a resolution can be effected. Indeed, under the appropriate conditions the chiral center can be race-mized and a dynamic kinetic resolution effected [57]. 

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