Hydrogen uptake

Transition metal hydrides. These are formed by hydrogen uptake by the metal. The phases are often non-stoicheiometric. 

In one isolated case the checkers found that no hydrogen uptake... 

The acetoxy dienone (218) gives phenol (220). Here, an alternative primary photoreaction competes effectively with the dienone 1,5-bonding expulsion of the lOjS-acetoxy substituent and hydrogen uptake from the solvent (dioxane). In the case of the hydroxy analog (219) the two paths are balanced and products from both processes, phenol (220) and diketone (222), are isolated. In the formation of the spiro compound (222) rupture of the 1,10-bond in the dipolar intermediate (221) predominates over the normal electron transmission in aprotic solvents from the enolate moiety via the three-membered ring to the electron-deficient carbon. While in protic solvents and in 10-methyl compounds this process is inhibited by the protonation of the enolate system in the dipolar intermediate [cf. (202), (203)], proton elimination from the tertiary hydroxy group in (221) could reverse the efficiencies of the two oxygens as electron sources. 

The rate of reaction is variable requiring from 1-4 days. Fresh catalyst is added whenever the rate of hydrogen uptake markedly decreases. Added catalyst must first be wet with solvent. The hydrogen must be well evacuated, for opening the mixture to the atmosphere without such evacuation will produce a mixture that may explode on contact with fresh catalyst, t A eutectic mixture of diphenyl and diphenyl ether, available from Dow Chemical Co. 

However, this behavior cannot be depended on. It is safer to monitor hydrogen uptake. Reduction of 4-oxo-slowed down at the ethylene stage although in other compounds, the same catalyst did give spontaneously selective reductions. 

Taqi, E. A. and Cottis, R. A., The influence of crevice chemistry on hydrogen uptake by structural steels , in Turnbull, A. (ed.). Corrosion Chemistry within Pits, Crevices and Cracks, National Physical Laboratory, London (1984)... 

Although laboratory tests (NACE TMO 169-76, and Reference 313) are obviously of value in selecting materials they cannot simulate conditions that occur in practice, and although an initial sorting may be made on the basis of these tests ultimate selection must be based on tests in the plant. This is particularly important where the process streams may contain small concentrations of unknown corrosive species whose influence cannot be assessed by laboratory trials. Testing is also important for monitoring various phenomena such as embrittlement, hydrogen uptake, corrosion rates, etc. which are considered in Section 19.3. 

When the desired hydrogen uptake had been achieved, the vessel was opened, catalyst separated by filtration, and the reaction solution analysed by chiral gas chromatography (column Cydex B, 50 m, SGE Ltd). Analysis gave conversion and enantiomeric excess Enantiomeric excess is defined as IR - SI /(R+S). 

The reaction of ADPA with acetone led to a solid product, presumably 4-(isopropylamino)diphenylamine (mp 72-76°C), with the hydrogen uptake leveling at 56 minutes. Since the product could not be analyzed by GC, the conversion was calculated from the amount of hydrogen consumed during the reaction. 

Figure 20.5. Rates of hydrogen uptake in the hydrogenation of 1-hexene at 36°C and 50 psig of hydrogen over different catalysts.

Semi-batch hydrogenation involves feeding the nitrile to an autoclave, containing a slurry of catalyst in the reaction product, for a specified time period after which time the nitrile feed is stopped. After the nitrile feed is stopped, the reaction will continue for a short period of time while the residual unreacted nitrile is consumed. Close monitoring of the hydrogen uptake during the time period after the nitrile feed is stopped provides insight into the rate at which... 

ADN, 19.4% CL, 1.3% ACAM, 2.2% CVAM and 30.9% others and was hydrogenated with Raney Co 2724 under identical conditions to the above. The reaction showed an initial hydrogen uptake rate of 7.8 psi (53.8 kPa)/minute. After 240 minutes, the reaction had consumed 525 psig (3.72 MPa) a sample was removed from the reactor for analysis. It comprised 39% HMD, 18% CL, and by-products. The reaction showed no evidence of catalyst deactivation. While the rate of hydrogen consumption was detectably larger in this experiment than in the Raney Co experiment with C02 and NH3, the differences are not sufficiently large to infer a mechanistic difference. 

This should be compared with the hydrogen uptake curve for the corresponding 10,000 TON homogeneously catalyzed reactions shown in Figure... 

Figure 1 Hydrogen uptake curves for 1-hexene hydrogenations run at 35°C and 50 psig of hydrogen in 10% toluene/EtOH with a stirring rate of 1700 rpm. a) AHC-Wilk catalyst b) Homogeneous Wilkinson s catalyst.

Table 1 Effect of stirring rate on the initial rate of hydrogen uptake (mmoles H2/mmole Rh/min) in the hydrogenation of 1-hexeneover AHC-Wilk.

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