Hemi-formals

Stepwise thermal- or base-eatalysed hydrolytic depolymerisation initiated from the hemi-formal chain end with the evolution of formaldehyde. The main reasons for end-capping and copolymerisation mechanisms described above are carried out in order to minimise this reaction. 

The reaction scheme was studied by carrying out tests at increasing reaction times, as shown in Figure 2. The guaiacol conversion increases as reaction time increases. The reaction scheme consists of parallel reactions for the formation of the three vanillol isomers and of by-products. The latter are mainly constituted of monoaryl products obtained by reaction between vanillols and methanol or hemi-formal. These are consecutive reactions from a chemical point of view, but are parallel ones from a kinetic point of view, since they occur extensively even at low guaiacol conversion therefore they likely occur in the zeolite pores before vanillols diffuse back into the liquid phase. Consecutive reactions mainly involve p-VA, with formation of by-products which consist of diaryl and of monoaryl compounds. 

Preparation of Solvents and Plasticizers. Just as derivatives oi ethylene oidde or its hydration pi oduct, ethylene glycol, have value as soh"ents and plasticizers, derivatives of methylene oxide (formaldehyde) oi its hydrate methylene glycol also have value for uses of this type. These derhutives include the formals and hemi-formal esters of formaldehyde. 

Stoddart and his coworkers have reported syntheses of the trans-syn-trans and the trans-anti-trans isomers of dicyclohexano-18-crown-6 The synthesis of these two compounds from trans-l,2-cyclohexanediol was accomplished in two stages. First, the diols were temporarily linked on one side by formation of the formal, and this was treated with diethylene glycol ditosylate and sodium hydride to form the hemi-crown formal. Removal of the formal protecting group, followed by a second cychzation completed the synthesis. The synthesis of the trans-anti-trans compound is illustrated below m Eq (3 12) and the structures of the five possible stereoisomers are shown as structures 1—5. 

As in the case of a reversible one-electron electrochemical reaction, the halfpeak width of the SWV does not depend on the electrode geometry. For a two-electron electrochemical reaction, Wy2 is only a function of the difference between formal potentials AE f and of the square wave amplitude sw. The evolution of the half-peak width Wy2 with AEcspherical electrodes has been plotted in Fig. 7.32. These curves give a very general criterion for the characterization of the EE process through Wy2. 

The values for parameters in the mass balances have to be determined either theoretically or experimentally. These parameters are equilibrium ratio (Kj), mass transfer coefficients (fcLi and kci), diffusion coefficients in the gas and liquid phases (Du and Da), volume ratios (Uy and Op), and liquid holdup (bl). The equilibrium ratio, Kj, can be estimated using thermodynamic theories [4] for gases with a low solubility, the equilibrium ratio, Ki, can sometimes be replaced by Hemy s constant [5]. In Refs. [6,7], some methods for estimating ku and ka are discussed. The correlation equations for ku and ka contain the diffusion coefficients, D i and Dci, in the gas and liquid phases, respectively. The mass transfer coefficients, ku and ka, are formally related to the diffusion coefficients by the film theory ... 

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