Alcohol-producing condensation

High-resolution Si NMR spectroscopy was used to study the hydrolysis and condensation kinetics of monomeric and dimeric species in the silicate sol-gel system. Peak assignments for the kinetics experiments were determined by comparing add-catalyzed reaction solutions prepared with limited amounts of water with the synthetically prepared dimeric, trimeric, and tetrameric species. Si NMR peaks were assigned for 5 of the 10 possible dimeric species. The temporal evolution of hydrolysis and condensation products has been compared with a kinetic model developed in our laboratory, and rate constants have been determined. The results indicate that the water-producing condensation of dimeric species is approximately 5 times slower than the water-producing condensation of the monomeric species. The alcohol-producing condensation of dimeric species is comparable with that of monomeric species. 

Analysis of Condensation Kinetics. An earlier publication (8) explained how, under certain limiting conditions, the observed rate constants for water- and alcohol-producing condensation reactions in the TMOS sol-gel reactions can be calculated. The limiting conditions are (1) hydrolysis occurs on a time scale short compared with condensation and (2) the concentration of Si-O-Si bonds formed by condensation reactions is small compared with the initial concentration of the methoxy functional group. If these conditions... 

The results of this analysis applied to the condensation of dimer are shown in Figure 3. The observed rate constants for alcohol-producing condensation are approximately the same (fc a 0.001 and 0.0007 L/mol-min for monomer and dimer, respectively), whereas the rate constants for water-producing condensation are significantly different k = 0.006 and 0.0011 L/mol-min, for monomer and dimer, respectively). 

Further work is needed to clarify the nature of the differences between the water- and alcohol-producing condensations of silicate species. But one conclusion is clear The two condensation reactions proceed by different reaction pathways (or at least have different transition states) and do not necessarily follow the same reactivity trends. 

Pouxviel et al. (12) demonstrated that higher W values (W = 10) enhance hydrolysis rates and thereby make hydrolysis under acid conditions nearly complete prior to condensation. At intermediate W (W = 4), hydrolysis and condensation proceed concurrently, and residual -OR groups remain alcohol-producing condensation reactions become important. At low W values (W = 1-2), -OH sites are limited thus reactions are retarded because both alcohol- and water-producing condensations require -OH groups. 

Kinetic measurements on the dimeric hexamethoxydisiloxane species generated values of a = 0.0007 L/mol-min and kw = 0.0011 L/mol-min, versus 0.001 and 0.006 L/mol-min for the monomer (46). The alcohol-producing condensation reaction is about the same for monomers and dimers, but the water-producing condensation is significantly lower for dimers. This difference is probably due to both increased steric crowding in the dimer and the inductive effect of an -OSi ligand retarding the reaction, as described earlier. Steric and inductive effects are important variables on reaction kinetics. 

There are several explanations for the large concentration of monomers present under neutral and basic conditions. From Fig, 21 we see that the rate of hydrolysis of siloxane bonds increases by over three orders of magnitude between pH 4 and 7. Because hydrolysis occurs preferentially at less highly condensed Q sites [1], monomers are the primary by-product of siloxane bond hydrolysis. In a related study, Klemperer and Ramamurthi [93] have shown that siloxane bonds are broken by redistribution reactions under basic conditions (Eq. 42) that produce unhydrolyzed monomers as a by-product. In addition, from the pH-dependence of the hydrolysis reaction (Fig. 9), we see that the hydrolysis rate is minimized at neutral pH. Because the rate constant of the alcohol-producing condensation reaction is less than that of the water-producing reaction [63,95], unhydrolyzed or partially hydrolyzed monomers may persist in solution past the gel point. Presumably these combined factors contribute to the large concentrations of monomers observed under neutral and basic conditions. 

C22-0122. As discussed in Chapter 13, a condensation reaction between a carboxyiic acid and an alcohol produces an ester with the eiimination of water ... 

Failure to remove the alcohols generated in either of the equilibrium condensation steps will reduce the efficiency of the polymerization process. This effect can be explained by Le Chatelier s principle, which was discussed in Chapter 3. The volatile alcohols produced during polymerization act as a chemical stress on the product side of the reaction, which inhibits polymerization. Another implication of the equilibrium nature of this polymerization process is seen in the molecular weight distribution of the final polymer. All polyesters contain a few percent of low molecular weight oligomers, regardless of the polymerization process. 

In a condensation reaction, two organic molecules combine to form a single organic molecule. A small molecule, usually water, is produced during the reaction. For example, a carboxylic acid and an alcohol can condense to form an ester. 

Acid added to an alcohol produces an ester (condensation reaction). 

Absolute alcohol is placed in the funnel and the tap arranged so as to deliver 36 c.cs. per hour. This is vaporised on dropping into the hot flask, and passes into contact with the bauxite. 50 gms. of ethylene are produced in 3 hours, any unchanged alcohol being condensed in the distilling flask. 

In a report dated 1915, Mrs Gertrude Maud Robinson (the first wife of Sir Robert Robinson) showed that the acid catalyzed condensation of veratryl alcohol produced, in excellent yield, a solid, m.p. 227 °C, which she considered to be 2,3,6,7-tetramethoxy-9,10-dihydroanthracene [1], In making this proposal (which, however, was not entirely accepted by her husband [1]), she was perhaps influenced by an earlier, hardly justified claim by Ewins (1909) that piperonyl alcohol, under acidic conditions, furnished a high-melting compound similar in many respects to hers described as being with considerable probability 2,3,6,7-dimethyl-enetetraoxy-9,10-dihydroanthracene [2]. 

The reaction of silicon tetrachloride with polyliydric alcohols produces a condensation polymer which may be called a silicon polyester. Some interesting condensation polymers related to these are produced by the reaction of ethyl silicate with glycols and phenols.40... 

Rh/PPhs-catalyzed hydroformylation is responsible for just over 50% of aU 0x0 alcohols produced. Propylene is the largest single alkene hydroformylated to produce butylaldehyde, which can be hydrogenated to produce butanol, or dimerized by an aldol condensation and then hydrogenated to form... 

The similarity of foi monomers and dimers is unexpected. If the reaction mechanism involves a protonated silanol, as in the water-producing condensation, subsequent reaction rate constants must be proportionately larger to give an observed of the same magnitude. However, the likely leaving group from a protonated silanol species is water, not an alcohol. 

The dehydration of alcohols is one of the oldest catalytic reactions and studied over kaolin, a-alumina, silica, silica-alumina, aluminium phosphate, W2O5 etc. The mononmolecular dehydration of ethyl alcohol produces ethylene and the bimolecular dehydration due to condensation of ethyl alcohol produces diethyl ether. The reported [11] apparent activation energies for the dehydration of ethyl alcohol to ethylene and condensation to diethyl ether are 13.8 and 14.2 K cal/mole respectively. These values have been precisely measured by pulse technique in the temperature range 190 to 275 C. Because of the closeness of the apparent activation energies, attainment of cent percent selectivity for ethylene by inhibition of the condensation reaction poses a great selectivity problem. However, the users are very rigid about cent percent selectivity for ethylene at a reactor temperature of 350 C. 

Alcohol- or water-producing condensation generates a three-dimensional network. Pohl and Osterholtz (6) showed that condensation of alkylsilane-triols is specific acid and base catalyzed. Above the isoelectric point of silica (about pH 2.5) condensation proceeds by nucleophilic attack of deprotonated silanols on neutral silicates (6, 23). Below the isoelectric point the reaction proceeds by protonation of silanols followed by electrophilic attack (2, 6). These reactions favor less highly condensed sites, because these are the most electron rich, and lead to more extended, ramified structures. Above pH 2.5, reactions favor more highly condensed sites. 

Hauser et al. (56) have reported the use of N-methylaminomethyl as an ortho-directing substituent. Treatment with acid of the secondary and tertiary alcohols produced by condensation of the 2-lithio intermediate— in this case with benzaldehyde and acetophenone, respectively—produced isoindolines (Reaction 43). 

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