Condensation reactions, definition mechanism

Although ethanol is dehydrogenated to acetaldehyde in the presence of zinc oxide at temperatures of 300° to 400° C. and atmospheric pressure, no aldehyde results when the reaction is conducted under sufficient hydrogen pressure. Instead a complex mixture including esters of acetic, butyric, and caproic acids and alcohols up to and higher than octyl is formed.01 Condensation reactions of acetaldehyde are used to account for the formation of these compounds but no definite proof has as yet been advanced to establish the mechanisms. 

In the previous case we found a pressure that finally stops and reverses the reaction, but it is not the mere mechanical pressure that is effective. An equal pressure of air—say 18 atmospheres—would not, in the case discussed, stop the action of zinc on sulphuric acid. What does oppose the reaction, and shows itself as pressure, is a definite concentration of hydrogen, like the definite concentration of water vapour in hydrates. So we must in the third place turn to cases more specially suited to measure affinity, in which a pressure stops and reverses the reaction indifferently, whether it be exerted by hydrogen or by a piston. These are the transformations taking place without evolution of gas in so-called condensed systems, such as that described (p. 26) for sulphur the latter consists in complete conversion in one direction or the other according to temperature,... 

Although these definitions were perfectly adequate at the time, it soon became obvious that notable exceptions existed and that a fundamentally sounder classification should be based on a description of the chain-growth mechanism. It is preferable to replace the term condensation with step-growth or step-reaction. Reclassification as step-growth polymerization now logically includes polymers such as polyurethanes, which grow by a step-reaction mechanism without elimination of a small molecule. 

Non-hydrolytic Protic Reactions. The categorization of non-hydrolytic sol-gel methods may be made difficult by the occurrence of complex mechanisms involving both hydrolytic and non-hydrolytic reactions. Thus the strict definition of non-hydrolytic conditions would preclude reactions in which water may be produced in situ. However, the addition of carboxylic acids to a tetraalkoxysilane is referred to as a non-hydrolytic method, even though it probably involves water formation by esterification (Eq. 26-1) besides non-hydrolytic hydroxylation (Eq. 26-2) and aprotic condensation (Eq. 26-3). 

The existence of the n-C2 to n-C4 mono- and dicarboxylic acids in hydro-thermal sedimentary environments depends upon the rates of their production and the rates of decomposition and/or oxidation. These two classes of acids exhibit very different rates and mechanisms of decarboxylation. Decarboxylation of acetic acid, and probably of other aliphatic monocar-boxylic acids, proceeds by a heterogeneous catalytic mechanism apparently very different from the homogeneous mechanism for decarboxylation of the dicarboxylic acids. Due to the limited amount of experimental information regarding the kinetics of oxidation or condensation for both classes of acids, no definitive mechanistic trends can be postulated for this process. Nevertheless, it is possible to place constraints on the kinetics and mechanism for the oxidation reaction(s) if this process were assumed to control the ultimate decomposition of acetic acid. Results from studies of mono- and dicarboxylic acid decarboxylation are summarized below. 

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