Acetalation of Starch

Aldehydes readily react with alcohols into hemiacetals and acetals. The first reaction involves simple addition, whereas water is liberated in the second reaction. The reactions are acid-catalyzed and reversible. Acid catalysis is, however, not a necessary condition for the reaction to proceed. Frequently, particularly in the case of aliphatic aldehydes, the reaction proceeds without a catalyst. If both alcohol and aldehyde are bifunctional reagents, it is possible that cyclic structures may form or that polymerization may occur. Whether or not one or the other of these reactions occur depends on the energy factors, steric accessibility of the reaction sites, and finally, as in all reversible reactions, on the position of the equilibrium according to the concentration of reagents and products in the solution. Starch, being a polyalcohol, also reacts with aldehydes. This subject was formerly reviewed by Roberts.1274 

Since aldehydes behave as bifunctional reagents, crosslinking of starch occurs. For example, Syniewski1275 reported the reaction of starch with formaldehyde. 

There are also patents for solid, water-insoluble products resulting from such a reaction.1276,1277 In addition there are reports of the strengthening of artificial silk, starches, and cellulose after contact with formaldehyde.1278 On the other hand model studies carried out by Tomasik and Schilling1279 on the reaction of maltodextrins with formaldehyde and pyruvaldehyde suggested that neither of these, in contrast to glyoxal and glutaraldehyde, acted as crosslinker. 

A wide variety of reaction conditions have been proposed for the reaction of starch with aldehydes. Early studies entailed use of alkaline solutions, perhaps in order to gelatinize starch.1276 Because of the known behavior of aldehydes in alkaline media (Cannizzaro disproportionation and aldol condensations of aldehydes with active active a-methylene groups), this reaction has no simple outcome. However, a process of acetalation in alkaline solutions was subsequendy patented.1293 An exothermic reaction was observed within 3 4 min using an excess of formaldehyde. It was also reported that a,(3-unsaturated aldehydes reacted readily in slightly basic media.1294 

Early patents reported formation of acetals with formaldehyde at pH 2.1295,1296 Interestingly, even when starch was pretreated with alkali,1295 the reaction is complete within 3-28 h at pH 1.6-2.5 and at a temperature below gelation. The concentration of either formaldehyde or acetaldehyde was 0.075-0.5 wt.% of starch in suspension. It was reported that this reaction carried out in acidic media could be shortened to 1 h and that the viscosity of the final product depended on the 

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Mercaptolysis of starch dialdehyde occurs upon reaction with thiolacetic acid.525 It is possible that acetalation and crosslinking are involved in making a size consisting of starch dialdehyde, starch, and boric acid.526... 

Preparation of a chlorostarch for improved pigment retention in paper has been reported.2278 Acetalation of chlorinated starches was described573 as a procedure for producing a high-strength paper size. 

Reaction conditions necessary to carry out the modifications described earlier in this section usually result in some decomposition of amylose and amylopectin, even when simple substitution, addition, or crosslinking are involved. As a rule, graft copolymerization produces derivatives of significantly increased molecular weight. Starch grafting usually entails etherification, acetalation, or esterification of starch with vinyl monomers to introduce a reaction site for the further formation of a copolymeric chain. Such a chain would typically consist of either identical or different vinyl monomers (block polymers), or it may be grafted onto another polymer altogether. 

Specific acetalation can be performed on starch dialdehyde. Unlike starch, acetalated by aldehydes, the aldehydo groups on starch dialdehyde lead to the formation of acetals with alcohols. Acetals are formed randomly along the macrochain, moreover, 1,4-dioxane rings (25 and 26) also formed as a result of new, hemiacetal bonds.425,426 In such a manner, an allylated derivative was prepared with the use... 

Starch dialdehyde acetalated with pullulan was applied as a cathode-mix binder in dry-cell batteries1363 and in contact lenses.1364 It was also reported that starch dialdehyde, after animation with ethylenediamine and crosslinking with 1,5-pentandial, immobilized trypsin.1411 This preparation was proposed as a component of wound dressing. 

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