Tollens condensation

Some reactions that belong in this category have been considered in earlier chapters. Among these are the Tollens condensation (16-46), the benzil-benzilic acid rearrangement (18-6), and the Wallach rearrangement (18-43). 

The Tollens condensation is a crossed aldolization employing formaldehyde and an aldehyde or a ketone in the presence of a weak base. Since formaldehyde has no -hydrogen atoms, it is an extremely reactive carbanion acceptor (p. 155). Consequently, reaction occurs readily with carbanions and leads to polymethylol derivatives 6... 

Historical. PE was first isolated by Tollens who was examining the effect of heat and reagents on formaldehyde. Apparently the crude formaldehyde he was working with contained a small % of acetaldehyde, which accounts for the PE formed. Although Tollens isolated it in 1882, it was not identified as PE until 1888 (Ref 2). Further details of the prepn and props of PE appeared in 1891 (Ref 3). The prepn is essentially a condensation betw 3 moles of formaldehyde and 1 of acetaldehyde to give an intermediate tris(hydroxymethyl)-acetaldehyde which is not isolated. An Intermolecular oxidn/redn then takes place betw this intermediate, and a 4th mole of formaldehyde, giving PE and formic ac (Ref 13, p 2). This type reaction is discussed under Cannizzaro Reaction in the Encycl (Vol 2, C25)... 

Condensation between formaldehyde and aldehydes or ketones (Tollens)... 

At a considerably later date. Frankland prepared ethyl methacrylate and methacrylic acid from ethyl a-hydroxyisobutyrate and phosphorus trichloride. Tollen prepared aciylate esters from 2,3-dibromopropionate esters and zinc. Otto Rohm, in 1901. described the structures of the liquid condensation products (including dimers and timers) obtained from the action of sodium alkoxides on methyl and ethyl acrylate. Shortly after World War 1, Rohm introduced a new acrylate synthesis, noting that an acrylate is formed in good yield from heating ethylene cyanohydrin and sulfuric acid and alcohol. A major incentive for the development of a clear, tough plastic acrylate was for use in the manufacture of safety glass. 

When aliphatic nitro compounds are used instead of aldehydes or ketones, no reduction occurs, and the reaction has been referred to as a Tollens reaction (see 16-43). However, the classical condensation of an aliphatic nitro compound with an aldehyde or ketone is usually called the Henry reaction or the Kamlet reaction, and is essentially a nitro aldol reaction. A variety of conditions have been reported, including the use of a silica catalyst, Mg—A1 hydrotalcite, a tetraalkylam-monium hydroxide,proazaphosphatranes, " or an ionic liquid.A solvent free Henry reaction was reported in which a nitroalkane and an aldehyde were reacted on KOH powder. Potassium phosphate has been used with nitromethane and aryl aldehydes. The Henry reaction has been done using ZnEt2 and 20%... 

Oxidation by Tollens reagent is useful chiefly for detecting aldehydes, and in particular for differentiating them from ketones (see Sec. 19.17). The reaction is of value in synthesis in those cases where aldehydes are more readily available than the corresponding acids in particular, for the synthesis of unsaturated acids from the unsaturated aldehydes obtained from the aldol condensation (Sec. 21.6), where advantage is taken of the fact that Tollens reagent does not attack carbon-carbon double bonds. 

The successful use of resins produced by the condensation of formaldehyde with phenol catalyzed the development of other condensation products of formaldehyde with urea and melamine. The urea resins were described by Tollens in 1884 and were patented by John in 1918. 

Whilst formylpyrroles behave in many ways as would be expected, some anomalous properties have attracted attention. Thus, 2-formylpyrrole gives an oxime, arylhydrazones and other typical carbonyl derivatives , and condenses with methyl ketones, hippuric acid and other reactive methylene compounds . 168, 169 It does not give the usual aldehyde reactions with Schiff s, Fehling s or Tollen s reagent ", nor does it form a cyanhydrin or take part in the Cannizzaro or Perkin reactions . ... 

Tollens described resins produced from the condensation of urea and formaldehyde in 1884, and John patented this polymer (UF) in 1918. In 1936, Henkel patented comparable resins (MF) based on melamine instead of urea. Both resins, which were referred to as amino resins, are insoluble in common solvents and used as additives for curing other coatings. For example, the addition of MF reduces the curing time of alkyd resins by 50%. Soluble amino resins, which can be used as coatings, are produced when modified by etherification with butanol. 

Amino-resins from condensation of urea and formaldehyde first produced by Tollens... 

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