Formaldehyde units

The stereochemistry of the Prins reaction is complex. In the transformation of cyclohexene and 2-butenes anti stereoselective addition was observed,67-69 whereas syn addition of two formaldehyde units takes place in the formation of 1,3-dioxanes from substituted styrenes.70 Most of the transformations are, however, nonstereo-selective,71 72 accounted for by carbocation 18. 

Polyformaldehyde. Polyformaldehyde or polyacetal is made by two different processes. Delrin is made from formaldehyde by anionic polymerization catalyzed by a tertiary amine. The homopolymer is end-capped with acetic anhydride. Celcon is made from trioxane cationic copolymerization using boron trifluoride catalyst and ethylene oxide (2-3%) as the comonomer. Boron trifluoride is a Lewis acid that associates with trioxane and opens up the six-membered ring. Ethylene oxide provides the end capping. Without an end cap, polyformaldehyde is thermally unstable and loses formaldehyde units. 

After cleavage of n + 1 formaldehyde units, a stable alcoholic end group (.. . —CHo—CH2—OH) is obtained. A few percent of dioxolane units is sufficient to render the bulk of the copolymer thermally stable. 

On the other hand copolymer with a trioxane unit at the cationic chain end (Pi+) may be converted intp P2+ by cleavage of several formaldehyde units. These side reactions change the nature of the active chain ends without participation of the actual monomers trioxane and dioxo-lane. Such reactions are not provided for in the kinetic scheme of Mayo and Lewis. In their conventional scheme, conversion of Pi+ to P2+ is assumed to take place exclusively by addition of monomer M2. Polymerization of trioxane with dioxolane actually is a ternary copolymerization after the induction period one of the three monomers—formaldehyde— is present in its equilibrium concentration. Being the most reactive monomer it still exerts a strong influence on the course of copolymerization (9). This makes it impossible to apply the conventional copolymerization equation and complicates the process considerably. 

Formaldehyde and acetaldehyde are the most common aldehydes. Formaldehyde is a gas at room temperature, so it is often stored and used as a 40% aqueous solution called formalin. When dry formaldehyde is needed, it can be generated by heating one of its solid derivatives, usually trioxane or paraformaldehyde. Trioxane is a cyclic trimer, containing three formaldehyde units. Paraformaldehyde is a linear polymer,... 

Biogenetically, this interesting variant on the heteroyohimbine theme could originate from tryptamine and either dihydroxyphenylalanine (Robinson) or prephenic acid (LXXV) (Wenkert) as example, a possible route from prephenic acid via the seco-prephenate-formaldehyde unit (LXXVI) (97) is outlined below. A vital feature in this proposal (and... 

CH20)3 or CH20CH20CH20. A trimer of formaldehyde, not to be confused with paraformaldehyde, which consists of eight or more formaldehyde units. Properties White crystals formaldehyde odor. Mp 62C, sublimes at 115C, flash p 113F (45C) (OC),... 

It was concluded that the ring expansion resulting from successive insertions of EO units (-CH2CH2O-) into cyclic formals issued from TOX is the main initial process of the reaaion during the induction period. The appearance of TXP and then DXL is delayed until most of EO is consumed. This conflicts with the previously proposed mechanism involving the prior formation of dioxolane by reaction of formaldehyde with the EO oxonium followed by the reversible insertion of formaldehyde unit (OCH2) into the DXL oxonium leading to TXP. 

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