formaldehyde - Sciencemadness Dot Org

Chapter 11
Reactions of Formaldehyde with Aldehydes
and Ketones
Reactions of formaldehyde with other aldehydes and with ketones may
be classified broadly as belonging to three generic types: (a) aldol condensations
resulting in the production of methylol derivatives, (b) condensations
leading to the formation of unsaturated methylene derivatives (vinyl
compounds.!, and (c) mixed Cannizzaro reactions in which formaldehyde
functions as a reducing agent. The first two types of inaction occur only
with aldehydes or ketones containing alpha-hydrogen atoms, and are
accordingly not encountered with aryl aldehydes, diaryl ketones, or substituted
compounds such as trimethylacetaldehyde.
Reactions of Formaldehyde with Other Aldehydes
Acetaldehyde: Reactions in Water Solution. When calcium hydroxide
is added to an aqueous solution containing one mol of acetaldehyde and at
least four jnols of formaldehyde, the tetrahydric alcohol pentaerythritol
(or pentaerythxite) and calcium formate are produced. This reaction was
apparently first observed by Toliens and Wiegand 55 in 1891.
CHsCHO 4 4CH20(aq) + £Ca(OH)2 -> C(CH2OH)i + §Ca(OOCH)a
The probable reaction mechanism is indicated in the following equations:
CHgCHO -f CH20(aq) -» CH2(CH2OH)CHO (a)
CH2(CH:.OH)CHO 4* CH,0(aq) -» CH(CH2OH)2CHO (b)
CH(CH,OH),CHO + CH,0(aq5 -* C(CH2OH)BCHO (c)
C{CH2OH)BCHO 4- CH20{aq) 4- *Ca{OH)2 -» CfCHaOH)* 4- iCa(OOCH)* (d)
Reactions (a), (b), and (c)T which lead to the formation of trimethylolacetaldehyde
or pentaerythrose, are aldol condensations in which calcium
hydroxide serves merely as a catalyst. Step (d) is a mixed Cannizzaro
reaction in -which alkali is consumed. Although isolation of the intermediates
formed in the reaction sequence is difficult because of their
instability and the rapidity with which further reaction takes place, the
presence of xnonomethylolacetaldehyde has been demonstrated, and pentaeryfchrose
itself has been prepared as a reaction product under special
circumstances. In general, however, this is possible only when weak
alkalies are employed as catalysts.
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