Formaldehyde, acid catalyzed procedure

Isoprene is the basic repeating unit in natural rubber and in the naturally occurring materials known as terpenoids. It is a diene like butadiene and is useful as a building block for synthetic polymers. The most frequently used synthetic procedure for making isoprene is acid-catalyzed reaction of formaldehyde with isobutylene, giving a dioxolane intermediate that is thermally cracked to isoprene (Fig. 22.37). Isoprene also can be recovered from petroleum refinery streams. A total of 370 million lb of isoprene was used in the United States in 1988. 

Alkyl vinyl ketones were synthesized in 1906 by heating (3-chloroethyl ketones with diethylaniline [327]. A large number of syntheses have been developed since that time, but only three or four have general applicability. Most syntheses are carried out at a low pH value to minimize the base-catalyzed condensation of the vinyl ketones. However, a rather elegant synthetic route for alkyl vinyl ketones involves a base catalyzed condensation of formaldehyde with methyl or ethyl ketones, respectively. Thermal dehydration of the p-ketoalcohol intermediates in the presence of weak acid catalysts produced a,p-unsaturated ketones in 50 to 60% yields. Several variations of this procedure have been reported [328]. 

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