Alkyde shift

The interconversion of possible CioH16 isomers to adamantane, the thermodynamically most stable isomer, is a very complex system. It involves practically thousands of carbocationic intermediates via 1,2-hydride, alkyde shifts. The interconversion possibilities were summarized in a complex map of rearrangement graph ( adamantaneland ).45,46 The adamantane rearrangement is now recognized to be general for polycyclic hydrocarbons with the formula C4 +6H4 +12 (n = 1,2,3). 

The formation of isomeric alkylated products is explained by the ability of carbocations to undergo rearrangements. Ion 2 may participate in a series of hydride and alkyde shifts, and each carbocation thus formed may react via hydride ion transfer to form isomeric alkanes (Scheme 5.1). 

Isomerization of n-hexane 6 in superacid proceeds by three steps formation of the carbenium ion (step 1, Scheme 5.10), isomerization of the carbenium ion via hydride shift, alkyde shift, and protonated cyclopropane (for the branching step) (step 2,... 

Formation and transformations of terpenoid natural products involve carbocation chemistry. Complicated multistep processes are generally interpreted by invoking hydride and alkyde shifts and the involvement of hypercarbon... 

In addition to the primary alkylated product formed according to the above mechanism (eqs. 48-51), various other products may also be formed during alkane-alkene alkylation. These may be isomeric alkylated products resulting from the rearrangements of carbocations. Carbocation 13 may participate in a series of hydride and alkyde shifts, and each carbocation thus formed may react via hydride ion transfer to form isomeric alkanes. In addition, alkenes often undergo isomerization prior to participating in alkylation. It was observed, for example, that in the alkylation of isobutane with n-butenes in the presence of protic acids product distributions are very similar. This may be explained by a fast equilibration of n-butenes prior to participating in the alkylation step. 

Conditions to be met in oven drying enamels depend also on the composition of the binder. Paint systems containing melamine-formaldehyde or urea-formaldehyde resins, for instance, harden by polycondensation with other resins, such as epoxy resins, short-oil alkyd or acrylic resins at elevated temperatures. Baking is carried out at temperatures between 100 and almost 200°C and may last from a few minutes to more than an horn. A general trend towards energy conservation has shifted public attention towards binders which require low baking temperatures. 

The use of biosustainable feedstocks is widely discussed, but little if any penetration within the coatings industry has been observed to date (alkyds and shellacs apart). The reason for this has been the generally poor performance when compared against synthetic oil based comparisons. However, performance criteria when weighted against environmental standards may shift, such that renewable resource based products may become practical. It is to be anticipated that this will become a major fruitful area of research within the... 

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