Many manifestations and applications of shape selective catalysis involve acid
catalyzed reactions such as isomeization, cracking, dehydration, etc. In
these reactions shape selectivity reverses the usual order of relative
carbocation reaction rates. Acid catalyzed reactivities of primary, secondary,
and tertiary carbons differ. Tertiary carbon atoms form carbocations easily;
therefore, they react much easier than secondary carbon atoms. Primary carbon
atoms do not form carbocations under ordinary conditions and therefore do not
react. Only secondary carbocations can form on normal paraff ins; whereas
tertiary carbocations can be generated on singly branched isoparaff ins.
Therefore, in most cases isoparaffins crack and isomerize much faster than
normal paraff ins. This order is reversed in most shape selctjye acid
atalysij that is, normal paraff ins react faster than branched ones, which
sometimes do not react at all. This is the essence of many applications of
shape selective acid catalysis.
Many manifestations and applications of shape selective catalysis involve acidcatalyzed reactions such as isomeization, cracking, dehydration, etc. Inthese reactions shape selectivity reverses the usual order of relativecarbocation reaction rates. Acid catalyzed reactivities of primary, secondary,and tertiary carbons differ. Tertiary carbon atoms form carbocations easily;therefore, they react much easier than secondary carbon atoms. Primary carbonatoms do not form carbocations under ordinary conditions and therefore do notreact. Only secondary carbocations can form on normal paraff ins; whereastertiary carbocations can be generated on singly branched isoparaff ins.Therefore, in most cases isoparaffins crack and isomerize much faster thannormal paraff ins. This order is reversed in most shape selctjye acidatalysij that is, normal paraff ins react faster than branched ones, whichsometimes do not react at all. This is the essence of many applications ofshape selective acid catalysis.
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