The mol. structure of the dehydration product I of pinacol under acidic condition was determine The results show that the dehydration reaction of pinacol gains an advantage over rearrangement reaction. The alkylation reaction of the formed substituted cyclopentadiene occurs only at fifth position carbon atom containing active hydrogen, whereas under harsh conditions, the alkylation occurs also at third position carbon atom. The reaction mechanisms of the dehydration and alkylation are discussed.

Key words: REACTION MECHANISM, PHENYL GROUP, ALKYLATION, MOLECULAR STRUCTURE, CYCLOPENTADIENE P, CYCLOPENTADIENE P, DEHYDRATION REACTION, PINACOL