Many alcohols undergo dehydration at 0°C when treated with phosphorus oxychloride (POCl3) in the mildly basic solvent pyridine. (Phosphorus oxychloride is the acid chloride of phosphoric acid, with chlorine atoms in place of the hydroxyl groups of phosphoric acid.)
(a) Propose a mechanism for the dehydration of cyclopentanol using POCl3 and pyridine. The first half of the mechanism, formation of a dichlorophosphate ester, is similar to the first half of the mechanism of reaction of an alcohol with thionyl chloride. Like a tosylate, the dichlorophosphate group is a good leaving group. The second half of the mechanism might be either first order or second order; draw both alternatives for now.
(b) When trans-2-methylcyclopentanol undergoes dehydration using POCl3 in pyridine, the major product is 3-methylcy-clopentene, and not the Zaitsev product. What is the stereochemistry of the dehydration? What does this stereochemistry imply about the correct mechanism in part (a)? Explain your reasoning.
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