1.

Cyclohexene is best prepared from cyclohexanol by which of the following reagents:

Answer»

conc. `H_(3)PO_(4)`
conc. `HCl//ZnCl_(2)`
conc. HCl
conc. `HBr`

Solution :to suppress substitution and to favour elimination, the acid used MUST have a non-nucleophilic counter ION. The counter ion of HCl or HCl/`ZnCl_(2)` or HBr, i.e., `Cl^(-)` or `Br^(-)` ion is a strong NUCLEOPHILE and HENCE forms either CYCLOHEXYL chloride or cyclohexyl bromide by substitution reaction. in contrast, the counter ion of `H_(3)PO_(4)`, i.e., `PO_(4)^(3-)` is non-nucleophilic and hence does not give substitution product but gives only the elimination product, i.e., cyclohexene.


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