Alcohols can also be prepared by oxymercuration-demercuration of alkenes, which gives the Markovnikov's products with anti -stereochemistry (Scheme 2).

Scheme 2

Diols can be prepared by syn -hydroxylation using osmium tetroxide (OsO₄). For example, the reaction of cyclopentene with OsO₄ gives cyclopentane-1,2-diol (Scheme 3).

Scheme 3

5.5 Reactions

5.5.1 Oxidation

Primary and secondary alcohols are easily oxidized by reagents such as chromium oxides, permanganate, and nitric acid. Chromic acid is used to oxidize a secondary alcohol to give a ketone (Scheme 4).

Scheme 4

The chromic acid reagent is prepared by dissolving sodium dichromate (Na₂Cr₂O₇) in a mixture of sulfuric acid and water, or by adding chromium trioxide (CrO₃) to dilute sulfuric acid (Scheme 5).

Scheme 5

The mechanism of chromic acid oxidation probably involves the formation of a chromate ester. In the elimination, the carbinol carbon retains its oxygen atom but loses its hydrogen and makes the second bond to oxygen (Scheme 6). One can follow the progress of a chromic acid oxidation by observing the color change. The chromium(IV) species formed reacts further to give the stable reduced chromium(III) species. Both sodium dichromate and chromic acid are orange, but chromic ion (Cr³⁺) is a deep blue.

Scheme 6

Chromium reagents produce hazardous byproducts that can be avoided by using simple oxidants such as sodium hypochlorite (Scheme 7).