Preparation of Ketones

A secondary alcohol is an alcohol with a hydroxyl group attached to a carbon that also has only one hydrogen atom attached to it. That is, the carbon has two R groups other than the alcohol. The alcohol reacts with an oxidizing agent, such as potassium dichromate or chromic acid, to form a chromate ester. Then a base removes the proton from the chromate ester to form a carbonyl with the loss of the chromium species. Ketones are prepared from ozonolysis of alkenes that are not monosubstituted (i.e., alkenes that have two alkyl groups attached to the same carbon of an alkene). Ozonolysis is a process in which ozone (O₃) is used to cleave a double bond and create two smaller carbonyl molecules. First, ozone is added to the double bond to form an ozonide. Then hydrolysis of the ozonide occurs, cleaving the molecule to form the two smaller molecules. Oxymercuration-demercuration hydration of terminal alkynes is used to prepare ketones. Oxymercuration is the reaction of an alkene with mercury(II) acetate in aqueous THF to form a three-membered mercurinium ion. Mercurinium is a positively charged mercury ion. Demercuration is the second step in the hydration of alkenes, where the acetoxymercury group is reduced with sodium borohydride to a hydrogen atom. Alkynes are hydrated to form enols that are immediately tautomerized to form ketones. Tautomers are compounds that differ only in the placement of a single proton. An acid, such as sulfuric acid, and a mercury salt are used as reagents. Protonation of the alkyne with an acid creates a stable carbocation intermediate. A carbocation is a carbon with a positive charge. The reaction occurs through a planar carbocation and is therefore not stereoselective. Water is the nucleophile that attacks the electrophilic carbocation. An acyl group (${\rm{RCO}}-$) becomes attached to an aromatic ring with Friedel-Crafts acylation, creating an aryl ketone, ${\rm{Ar{-}COR}}$. Friedel-Crafts acylation uses an acid chloride rather than the alkyl halides used in Friedel-Crafts alkylation. The Lewis acid reacts with the acyl halide to form an electrophilic acylium ion (${\rm{RCO}^+}$). The $\pi$ electrons in the aromatic ring act as a nucleophile and attack the positive carbon. The aromatic ring loses its aromaticity, forming a cyclohexadienyl cation intermediate. Deprotonation from the sp³ carbon reforms the aromatic ring.