In case of primary alcohols, the selective oxidation of primary alcohol to aldehydes can be accomplished using pyridinium chlorochromate (PCC) as a reagent (Scheme 8).
Scheme 8
While the Swern oxidation can convert primary alcohols to aldehydes in good yields. In this process, DMSO and oxalyl chloride are added to the alcohol at low temperature, followed by a hindered base such as triethylamine (Scheme 9). The byproducts of this reaction can be easily separated from the organic products.
Scheme 9
In the first step, DMSO reacts with oxalyl chloride to give an electrophilic sulfur compound and a chloride ion which then attacks the positively charge sulfur atom to give a chlorosulfonium ion (Scheme 10). The chlorosulfonium ion then reacts with alcohol to give a new sulfonium salt which is deprotonated by the base (Et3N).
Scheme 10
In contrast, the strong oxidizing agents such as permanganate and nitric acid oxidize secondary alcohols to ketones and primary alcohols to carboxylic acids (Scheme 11).
Scheme 11
5.5.2 Rearrangement
The pinacol rearrangement is a unique reaction of diol. Pinacol is heated to 100 °C in the presence of an acid to give pinacolone (Scheme 12).
Scheme 12
The first step involves the formation of a tertiary carbocation. Then, migration of a methyl group places the positive charge on the carbon atom bearing the second -OH group, where oxygen's nonbonding electrons stabilize the positive charge through resonance. Deprotonation of the resonance-stabilized cation gives pinacolone (Scheme 13).
Scheme 13
5.5.3 Esterification
A carboxylic ester is formed by replacing the - OH group of a carboxylic acid with the -OR group of an alcohol. In the following example, isopropyl alcohol is mixed with acetic acid to give an ester where a drop of sulfuric acid is added as a catalyst (Scheme 14). Good yields can be obtained by adding a large excess of the alcohol or the acid which move the equilibrium towards right.
Scheme 14