Aldehyde 8:

Problem

Solution

This sequence of reactions provides a method for converting an aldehyde 1 (benzaldehyde) in to a ketone 4 (ethyl phenyl ketone, propiophenone) via the dithiane 2 (2-phenyl-1,3-dithiane). The reaction can also be applied to aliphatic aldehydes. Because the polarity of the carbonyl carbon is positive, it is difficult to generate the anion of an aldehyde by deprotonation to form an acyl anion. This reversal of polarity is known as umpolung (Ger., pole reversal). The hydrogen at the 2-position of 2 has a pKa = ~ 29 (higher for aliphatic derivatives. Both aromatic and aliphatic dithianes are readily deprotonated by n-butyllithium. In this instance, anion 5 is formed. What is the approximate pKa of n-butyllithium? Here is a pKa table. Here is an answer. Anion 5 forms 3 by what type of reaction with ethyl iodide? Hydrolysis of the alkylated dithiane 3 liberates ketone 4. The mechanism for the formation of dithiane 2 is one that you should be able to handle if you known how acetals are formed. If not, review this concept with the solution to aldehyde #2. A more traditional route from aldehyde 2 to ketone 4 is accomplished by the addition of ethyl magnesium Grignard to 2 followed by oxidation of the intermediate alcohol to ketone 4. Both of these routes start with the same reactant and end with the same product. Therefore, the total electron change in both routes must be the same. Verify.