15. Aldehydes and Ketones

What would be the product of the following reaction?

Draw the intermediate in the following reaction.

Draw the structure of the product when one mole of cyclopentanone reacts with two moles of methanol in an acidic solution.

Determine whether the following compounds are acetals or ketals. Draw the structure of the aldehyde or ketone it came from.

c.

Draw the structures of the hemiacetals or hemiketals formed in these reactions: 

b.

Cyclic hemiacetals commonly form if a molecule has both an alcohol group and a carbonyl group elsewhere in the same molecule, especially if they are four or five carbons apart. What is the structure of the hydroxy aldehyde from which this hemiacetal might form? 

In Problem 15.24, you were given the structure of the free aldehyde form of glucose. Try to draw the two cyclic hemiacetal forms of glucose you would get if (a) the OH on C4 formed the ring and (b) the OH on C3 formed the ring.

For each compound shown next, determine whether it is a hemiacetal, a hemiketal, an acetal, or a ketal.

a.

b.

c.

d.

Glucose is the major sugar in mammalian blood. We often see it represented as either the "free aldehyde" or the cyclic hemiacetal forms shown here. Of the two forms of glucose, the cyclic hemiacetal is the preferred form found in blood. Can you suggest two reasons why?

Write the structures of the hemiacetal or hemiketal that result from reactions (a) and (b). Write the structures of the complete hydrolysis products of the acetal or ketal in (c) and (d).

a. Acetone + Ethanol → ?

What two products result from the complete hydrolysis of this cyclic acetal?

Acetals and ketals are usually made by reaction of an aldehyde or ketone with two molecules of a monoalcohol. If an aldehyde or ketone reacts with one molecule of a dialcohol, however, a cyclic acetal or ketal results.

b. Draw the cyclic ketal formed when the hemiketal from part (a) reacts with the ―OH labeled in blue.