Recognizing Hemi-acetals and Acetals
Hemi-acetals are compounds in which a sp3 hybridized carbon is connected to a hydroxy group (OH), an alkoxy group (OR), and:
**one alkyl group and one hydrogen. This is a hemi-acetal of an aldehyde.
**two alkyl groups. This is a hemi-acetal of a ketone (also called a hemi-ketal).
Acetals are compounds in which a sp3 hybridized carbon is connected to two alkoxy groups (OR) and:
**one alkyl group and one hydrogen. This is an acetal of an aldehdye.
**two alkyl groups. This is an acetal of a ketone (also called a ketal).
Once you have learned how to recognize an acetal or hemi-acetal and where the repsective parts of the molecule originate, it is then relatively easy to tackle some problems.
What is the hemi-acetal formed in the following reaction?
- The carbonyl compound is an aldehyde because the C=O is connected to an H and R = CH2C6H5.
- The problem asks for the structure of a hemi-acetal as product. This means the general structure is like the first one at the top of the page.
- The alcohol is CH3CH2OH, so the OR group is OCH2CH3.
- Fill in the respective groups for *R, H, and OR around the central C-OH to get the answer:
Note that the groups can be arranged around the central C in several ways, all equivalent for the purposes of answering the question.
What is the structure of the acetal formed in the following reaction?
- The carbonyl compound is a ketone because the C=O is connected to two alkyl groups: CH3 and CH2C6H5.
- The problem asks for the structure of an acetal as product, which will have the same general structure as the fourth structure at the top of the page.
- The alcohol is CH3OH, so the OR groups are OCH3.
- Fill in the respective groups for R1, R2, and OR around the central C to get the answer:
- Don't panic because you see a ring! Handle this situation like the non-cyclic cases.
- One carbon in the ring is connected to an OH group, an OR group and two alkyl groups (the two sides of the ring). This means we are looking at a hemi-acetal.
- The C connected to the OH and OR groups was the C of C=O. Since there are two alkyl groups connected to this C, the carbonyl compound was a ketone. In this case, the two alkyl groups are the rest of the ring, so we have a cycloketone.
- The OR group came from a alcohol. Here this must be CH3CH2OH.
- The reactants must have been:
What are the products of the following reaction?
- The organic compound is an acetal obtained from a ketone. Notice there is a C connected to two alkoxy (OR) groups and to two alkyl groups.
- The other reactant is water in the presence of acid (H+). Water and an acid catalyst hydrolyzes an acetal: reverses the acetal formation. The products are the starting carbonyl compound and alcohol which made the acetal.
- Another way to ask the question is then: what carbonyl compound and alcohol reacted to make the starting acetal?
- The alcohol came from the OR groups, so we have CH3CH2OH.
- The C connected to the two OR groups was the C of the C=O in the carbonyl compound. The other two groups were bonded to the C=O in a ketone.
- So the products are:
copyright 1998, Larry McGahey, The College of St Scholastica, Duluth MN.
All rights reserved.