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.
OR
**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.
OR
**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.

Example 1

What is the hemi-acetal formed in the following reaction?

Solution

  1. The carbonyl compound is an aldehyde because the C=O is connected to an H and R = CH2C6H5.
  2. 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.
  3. The alcohol is CH3CH2OH, so the OR group is OCH2CH3.
  4. 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.

    Example 2

    What is the structure of the acetal formed in the following reaction?

    Solution

    1. The carbonyl compound is a ketone because the C=O is connected to two alkyl groups: CH3 and CH2C6H5.
    2. 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.
    3. The alcohol is CH3OH, so the OR groups are OCH3.
    4. Fill in the respective groups for R1, R2, and OR around the central C to get the answer:

      Example 3

      Solution

      1. Don't panic because you see a ring! Handle this situation like the non-cyclic cases.
      2. 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.
      3. 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.
      4. The OR group came from a alcohol. Here this must be CH3CH2OH.
      5. The reactants must have been:

      Example 4

      What are the products of the following reaction?

      Solution

      1. 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.
      2. 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.
      3. Another way to ask the question is then: what carbonyl compound and alcohol reacted to make the starting acetal?
      4. The alcohol came from the OR groups, so we have CH3CH2OH.
      5. 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.
      6. So the products are:



      copyright 1998, Larry McGahey, The College of St Scholastica, Duluth MN.
      All rights reserved.