MCAT Organic Chemistry Review
1.3 Aldehydes and Ketones
Aldehydes and ketones are two classes of molecules that contain a carbonyl group, which is a carbon double-bonded to an oxygen. Aldehydes and ketones differ in the placement of the carbonyl group: aldehydes are chain-terminating, meaning that they appear at the end of a parent chain, while ketones are found in the middle of carbon chains. Aldehydes and ketones do not have any leaving groups connected to the carbonyl carbon; they are only connected to alkyl chains or, in the case of aldehydes, hydrogen atoms. As we’ll examine later, carboxylic acids and their derivatives do contain leaving groups connected to the carbonyl carbon.
Aldehydes have a carbonyl group found at the end of the carbon chain. Because this is a terminal functional group that takes precedence over many others, it is generally attached to carbon number 1. Aldehydes are named by replacing the –e of the parent alkane with the suffix –al. When the aldehyde is at position 1, as is usually the case, we do not need to include this number in the chemical name. Figure 1.7 shows the IUPAC nomenclature for two aldehydes.
Figure 1.7. Naming Aldehydes The carbonyl group of the aldehyde usually does not receive a number in the name because it is a terminal functional group.
Methanal, ethanal, and propanal are referred to almost exclusively by their common names, formaldehyde, acetaldehyde, and propionaldehyde, rather than their IUPAC names. These molecules are shown in Figure 1.8.
Figure 1.8. Common Names of Aldehydes
Ketones contain a carbonyl group somewhere in the middle of the carbon chain. Because this is the case, we will always have to assign a number to the carbonyl carbon when naming ketones (except propanone, which must have the ketone on carbon 2 by default). Ketones are named by replacing –ane in the name of the parent alkane with the suffix –one. Just as when naming other compounds, be sure to give the carbonyl the lowest possible number if it is the highest-priority group.
Sugars are classified as either aldoses (aldehyde sugars) or ketoses (ketone sugars). Understanding nomenclature can help you to identify the structure of a sugar as well as some of its physical properties. Carbohydrate structure is discussed in Chapter 4 of MCAT Biochemistry Review.
Ketones are commonly named by listing the alkyl groups in alphabetical order, followed by ketone, such as ethylmethylketone. Acetone is the smallest possible ketone molecule, and it defies traditional naming conventions because it has three rather than two carbons (acet–, like eth– generally refers to two carbons). Figure 1.9 includes IUPAC and common names for a number of ketones.
Figure 1.9. Naming Ketones Common names are included in parentheses.
As is the case for alcohols, it is important to know both the common names and IUPAC names for common aldehydes and ketones. Make sure that you know what formaldehyde, acetaldehyde, and acetone are!
In a more complex molecule with a higher-priority group that takes precedence over the carbonyl, we name aldehydes and ketones as substituents, using the prefix oxo–. This is in reference to the carbonyl oxygen and applies for both ketones and aldehydes. Sometimes ketones may also be indicated with the prefix keto–.
Another convention that you may see on the MCAT is naming carbons relative to the carbonyl group. By this convention, the carbon adjacent to the carbonyl carbon is indicated by alpha (α). Moving away from the carbonyl, the successive carbons are referred to as beta (β), gamma (γ), and delta (δ) carbons. This applies on both sides of the carbonyl in the same fashion, so the carbons on both sides of a ketone are considered alpha carbons. This will become important when we discuss α-hydrogen acidity in Chapter 7 of MCAT Organic Chemistry Review.
MCAT Concept Check 1.3:
Before you move on, assess your understanding of the material with these questions.
1. What is the difference between an aldehyde and a ketone?
2. What suffixes are used for aldehydes and ketones; how are carbonyl groups named as a substituent?
· Aldehyde suffix: ; substituent prefix:
· Ketone suffix: ; substituent prefix:
3. Fill in the common names in the following chart.
4. For a molecule with a double bond, an aldehyde, and an alcohol, which functional group would determine the suffix when naming?