Chapter 24 – Summary

24.1 The Carbonyl Group

Functional groups related to the carbonyl group include the –CHO group of an aldehyde, the –CO– group of a ketone, the –CO₂H group of a carboxylic acid, and the –CO₂R group of an ester (these latter two will be covered in the next chapter). The carbonyl group, a carbon-oxygen double bond, is the key structure in these classes of organic molecules: Aldehydes contain at least one hydrogen atom attached to the carbonyl carbon atom, while ketones contain two carbon groups attached to the carbonyl carbon atom. These compounds contain oxidized carbon atoms relative to the carbon atom of an alcohol group.

24.2 Naming Aldehydes and Ketones

The common names of aldehydes are taken from the names of the corresponding carboxylic acids: formaldehyde, acetaldehyde, and so on. The common names of ketones, like those of ethers, consist of the names of the groups attached to the carbonyl group, followed by the word ketone. The official IUPAC naming system uses the stem names of aldehydes and ketones are derived from those of the parent alkanes, using an –al ending for an aldehydes and an –one ending for a ketone.  The steps are 1) the stem names of aldehydes and ketones are derived from those of the parent alkanes, defined by the longest continuous chain (LCC) of carbon atoms that contains the functional group. 2) For an aldehyde, drop the –e from the alkane name and add the ending –al.  3) For a ketone, drop the –e from the alkane name and add the ending –one. 4) To indicate the position of a substituent on an aldehyde, the carbonyl carbon atom is always considered to be C1; it is unnecessary to designate this group by number. 5) To indicate the position of a substituent on a ketone, number the chain in the manner that gives the carbonyl carbon atom the lowest possible number. In cyclic ketones, it is understood that the carbonyl carbon atom is C1.

24.3 Physical Properties of Aldehydes and Ketones

The carbonyl group found in aldehydes and ketones is very polar, with the oxygen pulling electrons from the carbon. This polar carbon-to-oxygen double bond causes aldehydes and ketones to have higher boiling points than those of ethers and alkanes of similar molar masses but lower than those of comparable alcohols, since alcohols can engage in intermolecular hydrogen bonding. Aldehydes and ketones with four or fewer carbons tend to be soluble in water, while those with higher molar masses are insoluble due to increased dispersion forces present in the molecules. As well, lower molar mass aldehydes have a sharp, disagreeable odours, while higher molar mass aldehydes and ketones are much more fragrant and are often found as ingredients in food flavourings and perfumes. Formaldehyde (methanal) is a major industrial product produced from methanol by air oxidation, and is used predominantly in the manufacture of polymers. Acetaldehyde (ethanal) is very important as the industrial starting material to produce acetic acid and 1-butanol, and is the biological byproduct the fermentation of sugars and the detoxification of alcohol in the liver. Acetone (propanone) and methyl ethyl ketone (2-butanone) are created by oxidating the secondary alcohols 2-propanol and 2-butanol respectively. These products are mainly used as industrial solvents, and are commonly used to produce varnishes, paints, lacquers, and nail polish remover.

24.4 Chemical Properties of Aldehydes and Ketones

Primary alcohols are oxidized to form aldehydes whereas secondary alcohols are oxidized to form ketones.  Aldehydes are readily oxidized to carboxylic acids, whereas ketones resist oxidation under similar circumstances. The Tollens’ reaction is a test for presence of aldehydes where the reaction vessel gets a silver mirror finish with a positive result.  The Benedicts’ reagent and Fehling test also test for the presence of aldehydes through oxidation using copper ions with the solution changing from a blue colour and producing a brick red precipitate. Aldehydes and ketones can be reduced to their corresponding alcohols (opposite of oxidation).  Aldehydes and ketones undergo addition at the carbon-oxygen double bond site.  Alcohol, water and HCN can be added to produce hemiacetal/hemiketal, hydrate and cyanohydrin respectively.

Attribution & References

Adapted by Gregory A. Anderson and Samantha Sullivan Sauer as follows:

• “24.1 The Carbonyl Group” summary is adapted from “18.3 Aldehydes, Ketones, Carboxylic Acids, and Esters” In General Chemistry 1 & 2 by Rice University, a derivative of Chemistry (Open Stax) by Paul Flowers, Klaus Theopold, Richard Langley & William R. Robinson and is licensed under CC BY 4.0. ​Access for free at Chemistry (OpenStax)​
• “24.2 Naming Aldehydes and Ketones” is adapted from “15.2: Naming Aldehydes and Ketones” In Map: Fundamentals of General Organic and Biological Chemistry (McMurry et al.), CC BY-NC-SA 3.0, a remixed version of Basics of GOB (Ball et al.), CC BY-NC-SA 4.0 which is a LibreTexts version of Introduction to Chemistry: GOB (v. 1.0), CC BY-NC-SA 3.0
• “24.3 Physical and Chemical Properties of Aldehydes and Ketones” is adapted from “15.3: Properties of Aldehydes and Ketones” In Map: Fundamentals of General Organic and Biological Chemistry (McMurry et al.), CC BY-NC-SA 3.0, a derivative of “14.10: Properties of Aldehydes and Ketones” In Basics of GOB (Ball et al.), CC BY-NC-SA 4.0 which is the LibreText version of Introduction to Chemistry: GOB (v. 1.0), CC BY-NC 3.0
• “24.4 Some Common Aldehydes and Ketones” is adapted from “15.4: Some Common Aldehydes and Ketones” In Map: Fundamentals of General Organic and Biological Chemistry (McMurry et al.), CC BY-NC-SA 3.0, a remixed version of Basics of GOB (Ball et al.), CC BY-NC-SA 4.0 which is a LibreTexts version of Introduction to Chemistry: GOB (v. 1.0), CC BY-NC-SA 3.0