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20.09: Aldehydes and Ketones

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  • There's nothing like the smell of a fresh cinnamon roll; the taste is even better. What is that delicious taste owed to? This flavoring comes from the bark of a tree (actually, several different kinds of trees). One of the major compounds responsible for the taste and odor of cinnamon is cinnamaldehyde. Cinnamon has been widely used through centuries to treat a number of different disorders. In ancient times, doctors believed it could cure snakebite poisoning, freckles, and the common cold. Today, there are several research studies being carried out on the health benefits of cinnamon. So, enjoy that cinnamon roll—it just might have a health benefit or two.

    Aldehydes and Ketones

    Aldehydes and ketones are two related categories of organic compounds that both contain the carbonyl group, shown below.

    The difference between aldehydes and ketones is the placement of the carbonyl group within the molecule. An aldehyde is an organic compound in which the carbonyl group is attached to a carbon atom at the end of a carbon chain. A ketone is an organic compound in which the carbonyl group is attached to a carbon atom within the carbon chain. The general formulas for each are shown below.

    For aldehydes, the \(\ce{R}\) group may be a hydrogen atom or any length carbon chain. Aldehydes are named by finding the longest continuous chain that contains the carbonyl group. Change the -e at the end of the name of the alkane to -al.

    For ketones, \(\ce{R}\) and \(\ce{R'}\) must be carbon chains, of either the same or different lengths. The steps for naming ketones, followed by two examples, are shown below.

    1. Name the parent compound by finding the longest continuous chain that contains the carbonyl group. Change the -e at the end of the name of the alkane to -one.
    2. Number the carbon atoms in the chain in a way so that the carbonyl group has the lowest possible number.
    3. Add the numerical prefix into the name before the name of the ketone.
    4. Use a hyphen between the number and the name of the ketone.

    Properties of Aldehydes and Ketones

    Aldehydes and ketones can work weak hydrogen bonds with water through the carbonyl oxygen atom. The lower members of both series (3 carbons or fewer) are soluble in water in all proportions. As the length of the carbon chain increases, water solubility decreases. Similar to ethers, neither aldehydes nor ketones can hydrogen bond with themselves. As a result, their boiling points are generally lower than those of alcohols. Unlike alkanes however, aldehydes and ketones are polar molecules due to the more electronegative oxygen atom. The dipole-dipole interactions are stronger than the dispersion forces present in alkanes. The boiling points of aldehydes and ketones are intermediate between those of alkanes and alcohols. For example, the boiling point of ethane is \(-89^\text{o} \text{C}\), ethanal is \(20^\text{o} \text{C}\), and ethanol is \(78^\text{o} \text{C}\).

    Methanal, commonly known as formaldehyde, was once commonly used as a biological preservative for dead animals. In recent years, formaldehyde has been shown to be a carcinogen, and so has been replaced by safer alternatives for preservation purposes. Aldehydes are currently used in the production of resins and plastics. The simplest ketone, propanone, is commonly called acetone. Acetone is a common organic solvent that was once used in most nail polish removers, but has largely been replaced by other solvents.

    Summary

    • Aldehydes and ketones are two related categories of organic compounds that both contain the carbonyl group.
    • An aldehyde is an organic compound in which the carbonyl group is attached to a carbon atom at the end of a carbon chain.
    • A ketone is an organic compound in which the carbonyl group is attached to a carbon atom within the carbon chain.
    • Aldehydes and ketones generally have lower boiling points than those of alcohols.

    Contributors and Attributions

    • CK-12 Foundation by Sharon Bewick, Richard Parsons, Therese Forsythe, Shonna Robinson, and Jean Dupon.

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