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19.1: Naming Aldehydes and Ketones

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    448751
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    Aldehydes are named by replacing the terminal -e of the corresponding alkane name with -al. The parent chain must contain the –CHO group, and the –CHO carbon is numbered as carbon 1. Note in the following examples that the longest chain in 2-ethyl-4-methylpentanal is actually a hexane, but this chain does not include the –CHO group and thus is not the parent.

    The structures of ethanal (also called acetaldehyde), propanal (also called propionaldehyde) and 2-ethyl-4-methylpentanal. The carbon atoms are numbered in 2-ethyl-4-methylpentanal.

    For cyclic aldehydes in which the –CHO group is directly attached to a ring, the ring name followed by carbaldehyde is used.

    The structures of cyclohexanecarbaldehyde and 2-naphthalenecarbaldehyde. The C H O groups are on C 1 and C 2 positions of cyclohexanecarbaldehyde and 2-naphthalenecarbaldehyde, respectively.

    A few simple and well-known aldehydes have common names that are recognized by IUPAC. Several that you might encounter are listed in Table \(\PageIndex{1}\).

    Table \(\PageIndex{1}\): Common Names of Some Simple Aldehydes
    Formula Common name Systematic name
    HCHO Formaldehyde Methanal
    CH3CHO Acetaldehyde Ethanal
    \(\ce{H2C═CHCHO}\) Acrolein Propenal
    \(\ce{CH3CH═CHCHO}\) Crotonaldehyde 2-Butenal
    The structure of benzaldehyde in which a benzene ring with the first carbon is linked to an aldehyde (C H O). Benzaldehyde Benzenecarbaldehyde

    Ketones are named by replacing the terminal -e of the corresponding alkane name with -one. The parent chain is the longest one that includes the ketone group, and the numbering begins at the end nearer the carbonyl carbon. As with alkenes (Section 7.3) and alcohols (Section 17.1), the locant is placed before the parent name using older rules but before the suffix with the newer IUPAC guidelines. For example:

    The structures of 3-hexanone (new name: hexan-3-one), 4-hexen-2-one (new name: hex-4-en-2-one), and 2,4-hexanedione (new name: hexane-2,4-dione). The carbon atoms in each structure are numbered.

    A few ketones are allowed by IUPAC to retain their common names.

    The structures of acetone (carbonyl with two methyls), acetophenone (carbonyl with methyl and phenyl), and benzophenone (carbonyl with two phenyls).

    When it’s necessary to refer to the R–C═OR–C═O group as a substituent, the name acyl (a-sil) group is used and the name ending -yl is attached. Thus, –COCH3 is an acetyl group, –CHO is a formyl group, –COAr is an aroyl group, and –COC6H5 is a benzoyl group.

    The structures of acyl (C O R), acetyl (C O C H 3), formyl (C H O), and benzoyl (C O Ph) groups.

    If other functional groups are present and the double-bonded oxygen is considered a substituent on a parent chain, the prefix oxo- is used. For example:

    The structure of 3-oxohexanal comprises a six-carbon chain. C 1 is a C H O group, and C 3 is double-bonded to O.
    Exercise \(\PageIndex{1}\)

    Name the following aldehydes and ketones:

    1. a=
    2. A three-carbon chain in which (from left to right) first carbon has a phenyl group and third is part of C H O.
    3. An eight-carbon chain in which (from left to right) second and sixth carbons have oxo groups.
    4. Cyclohexane ring in which one carbon has methyl (wedge) and H (dash) subtituents, and adjacent (clockwise) carbon has hydrogen (wedge) and C H O (dash) groups.
    5. A six-carbon chain in which (from left to right) second and third carbons are double-bonded and sixth carbon is part of C H O.
    6. Cyclohexane ring in which one carbon has methyl (wedge) and H (dash) substituents, adjacent (clockwise) carbon has oxo, and two carbons further clockwise has methyl (wedge) and H (dash) groups.
    Answer
    1. 2-Methyl-3-pentanone
    2. 3-Phenylpropanal
    3. 2,6-Octanedione
    4. trans-2-Methylcyclohexanecarbaldehyde
    5. 4-Hexenal
    6. cis-2,5-Dimethylcyclohexanone
    Exercise \(\PageIndex{2}\)

    Draw structures corresponding to the following names:

    1. 3-Methylbutanal
    2. 4-Chloro-2-pentanone
    3. Phenylacetaldehyde
    4. cis-3-tert-Butylcyclohexanecarbaldehyde
    5. 3-Methyl-3-butenal
    6. 2-(1-Chloroethyl)-5-methylheptanal
    Answer
    1. The structure of 3-methylbutanal. A four-carbon chain with an aldehyde group on the first carbon and a methyl group attached to the third carbon.
    2. The structure of 4-chloro-2-pentanone. A five-carbon chain with a chlorine atom attached to the fourth carbon and a carbonyl group is on the second position.
    3. The structure of phenylacetaldehyde. A benzene ring with the first carbon attached to C H 2 C H O group.
    4. Cyclohexane ring in which one carbon has t-butyl (wedge) and H (dash) substituents, and two carbons away (clockwise) has C H O (wedge) and H (dash) groups.
    5. The structure of 3-methyl-3-butenal. A four-carbon chain with a methyl group on the third carbon, sharing a double bond between the third and fourth carbons with aldehyde on first position.
    6. A seven-carbon chain with a chloroethyl group attached to the second carbon by chloroethyl C 1, methyl attached to the fifth carbon, and aldehyde group attached to the first carbon.

    This page titled 19.1: Naming Aldehydes and Ketones is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by OpenStax via source content that was edited to the style and standards of the LibreTexts platform.