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14.9: Aldehydes and Ketones- Structure and Names

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    16049
    • Contributed by Anonymous
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    Learning Objectives

    • Identify the general structure for an aldehyde and a ketone.
    • Use common names to name aldehydes and ketones.
    • Use the IUPAC system to name aldehydes and ketones.

    The next functional group we consider, the carbonyl group, has a carbon-to-oxygen double bond.

    Carbonyl groups define two related families of organic compounds: the aldehydes and the ketones.

    The carbonyl group is ubiquitous in biological compounds. It is found in carbohydrates, fats, proteins, nucleic acids, hormones, and vitamins—organic compounds critical to living systems.

    In a ketone, two carbon groups are attached to the carbonyl carbon atom. The following general formulas, in which R represents an alkyl group and Ar stands for an aryl group, represent ketones.

    Three general structural formulas are shown. The first one shows the carbonyl carbon bonded to two R groups, the second shows one R group and one Ar group and the third one shows bonding to two Ar groups.

    In an aldehyde, at least one of the attached groups must be a hydrogen atom. The following compounds are aldehydes:

    Three general structural formula are shown. The first shows the carbonyl carbon bonded to a H atom. The second shows this carbon bonded to a R group and the third one shows bonding to an Ar group.

    In condensed formulas, we use CHO to identify an aldehyde rather than COH, which might be confused with an alcohol. This follows the general rule that in condensed structural formulas H comes after the atom it is attached to (usually C, N, or O).

    An aldehyde is shown to have structural formula CHO and ketone CO.

    The carbon-to-oxygen double bond is not shown but understood to be present. Because they contain the same functional group, aldehydes and ketones share many common properties, but they still differ enough to warrant their classification into two families.

    • Naming Aldehydes and Ketones

      Both common and International Union of Pure and Applied Chemistry (IUPAC) names are frequently used for aldehydes and ketones, with common names predominating for the lower homologs. The common names of aldehydes are taken from the names of the acids into which the aldehydes can be converted by oxidation.

      Formaldehyde and acetaldehyde forms formic acid and acetic acid respectively upon oxidation.

      The stems for the common names of the first four aldehydes are as follows:

      • 1 carbon atom: form-
      • 2 carbon atoms: acet-
      • 3 carbon atoms: propion-
      • 4 carbon atoms: butyr-

      Because the carbonyl group in a ketone must be attached to two carbon groups, the simplest ketone has three carbon atoms. It is widely known as acetone, a unique name unrelated to other common names for ketones.

      Generally, the common names of ketones consist of the names of the groups attached to the carbonyl group, followed by the word ketone. (Note the similarity to the naming of ethers.) Another name for acetone, then, is dimethyl ketone. The ketone with four carbon atoms is ethyl methyl ketone.

      Example \(\PageIndex{1}\)

      Classify each compound as an aldehyde or a ketone. Give the common name for each ketone.

      1. A carbonyl carbon is bonded to a propyl group and 1 H atom.
      2. A carbonyl carbon is bonded to two identical propyl groups.
      3. A carbonyl carbon is bonded to a methyl group and an isopropyl group.

      Solution

      1. This compound has the carbonyl group on an end carbon atom, so it is an aldehyde.
      2. This compound has the carbonyl group on an interior carbon atom, so it is a ketone. Both alkyl groups are propyl groups. The name is therefore dipropyl ketone.
      3. This compound has the carbonyl group between two alkyl groups, so it is a ketone. One alkyl group has three carbon atoms and is attached by the middle carbon atom; it is an isopropyl group. A group with one carbon atom is a methyl group. The name is therefore isopropyl methyl ketone.

      Exercise \(\PageIndex{1}\)

      Classify each compound as an aldehyde or a ketone. Give the common name for each ketone.

      1. A carbonyl carbon is bonded to an ethyl group and a propyl group.
      2. A carbonyl carbon is bonded to 1 H atom and a 1 methylpropyl group.
      3. A carbonyl carbon is bonded to a cyclobutyl group and a methyl group.

      Here are some simple IUPAC rules for naming aldehydes and ketones:

      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. Methanal is the IUPAC name for formaldehyde, and ethanal is the name for acetaldehyde.
      3. For a ketone, drop the -e from the alkane name and add the ending -one. Propanone is the IUPAC name for acetone, and butanone is the name for ethyl methyl ketone.
      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.

      Example \(\PageIndex{2}\)

      Give the IUPAC name for each compound.

      1. A carbonyl carbon is bonded to 1 H atom and a 1 methylbutyl group.
      2. A carbonyl carbon is bonded to two identical isopropyl groups.
      3. A carbonyl carbon forms a six carbon cyclic structure.

      Solution

      1. There are five carbon atoms in the LCC. The methyl group (CH3) is a substituent on the second carbon atom of the chain; the aldehyde carbon atom is always C1. The name is derived from pentane. Dropping the -e and adding the ending -al gives pentanal. The methyl group on the second carbon atom makes the name 2-methylpentanal.
      2. There are five carbon atoms in the LCC. The carbonyl carbon atom is C3, and there are methyl groups on C2 and C4. The IUPAC name is 2,4-dimethyl-3-pentanone.
      3. There are six carbon atoms in the ring. The compound is cyclohexanone. No number is needed to indicate the position of the carbonyl group because all six carbon atoms are equivalent.

      Exercise

      Give the IUPAC name for each compound.

      1. A carbonyl carbon is bonded to 1 H atom and a 2 2 dimethylpropyl group.
      2. A carbonyl carbon is bonded to a propyl group and an isopropyl group.
      3. A carbonyl carbon forms a four carbon cyclic structure.

      Example \(\PageIndex{3}\)

      Draw the structure for each compound.

      1. 7-chlorooctanal
      2. 4-methyl–3-hexanone

      Solution

      1. The octan- part of the name tells us that the LCC has eight carbon atoms. There is a chlorine (Cl) atom on the seventh carbon atom; numbering from the carbonyl group and counting the carbonyl carbon atom as C1, we place the Cl atom on the seventh carbon atom.
      2. The hexan- part of the name tells us that the LCC has six carbon atoms. The 3 means that the carbonyl carbon atom is C3 in this chain, and the 4 tells us that there is a methyl (CH3) group at C4:

      Exercise

      Draw the structure for each compound.

      1. 5-bromo-3-iodoheptanal
      2. 5-bromo-4-ethyl-2-heptanone

      Concept Review Exercises

      1. Give the structure and IUPAC name for the compound that has the common name m-bromobenzaldehyde.

      2. Give the IUPAC name for glyceraldehyde, (HOCH2CHOHCHO). (Hint: as a substituent, the OH group is named hydroxy.)

      Summary

      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. 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.

      Answers

      1. A carbonyl carbon is attached to 1 H atom and a hexagonal structure with a circle drawn in the center. The bromine bonded to the benzene ring is on the third carbon from the site of attachment to the carbonyl carbon.

        3-bromobenzaldehyde

      2. 2,3-dihydroxypropanal

      Exercises

      1. Name each compound.

        1. A carbonyl carbon is bonded to 1 H atom and a ethyl group.
        2. A carbonyl carbon is bonded to 1 H atom and a propyl group.
        3. A carbonyl carbon is bonded to two identical ethyl groups.
        4. A carbonyl carbon is bonded to 1 H atom and a benzene ring.
      2. Name each compound.

        1. A carbonyl carbon is bonded to a methyl group and an isopropyl group.
        2. CH3CH2CH2CH2CH2CHO
        3. A carbonyl carbon is bonded to a 3 bromine butyl group and 1 H atom.
        4. A carbonyl carbon is bonded to a butyl group and an ethyl group.
      3. Draw the structure for each compound.

        1. butyraldehyde
        2. 2-hexanone
        3. p-nitrobenzaldehyde
      4. Draw the structure for each compound.

        1. 5-ethyloctanal
        2. 2-chloropropanal
        3. 2-hydroxy-3-pentanone

      Answers

        1. propanal or propionaldehyde
        2. butanal or butyraldehyde
        3. 3-pentanone or diethyl ketone
        4. benzaldehyde
        1. CH3CH2CH2CHO
        2. A carbonyl carbon is bonded to a butyl group and a methyl group.
        3. A carbonyl carbon is bonded to 1 H atom and a benzene ring with a N O subscript 2 group attached to the fourth carbon from the site of attachment to the carbonyl carbon.
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