19.2: Naming Aldehydes and Ketones

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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 19.1.

Table 19.1 Common Names of Some Simple Aldehydes
Formula	Common name	Systematic name
HCHO	Formaldehyde	Methanal
CH₃CHO	Acetaldehyde	Ethanal
$H_{2} C═CHCHO$	Acrolein	Propenal
${CH}_{3} CH═CHCHO$	Crotonaldehyde	2-Butenal
	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 $R–C═O$ 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.

(b)

A three-carbon chain in which (from left to right) first carbon has a phenyl group and third is part of C H O. (c)

An eight-carbon chain in which (from left to right) second and sixth carbons have oxo groups. (d)

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. (e)

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. (f)

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.

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