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The Chemistry of Aldehydes and Ketones. Carbonyl-Addition Reactions

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The Chemistry of Aldehydes and Ketones. Carbonyl-Addition Reactions 19_BRCLoudon_pgs5-0.qxd 12/9/08 11:41 AM Page 888 19 19 The Chemistry of Aldehydes and Ketones. Carbonyl-Addition Reactions This chapter begins the study of carbonyl compounds—compounds containing the carbonyl group, CAO. Aldehydes, ketones, carboxylic acids, and the carboxylic acid derivatives (es- ters, amides, anhydrides, and acid chlorides) are all carbonyl compounds. This chapter focuses on the nomenclature, properties, and characteristic carbonyl-group reactions of aldehydes and ketones. Chapters 20 and 21 consider carboxylic acids and carboxylic acid derivatives, respectively. Chapter 22 deals with ionization, enolization, and condensation reactions, which are common to the chemistry of all classes of carbonyl compounds. Aldehydes and ketones have the following general structures: O O S carbonyl group S RHC RRC Ј L L L L aldehyde ketone Examples: O O S S H3CHC H3CC CH3 L L L L acetaldehyde acetone (an aldehyde) (a ketone) 888 19_BRCLoudon_pgs5-0.qxd 12/9/08 11:41 AM Page 889 THE CHEMISTRY OF ALDEHYDES AND KETONES. CARBONYL-ADDITION REACTIONS 889 C—O s bond C—O p bond (bonding p MO) Figure 19.1 Bonding in formaldehyde, the simplest aldehyde, is typical of bonding in aldehydes and ketones. The carbonyl group and the two hydrogens lie in the same plane, and both a s bond and a p bond connect the carbonyl carbon and oxygen.The p bond is a bonding p molecular orbital (MO) occupied by two electrons. In a ketone, the groups bound to the carbonyl carbon (R and RЈ in the preceding structures) are alkyl or aryl groups. In an aldehyde, at least one of the groups at the carbonyl carbon atom is a hydrogen, and the other may be alkyl, aryl, or a second hydrogen. The carbonyl carbon of a typical aldehyde or ketone is sp2-hybridized with bond angles ap- proximating 120 . The carbon–oxygen double bond consists of a s bond and a p bond, much like the double bond° of an alkene (Fig. 19.1). Just as C O single bonds are shorter than C C single bonds, CAO bonds are shorter than CAC bondsL (Sec. 1.3B). The structures of someL simple aldehydes and ketones are given in Fig. 19.2. 1.21 Å 1.21 Å 1.34 Å 124° 1.21 Å 125° 118° H3CH1.51 Å 3CH116° 3C 1.50 Å Figure 19.2 Structures of aldehydes and ketones.(a) The structures of formaldehyde,acetaldehyde,and acetone are compared with the structure of propene.The CAO bonds are shorter than the CAC bond, and the carbonyl carbon is trigonal planar with bond angles very close to 120 .(b) A ball-and-stick model of acetone.(c) A space- filling model of acetone. ° 19_BRCLoudon_pgs5-0.qxd 12/9/08 11:41 AM Page 890 890 CHAPTER 19 • THE CHEMISTRY OF ALDEHYDES AND KETONES. CARBONYL-ADDITION REACTIONS 19.1 NOMENCLATURE OF ALDEHYDES AND KETONES A. Common Nomenclature Common names are almost always used for the simplest aldehydes. In common nomenclature the suffix aldehyde is added to a prefix that indicates the chain length. A list of prefixes is given in Table 19.1. H2C A O H3C CH2 CH2 CH AO L L L formaldehyde butyr aldehyde butyraldehyde + = Acetone is the common name for the simplest ketone, and benzaldehyde is the simplest aro- matic aldehyde. O S CH A O H3CC CH3 c L L L acetone benzaldehyde Certain aromatic ketones are named by attaching the suffix ophenone to the appropriate prefix from Table 19.1. O O S S H3C C C L L c v LL v acet ophenone acetophenone benzophenone + = The common names of some ketones are constructed by citing the two groups on the car- bonyl carbon followed by the word ketone. O O S S C C M M M M cyclohexyl phenyl ketone dicyclohexyl ketone TABLE 19.1 Prefixes Used in Common Nomenclature of Carbonyl Compounds R in R CHAORin R CHAO Prefix orL R COL2H Prefix orL R COL2H L L form H isobutyr (CH3)2CH L L acet H3C valer CH3CH2CH2CH2 L L propion, propi* CH3CH2 isovaler (CH3)2CHCH2 L L † butyr CH3CH2CH2 benz, benzo Ph L L *Used in phenone nomenclature as discussed in the text. †Used in carboxylic acid nomenclature (Sec. 20.1A). 19_BRCLoudon_pgs5-0.qxd 12/9/08 11:41 AM Page 891 19.1 NOMENCLATURE OF ALDEHYDES AND KETONES 891 Simple substituted aldehydes and ketones can be named in the common system by desig- nating the positions of substituents with Greek letters, beginning at the position adjacent to the carbonyl group. O O g baS S CH2 CH2 CH2 C BrCH2CH2CH L L L L L b-bromopropionaldehyde As suggested by this nomenclature, a carbon adjacent to the carbonyl group is called the a-carbon, and the hydrogens on the a-carbon are called a-hydrogens. Many common carbonyl-containing substituent groups are named by a simple extension of the terminology in Table 19.1: the suffix yl is added to the appropriate prefix. The following names are examples: O O O O S S S S HC H3CC CH3CH2 C Ph C L L L L L L L L formyl group acetyl group propionyl group benzoyl group Such groups are called in general acyl groups. (This is the source of the term acylation, used in Sec. 16.4F.) To be named as an acyl group, a substituent group must be connected to the re- mainder of the molecule at its carbonyl carbon. O O S S H3C C C H L L c L L p-acetylbenzaldehyde Be careful not to confuse the benzoyl group, an acyl group, with the benzyl group, an alkyl group. The benzoyl group has an “o” in both the name and the structure. O S Ph C Ph CH2 L L L L benzoyl group benzyl group A great many aldehydes and ketones were well known long before any system of nomencla- ture existed. These are known by the traditional names illustrated by the following examples: CH A O Ph CHA CH CH A O O L H2C A CH CH A O LL L cinnamaldehyde furfural acrolein 19_BRCLoudon_pgs5-0.qxd 12/9/08 11:41 AM Page 892 892 CHAPTER 19 • THE CHEMISTRY OF ALDEHYDES AND KETONES. CARBONYL-ADDITION REACTIONS B. Substitutive Nomenclature The substitutive name of an aldehyde is constructed from a prefix indicating the length of the carbon chain followed by the suffix al. The prefix is the name of the corresponding hydrocarbon without the final e. CH3CH2CH2CH A O butane al butanal + = In numbering the carbon chain of an aldehyde, the carbonyl carbon receives the number one. 4321 CH3CH2CHCH A O "CH3 2-methylbutanal Note carefully the difference in chain numbering of aldehydes in common and substitutive nomenclature. In common nomenclature, numbering begins at the carbon adjacent to the car- bonyl (the a-carbon); in substitutive nomenclature, numbering begins at the carbonyl carbon itself. As with diols, the final e is not dropped when the carbon chain has more than one aldehyde group. OA CH CH2CH2CH2CH2 CH A O LL hexanedial When an aldehyde group is attached to a ring, the suffix carbaldehyde is appended to the name of the ring. (In older literature, the suffix carboxaldehyde was used.) CH A O 0L cyclohexanecarbaldehyde In aldehydes of this type, carbon-1 is not the carbonyl carbon, but rather the ring carbon attached to the carbonyl group. CH3 3 2 ) 1 4 CH A O 0L 2-methylcyclohexanecarbaldehyde The name benzaldehyde (Sec. 19.1A) is used in both common and substitutive nomenclature. A ketone is named by giving the hydrocarbon name of the longest carbon chain containing the carbonyl group, dropping the final e, and adding the suffix one. The position of the car- bonyl group is given the lowest possible number. 19_BRCLoudon_pgs5-0.qxd 12/9/08 11:41 AM Page 893 19.1 NOMENCLATURE OF ALDEHYDES AND KETONES 893 O S H3CCH2 CH2 C CH3 5 L 4 L 321L L five-carbon chain: pentane one pentanone position of carbonyl: 2-pentanone+ = O O A S 1 2 M 3 M CH3 0 CH3 cyclohexane one cyclohexanone + = 3,3-dimethylcyclohexanone As with diols and dialdehydes, the final e of the hydrocarbon name is not dropped in the nomenclature of diones, triones, and so on. O O S S H3C C CH2 C CH2 CH3 L L L L L six-carbon chain: hexane dione hexanedione positions of carbonyls: 2,4-hexanedione+ = Aldehyde and ketone carbonyl groups receive higher priority than OH or SH groups for citation as principal groups (Sec. 8.1B). L L Priority for citation as principal group: O O S S CH (aldehyde) > C (ketone) > OH > SH (19.1) L L L L L Study Problem 19.1 Provide a substitutive name for the following compound. (The numbers are used in the solution that follows.) O OH S 5 H3C C CH2 "CH CH CH2 CH2 CH3 1 L 2 L 3 L 4 L (L L L "C 6 H3C CH2 M 7 Solution To name this compound, use the nomenclature rules in Sec. 8.1B. First, identify the principal group. Possible candidates are the carbonyl group at carbon-2 and the hydroxy group at carbon-4. Because ketones have a higher citation priority than hydroxy groups (Eq. 19.1), the compound is named as a ketone with the suffix one. Next, identify the principal chain. This is the longest carbon chain containing the principal group and the greatest number of double and triple bonds. This chain (numbered in the preceding structure) contains seven carbons.
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