CHAPTER 17 ALDEHYDES AND KETONES: NUCLEOPHILIC ADDITION TO THE CARBONYL GROUP A ldehydes and ketones contain an acyl group RC- bonded either to hydrogen or to another carbon HCH RCH RCR′ Although the present chapter includes the usual collection of topics designed to acquaint us with a particular class of compounds, its central theme is a fundamental reaction typ cleophilic addition to carbonyl groups. The principles of nucleophilic addition to alde des and ketones developed here will be seen to have broad applicability in later chap- ters when transformations of various derivatives of carboxylic acids are discussed 17.1 NOMENCLATURE The longest continuous chain that contains the -CH group provides the base name for aldehydes. The -e ending of the corresponding alkane name is replaced by -al, and sub- stituents are specified in the usual way. It is not necessary to specify the location of the -CH group in the name, since the chain must be numbered by starting with this group as C-1. The suffix -dial is added to the appropriate alkane name when the com pound contains two aldehyde functions. The -e ending of an alkane name is dropped before a suffix beginning with a vowel (-aln) and retained be- fore one beginning with a consonant (-dial Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
654 CHAPTER 17 ALDEHYDES AND KETONES: NUCLEOPHILIC ADDITION TO THE CARBONYL GROUP Aldehydes and ketones contain an acyl group bonded either to hydrogen or to another carbon. Although the present chapter includes the usual collection of topics designed to acquaint us with a particular class of compounds, its central theme is a fundamental reaction type, nucleophilic addition to carbonyl groups. The principles of nucleophilic addition to aldehydes and ketones developed here will be seen to have broad applicability in later chapters when transformations of various derivatives of carboxylic acids are discussed. 17.1 NOMENCLATURE The longest continuous chain that contains the group provides the base name for aldehydes. The -e ending of the corresponding alkane name is replaced by -al, and substituents are specified in the usual way. It is not necessary to specify the location of the group in the name, since the chain must be numbered by starting with this group as C-1. The suffix -dial is added to the appropriate alkane name when the compound contains two aldehyde functions.* ±CH O X ±CH O X RCH O X Aldehyde HCH O X Formaldehyde RCR O X Ketone RC± O X * The -e ending of an alkane name is dropped before a suffix beginning with a vowel (-al) and retained before one beginning with a consonant (-dial). Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
CHaCCHCHCH CH=CHCHCHoCHCH HCCHCH 4, 4-Dimethylpentanal 5-Hexenal 2-Phenylpropanedial When a formyl group(-CH-O)is attached to a ring, the ring name is followe by the suffix -carbaldehyde CH CH yclopentanecarbaldehyde nthalenecarbaldehyde Certain common names of familiar aldehydes are acceptable as IUPAC few examples include O HCH Benzaldehyde (ethanal) (benzenecarbaldehyde) PROBLEM 17.1 The common names and structural formulas of a few aldehydes follow Provide an iuPac name (a)(CH3)2CHCH (C)C6HsCH=CHCH (isobutyraldehyde) (cinnamaldehyde) (b)HCCH2 CH, CH2 CH d)Ho一 (glutaraldehyde) CHO SAMPLE SoLUTION (a) Don ' t be fooled by the fact that the common name is isobutyraldehyde the longest continuous chain has three carbons, and so the base name is propanal. there is a methyl group at C-2; thus the compound is 2-methyl- CH3 CHCH (isobutyraldehy Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
When a formyl group (±CHœO) is attached to a ring, the ring name is followed by the suffix -carbaldehyde. Certain common names of familiar aldehydes are acceptable as IUPAC names. A few examples include PROBLEM 17.1 The common names and structural formulas of a few aldehydes follow. Provide an IUPAC name. (a) (c) (b) (d) SAMPLE SOLUTION (a) Don’t be fooled by the fact that the common name is isobutyraldehyde. The longest continuous chain has three carbons, and so the base name is propanal. There is a methyl group at C-2; thus the compound is 2-methylpropanal. 2-Methylpropanal (isobutyraldehyde) CH3CHCH O CH3 3 2 1 HO CH CH3O O (vanillin) HCCH2CH2CH2CH O O (glutaraldehyde) C6H5CH CHCH O (cinnamaldehyde) (CH3)2CHCH O (isobutyraldehyde) HCH O Formaldehyde (methanal) CH3CH O Acetaldehyde (ethanal) CH O Benzaldehyde (benzenecarbaldehyde) CH O Cyclopentanecarbaldehyde CH O 2-Naphthalenecarbaldehyde CH3CCH2CH2CH CH3 O CH3 4,4-Dimethylpentanal CHCH2CH2CH2CH O CH2 5-Hexenal HCCHCH O O 2-Phenylpropanedial 17.1 Nomenclature 655 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
CHAPTER SEVENTEEN Aldehydes and Ketones: Nucleophilic Addition to the Carbonyl Group with ketones, the -e ending of an alkane is replaced by -one in the longest con- tinuous chain containing the carbonyl group The chain is numbered in the direction that provides the lower number for this group CH3CH,CCH-CH,CH3 CH3CHCH_ CCH3 CH3 3-Hexanone 4-Methyl-2-pentanone 4-Methylcyclohexanone Although substitutive names of the type just described are preferred, the IUPAC rules also permit ketones to be named by functional class nomenclature. The groups attached to the carbonyl group are named as separate words followed by the word ketone. The groups are listed alphabetically O CHa,CCH,CH,CH3 CH,CCH, CH3 CH=CHCCH=CH2 Ethyl propy Benzyl ethyl ketone Divinyl ketone PROBLEM 17.2 Convert each of the following functional class IUPAC names to a substitutive name (b)Ethyl isopropyl ketone (cMethyl 2, 2-dimethylpropyl ketone d) Allyl methyl ketone SAMPLE SOLUTION (a)First write the structure corresponding to the name Dibenzyl ketone has two benzyl groups attached to a carbonyl CH2CCH benzyl ketone The longest continuous chain contains three carbons, and C-2 is the carbon of the carbonyl group. The substitutive IUPAC name for this ketone is 1, 3-diphenyl-2- A few of the common names acceptable for ketones in the IUPAC system are CH3CCH3 CCH Acetophenone Benzophenone (The suffix -phenone indicates that the acyl group is attached to a benzene ring. Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
With ketones, the -e ending of an alkane is replaced by -one in the longest continuous chain containing the carbonyl group. The chain is numbered in the direction that provides the lower number for this group. Although substitutive names of the type just described are preferred, the IUPAC rules also permit ketones to be named by functional class nomenclature. The groups attached to the carbonyl group are named as separate words followed by the word “ketone.” The groups are listed alphabetically. PROBLEM 17.2 Convert each of the following functional class IUPAC names to a substitutive name. (a) Dibenzyl ketone (b) Ethyl isopropyl ketone (c) Methyl 2,2-dimethylpropyl ketone (d) Allyl methyl ketone SAMPLE SOLUTION (a) First write the structure corresponding to the name. Dibenzyl ketone has two benzyl groups attached to a carbonyl. The longest continuous chain contains three carbons, and C-2 is the carbon of the carbonyl group. The substitutive IUPAC name for this ketone is 1,3-diphenyl-2- propanone. A few of the common names acceptable for ketones in the IUPAC system are (The suffix -phenone indicates that the acyl group is attached to a benzene ring.) CH3CCH3 O Acetone CCH3 O Acetophenone C O Benzophenone CH2CCH2 O 1 23 Dibenzyl ketone CH3CH2CCH2CH2CH3 O Ethyl propyl ketone CH2CCH2CH3 O Benzyl ethyl ketone CH2 O CHCCH CH2 Divinyl ketone CH3CH2CCH2CH2CH3 O 3-Hexanone CH3CHCH2CCH3 O CH3 4-Methyl-2-pentanone CH3 O 4-Methylcyclohexanone 656 CHAPTER SEVENTEEN Aldehydes and Ketones: Nucleophilic Addition to the Carbonyl Group Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
17.2 Structure and Bonding: The Carbonyl Group 17.2 STRUCTURE AND BONDING: THE CARBONYL GROUP Two notable aspects of the carbonyl group are its geometry and its polarity. The car bonyl group and the atoms directly attached to it lie in the same plane. Formaldehyde, for example, is planar. The bond angles involving the carbonyl group of aldehydes and ketones are close to 120 121.7°121.7° 1239°1186° 121.4° making erify their gemalds by tone. Make sure you execute the H H HC H 116.5° 1175° Formaldehyde Acetaldehyde Aceton At 122 pm, the carbon-oxygen double bond distance in aldehydes and ketones is sig- nificantly shorter than the typical carbon-oxygen single bond distance of 141 pm in alco- ols and ethers The carbonyl group makes aldehydes and ketones rather polar, with molecular dipole moments that are substantially larger than those of comparable compounds that contain carbon-carbon double bonds CH,CH=CH CH3CH,CH=O 1-Butene Propanal Dipole moment: 0.3D Dipole moment: 2.5D Bonding in formaldehyde can be described according to an sp 2 hybridization model on Learning By Mo analogous to that of ethylene, as shown in Figure 17.1 Figure 17. 2 compares the electrostatic potential surfaces of ethylene and formalde hyde and vividly demonstrates how oxygen affects the electron distribution in formalde- hyde. The electron density in both the o and T components of the carbon-oxygen dou ble bond is displaced toward oxygen. The carbonyl group is polarized so that carbon is partially positive and oxygen is partially negative FIGURE 17.1 Similari- ties between the orbital idization bonding in (a)ethylene and In resonance terms, electron delocalization in the carbonyl group is represented by (b) formaldehyde. Both mol contributions from two principal resonance structures z-hybridized in both I carbons is replaced by an sp hybridized xvaen in red). Oxygen has two hybridized orbital. Like the carbon-carbon double bond formaldehyde is composed of a two-electron nent and a two-electron TT (b) Formaldehyde Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
17.2 STRUCTURE AND BONDING: THE CARBONYL GROUP Two notable aspects of the carbonyl group are its geometry and its polarity. The carbonyl group and the atoms directly attached to it lie in the same plane. Formaldehyde, for example, is planar. The bond angles involving the carbonyl group of aldehydes and ketones are close to 120°. At 122 pm, the carbon–oxygen double bond distance in aldehydes and ketones is significantly shorter than the typical carbon–oxygen single bond distance of 141 pm in alcohols and ethers. The carbonyl group makes aldehydes and ketones rather polar, with molecular dipole moments that are substantially larger than those of comparable compounds that contain carbon–carbon double bonds. Bonding in formaldehyde can be described according to an sp2 hybridization model analogous to that of ethylene, as shown in Figure 17.1. Figure 17.2 compares the electrostatic potential surfaces of ethylene and formaldehyde and vividly demonstrates how oxygen affects the electron distribution in formaldehyde. The electron density in both the and components of the carbon–oxygen double bond is displaced toward oxygen. The carbonyl group is polarized so that carbon is partially positive and oxygen is partially negative. In resonance terms, electron delocalization in the carbonyl group is represented by contributions from two principal resonance structures: C O or C O CH3CH2CH CH2 1-Butene Dipole moment: 0.3 D CH3CH2CH O Propanal Dipole moment: 2.5 D C O H H 116.5° 121.7° 121.7° Formaldehyde C O H3C H 117.5° 123.9° 118.6° Acetaldehyde C O H3C CH3 117.2° 121.4° 121.4° Acetone 17.2 Structure and Bonding: The Carbonyl Group 657 (a) Ethylene (b) Formaldehyde FIGURE 17.1 Similarities between the orbital hybridization models of bonding in (a) ethylene and (b) formaldehyde. Both molecules have the same number of electrons, and carbon is sp2 -hybridized in both. In formaldehyde, one of the carbons is replaced by an sp2 - hybridized oxygen (shown in red). Oxygen has two unshared electron pairs; each pair occupies an sp2 - hybridized orbital. Like the carbon–carbon double bond of ethylene, the carbon–oxygen double bond of formaldehyde is composed of a two-electron component and a two-electron component. Verify their geometries by making models of formaldehyde, acetaldehyde, and acetone. Make sure you execute the minimization routine. Compare the dipole moments and electrostatic potential maps of 1-butene and propanal on Learning By Modeling. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
CHAPTER SEVENTEEN Aldehydes and Ketones: Nucleophilic Addition to the Carbonyl Group FIGURE 17.2 Differ- ences in the electron distribu tion of (a) ethylene and (b)formaldehyde. The region of highest electrostatic tential (red )in ethylene lies above and below the plane of the atoms and is associated with the electrons. the re- gion close to oxygen is the site of highest electrostatic potential in formaldehyde (a) Ethylene (b) Formaldehyde stry of the hat carbon is partially posi- A tive(has carbocation charac- ter)and oxygen is partially Of these two, A, having one more covalent bond and avoiding the separation of positive and negative charges that characterizes B, better approximates the bonding in a carbonyl group. Alkyl substituents stabilize a carbonyl group in much the same way that they sta bilize carbon-carbon double bonds and carbocations-by releasing electrons to sp hybridized carbon. Thus, as their heats of combustion reveal, the ketone 2-butanone more stable than its aldehyde isomer butanal CH3 CH,CH,CH CH3 CH, CCH3 Butanal 2-Butanone Heat of combustion: 2475 kJ/mol(592 kcal/mol) 2442 k/mol (584 kcal/mol) The carbonyl carbon of a ketone bears two electron-releasing alkyl groups; an aldehyde carbonyl group has only one. Just as a disubstituted double bond in an alkene is more stable than a monosubstituted double bond, a ketone carbonyl is more stable than an aldehyde carbonyl. We'll see later in this chapter that structural effects on the relative stability of carbonyl groups in aldehydes and ketones are an important factor in their rel ative reactivity. 17.3 PHYSICAL PROPERTIES In general, aldehydes and ketones have higher boiling points than alkenes because they int, boiling point, are more polar and the dipole-dipole attractive forces between molecules are stronger. ty in water are ected for a variety of But they have lower boiling points than alcohols because, unlike alcohols, two carbonyl aldehydes and ketones in groups can't form hydrogen bonds to each other. Appendix 1 CH3CH, CH=CH, CH3 CH,CH=O CH3 CH,CH,OH 1-Butene bp(I atm) Solubility in Negligible Miscible in alll water(g/100 mL) Aldehydes and ketones can form hydrogen bonds with the protons of oH groups. This makes them more soluble in water than alkenes. but less soluble than alcohols Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
Of these two, A, having one more covalent bond and avoiding the separation of positive and negative charges that characterizes B, better approximates the bonding in a carbonyl group. Alkyl substituents stabilize a carbonyl group in much the same way that they stabilize carbon–carbon double bonds and carbocations—by releasing electrons to sp2 - hybridized carbon. Thus, as their heats of combustion reveal, the ketone 2-butanone is more stable than its aldehyde isomer butanal. The carbonyl carbon of a ketone bears two electron-releasing alkyl groups; an aldehyde carbonyl group has only one. Just as a disubstituted double bond in an alkene is more stable than a monosubstituted double bond, a ketone carbonyl is more stable than an aldehyde carbonyl. We’ll see later in this chapter that structural effects on the relative stability of carbonyl groups in aldehydes and ketones are an important factor in their relative reactivity. 17.3 PHYSICAL PROPERTIES In general, aldehydes and ketones have higher boiling points than alkenes because they are more polar and the dipole–dipole attractive forces between molecules are stronger. But they have lower boiling points than alcohols because, unlike alcohols, two carbonyl groups can’t form hydrogen bonds to each other. Aldehydes and ketones can form hydrogen bonds with the protons of OH groups. This makes them more soluble in water than alkenes, but less soluble than alcohols. CH3CH2CH CH2 1-Butene 6°C Negligible bp (1 atm) Solubility in water (g/100 mL) CH3CH2CH O Propanal 49°C 20 CH3CH2CH2OH 1-Propanol 97°C Miscible in all proportions CH3CH2CH2CH O Butanal 2475 kJ/mol (592 kcal/mol) CH3CH2CCH3 O 2-Butanone Heat of combustion: 2442 kJ/mol (584 kcal/mol) C O B C O A 658 CHAPTER SEVENTEEN Aldehydes and Ketones: Nucleophilic Addition to the Carbonyl Group The chemistry of the carbonyl group is considerably simplified if you remember that carbon is partially positive (has carbocation character) and oxygen is partially negative (weakly basic). Physical constants such as melting point, boiling point, and solubility in water are collected for a variety of aldehydes and ketones in Appendix 1. (a) Ethylene (b) Formaldehyde FIGURE 17.2 Differences in the electron distribution of (a) ethylene and (b) formaldehyde. The region of highest electrostatic potential (red) in ethylene lies above and below the plane of the atoms and is associated with the electrons. The region close to oxygen is the site of highest electrostatic potential in formaldehyde. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website