19.3 Spectroscopy of Aldehydes and Ketones 895

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d– O 33S C d+ % % bond dipole of the C O bond EPM of acetone

Because of their polarities, aldehydes and ketones have higher boiling points than alkenes or alkanes with similar molecular masses and shapes. But because aldehydes and ketones are not hydrogen-bond donors, their boiling points are considerably lower than those of the corre- sponding alcohols.

CH3CH A CH2 CH3CH AO CH3CH2OH boiling point 47.4 °C 20.8 °C 78.3 °C dipole moment -0.4 D 2.7 D 1.7 D

CH2 O OH S S C C "CH H C CH H C CH H C CH 3 % % 3 3 % % 3 3 M % 3 boiling point 6.9 °C 56.5 °C 82.3 °C dipole moment -0.5 D 2.7 D 1.7 D

Aldehydes and ketones with four or fewer carbons have considerable solubilities in water because they can accept hydrogen bonds from water at the carbonyl oxygen.

HOH H OH L O L 33S H3C C CH3 LL Acetaldehyde and acetone are miscible with water (that is, soluble in all proportions). The water solubility of aldehydes and ketones along a series diminishes rapidly with increasing molecular mass. Acetone and 2-butanone are especially valued as solvents because they dissolve not only water but also a wide variety of organic compounds. These solvents have sufﬁciently low boiling points that they can be easily separated from other less volatile compounds. Acetone, with a dielectric constant of 21, is a polar aprotic solvent and is often used as a solvent or co-solvent for nucleophilic substitution reactions.

19.3 SPECTROSCOPY OF ALDEHYDES AND KETONES

A. IR Spectroscopy The principal infrared absorption of aldehydes and ketones is the CAO stretching absorption, 1 a strong absorption that occurs in the vicinity of 1700 cm_ . In fact, this is one of the most important of all infrared absorptions. Because the CAO bond is stronger than the CAC bond, the stretching frequency of the CAO bond is greater. 19_BRCLoudon_pgs5-0.qxd 12/9/08 11:41 AM Page 896

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wavelength, micrometers 2.6 2.83 3.5 4 4.5 5 5.5 6 7 8 9 10 11 12 13 14 1516 100

60 O 40 H C stretch 20 CO percent transmittance stretch CH3CH2CH2CH O 0 3800 3400 3000 2600 2200 2000 1800 1600 1400 1200 1000 800 600 wavenumber, cm 1 _

Figure 19.3 The infrared spectrum of butyraldehyde with key absorptions indicated in red.

The position of the CAO stretching absorption varies predictably for different types of 1 carbonyl compounds. It generally occurs at 1710–1715 cm_ for simple ketones and at 1 1720–1725 cm_ for simple aldehydes. The carbonyl absorption is clearly evident, for example, in the IR spectrum of butyraldehyde (Fig. 19.3). The stretching absorption of the car- 1 bonyl–hydrogen bond of aldehydes near 2710 cm_ is another characteristic absorption; however, NMR spectroscopy provides a more reliable way to diagnose the presence of this type of hydrogen (Sec. 19.3B). Compounds in which the carbonyl group is conjugated with aromatic rings, double bonds, or triple bonds have lower carbonyl stretching frequencies than unconjugated carbonyl compounds.

O O O S S S C CH3 H2C A CH C CH3 compare: H2C A CHCH2CH3 H3C C CH2CH3 c LL LL LL 3-buten-2-one 1-butene 2-butanone acetophenone

$ A 1 1 1

$C O 1685 cm_ 1670 cm_ — 1715 cm_ $

A $ 1 1 1 $C C $ 1600 cm_ 1613 cm_ 1642 cm_ — (19.2)

The carbon–carbon double-bond stretching frequencies are also lower in the conjugated mol- ecules. These effects can be explained by the resonance structures for these compounds. Be- cause the CAO and CAC bonds have some single-bond character, as indicated by the following resonance structures, they are somewhat weaker than ordinary double bonds, and therefore absorb in the IR at lower frequency. 19_BRCLoudon_pgs5-0.qxd 12/9/08 11:41 AM Page 897

19.3 SPECTROSCOPY OF ALDEHYDES AND KETONES 897

single-bond character (19.3) O O _ 33S 332 H2C A CH C CH3 H2C| CH A"C CH3 LL L L In cyclic ketones with rings containing fewer than six carbons, the carbonyl absorption frequency increases signiﬁcantly as the ring size decreases. (See Further Exploration 19.1.)

Further Exploration 19.1 S S IR Absorptions of S V S Cyclic Ketones O O O O H2C AC AO cyclohexanone cyclopentanone cyclobutanone cyclopropanone ketene 1715 cm 1 1745 cm 1 1780 cm 1 1850 cm 1 2150 cm 1 (normal)_ _ _ _ _ (19.4)

PROBLEM 19.3 Explain how IR spectra could be used to differentiate the isomers within each of the following pairs. (a) cyclohexanone and hexanal (b) 3-cyclohexenone and 2-cyclohexenone (c) 2-butanone and 3-buten-2-ol

B. Proton NMR Spectroscopy The characteristic NMR absorption common to both aldehydes and ketones is that of the protons on the carbons adjacent to the carbonyl group: the a-protons. This absorption is in the d 2.0–2.5 region of the spectrum (see also Fig. 13.4 on p. 580). This absorption is slightly far- ther downﬁeld than the absorptions of allylic protons because the CAO group is more electronegative than the CAC group. In addition, the absorption of the aldehydic proton is quite distinctive, occurring in the d 9–10 region of the NMR spectrum, at lower ﬁeld than most other NMR absorptions.

a-protons a-protons aldehydic a-protons proton O O O S S S H3CCC H3 H3CCC H2 CH3 H3C C H d 2.17 L Ld 2.17 d 2.15 L Ld 2.40Ld 0.99 d 2.2 L Ld 9.8

In general, aldehydic protons have very large chemical shifts. The explanation is the same as that for the large chemical shifts of protons on a carbon–carbon double bond (Sec. 13.7A). However, the carbonyl group has a greater effect on chemical shift than a carbon–carbon double bond because of the electronegativity of the carbonyl oxygen. 19_BRCLoudon_pgs5-0.qxd 12/9/08 11:41 AM Page 898

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chemical shift, Hz 2400 2100 1800 1500 1200 900 600 300 0 Hz

6H J 7.1 Hz = J 7.1 Hz J 1.2 Hz = =

d 9.65 J (off scale) 1.2= Hz 1H

8 7 6 5 4 3 2 1 0 chemical shift, ppm (d)

Figure 19.4 The NMR spectrum for Problem 19.4(a). The relative integrals are indicated in red over their respective resonances.

PROBLEM 19.4 Deduce the structures of the following compounds. 1 (a) C4H8O: IR 1720, 2710 cm_ NMR in Fig. 19.4 1 (b) C4H8O: IR 1717 cm_ NMR d 0.95 (3H, t, J 8 Hz); d 2.03 (3H, s); d 2.38 (2H, q, J 8 Hz) = = 1 (c) A compound with molecular mass 70.1, IR absorption at 1780 cm_ , and the following NMR spectrum: d 2.01 (quintuplet,= J 7 Hz); d 3.09 (t, J 7 Hz). The inte- gral of the d 3.09 resonance is twice as large as that= of the d 2.01 resonance.= 1 1 (d) C10H12O2: IR 1690 cm_ , 1612 cm_ NMR d 1.4 (3H, t, J 8 Hz); d 2.5 (3H, s); d 4.1 (2H, q, J 8 Hz); d 6.9 (2H, d, J 9 Hz); d 7.9 (2H=, d, J 9 Hz) = = =

C. Carbon NMR Spectroscopy The most characteristic absorption of aldehydes and ketones in 13C NMR spectroscopy is that of the carbonyl carbon, which occurs typically in the d 190–220 range (see Fig. 13.20, p. 624). This large downﬁeld shift is due to the induced electron circulation in the p bond, as in alkenes (Fig. 13.14, p. 613), and to the additional chemical-shift effect of the electronegative carbonyl oxygen. Because the carbonyl carbon of a ketone bears no hydrogens, its 13C NMR absorption, like that of other quaternary carbons, is characteristically rather weak (Sec. 13.9). This effect is evident in the 13C NMR spectrum of propiophenone (Fig. 19.5). The a-carbon absorptions of aldehydes and ketones show modest downﬁeld shifts, typically in the d 30–50 range, with, as usual, greater shifts for more branched carbons. The a-carbon shift of propiophenone, 31.7 ppm (Fig. 19.5, carbon b) is typical. Because shifts in this range are also observed for other functional groups, these absorptions are less useful than the carbonyl carbon resonances for identifying aldehydes and ketones. 19_BRCLoudon_pgs5-0.qxd 12/9/08 11:41 AM Page 899

19.3 SPECTROSCOPY OF ALDEHYDES AND KETONES 899

c,d

d c O f b a e C CH CH g 2 3 (offset) e d c d 200.1 b a f g

200 190 180 170 160 150 140 130 120 110 100 90 80 70 60 50 40 30 20 10 0 chemical shift, ppm (d)

Figure 19.5 The 13C NMR spectrum of propiophenone.Two particularly important features of the spectrum are the large downﬁeld shift of the carbonyl carbon g,and the small resonances for the two carbons (f and g) that bear no protons. Recall from Sec. 13.9 that absorption intensities in 13C NMR spectra generally do not accurately corre- spond to numbers of carbons,and quaternary carbons generally have considerably weaker absorptions than proton-bearing carbons.

PROBLEMS 1 19.5 Propose a structure for a compound C6H12O that has IR absorption at 1705 cm_ , no proton NMR absorption at a chemical shift greater than d 3, and the following 13C NMR spectrum: d 24.4, d 26.5, d 44.2, and d 212.6. The resonances at d 44.2 and d 212.6 have very low intensity. 19.6 The 13C NMR spectrum of 2-ethylbutanal consists of the following absorptions: d 11.5, d 21.7, d 55.2, and d 204.7. Draw the structure of this aldehyde, label each chemically non- equivalent set of carbons, and assign each absorption to the appropriate carbon(s).

D. UV Spectroscopy The p p* absorptions (Sec. 15.2B) of unconjugated aldehydes and ketones occur at about 150 nm,T a wavelength well below the operating range of common UV spectrometers. Simple aldehydes and ketones also have another, much weaker, absorption at higher wavelength, in the 260–290 nm region. This absorption is caused by excitation of the unshared electrons on oxygen (sometimes called the n electrons). This high-wavelength absorption is usually re- ferred to as an n p* absorption. T

(CH3)2C A O n p* 271 nm (e = 16) (in ethanol) 2 2 n electrons

2 This absorption is easily distinguished from a p p* absorption because it is only 10_ to 3 T 10_ times as strong. However, it is strong enough that aldehydes and ketones cannot be used as solvents for UV spectroscopy. 19_BRCLoudon_pgs5-0.qxd 12/9/08 11:41 AM Page 900

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1.0 0.9 O 0.8 C CH3 0.7 803 concentration

0.6 p p* 0.5 n p*

absorbance 0.4 0.3 0.2 0.1 0 200 220 240 260 280 300 320 340 350 wavelength (l), nm

Figure 19.6 The ultraviolet spectrum of 1-acetylcyclohexene [1-(cyclohexen-1-yl)ethanone] in methanol. The spectrum of a more concentrated solution (red) reveals the “forbidden”n p* absorption, which is so weak that it is not apparent in the spectrum taken on a more dilute solution (black).T

Like conjugated dienes, the p electrons of compounds in which carbonyl groups are conjugated with double or triple bonds have strong absorption in the UV spectrum. The spectrum of 1-acetylcyclohexene (Fig. 19.6) is typical. The 232-nm peak is due to light absorption by the conjugated p-electron system and is thus a p p* absorption. It has a very large extinction coefﬁcient, much like that of a conjugated diene.T The weak 308-nm absorption is an n p* absorption. T

O S l 232 nm (e 13,200) conjugated 33 max p-electron system C CH3 lmax = 308 nm (e = 150) L L = (in methanol)=

1-acetylcyclohexene [1-(cyclohexen-1-yl)ethanone]

The lmax of a conjugated aldehyde or ketone is governed by the same variables that affect the lmax values of conjugated dienes: the number of conjugated double bonds, substitution on the double bond, and so on. When an aromatic ring is conjugated with a carbonyl group, the typical aromatic absorptions are more intense and shifted to higher wavelengths than those of benzene. O S C CH3 c c LL lmax 204 (e 7900) lmax 240 (e 13,000) = 254 (e = 212) = 278 (e = 1100) = 319 (e = 50) (n p *) = 19_BRCLoudon_pgs5-0.qxd 12/9/08 11:41 AM Page 901

19.3 SPECTROSCOPY OF ALDEHYDES AND KETONES 901

The p p* absorptions of conjugated carbonyl compounds, like those of conjugated alkenes, ariseT from the promotion of a p electron from a bonding to an antibonding (p*) molecular orbital (Sec. 15.2B). An n p* absorption arises from promotion of one of the n (unshared) electrons on a carbonyl oxygenT to a p* molecular orbital. As stated previously in this section, n p* absorptions are weak. Spectroscopists say that these absorptions are forbidden. This termT refers to certain physical reasons for the very low intensity of these absorptions. The 254-nm absorption of benzene, which has a very low extinction coefﬁcient of 212, is another example of a “forbidden” absorption.

PROBLEMS 19.7 Explain how the compounds within each set can be distinguished using only UV spectroscopy. (a) 2-cyclohexenone and 3-cyclohexenone (b) O O ( and (

(c) 1-phenyl-2-propanone and p-methylacetophenone 19.8 In neutral alcohol solution, the UV spectra of p-hydroxyacetophenone and p-methoxy-

acetophenone are virtually identical. When NaOH is added to the solution, the lmax of p-hydroxyacetophenone increases by about 50 nm, but that of p-methoxyacetophenone is unaf- fected. Explain these observations. 19.9 Which one of the following compounds should have a p p* UV absorption at the greater

lmax when the compound is dissolved in NaOH solution? TExplain.

CH3O HO

HOCH O CH3O CH O

vanillin isovanillin A B

E. Mass Spectrometry Important fragmentations of aldehydes and ketones are illustrated by the electron-impact (EI) mass spectrum of 5-methyl-2-hexanone (Fig. 19.7, p. 902). The three most important peaks occur at mÜz 71, 58, and 43. The peaks at mÜz 71 and mÜz 43 arise from cleavage of the molecular= ion at the bond between the carbonyl= group and an adjacent= carbon atom by two mechanisms that were discussed in Sec. 12.6C: inductive cleavage and a-cleavage. Inductive cleavage accounts for the mÜz 71 peak. In this cleavage, the alkyl fragment carries the charge and the carbonyl fragment= carries the unpaired electron.

inductive O| O| cleavage 3S8 33 (CH3)2CHCH2CH2 C CH3 (CH3)2CHCH2CH2 "C CH3 L L LL molecular ion from loss of unshared electron 8 O 33S (CH3)2CHCH2CH| 2 C CH3 (19.5) m͞z 71 + 8 L = 19_BRCLoudon_pgs5-0.qxd 12/9/08 11:41 AM Page 902

902 CHAPTER 19 • THE CHEMISTRY OF ALDEHYDES AND KETONES. CARBONYL-ADDITION REACTIONS

100 43 80

40 58 O

20 (CH3)2CHCH2CH2CCH3 relative abundance 71 0 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 mass-to-charge ratio m/z

Figure 19.7 The EI mass spectrum of 5-methyl-2-hexanone. The odd-electron ion at mÜz 58 results from a McLafferty rearrangement of the molecular ion. =

a-Cleavage accounts for the mÜz 43 peak. In this case the same molecular ion undergoes fragmentation in such a way that the= carbonyl fragment carries the charge and the alkyl fragment carries the unpaired electron:

O| a-cleavage 3S8 (CH ) CHCH CH C CH (CH ) CHCH CH O| ' C CH (19.6) 3 2 2 2 3 3 2 2 8 2 3 L L + 3 m͞z L 43 = An analogous cleavage at the carbon–hydrogen bond accounts for the fact that many aldehydes show a strong M 1 peak. What accounts for the- mÜz 58 peak? A common mechanism for the formation of odd- electron ions is hydrogen transfer= followed by loss of a stable neutral molecule (p. 564). In this case, the oxygen radical in the molecular ion abstracts a hydrogen atom from a carbon ﬁve atoms away, and the resulting radical then undergoes a-cleavage.

McLafferty rearrangement H H O| C(CH ) O| C(CH ) O|H C(CH ) % 3 2 % 3 2 3 2 3S8 3S 8 3S S H3CC "CH2 H3CC "CH2 H3CC CH2 (19.7) L L L CH + %CH2% %CH2% % 8 2 hydrogen transfer a-cleavage m͞z 58 = If we count the hydrogen that is transferred, the ﬁrst step occurs through a transient six-mem- bered ring. This process is called a McLafferty rearrangement, after Professor Fred McLaf- ferty of Cornell University, who investigated this type of fragmentation extensively. The McLafferty rearrangement and subsequent a-cleavage constitute a common mechanism for the production of odd-electron fragment ions in the mass spectrometry of carbonyl compounds. As illustrated by Fig. 19.7, the EI mass spectra of many aldehydes and ketones have weak molecular ions, because relatively stable fragment ions can be formed. However, as we’ll learn in Sec. 19.6, the carbonyl oxygen can be protonated. As a result, chemical-ionization (CI) mass spectra (Sec. 12.6D) of aldehydes and ketones show very strong M 1 ions, from which accurate molecular masses can be determined. + 19_BRCLoudon_pgs5-0.qxd 12/9/08 11:41 AM Page 903

19.5 INTRODUCTION TO ALDEHYDE AND KETONE REACTIONS 903

PROBLEMS 19.10 Explain each of the following observations resulting from a comparison of the mass spectra of 2-hexanone (A) and 3,3-dimethyl-2-butanone (B). (a) The mÜz 57 fragment peak is much more intense in the spectrum of B than it is in the spectrum= of A. (b) The spectrum of compound A shows a fragment at mÜz 58, but that of compound B does not. = 19.11 Using only mass spectrometry, how would you distinguish 2-heptanone from 3-heptanone?

19.4 SYNTHESIS OF ALDEHYDES AND KETONES

Several reactions presented in previous chapters can be used for the preparation of aldehydes and ketones. The four most important of these are:

1. Oxidation of alcohols (Secs. 10.6A and 17.5A). Primary alcohols can be oxidized to aldehydes, and secondary alcohols can be oxidized to ketones. 2. Friedel–Crafts acylation (Sec. 16.4F). This reaction provides a way to synthesize aryl ketones. It also involves the formation of a carbon–carbon bond, the bond between the aryl ring and the carbonyl group. 3. Hydration of alkynes (Sec. 14.5A) 4. Hydroboration–oxidation of alkynes (Sec. 14.5B)

Two other reactions have been discussed that give aldehydes or ketones as products, but these are less important as synthetic methods:

1. Ozonolysis of alkenes (Sec. 5.5) 2. Periodate cleavage of glycols (Sec. 11.5B)

Ozonolysis and periodate cleavage are reactions that break carbon–carbon bonds. Because an important aspect of organic synthesis is the making of carbon–carbon bonds, use of these reactions in effect wastes some of the effort that goes into making the alkene or glycol start- ing materials. Nevertheless, these reactions can be used synthetically in certain cases. Other important methods of preparing aldehydes and ketones start with carboxylic acid de- rivatives; these methods are discussed in Chapter 21. Appendix V gives a summary of all of the synthetic methods for aldehydes and ketones, arranged in the order in which they appear in the text.

19.5 INTRODUCTION TO ALDEHYDE AND KETONE REACTIONS

The reactions of aldehydes and ketones can be grouped into two categories: (1) reactions of the carbonyl group, which are considered in this chapter; and (2) reactions involving the a- carbon, which are presented in Chapter 22. The great preponderance of carbonyl-group reactions of aldehydes and ketones fall into three categories:

1. Reactions with acids. The carbonyl oxygen is weakly basic and thus reacts with Lewis

and Brønsted acids. With E| as a general electrophile, this reaction can be represented as follows: