19.3 SPECTROSCOPY OF ALDEHYDES AND KETONES

Transcription

1 19.3 PETRPY F ALDEHYDE AND KETNE 895 d 33 d+ bond dipole of the bond EPM of aetone Beause of their polarities, aldehydes and ketones have higher boiling points than alkenes or alkanes with similar moleular masses and shapes. But beause aldehydes and ketones are not hydrogen-bond donors, their boiling points are onsiderably lower than those of the orresponding alohols. boiling point dipole moment H 3 H A H D H 3 H A D H 3 H 2 H D boiling point dipole moment H 2 H 3 H D H 3 H D H " M H 3 H H D Aldehydes and ketones with four or fewer arbons have onsiderable solubilities in water beause they an aept hydrogen bonds from water at the arbonyl oxygen. H L H H L H 33 H 3 L LH 3 Aetaldehyde and aetone are misible with water (that is, soluble in all proportions). The water solubility of aldehydes and ketones along a series diminishes rapidly with inreasing moleular mass. Aetone and 2-butanone are espeially valued as solvents beause they dissolve not only water but also a wide variety of organi ompounds. These solvents have suffiiently low boiling points that they an be easily separated from other less volatile ompounds. Aetone, with a dieletri onstant of 21, is a polar aproti solvent and is often used as a solvent or o-solvent for nuleophili substitution reations PETRPY F ALDEHYDE AND KETNE A. IR petrosopy The prinipal infrared absorption of aldehydes and ketones is the A strething absorption, a strong absorption that ours in the viinity of 1700 m _1. In fat, this is one of the most important of all infrared absorptions. Beause the A bond is stronger than the A bond, the strething frequeny of the A bond is greater.

2 896 HAPTER 19 THE HEMITRY F ALDEHYDE AND KETNE. ARBNYL-ADDITIN REATIN 100 wavelength, mirometers perent transmittane H streth streth H 3 H 2 H 2 H wavenumber, m _1 Figure 19.3 The infrared spetrum of butyraldehyde with key absorptions indiated in red. The position of the A strething absorption varies preditably for different types of arbonyl ompounds. It generally ours at m _1 for simple ketones and at m _1 for simple aldehydes. The arbonyl absorption is learly evident, for example, in the IR spetrum of butyraldehyde (Fig. 19.3). The strething absorption of the arbonyl hydrogen bond of aldehydes near 2710 m _1 is another harateristi absorption; however, NMR spetrosopy provides a more reliable way to diagnose the presene of this type of hydrogen (e. 19.3B). ompounds in whih the arbonyl group is onjugated with aromati rings, double bonds, or triple bonds have lower arbonyl strething frequenies than unonjugated arbonyl ompounds. L LH 3 H 2 A H L LH 3 ompare: H 2 A HH 2 H 3 H 3 L LH 2 H 3 aetophenone 3-buten-2-one 1-butene 2-butanone $ A $ 1685 m _ m _ m _1 $ A $ $ $ 1600 m _ m _ m _1 (19.2) The arbon arbon double-bond strething frequenies are also lower in the onjugated moleules. These effets an be explained by the resonane strutures for these ompounds. Beause the A and A bonds have some single-bond harater, as indiated by the following resonane strutures, they are somewhat weaker than ordinary double bonds, and therefore absorb in the IR at lower frequeny.

3 19.3 PETRPY F ALDEHYDE AND KETNE H 2 A H L LH 3 single-bond harater _ H 2 " L H A LH 3 (19.3) In yli ketones with rings ontaining fewer than six arbons, the arbonyl absorption frequeny inreases signifiantly as the ring size dereases. (ee Further Exploration 19.1.) Further Exploration 19.1 IR Absorptions of yli Ketones V H 2 A A ylohexanone 1715 m _1 (normal) ylopentanone 1745 m _1 ylobutanone 1780 m _1 ylopropanone 1850 m _1 ketene 2150 m _1 (19.4) PRBLEM 19.3 Explain how IR spetra ould be used to differentiate the isomers within eah of the following pairs. (a) ylohexanone and hexanal (b) 3-ylohexenone and 2-ylohexenone () 2-butanone and 3-buten-2-ol B. Proton NMR petrosopy The harateristi NMR absorption ommon to both aldehydes and ketones is that of the protons on the arbons adjaent to the arbonyl group: the a-protons. This absorption is in the d region of the spetrum (see also Fig on p. 580). This absorption is slightly farther downfield than the absorptions of allyli protons beause the A group is more eletronegative than the A group. In addition, the absorption of the aldehydi proton is quite distintive, ourring in the d 9 10 region of the NMR spetrum, at lower field than most other NMR absorptions. a-protons H 3 L L H 3 d 2.17 d 2.17 a-protons a-protons aldehydi proton H 3 L L H 2 L H 3 H 3 L L H d 2.15 d 2.40 d 0.99 d 2.2 d 9.8 In general, aldehydi protons have very large hemial shifts. The explanation is the same as that for the large hemial shifts of protons on a arbon arbon double bond (e. 13.7A). However, the arbonyl group has a greater effet on hemial shift than a arbon arbon double bond beause of the eletronegativity of the arbonyl oxygen.

4 898 HAPTER 19 THE HEMITRY F ALDEHYDE AND KETNE. ARBNYL-ADDITIN REATIN hemial shift, Hz Hz J = 1.2 Hz J = 7.1 Hz 6H J = 7.1 Hz d 9.65 (off sale) 1H J = 1.2 Hz 1H hemial shift, ppm (d) Figure 19.4 The NMR spetrum for Problem 19.4(a). The relative integrals are indiated in red over their respetive resonanes. PRBLEM 19.4 Dedue the strutures of the following ompounds. (a) 4 H 8 : IR 1720, 2710 m _1 NMR in Fig (b) 4 H 8 : IR 1717 m _1 NMR d 0.95 (3H, t, J = 8 Hz); d 2.03 (3H, s); d 2.38 (2H, q, J = 8 Hz) () A ompound with moleular mass = 70.1, IR absorption at 1780 m _1, and the following NMR spetrum: d 2.01 (quintuplet, J = 7 Hz); d 3.09 (t, J = 7 Hz). The integral of the d 3.09 resonane is twie as large as that of the d 2.01 resonane. (d) 10 H 12 2 : IR 1690 m _1, 1612 m _1 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). arbon NMR petrosopy The most harateristi absorption of aldehydes and ketones in 13 NMR spetrosopy is that of the arbonyl arbon, whih ours typially in the d range (see Fig , p. 624). This large downfield shift is due to the indued eletron irulation in the p bond, as in alkenes (Fig , p. 613), and to the additional hemial-shift effet of the eletronegative arbonyl oxygen. Beause the arbonyl arbon of a ketone bears no hydrogens, its 13 NMR absorption, like that of other quaternary arbons, is harateristially rather weak (e. 13.9). This effet is evident in the 13 NMR spetrum of propiophenone (Fig. 19.5). The a-arbon absorptions of aldehydes and ketones show modest downfield shifts, typially in the d range, with, as usual, greater shifts for more branhed arbons. The a-arbon shift of propiophenone, 31.7 ppm (Fig. 19.5, arbon b) is typial. Beause shifts in this range are also observed for other funtional groups, these absorptions are less useful than the arbonyl arbon resonanes for identifying aldehydes and ketones.

5 19.3 PETRPY F ALDEHYDE AND KETNE 899,d e d f g b a H 2 H 3 (offset) d e d b a g f hemial shift, ppm (d) Figure 19.5 The 13 NMR spetrum of propiophenone.two partiularly important features of the spetrum are the large downfield shift of the arbonyl arbon g,and the small resonanes for the two arbons (f and g) that bear no protons. Reall from e that absorption intensities in 13 NMR spetra generally do not aurately orrespond to numbers of arbons, and quaternary arbons generally have onsiderably weaker absorptions than proton-bearing arbons. PRBLEM 19.5 Propose a struture for a ompound 6 H 12 that has IR absorption at 1705 m _1, no proton NMR absorption at a hemial shift greater than d 3, and the following 13 NMR spetrum: d 24.4, d 26.5, d 44.2, and d The resonanes at d 44.2 and d have very low intensity The 13 NMR spetrum of 2-ethylbutanal onsists of the following absorptions: d 11.5, d 21.7, d 55.2, and d Draw the struture of this aldehyde, label eah hemially nonequivalent set of arbons, and assign eah absorption to the appropriate arbon(s). D. UV petrosopy The p T p* absorptions (e. 15.2B) of unonjugated aldehydes and ketones our at about 150 nm, a wavelength well below the operating range of ommon UV spetrometers. imple aldehydes and ketones also have another, muh weaker, absorption at higher wavelength, in the nm region. This absorption is aused by exitation of the unshared eletrons on oxygen (sometimes alled the n eletrons). This high-wavelength absorption is usually referred to as an n T p* absorption. (H 3 ) 2 A 2 2 n eletrons n p* 271 nm (e = 16) (in ethanol) This absorption is easily distinguished from a p T p* absorption beause it is only 10 _2 to 10 _3 times as strong. However, it is strong enough that aldehydes and ketones annot be used as solvents for UV spetrosopy.

6 900 HAPTER 19 THE HEMITRY F ALDEHYDE AND KETNE. ARBNYL-ADDITIN REATIN absorbane H onentration p p* n p* wavelength (l), nm Figure 19.6 The ultraviolet spetrum of 1-aetylylohexene [1-(ylohexen-1-yl)ethanone] in methanol. The spetrum of a more onentrated solution (red) reveals the forbidden n T p* absorption, whih is so weak that it is not apparent in the spetrum taken on a more dilute solution (blak). Like onjugated dienes, the p eletrons of ompounds in whih arbonyl groups are onjugated with double or triple bonds have strong absorption in the UV spetrum. The spetrum of 1-aetylylohexene (Fig. 19.6) is typial. The 232-nm peak is due to light absorption by the onjugated p-eletron system and is thus a p T p* absorption. It has a very large extintion oeffiient, muh like that of a onjugated diene. The weak 308-nm absorption is an n T p* absorption. onjugated p-eletron system 33 L LH 3 l max = 232 nm (e = 13,200) l max = 308 nm (e = 150) (in methanol) 1-aetylylohexene [1-(ylohexen-1-yl)ethanone] The l max of a onjugated aldehyde or ketone is governed by the same variables that affet the l max values of onjugated dienes: the number of onjugated double bonds, substitution on the double bond, and so on. When an aromati ring is onjugated with a arbonyl group, the typial aromati absorptions are more intense and shifted to higher wavelengths than those of benzene. L LH 3 l max = 204 (e = 7900) 254 (e = 212) l max = 240 (e = 13,000) 278 (e = 1100) 319 (e = 50) (n p*)

7 19.3 PETRPY F ALDEHYDE AND KETNE 901 The p T p* absorptions of onjugated arbonyl ompounds, like those of onjugated alkenes, arise from the promotion of a p eletron from a bonding to an antibonding (p*) moleular orbital (e. 15.2B). An n T p* absorption arises from promotion of one of the n (unshared) eletrons on a arbonyl oxygen to a p* moleular orbital. As stated previously in this setion, n T p* absorptions are weak. petrosopists say that these absorptions are forbidden. This term refers to ertain physial reasons for the very low intensity of these absorptions. The 254-nm absorption of benzene, whih has a very low extintion oeffiient of 212, is another example of a forbidden absorption. PRBLEM 19.7 Explain how the ompounds within eah set an be distinguished using only UV spetrosopy. (a) 2-ylohexenone and 3-ylohexenone (b) ( ( and () 1-phenyl-2-propanone and p-methylaetophenone 19.8 In neutral alohol solution, the UV spetra of p-hydroxyaetophenone and p-methoxyaetophenone are virtually idential. When NaH is added to the solution, the l max of p-hydroxyaetophenone inreases by about 50 nm, but that of p-methoxyaetophenone is unaffeted. Explain these observations Whih one of the following ompounds should have a p T p* UV absorption at the greater l max when the ompound is dissolved in NaH solution? Explain. H 3 H H H H 3 H vanillin A isovanillin B E. Mass petrometry Important fragmentations of aldehydes and ketones are illustrated by the eletron-impat (EI) mass spetrum of 5-methyl-2-hexanone (Fig. 19.7, p. 902). The three most important peaks our at müz = 71, 58, and 43. The peaks at müz = 71 and müz = 43 arise from leavage of the moleular ion at the bond between the arbonyl group and an adjaent arbon atom by two mehanisms that were disussed in e. 12.6: indutive leavage and a-leavage. Indutive leavage aounts for the müz = 71 peak. In this leavage, the alkyl fragment arries the harge and the arbonyl fragment arries the unpaired eletron " (H 3 ) 2 HH 2 H 2 L LH 3 (H 3 ) 2 HH 2 H 2 L LH 8 3 moleular ion from loss of unshared eletron indutive leavage 33 (H 3 ) 2 HH 2 H LH 3 m z = 71 (19.5)

8 902 HAPTER 19 THE HEMITRY F ALDEHYDE AND KETNE. ARBNYL-ADDITIN REATIN relative abundane (H 3 ) 2 HH 2 H 2 H mass-to-harge ratio m/z Figure 19.7 The EI mass spetrum of 5-methyl-2-hexanone. The odd-eletron ion at müz = 58 results from a MLafferty rearrangement of the moleular ion. a-leavage aounts for the müz = 43 peak. In this ase the same moleular ion undergoes fragmentation in suh a way that the arbonyl fragment arries the harge and the alkyl fragment arries the unpaired eletron: 3 8 a-leavage (H 3 ) 2 HH 2 H 2 L LH 3 8 (H 3 ) 2 HH 2 H ' LH 3 m z = 43 (19.6) An analogous leavage at the arbon hydrogen bond aounts for the fat that many aldehydes show a strong M - 1 peak. What aounts for the müz = 58 peak? A ommon mehanism for the formation of oddeletron ions is hydrogen transfer followed by loss of a stable neutral moleule (p. 564). In this ase, the oxygen radial in the moleular ion abstrats a hydrogen atom from a arbon five atoms away, and the resulting radial then undergoes a-leavage. 3 H 8 H 3 L H 2 H 2 (H 3 ) 2 " hydrogen transfer 3 H 8(H 3 ) 2 " H 3 L H 2 H 2 a-leavage MLafferty rearrangement 3H (H 3 ) 2 H 3 L 8 + H 2 H2 m z = 58 (19.7) If we ount the hydrogen that is transferred, the first step ours through a transient six-membered ring. This proess is alled a MLafferty rearrangement, after Professor Fred MLafferty of ornell University, who investigated this type of fragmentation extensively. The MLafferty rearrangement and subsequent a-leavage onstitute a ommon mehanism for the prodution of odd-eletron fragment ions in the mass spetrometry of arbonyl ompounds. As illustrated by Fig. 19.7, the EI mass spetra of many aldehydes and ketones have weak moleular ions, beause relatively stable fragment ions an be formed. However, as we ll learn in e. 19.6, the arbonyl oxygen an be protonated. As a result, hemial-ionization (I) mass spetra (e. 12.6D) of aldehydes and ketones show very strong M + 1 ions, from whih aurate moleular masses an be determined.

9 19.5 INTRDUTIN T ALDEHYDE AND KETNE REATIN 903 PRBLEM Explain eah of the following observations resulting from a omparison of the mass spetra of 2-hexanone (A) and 3,3-dimethyl-2-butanone (B). (a) The müz = 57 fragment peak is muh more intense in the spetrum of B than it is in the spetrum of A. (b) The spetrum of ompound A shows a fragment at müz = 58, but that of ompound B does not Using only mass spetrometry, how would you distinguish 2-heptanone from 3-heptanone? 19.4 YNTHEI F ALDEHYDE AND KETNE everal reations presented in previous hapters an be used for the preparation of aldehydes and ketones. The four most important of these are: 1. xidation of alohols (es. 10.6A and 17.5A). Primary alohols an be oxidized to aldehydes, and seondary alohols an be oxidized to ketones. 2. Friedel rafts aylation (e. 16.4F). This reation provides a way to synthesize aryl ketones. It also involves the formation of a arbon arbon bond, the bond between the aryl ring and the arbonyl group. 3. Hydration of alkynes (e. 14.5A) 4. Hydroboration oxidation of alkynes (e. 14.5B) Two other reations have been disussed that give aldehydes or ketones as produts, but these are less important as syntheti methods: 1. zonolysis of alkenes (e. 5.5) 2. Periodate leavage of glyols (e. 11.5B) zonolysis and periodate leavage are reations that break arbon arbon bonds. Beause an important aspet of organi synthesis is the making of arbon arbon bonds, use of these reations in effet wastes some of the effort that goes into making the alkene or glyol starting materials. Nevertheless, these reations an be used synthetially in ertain ases. ther important methods of preparing aldehydes and ketones start with arboxyli aid derivatives; these methods are disussed in hapter 21. Appendix V gives a summary of all of the syntheti methods for aldehydes and ketones, arranged in the order in whih they appear in the text INTRDUTIN T ALDEHYDE AND KETNE REATIN The reations of aldehydes and ketones an be grouped into two ategories: (1) reations of the arbonyl group, whih are onsidered in this hapter; and (2) reations involving the a- arbon, whih are presented in hapter 22. The great preponderane of arbonyl-group reations of aldehydes and ketones fall into three ategories: 1. Reations with aids. The arbonyl oxygen is weakly basi and thus reats with Lewis and Brønsted aids. With E as a general eletrophile, this reation an be represented as follows: