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19.3 Spectroscopy of Aldehydes and Ketones 895 19_BRCLoudon_pgs5-0.qxd 12/9/08 11:41 AM Page 895 19.3 SPECTROSCOPY OF ALDEHYDES AND KETONES 895 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 boil- ing 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-sol- vent 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 im- portant 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 896 896 CHAPTER 19 • THE CHEMISTRY OF ALDEHYDES AND KETONES. CARBONYL-ADDITION REACTIONS 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 80 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; how- ever, 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 com- pounds. 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 $C A O 1685 cm 1 1670 cm 1 — 1715 cm 1 _ _ _ $ 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 fol- lowing 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 fre- quency 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 pro- tons 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 elec- tronegative 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 dou- ble bond because of the electronegativity of the carbonyl oxygen. 19_BRCLoudon_pgs5-0.qxd 12/9/08 11:41 AM Page 898 898 CHAPTER 19 • THE CHEMISTRY OF ALDEHYDES AND KETONES. CARBONYL-ADDITION REACTIONS 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 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 absorp- tion, 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.
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