Carbonyl Compounds: Ketones, Aldehydes, Carboxylic Acids, Lecture notes of Organic Chemistry

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(7-9/94)(10/96)(06,9-11/04) Neuman Chapter 13 1 13: Carbonyl Compounds: Ketones, Aldehydes, Carboxylic Acids 13.1 Carbonyl Compounds 13-3 The Carbonyl Group (C=O) (13.1A) 13-3 Carbonyl Compounds from Alcohol Oxidation (13.1B) 13-4 13.2 Nomenclature 13-4 Ketones (Alkanones) (13.2A) 13-4 Aldehydes (Alkanals) (13.2B) 13-5 Carboxylic Acids (Alkanoic Acids) (13.2C) 13-7 Carboxylate Ions Common Nomenclature (13.2D) 13-8 Carboxylic Acids Aldehydes Ketones Acyl Groups 13.3 Oxidation and Reduction 13-12 General Features (13.3A) 13-13 Oxidation States of Organic Compounds (13.3B) 13-13 Bond Order of Carbon Atoms Oxidation Numbers Aldehydes from Oxidation of 1° Alcohols (13.3C) 13-16 Pyridinium Chlorochromate (PCC) Other Cr(VI) Reagents Ketones from Oxidation of Secondary Alcohols (13.3D) 13-16 Oxidation of Tertiary Alcohols is Not a Useful Reaction Carboxylic Acids from Oxidation of Aldehydes (13.3E) 13-18 Oxidation of Ketones is Not a Useful Reaction Comparative Oxidation States (13.3F) 13-20 13.4 Structure and Reactivity of Groups with C=O 13-20 Bonding and Structure of Ketones and Aldehydes (13.4A) 13-20 Bonding Polarity Bonding and Structure of Carboxylic Acids (13.4B) 13-21 Bonding and Polarity Hydrogen Bonding Reactivity and Selectivity of C=O Groups (13.4C) 13-22 Addition of Electrophiles Addition of Nucleophiles Conjugate Addition of Nucleophiles (continued) (7-9/94)(10/96)(06,9-11/04) Neuman Chapter 13 2 13.5 C=O Influence on Reactivity of Neighboring Atoms 13-24 Acidity of Carboxylic Acids (Y is O) (13.5A) 13-24 Acidity Constants Resonance Effects Acidity of Hydrogens on α-Carbons (Y is CR2) (13.5B) 13-26 C-H Ka Values Enolate Ions and Enols Keto-Enol Tautomerization 13.6 Spectrometric Properties of Carbonyl Compounds 13-29 Ultraviolet-Visible Spectrometry (13.6A) 13-29 π→π* Excitation n→π* Excitation Infrared Spectrometry (13.6B) 13-30 C=O Stretch C-H Stretch in Aldehydes O-H Stretch in Carboxylic Acids C=O Bands in Carboxylic Acids NMR Spectrometry (13.6C) 13-33 13C NMR 1H NMR (7-9/94)(10/96)(06,9-11/04) Neuman Chapter 13 5 Figure 13.6 The longest continuous carbon chain containing the C=O group provides the root name of the compound. Systematic names for ketones are analogous to those for structurally similar 2° alcohols because ketones are formed by oxidation of the corresponding 2° alcohol. We show some pairs of.ketones and their 2° alcohol precursors in Figure 13.7 Figure 13.7 You can see that the ending "ol" in the name of the precursor alcohol is simply replaced with the ending "one" in the product ketone. We designate the position of the C of the C=O group in the chain using a number in front of the root "alkanone" name just as we designate the position of the C of C-OH in the precursor 2° alcohol. Aldehydes (Alkanals) (13.2B) The general structure of an aldehyde looks like that of a ketone. However, aldehydes must have at least one H bonded to the C=O (Figure 13.8). Figure 13.8 As a result, the C=O group of aldehydes is always at the end of an alkane chain. In contrast, the C=O group of a ketone can never be at the end of an alkane chain and must always be part of the interior of that chain. (7-9/94)(10/96)(06,9-11/04) Neuman Chapter 13 6 We systematically name aldehydes as "alkanals" and show simple examples in Figure 13.9. Figure 13.9 The systematic nomenclature for aldehydes derives from the systematic nomenclature for 1° alcohols because oxidation of 1° alcohols gives aldehydes. The ending "ol" in the name of the precursor 1° alcohol is replaced by the ending "al" in the aldehyde product. Since the C of the C=O group is always at the end of the chain, we do not need to use a number to designate the position of the C=O group. We show some examples of pairs of aldehydes and their precursor 1° alcohols in Figure 13.10. Figure 13.10 However remember that we must include the number "1" in the 1° alcohol name because we name all alcohols as "alkanols" whether the OH group is at C1 or C2 or on any other carbon of the chain. When the R group of an aldehyde is a cycloalkyl group, we name these compounds in the manner shown in Figure 13.11. Figure 13.11 (7-9/94)(10/96)(06,9-11/04) Neuman Chapter 13 7 We can also replace the ending carboxaldehyde with the ending carbaldehyde as we show in that figure. Aldehydes versus Ketones. Why are aldehydes and ketones placed in separate classes while 1° and 2° alcohols are not? This is probably the result of early chemists' observations that aldehydes undergo some important chemical reactions not available to ketones. For example we have already mentioned that aldehydes are oxidized to carboxylic acids of the same chain length. However, oxidation of ketones is very difficult, and when it occurs an R group is generally lost and the length of the chain containing the C=O group decreases (Figure 13.12). Figure 13.12 The chain length remains the same during oxidation of aldehydes because the C=O group is at the very end of the chain. However, since most other chemical reactions of aldehydes and ketones are similar, it is likely that if they had been discovered recently they would not have been separated into two separate classes. Proposals have been made to use the ketone nomenclature system for both aldehydes and ketones (aldehydes would be named as 1-alkanones) however these proposals have not yet prevailed. Carboxylic Acids (Alkanoic Acids) (13.2C) We can always call the C=O group a carbonyl group, but when it is attached to an OH, the resulting C(=O)OH structure is a new functional group called the carboxy group or carboxylic acid group found in carboxylic acids whose general structure we show again in Figure 13.13 where the R group can be H or contain a C directly bonded to C=O. Figure 13.13 You can see that carboxylic acids look like aldehydes in which an O has been inserted between the H attached to the C=O to give an OH group. This comparison is useful since (7-9/94)(10/96)(06,9-11/04) Neuman Chapter 13 10 as our example, we can say that the fluorine atom is "beta" (β) to the carboxylic acid group whether we name the acid with common or systematic nomenclature. Figure 13.20 Common names of carboxylate ions are usually derived from the common names of carboxylic acids by dropping the ending "ic acid" and replacing it with "ate" as in Figure 13.21. Figure 13.21 Aldehydes. Simple aldehydes have common names that are directly related to those of the carboxylic acids (Figure 13.22) that they form on oxidation. Their systematic names are included in parentheses in this figure. Figure 13.22 We use Greek letters to designate positions of substituents on aldehydes in the same ways as we did for carboxylic acids. The asterix (*) shows aldehydes whose common names are virtually always used in place of their systematic names. (7-9/94)(10/96)(06,9-11/04) Neuman Chapter 13 11 Ketones. Ketones do not have common names like those we have just described for aldehydes and carboxylic acids. However simple ketones have other types of common names that are frequently used in place of their systematic names. One type is similar to that described for ethers in Chapter 3. The two R groups attached to the C=O group are each named as an alkyl group and those alkyl group names are placed in front of the word "ketone" as we show in Figure 13.23. Figure 13.23 This method is particularly useful for naming simple ketones with cycloalkyl groups directly attached to the C=O group. A second type is used specifically for ketones with the general structure Ar-C(=O)-R where Ar represents a benzene or arene ring attached to the carbonyl group as we show in Figure 13.24 Systematic names are again shown in parentheses. Figure 13.24 In these examples, the arene portion of the molecule is designated by the endings phenone or naphthone as appropriate. The other group attached to C=O (R-C=O) is indicated in the name by a prefix derived from the common name of the corresponding carboxylic acid R- C(=O)-OH. (7-9/94)(10/96)(06,9-11/04) Neuman Chapter 13 12 For example, the prefix aceto in acetophenone (see figure above) is derived from the prefix acet in acetic acid. Similarly that prefix acet is used in the common name acetone, that belongs to the very familiar ketone in Figure 13.25. Figure 13.25 Acyl Groups. Organic chemists often find it useful to refer to the whole R-C(=O) group by a single name. One example is when such a group is attached to an aromatic ring as we show in Figure 13.26. Figure 13.26 For example we can refer to the R-C(=O) group on anthracene in Figure 13.26 as an acyl group or alkanoyl group. We show specific names of various acyl groups with different R's in Figure 13.27. Figure 13.27 13.3 Oxidation and Reduction Because oxidation reactions are important synthetic routes to ketones, aldehydes, and carboxylic acids we describe these reactions and the reverse reduction reactions in more detail in this section. (7-9/94)(10/96)(06,9-11/04) Neuman Chapter 13 15 in Chapter 10 using hydrogenation of alkenes that is a reduction reaction. Although they are not routinely used by organic chemists, we use them to compare the oxidation states of methanol and methanal in Figure 13.33. Figure 13.33 Details of Oxidation Number Calculations. The rules used to calculate these numbers are similar to those used for inorganic molecules. The H atoms are assigned an oxidation number of +1 when they are bonded to C or to more electronegative atoms. In contrast, oxygen atoms, whether singly or doubly bonded, are assigned the oxidation number -2 except when they are bonded to another oxygen. We designate the unknown oxidation number of C in each molecule as X. Since the sum of the oxidation numbers of all atoms in a particular compound must equal the net ionic charge on that compound, and the ionic charge on both methanol and methanal is zero (0), we obtain the algebraic equations shown below each molecule in Figure 13.33. Solution of these equations gives the oxidation number X = -2 for the C in methanol (CH3OH) and the more positive (greater) oxidation number X = 0 for the C in methanal (H2C=O). Conversion of methanol to methanal causes the oxidation number for the C bonded to O to become more positive (an increase from -2 to 0) signifying that the C has been oxidized. (7-9/94)(10/96)(06,9-11/04) Neuman Chapter 13 16 Aldehydes from Oxidation of 1° Alcohols (13.3C) Any 1° alcohol (RCH2OH) can be oxidized to an aldehyde as we show in the general equation, and some specific examples, in Figure 13.34. Figure 13.34 Pyridinium Chlorochromate (PCC). We show a reagent and solvent for this oxidation in the reaction in Figure 13.35. Figure 13.35 The oxidizing agent pyridinium chlorochromate (PCC) (Figures 13.35 and 13.36) contains Cr(VI) that is reduced to a lower oxidation state as it oxidizes the alcohol to the aldehyde. Figure 13.36 Other Cr(VI) Reagents. PCC is a specialized oxidizing reagent that is less powerful than other Cr(VI) reagents such as Na2Cr2O7, K2Cr2O7, or CrO3, that are also used to oxidize alcohols. These more powerful reagents not only oxidize 1° alcohols to aldehydes, but further oxidize aldehydes to carboxylic acids. As a result, PCC is a reagent of choice when an aldehyde is the desired product. All of these Cr(VI) reagents and their chemistry are discussed in more detail in Chapter 17 Ketones from Oxidation of Secondary Alcohols (13.3D) Oxidation of 2° alcohols leads to the formation of ketones (Figure 13.37). Figure 13.37 We can see this in the formula where H's "disappear" from the C and the O converting the CH-OH group into the C=O group. Since the C in C=O is doubly bonded to the oxygen, that (7-9/94)(10/96)(06,9-11/04) Neuman Chapter 13 17 C is in a higher oxidation state than the C in the CH-OH group and you can verify this using oxidation numbers. Comparative Oxidation Numbers for 2° Alcohols and Ketones. We can simplify calculations of C oxidation numbers because the R groups do not change as the secondary alcohol is oxidized to the ketone. We do not need to include R groups in our calculations to show that C in CH-OH has a lower oxidation number than C in the C=O group (Figure 13.38). Figure 13.38 When we calculate C oxidation numbers by this shorthand approach without considering the R groups, the absolute values of those C oxidation numbers have no significance. They have meaning only in their comparison or difference. Similarly, we can ignore the R groups when we compare the oxidation states of C in primary alcohols with those of their product aldehydes. We can use PCC in CH2Cl2 to convert 2° alcohols to ketones as we did for oxidation of 1° alcohols to aldehydes. However, we can also use more powerful oxidizing agents made by dissolving sodium or potassium dichromate (Na2Cr2O7 or K2Cr2O7) or chromium trioxide (CrO3) in aqueous solutions of sulfuric or acetic acid (Figure 13.39) [next page]. In contrast with aldehydes, ketones are not further oxidized by these more powerful oxidizing agents. (7-9/94)(10/96)(06,9-11/04) Neuman Chapter 13 20 Comparative Oxidation States (13.3F) We summarize and compare the oxidation states/levels of alcohols, ketones and aldehydes, and carboxylic acids in the diagram shown here. Increasing Oxidation State → Alcohols Aldehydes and Ketones Carboxylic Acids RCH2-OH RC(H)=O RCO2H R2CH-OH R2C=O R3C-OH <--- (Less Oxidized) (More Oxidized)---> <---(More Reduced) (Less Reduced)----> Alcohols are at lower oxidation levels than those of aldehydes or ketones. Similarly, aldehydes and ketones are at lower oxidation levels than that of carboxylic acids. It is important that you remember these general comparisons since oxidation and reduction reactions of these functional groups are very important in organic chemistry. 13.4 Structure and Reactivity of Groups with C=O This section describes bonding, geometry, polarity, and reactivity of C=O groups. Bonding and Structure of Ketones and Aldehydes (13.4A) C=O groups in ketones and aldehydes have the same bonding and polarity. Bonding. We described molecular orbitals of C=O double bonds in Chapter 1. We assign sp2 hybridization to both the C and O so that a C=O group has an sp2-sp2 σ bond, as well as a 2p-2p π bond between the C and O atoms (Figure 13.45). Figure 13.45 The sp2 hybridization of C agrees with the observed planarity of the carbonyl groups and the approximately120° angles between their attached groups. Actual bond angles vary with the size of the attached groups as we see in the calculated structures that we show in Figure 13.46 [next page]. (7-9/94)(10/96)(06,9-11/04) Neuman Chapter 13 21 Figure 13.46 Polarity. O is significantly more electronegative than C, so C=O double bonds are polarized with a partially positive C and a partially negative O (Figure 13.47). Figure 13.47 Since the C=O group has a π bond, we can represent this polarity using the resonance structure with a positive charge on C and a negative charge on O that we have labelled as "good" (Figure 13.48). Figure 13.48 The "bad" resonance structure with the opposite polarity and the large "X" drawn through the resonance arrow is not a significant resonance contributor because its atom polarities are inconsistent with the relative electronegativities of C and O. As a result of this polarity, C=O has a significant dipole moment (see Figure 13.47). Bonding and Structure of Carboxylic Acids (13.4B) There are similarities and differences between C=O groups in carboxylic acids and those in aldeydes and ketones. Bonding and Polarity. The C(=O)OH group (CO2H group) of carboxylic acids is also planar and the C is sp2 hybridized as it is in ketones and aldehydes (Figure 13.49). Figure 13.49 The C=O in CO2H is polar as in ketones and aldehydes, but the unshared electron pairs on OH permit an additional resonance structure to be drawn where one of those pairs is delocalized into the C=O group (Figure 13.50) [next page]. (7-9/94)(10/96)(06,9-11/04) Neuman Chapter 13 22 Figure 13.50 Hydrogen Bonding. In contrast with ketones and aldehydes, the OH group in carboxylic acids permits hydrogen bonding between RCO2H molecules. As a result, H-bonded dimers composed of two carboxylic acid molecules (Figure 13.51) are relatively stable. Figure 13.51 These dimers are the predominant way that RCO2H molecules exist in pure carboxylic acids or when carboxylic acids are dissolved in nonpolar solvents. When a carboxylic acid is dissolved in a polar protic solvent such as water, the dimeric structure is replaced by H-bonding between a carboxylic acid molecule and water molecules (Figure 13.52). This H-bonded structure is like that we would expect for solutions of aldehydes or ketones in water (Figure 13.53). Reactivity and Selectivity of C=O Groups (13.4C) The polarity of C=O groups affects the selectivity of its addition reactions. This is a brief overview of the detailed discussion in Chapter 16. Addition of Electrophiles. Electrophiles readily add to C=O bonds, just as they do to C=C bonds. However electrophiles react with C=O bonds exclusively at the O atom. A very important example of electrophilic addition to a C=O group is protonation (Figure 13.54). Figure 13.54 Protonization of O leads to the formation of a cationic species where the positive charge is delocalized on both O and C (Figure 13.54). (7-9/94)(10/96)(06,9-11/04) Neuman Chapter 13 25 Table 13.1. Comparative Acidities of Simple Alcohols and Carboxylic Acids Alcohol Ka pKa Carboxylic Acid Ka pKa H(CH2)OH 10-16 16 H(C=O)OH 1.8 x 10-4 3.8 Me(CH2)OH* 10-16 16 Me(C=O)OH** 1.8 x 10-5 4.8 Et(CH2)OH 10-16 16 Et(C=O)OH 1.3 x 10-5 4.9 Pr(CH2)OH 10-16 16 Pr(C=O)OH 1.5 x 10-5 4.8 Bu(CH2)OH 10-16 16 Bu(C=O)OH 1.5 x 10-5 4.8 *Ethanol, CH3CH2OH **Acetic acid, CH3CO2H Acidity, Ka, and pKa. You should review the discussions in Chapter 3 on the use of Ka and pKa values to measure acidity We will continue to use concepts of acidity and basicity throughout this text. They will be important in discussions of amino acids, nucleic acids, and fatty acids, in later chapters of this text and in your biology and biochemistry classes. Ka and pKa values of these molecules are important physical properties that not only characterize them as chemical systems, but are crucial to understanding their biological function. Acidic and basic functional groups are important in biological systems because they can exist in the same environment with more than one charge type. For example, carboxylic acids can simultneously exist as neutral or negatively charged forms as we show in Figure 13.62. Figure 13.62 Similarly, amines can simultaneously be neutral or positively charged forms as we show in the same figure. We will learn that different charge types of such functional groups are of critical importance in the maintenance of the structure of biological macromolecules. Resonance Effects. The dramatically greater acidity of each carboxylic acid R(C=O)OH compared to its corresponding alcohol R(CH2)OH is because C=O resonance stabilizes the negative charge on the carboxylate ion (R(C=O)O-) that arises by loss of the OH proton of a carboxylic acid (Figure 13.63). Figure 13.63 Comparable resonance stabilization is not possible for alkoxide ions (R(CH2)O-) that form from loss of an OH proton from an alcohol. (7-9/94)(10/96)(06,9-11/04) Neuman Chapter 13 26 Resonance Stabilization of Oxyegen Anions. We used similar resonance stabilization of an oxygen anion to explain the relatively high acidity of arene OH groups as we described for phenol (PhOH) in Chapter 12 (Figure 13.64). Figure 13.64 The data in Table 13.2 show that a C=O group has an effect on O- stability that is even greater than that of a benzene ring. Table 13.2. A Comparison of Acidities of Phenol, Alcohols, and Carboxylic Acids Compound Approximate Ka R-OH 10-16 Ph-OH 10-10 R-C(=O)-OH 10-5 The difference between stabilization by a benzene ring and a C=O is due partly to the fact that the two resonance structures for a carboxylate anion ion are completely identical indicating that both are very favorable. In contrast, those for a phenoxide ion include less favorable structures in which the negative charge is moved onto carbon atoms in the aromatic rings as shown in Figure 13.65. Figure 13.65 Acidity of Hydrogens on α-Carbons (Y is CR2) (13.5B) The carbonyl group also increases the acidity of H's on C's that are directly bonded to the C=O (Figure 13.66). Figure 13.66 C's directly bonded to C=O groups are called α-carbons and their H's are called α-hydrogens or α-H's. (7-9/94)(10/96)(06,9-11/04) Neuman Chapter 13 27 C-H Ka Values. The approximate Ka for an α-H on a CH3 of acetone (2-propanone) is 10-20. In contrast the estimated Ka for a CH3 proton of propane is 10-50 (Figure 13.67). Figure 13.67 These numbers show that replacing the central CH2 of propane (in the box) with a C=O group (in the box) increases the Ka value of a CH3 proton by a factor of 1030! We explain this enormous increase in Ka by the presence of C=O resonance stabilization of the CH2 anion formed from acetone, and the absence of such stabilization for the CH2 anion formed from propane (Figure 13.68). Figure 13.68 The Meaning of Ka = 10-50. The estimated Ka value of 10-50 for C-H's in propane has very little absolute significance. It is an attempt to compare a hypothetical acid strength of the CH3 protons on propane (that do not react at all with water), with those of other acidic H's that do react with water. The major significant aspects of this number are (1) it is very small, (2) it is much smaller than Ka's for CH3 protons in acetone, and (3) it shows that propane C-H's are not acidic. Even though the C=O group dramatically increases the acidity of protons on α-carbons in acetone compared to those on propane, acetone is by no means an "acid" since it has a much smaller Ka value (Ka = 10-20) than water (Ka = 10-16). Enolate Ions and Enols. Although their Ka values are small, α-H's on acetone and other carbonyl compounds can be removed as protons by strong bases such as hydride ion (H-) or diisopropylamide ion ((i-Pr)2N-) that we described earlier in this text. The resulting carbanions formed α to C=O groups are called enolate ions that are resonance delocalized species that we show in Figure 13.69. Figure 13.69 (7-9/94)(10/96)(06,9-11/04) Neuman Chapter 13 30 n→π* Excitation. In contrast with polyenes that have no unshared electrons, an unshared electron on O of the C=O group can be excited by UV electromagnetic radiation into the lowest antibonding π MO of the C=O group. This so-called n→π* absorption has low ε values so n→π* absorption bands are quite weak. The λmax for this n→π* absorption is in the normal UV range for all compounds containing carbonyl groups (Figure 13.73). Figure 13.73 You can see that these n→π* λmax values shift to longer wavelengths with increasing conjugation of the π systems analogous to the π→π* absorption band. The weakness of this n→π* absorption band makes it relatively useless as an aid in structure identification, but it has great chemical significance. The electronically excited state that forms by excitation of an n electron into a π antibonding orbital participates in a variety of important chemical reactions. Infrared Spectrometry (13.6B) Carbonyl compounds have a number of distinctive IR spectral features. We report IR spectral data here using wavenumbers (units of cm-1) that increase with increasing frequency (energy) and decrease with increasing wavelength of the electromagnetic radiation. C=O Stretch. All carbonyl compounds show a strong IR absorption band corresponding to a C=O bond stretching vibration. The position of this band depends on the type of carbonyl compound as we see in data for pure samples of these compounds in Table 13.4. Table 13.4. Some Typical C=O IR Stretching Bands Compound Wavenumber (cm-1) Ketone 1715 Aldehyde 1740 to 1720 Carboxylic acid (monomer form) 1760 (dimer form) 1720-1706 We show IR specta in Figures 13.74a-d [next pages] that show these strong bands for two ketones, an aldehyde and a carboxylic acid. (7-9/94)(10/96)(06,9-11/04) Neuman Chapter 13 31 Figure 13.74a-d (Figures 13.74a-d are from: Silverstein, Bassler, and Morrill, "Spectrometric Identification of Organic Compounds", 5th Edition, John Wiley and Sons, Inc., 1991) Figure 13.74d [next page] (7-9/94)(10/96)(06,9-11/04) Neuman Chapter 13 32 Figure 13.74d The frequency maximum (wavenumber value) of these C=O bands depends on solvent. Polar solvents generally cause small decreases in the wavenumber values for the C=O stretch maximum, while these bands appear at slightly higher wavenumber values in nonpolar solvents. The C=O absorption frequency also depends on the R-C-R bond angle in R- C(=O)-R as we can see in the IR data for several cycloalkanones (Figure 13.75). Figure 13.75 C-H Stretch in Aldehydes. The C-H streching band of the aldehyde group appears at slightly lower wavenumber values than other C-H stretching bands (e.g. Figure 13.74c). O-H Stretch in Carboxylic Acids. IR spectra of carboxylic acids (e.g. Figure 13.74d) show a very broad OH stretching band between 3500 and 2200 cm-1. These broad OH bands of carboxylic acids are often centered at about 3000 cm-1 as is the case in this spectrum. You can also see that the C-H stretching bands for the compound are superimposed on this broad OH band. C=O Bands in Carboxylic Acids. Carboxylic acids sometimes show two C=O stretching bands. In the carboxylic acid spectrum in Figure 13.74d, there is a weak shoulder at about 1750 cm-1 on the intense carbonyl band that is centered at about 1710 cm-1. The shoulder probably corresponds to the small amount of monomeric form of the carboxylic acid, while the strong band is that for the predominant dimeric form (see Table 13.4).