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26 Ketones

DOI: 10.1055/sos-SD-026-00001

Cossy, J.Science of Synthesis, (2005261.

Ketones are among the most important organic compounds in terms of their occurrence and utility in chemical and biochemical transformations. This volume of Science of Synthesis covers the synthesis of ketones. Each author has described the most general and useful methods for the synthesis of a particular class of ketones. The parent ketones that are covered in this volume are shown in Table 1 together with the sections in which they appear.

Table 1 Classes of Ketones Covered in Volume 26

Product Class and Method Structural Formula Section
aliphatic and alicyclic ketones R1C(=O)R2 26.1
by oxidation of heterosubstituted alkanes R1CHXR2 26.1.1
by oxidation of alkanes and alkenes 26.1.2
by reduction of 1,2-diketones and derivatives R1C(=O)C(=X)R2, α,α-diheterosubstituted ketones R1C(=O)CX2R2, α-heterosubstituted ketones R1C(=O)CHXR2, enones, and ynones 26.1.3
from carboxylic acids and derivatives R1C(=X)Y 26.1.4
from aldehydes R1CHO 26.1.5
from thioketones R1C(=S)R2, acetals R1C(OR3)2R2, cyanohydrins, enol ethers R1C(OR2)=CR3R4, enamines, other ene derivatives, and related compounds 26.1.6
from alkynes, allenes, and ketenes 26.1.7
by fragmentation or rearrangement of epoxides, diols, and allyl alcohols 26.1.8
from ketones R1C(=O)R2 26.1.9
from enones R1R2C=C(R3)C(=O)R4 26.1.10
cyclobutanones 26.2
cyclopropanones 26.3
1,2-diketones and related compounds R1C(=O)C(=X)R2 26.4
α,α-diheterosubstituted ketones R1C(=O)CX2R2 26.5
α-heterosubstituted ketones R1C(=O)CHXR2 26.6
ynones R1CCC(=O)R2 and R1CC(CH2)nC(=O)R2 26.7
aryl ketones Ar1C(=O)R1 26.8
enones R1CH=CC(=O)R2 and R1CH=C(CH2)nC(=O)R2 26.9
β- and more remotely functionalized cyano ketones, oxo ketones, and carboxy ketones and derivatives R1C(=O)(C)nCN, R1C(=O)(C)nC(=X)R2, and R1C(=O)(C)nC(=X)Y; n > 1 26.10
β- and more remotely functionalized ketones R1C(=O)(C)nCHXR2; n > 1 26.11

Within each product class, the synthetic methods are described and the number of methods vary depending on the ketone class. The scope of the methods are described with representative examples, safety concerns, reaction schemes, and selected experimental procedures. Many methods are further subdivided into variations.

Section 26.1 is an introduction dealing with the general chemical properties of aliphatic and alicyclic ketones; the synthesis of these ketones is covered in Sections 26.1.126.1.10. Topics covered in the introduction of Section 26.126.1 include thermochemistry, acid/base properties, hydrogenation, oxidation, hydration, the oxa-ene and DielsAlder reactions, the frangomeric effect, regioselective pinacol-type coupling, bond dissociation energies, and radical chemistry, including Norrish type I and II processes.

The synthesis of ketones by oxidation of secondary alcohols is described in Section 26.1.1. The most frequently used reagents are chromium(VI) oxide, pyridinium chlorochromate (Scheme 1),[‌1‌] pyridinium dichromate, manganese(IV) oxide, tetrapropylammonium perruthenate, the DessMartin periodinane, and oxidants based on dimethyl sulfoxide (Swern oxidation). Also covered in Section 26.1.1 are oxidations of other functions such as halide, nitro (Nef reaction), and amino.

Scheme 1 Oxidation of Cycloheptanol to Cycloheptanone by Pyridinium Chlorochromate[‌1‌]

The focus of Section 26.1.2 is on oxidation of alkenes and alkanes, excluding allylic and benzylic oxidations. The oxidation of alkenes to ketones can be achieved without the cleavage of the skeleton as in the Wacker oxidation or by the cleavage of the skeleton by using ozone or the osmium(VIII) oxide/sodium periodate procedure. The use of molecular oxygen for the oxidation of alkanes is important in both industrial and synthetic aspects. Substituted porphyrins can be used for the aerobic oxidation of alkanes in the presence of acetaldehyde (Scheme 2).[‌2‌]

Scheme 2 Oxidation of an Alkane Using a PorphyrinMetal Complex[‌2‌]

Section 26.1.3 describes reduction of α-heterosubstituted ketones, enones, and ynones. 1,2-Diketones and related compounds can be transformed to ketones by reduction under acidic conditions, while reductive processes for α-hetero- and α,α-diheterosubstituted ketones include the use of active metals, low-valent metal ions, electrolysis, or hydride reagents. Enones and ynones are transformed into ketones by catalytic hydrogenation, by conjugate reduction using transition-metal hydrides, or by dissolving metals. The synthesis of ketones from carboxylic acids and derivatives is covered in Section 26.1.4. Substrates include acyl halides, carboxylic acids, acid anhydrides, esters, lactones, amides, nitriles, and dihydroimidazoles and, with careful selection of the organometallic reagent and control of the reaction conditions, ketones can be obtained in good yields without double addition of the nucleophile (Scheme 3).[‌3‌]

Scheme 3 Preparation of Ketones from Acyl Chlorides and Grignard Reagents[‌3‌]

Synthesis from aldehydes by substitution of the aldehyde hydrogen is discussed in Section 26.1.5. This substitution is achieved mainly either by generating an acyl radical which then undergoes addition to an alkene, or by transition-metal-catalyzed hydroacylation. Syntheses from chalcogenoketones, acetals, enol ethers, enamines, and derivatives are found in Section 26.1.6. These compounds often function as protected ketones and/or valuable reactive umpolung synthons. Formation of ketones by transformation of alkynes, allenes, and ketenes is described in Section 26.1.7. Alkynes and allenes can be transformed to ketones by metal-catalyzed hydration or by a hydroborationoxidation sequence, and ketenes to ketones by nucleophilic addition followed by hydrolysis. Methods involving fragmentation and rearrangement (Section 26.1.8) are important due to the large number of applications of these transformations in the synthesis of natural products. Chemical transformations such as oxidative cleavage of alkenes and glycol derivatives, fragmentation of 1,3-dihetero-functionalized compounds (e.g., the Grob fragmentation,[‌4‌] Scheme 4), the Eschenmoser fragmentation, and some photochemical fragmentations such as the Norrish type II are described. Important rearrangement reactions covered include the Claisen, oxy-Cope, pinacol, and semi-pinacol transformations, as well as isomerizations of allylic alcohols and epoxides.

Scheme 4 Synthesis of a Medium-Sized-Ring Ketone by Grob Fragmentation[‌4‌]

In Section 26.1.9, dealing with the synthesis of ketones from other ketones, the emphasis is on the formation of CH and CC bonds with chemo-, regio-, stereo-, and enantioselectivity. The synthesis of ketones from enones by formation of CC bonds in a 1,4-addition process is covered in Section 26.1.10. Suitable Michael donors include Grignard reagents, cuprates, malonates (and derivatives), β-oxo esters, β-diketones, nitroalkanes, sulfones, and phosphonates. In this section, emphasis is placed upon asymmetric conjugated addition which has been dominated by metal catalysis as well as upon organocatalysis (Scheme 5).[‌5‌]

Scheme 5 Enantioselective Conjugate Addition of Organosiloxanes[‌5‌]

Sections 26.2 and 26.3 deal respectively with the synthesis of cyclobutanones and cyclopropanones as well as their derivatives. Cyclobutanones are synthetically useful building blocks for the synthesis of a wide range of natural and nonnatural complex molecules, and can be synthesized by cyclodialkylation of protected carbonyl groups or by carbon­ylation of metallacyclobutane complexes, but the most common methodology is probably the [2+2] cycloaddition between a ketene and an alkene (Scheme 6).[‌6‌] In general, cyclopropanones are not isolated since they readily undergo ring opening or polymerization in the presence of acids, bases, electrophiles, or nucleophiles. Commonly, cyclopropanone chemistry has been performed from intermediates such as cyclopropanone hemiacetals and acetals, and Section 26.3 focuses on the preparation of such compounds. In contrast, cyclopropenones, also covered here, are stable compounds.

Scheme 6 [2+2] Cycloaddition of a Ketene to Cyclopenta-1,3-diene[‌6‌]

The synthesis of ketones with an α-heteroatom such as 1,2-diketones (and derivatives), α,α-diheterosubstituted ketones, and α-monoheterosubstituted ketones are covered in Sections 26.4, 26.5, and 26.6, respectively. Symmetrical 1,2-diketones are efficiently prepared reductively from carboxylic acids, while the unsymmetrical compounds can be prepared by nucleophilic acylation of acid derivatives by acylmetal reagents. Syntheses of α,α-diheterosubstituted ketones covered in Section 26.5 include direct replacement of one or two hydrogens adjacent to the carbonyl, as well as more specific methods such as the Pummerer rearrangement, a very useful method of introducing a sulfur and an oxygen at the α-position of a ketone (Scheme 7).[‌7‌] Section 26.6 focuses principally upon indirect methods of preparation of α-substituted ketones, many of which are extremely important synthetic intermediates, notably from acetate enol ethers, enols, enolates, silyl ethers, enamines, or enamides.

Scheme 7 A Pummerer Rearrangement[‌7‌]

Ynones (Section 26.7) are valuable intermediates for heterocycle synthesis. Important methods covered include the acylation of metalated alkynes and the oxidation of alkynyl alcohols, as well as the Eschenmoser fragmentation of α,β-epoxyketones. The most important synthesis of aryl ketones (Section 26.8) is probably the FriedelCrafts acylation but acylation of organometallic reagents is an attractive alternative. Other useful routes to aryl ketones include benzylic oxidation and aromatization of various cycloadducts. Section 26.9 deals with enones which appear as structural motifs in a diverse range of naturally occurring molecules. α,β-Unsaturated ketones, in particular, are widely used as building blocks in synthesis and comprise the main body of this section. Methods covered include allylic oxidation (Scheme 8),[‌8‌] oxidation of allylic alcohols, acylation of organometallic reagents, as well as the aldol, Wittig, BaylisHillman, and PausonKhand reactions.

Scheme 8 Allylic Oxidation with a Selenium Reagent[‌8‌]

In Sections 26.10 and 26.11, which cover the synthesis of β- and more remotely functionalized ketones, only the formation of the ketone function is described. Section 26.10 reviews the synthesis of ketones with an additional carbonyl, nitrile, or carboxy substituent, or equivalent, while Section 26.11 covers halo, hydroxy, sulfanyl, amino, and phosphono ketones, and derivatives. An overview of the more popular methods is presented without neglecting routes that can bring solutions to specific problems. For example, the ring opening of 1-methylpiperidin-4-one affords an α,β-unsaturated ketone with an amino group at a remote position (Scheme 9).[‌9‌]

Scheme 9 Ring Opening of Cyclic Amino Ketones[‌9‌]

The R1 and R2 groups in ketones (R1COR2) can be alkyl, alkenyl, alkynyl, or aryl. The simplest member is named trivially as acetone. Higher ketones can be named using functional class nomenclature, indicating the alkyl, alkenyl, alkynyl, and aryl groups associated with the carbonyl function in alphabetical order, e.g. methyl propyl ketone (MeCOPr). However, the preferred IUPAC nomenclature is substitutive, in which ketones are named as a derivative of the appropriate hydrocarbon by adding the suffix one, e.g. pentan-2-one (MeCOPr). If a higher priority substituent is also present, the prefix oxo is used, e.g. 2-oxopropanoic acid (MeCOCO2H).

Acetone is present in blood and urine and in large amounts in the urine of diabetics. It can be obtained by pyrolysis of tartaric and citric acids and of carbohydrates. It can be made from wood alcohol, from acetic acid, or by fermentation processes. Industrial methods to produce acetone include: (a) the catalytic dehydrogenation of propan-2-ol which is available from propene,[‌10‌,‌11‌] (b) processes based on acetylene which, with steam, is passed over a heated mixed oxide catalyst, (c) the cumene hydroperoxide route to phenol,[‌12‌] and (d) hydrolysis of an alkenepalladium salt complex with reoxidation of the palladium.[‌13‌]

Acetone is also formed by hydration of propyne, by hydrolysis of 2-bromopropene, by hydrolysis of 2,2-dihalopropanes, from 2-hydroxy-2-methylpropanoic acid by oxidation, from acetyl chloride by action of zinc or dimethylcadmium(II), or from calcium acetate by heating.

Acetone is a colorless, water-soluble liquid (bp 56.2°C, mp 94.9°C, d20 0.796, nD 1.3602) of characteristic smell. It is highly inflammable and forms explosive mixtures with air or oxygen. It is miscible in all proportions with other organic solvents such as alcohols and ethers. Acetone is a good organic solvent and can dissolve some inorganic salts such as potassium iodide and potassium manganate(VII).

Acetone gives, with sodium nitroprusside solution followed by the addition of alkali, a red color which changes to violet on addition of acetic acid.[‌14‌] Acetone and other ketones can be detected by chromatography as derivatives such as the 2,4-dinitrophenylhydrazone.[‌15‌]

The simplest ketones are water-soluble liquids with pleasant odors and the higher members are low-melting solids. The physical properties of the simpler aliphatic symmetrical ketones, are listed in Table 2, those of cyclic ketones in Table 3, those of unsymmetrical ketones in Table 4, those of aryl ketones in Table 5, and those of unsaturated ketones in Table 6.

The unsaturation can be a C=C or CC bond conjugated with the ketone functionality.

Table 2 Physical Properties of Symmetric Aliphatic Ketones[‌16‌]

Compound Formula mp (°C) bp (°C) d (g·mL1) Refractive ­Index n20 Ref
pentan-3-one EtCOEt 42 102 d20 0.8138 1.3922 [‌16‌]
heptan-4-one PrCOPr 33 144 d20 0.8145 1.4069 [‌16‌]
2,4-dimethylpentan-3-one iPrCOiPr 124 d20 0.8108 1.4001 [‌16‌]
nonan-5-one BuCOBu 6 186 d15 0.818 1.4210 [‌16‌]
2,6-dimethylheptan-4-one iBuCOiBu 168 d20 0.8053 1.4121 [‌16‌]
3,5-dimethylheptan-4-one s-BuCOs-Bu 162 d14 0.8260 1.4193 [‌16‌]
2,2,4,4-tetramethylpentan-3-one t-BuCOt-Bu 152 d18 0.8240 1.4195 [‌16‌]
undecan-6-one Me(CH2)4CO(CH2)4Me 14 226 d20 0.8262 [‌16‌]
2,8-dimethylnonan-5-one iPr(CH2)2CO(CH2)2iPr 226 [‌16‌]
3,3,5,5-tetramethylheptan-4-one EtMe2CCOCMe2Et 196 [‌16‌]
3,5-diethylheptan-4-one Et2CHCOCHEt2 203 [‌16‌]
tridecan-7-one Me(CH2)5CO(CH2)5Me 31 264 d30 0.825 [‌16‌]
pentadecan-8-one Me(CH2)6CO(CH2)6Me 41 [‌16‌]
heptadecan-9-one Me(CH2)7CO(CH2)7Me 53 [‌16‌]
nonadecan-10-one Me(CH2)8CO(CH2)8Me 58 [‌16‌]
tricosan-12-one Me(CH2)10CO(CH2)10Me 69 d90 0.7888 [‌16‌]
heptacosan-14-one Me(CH2)12CO(CH2)12Me 78 d81 0.7986 [‌16‌]
hentriacontan-16-one (palmitone) Me(CH2)14CO(CH2)14Me 83 d90 0.7947 [‌16‌]
pentatriacontan-18-one (stearone) Me(CH2)16CO(CH2)16Me 88 d95 0.7932 [‌16‌]

Table 3 Physical Properties of Symmetric Cyclic Ketones[‌16‌]

Ketone mp (°C) bp (°C) d20 (g·mL1) Refractive Index nD20 Ref
cyclobutanone 99 0.938 1.4210 [‌16‌]
cyclopentanone 51 130131 0.997 1.4370 [‌16‌]
cyclohexanone 47 155 0.947 1.4500 [‌16‌]
cycloheptanone 179 0.951 1.4610 [‌16‌]
cyclooctanone 3941 195197 0.958 [‌16‌]

Table 4 Physical Properties of Unsymmetric Aliphatic Ketones[‌16‌]

Compound Formula mp (°C) bp (°C) d (g·mL1) Refractive Index nD Ref
ethyl methyl ketone (butan-2-one) MeCOEt 86 79.6 d20 0.8058 nD20 1.7880 [‌16‌]
pentan-2-one MeCOPr 78 102 d20 0.8089 nD20 1.3902 [‌16‌]
3-methylbutan-2-one MeCOiPr 95 d16 0.8046 nD16 1.3879 [‌16‌]
hexan-2-one MeCOBu 57 127 d20 0.8095 nD20 1.4007 [‌16‌]
hexan-3-one EtCOPr 125 d20 0.8118 nD20 1.4007 [‌16‌]
4-methylpentan-2-one MeCOiBu 85 117 d20 0.7908 nD20 1.3956 [‌16‌]
3-methylpentan-2-one MeCOs-Bu 118 d20 0.8145 nD20 1.4002 [‌16‌]
3,3-dimethylbutan-2-one MeCOt-Bu 106 d16 0.7999 [‌16‌]
heptan-2-one MeCO(CH2)4Me 35 151 d20 0.8111 n20 1.4086 [‌16‌]
3-ethylpentan-2-one MeCOCHEt2 138 [‌16‌]
4-methylhexan-2-one MeCOCH2s-Bu 146 [‌16‌]
heptan-3-one EtCOBu 39 149 [‌16‌]
octan-2-one MeCO(CH2)5Me 16 173 d20 0.8179 nD20 1.4154 [‌16‌]
nonan-2-one MeCO(CH2)6Me 15 194 d22 0.8188 nD22 1.4175 [‌16‌]
decan-2-one MeCO(CH2)7Me 14 209 d22 0.8230 nD22 1.4263 [‌16‌]
undecan-2-one MeCO(CH2)8Me 15 225 d17 0.8295 nD17 1.4300 [‌16‌]
dodecan-2-one MeCO(CH2)9Me 21 246/100Torr [‌16‌]
tridecan-2-one MeCO(CH2)10Me 29 250/100Torr d28 0.8229 [‌16‌]
tetradecan-2-one MeCO(CH2)11Me 34 207/100Torr [‌16‌]
pentadecan-2-one MeCO(CH2)12Me 39 294 d39 0.8182 [‌16‌]
hexadecan-2-one MeCO(CH2)13Me 43 230/100Torr [‌16‌]
heptadecan-2-one MeCO(CH2)14Me 48 246/110Torr d48 0.8140 [‌16‌]
octadecan-2-one MeCO(CH2)15Me 52 251/100Torr [‌16‌]
nonadecan-2-one MeCO(CH2)16Me 55 266/110Torr d56 0.8180 [‌16‌]

Table 5 Physical Properties of Aryl Ketones[‌16‌]

Compound mp (°C) bp (°C) d20 (g·mL1) Refractive Index n20 Ref
acetophenone 20.5 202 1.030 1.5320 [‌16‌]
propiophenone 18 218 1.009 1.5250 [‌16‌]
benzophenone 4849 305 [‌16‌]
4-(dimethylamino)benzophenone 8890 [‌16‌]
4,4-bis(dimethylamino)benzophenone (Michler's ketone) 174176 [‌16‌]

Table 6 Physical Properties of Unsaturated Ketones[‌16‌]

Compound mp (°C) bp (°C) d20 (g·mL1) Refractive Index n20 Ref
but-3-en-2-one 36.536.8/145Torr 0.842 1.4110 [‌16‌]
pent-3-en-2-one 121124 0.862 1.4370 [‌16‌]
cyclopent-2-enone 6465/19Torr 0.980 1.4810 [‌16‌]
cyclohex-2-enone 53 168 0.993 1.4880 [‌16‌]
cyclohept-2-enone 0.993 1.4940 [‌16‌]
but-3-yn-2-one 85 0.870 1.4060 [‌16‌]

The length of the C=O bond in ketones is about 1.20 Å according to the determination by electron diffraction and by microwave spectroscopy. The length of the bond increases as its polarity decreases.[‌17‌] The conjugation of the C=O bond with a C=C, CC, or CN bond has only a small influence on the C=O distance. For example the C=O bond length in acetone is 1.22 Å[‌17‌] and that in 2-oxopropanenitrile is 1.226 Å.[‌18‌]

The strong C=O bonds have relatively short lengths. In acetone the bond energy is 160.0 kcal·mol1.[‌19‌]

Dipole moments are usually determined by studying the dielectric constant of solutions and by measurement of the Stark effect on microwave transitions.[‌20‌] The dipole moments of some carbonyl compounds are reported in Table 7.[‌21‌‌23‌]

Table 7 Dipole Moments of Ketones[‌21‌‌23‌]

Compound Dipole Moment Method Ref
acetone 2.90 Stark effect on microwave spectra [‌21‌]
acetophenone 2.97 dielectric-constant method [‌22‌]
cyclohexanone 3.08 computation from experimental values [‌23‌]

Adjacent alkyl groups increase the polarity of the C=O bond. The conjugation of the carbonyl bond with a C=C or CC bond has only a small influence on the C=O dipole moment as in the case of the bond lengths. As a first approximation, one might expect that the dipole moment of an aromatic ketone and that of the corresponding aliphatic compound would be nearly the same but, because of the greater polarizability of the phenyl group in the near neighborhood of a dipolar group, the dipole moment of the aromatic compound is slightly larger. This is true for meta-directing substituents, but not for ortho- and para-directing substituents.

Ionization energies can be determined by extrapolation of the Rydberg series in the vacuum ultraviolet spectra of the molecules, or from the appearance potentials of the ions in a mass spectrometer, or by studying the photoionization efficiency curves.[‌24‌] The first of these methods yields the energy difference between the ground states of the ion and of the molecule, both of these states being at the zeroth vibrational level; this spectroscopic value is an adiabatic ionization energy. In the second method the transition is considered to be so fast that the nuclei do not change their position during the transition. The ion is in a vibrational level frequently above that of the zeroth level; the electron-impact method is supposed to yield the ionization energy. The vertical ionization energies (Iv) are equal or greater in magnitude than the adiabatic energies (Ia). Both Iv and Ia for different ketones are reported in Table 8.

Table 8 Ionization Energies of Ketones[‌25‌‌30‌]

Compound Ionization Energy (eV)a Method Ref
acetone 9.69 photoionization [‌25‌,‌26‌]
9.89 impact [‌27‌]
9.92 impact [‌28‌]
ethyl methyl ketone 9.58 impact [‌29‌]
Iv=9.52, Ia=9.48 photoionization [‌30‌]
pentan-2-one Iv=9.41, Ia=9.37 photoionization [‌30‌]

a Iv=vertical ionization energy; Ia=adiabatic ionization energy.

Only the carbonyl stretching vibration, which is typical of infrared spectra of ketones, will be considered here. All compounds containing a carbonyl group show a very strong band in the region 16501850cm1 (Table 9). The region in which this vibration appears is very narrow for a series of similar compounds. The position of the C=O band depends on the physical state of the compound, the inductive effect, electronic effects of neighboring substituents, ring strain, vibrational coupling between neighboring carbonyl groups, hydrogen bonding, enolization, and solvent effects.

Groups with a strongly electron-attracting inductive effect decrease the negative charge on the oxygen and the vibrational frequency increases relative to that of other ketones. The electron-repelling inductive effect of the ethyl group is larger than that of the methyl group and increases the negative charge on oxygen. This effect is responsible for the shift in stretching vibration from 1689cm1 for acetophenone to 1694cm1 for propiophenone. Other effects can add their influence to the inductive effect such as the variation of the force constant of adjacent bonds or the action of an electrostatic field created by a neighboring electronic cloud.

The mesomeric effect occurs when the C=O bond is conjugated either with another double bond or with a lone pair of electrons. Usually in this case, the carbonyl bond is more polarized, the force constant is smaller, and the stretching bands are shifted toward lower frequencies. For example, the carbonyl stretching frequency shifts from 1717cm1 in cyclohexanone to 1675cm1 in cyclohex-2-enone. If partial enolization occurs in a saturated ketone, the C=O stretching band intensity decreases and OH and C=C stretching bands appear. A new C=O stretching band might also appear as a consequence of hydrogen bonding between the ketonic and enolic species. It should be pointed out that solvent effects are important and are characteristic for each ketone. Carbonyl stretching frequencies of some ketones in solution are reported in Table 9.

Table 9 Carbonyl Stretching Frequencies of Ketones[‌31‌‌36‌]

Compound ν̃ (C=O) (cm1) Ref
2,2,4,4-tetramethylpentan-3-one 1685 [‌31‌]
2,4-dimethylheptan-4-one 1695 [‌31‌]
cyclohexanone 1717 [‌32‌,‌33‌]
cyclopentanone 1750 [‌34‌]
cyclobutanone 1775 [‌31‌]
acetophenone 1689 [‌35‌]
propiophenone 1694 [‌35‌]
4-aminoacetophenone 1677 [‌36‌]
4-methylacetophenone 1687 [‌36‌]
4-fluoroacetophenone 1692 [‌36‌]
4-nitroacetophenone 1700 [‌36‌]
diaryl ketones 1665 [‌31‌]
cyclohex-2-enone 1675 [‌31‌]
α-diketones 17201760 [‌31‌]
enolized α-diketones 1675 [‌31‌]
β-diketones 1720 [‌31‌]
β-diketones enol form 1650 [‌31‌]

Simple carbonyl compounds present several regions of absorption below the Rydberg bands in the ultraviolet region. The first band is located near 4000 Å and corresponds to a very weak absorption (εmax ~103), the second band near 3000 Å corresponds to a weak absorption (εmax ~10). The third and the fourth bands have a moderate to strong intensity and are near 1800 Å, and the fifth band with a very strong intensity is at shorter wavelength (1600 Å). The first two bands correspond to a transition from an oxygen 2p lone-pair orbital to the carbonyl antibonding π-orbital. The third and the fourth bands correspond to a transition of an oxygen 2p lone-pair electron to the carbonyl antibonding σ-orbital. The fifth band corresponds to a transition of a π electron to the antibonding π*-orbital.

The band around 4000 Å is due to a nπ* transition where the ground state is a singlet and the excited state is a triplet. The intensity of this first band is small since these bands involve two states of different multiplicity and are therefore forbidden. If the C=O bond is conjugated with a double bond or with a conjugated system, both the singletsinglet and singlettriplet nπ* transitions will shift to longer wavelengths (red shift). In contrast to the effect of a double bond, alkyl groups cause a blue shift of the nπ* transition of carbonyl compounds. It must be noted that these nπ* bands have a very low intensity and that their wavelengths depend very much on the solvent. There is a large blue shift on going from a nonpolar solvent to a polar one.[‌37‌] The singletsinglet nπ* transitions and singlettriplet nπ* of some ketones are reported in Table 10.

The promotion of an n-orbital electron to a π-antibonding orbital leaves one electron in the n-orbital, so the nπ* excited state may have a radical-like behavior. For example, ketones are well-known for their rearrangement, reduction, or cycloaddition on irradiation. Many other examples show that the photochemistry arising from the nπ* excited states demands interpretation based on radical-like intermediates.

Table 10 Singlet and Triplet nπ* Transitions[‌38‌‌41‌]

Compound SingletSinglet nπ* SingletTriplet nπ* Ref
Transition ­Wavelength (Å) εmax Solvent Transition ­Wavelength (Å) εmax Solvent
acetone 2750 22 cyclohexane [‌38‌]
cyclopentanone 2780 18 MeOH [‌39‌]
cyclohexanone 2820 15 MeOH [‌39‌]
cycloheptanone 2838 20 MeOH [‌39‌]
acetophenone 3628 78 hexane 3885 0.03 hexane [‌40‌]
benzophenone 3787 337 hexane 4121 0.03 hexane [‌40‌,‌41‌]

The absorption at about 1500 Å in the spectra of simple ketones is very strong (εmax ~20000). This band corresponds to a ππ* transition. An electron-donating substituent shifts the absorption to longer wavelengths, owing to resonance interaction between the π-electron system and the substituent. The conjugation of a double bond with a carbonyl group leads to intense absorption (εmax ~15000) and to an important red shift. This band corresponds to the transfer of a π-electron from a π-molecular orbital to a π*-molecular orbital and this band has a charge-transfer character. The prediction of the maxima of the ππ* of α,β-unsaturated ketones can be made by Woodward's rules or by an extension of these rules.[‌42‌]

A main difference between the nπ* and the ππ* bands consists in the solvent effect. In going from nonpolar solvent to polar solvent, the ππ* bands shift to longer wavelengths. The interaction with the solvent stabilizes the ground and the excited states in a different way and induces the red shift of the ππ* transition. Singlet ππ* transitions for some ketones are reported in Table 11.

Table 11 Singlet ππ* Transitions[‌43‌‌46‌]

Compound Transition ­Wavelength (Å) Phase Ref
acetone 1800 900 vapor [‌43‌]
but-2-en-3-one 2190 3600 EtOH [‌44‌]
1-methylpent-2-en-4-one 2490 2490 EtOH [‌44‌]
but-3-yn-2-one 2150 5000 EtOH [‌45‌]
benzophenone 2600 EtOH [‌46‌]

References


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