Ketones, absorption bands

Liquid acetoin stored for a long period of time has a well-developed ketonic absorption band at 275 nm, indicating the prevalent form (A), whereas liquid acetoin freshly prepared by melting or dissolving the dimer shows no such absorption, which indicates that based on the dimer structure the latter liquid can be expected to consist of the oxide form (B), and that the equilibrium between the keto form and the oxide form is reached very slowly (in a matter of days). 

Absorption Spectrum and Cross Sections of HCHO For carbonyl compounds such as aldehydes and ketones, absorption bands due to the electronic transition called n — r transition, in which the isolated pair of lone-pair electrons on the O atom of carbonyl group (—C = O) is excited to the excited Jt orbital of the double bond, appears around 300 nm. Since this transition is a forbidden transition, the absorption cross sections are not very large ( 10 cm molecule ) in general. However, since the absorption bands extend to near 350 nm where solar actinic flux grows, their photolyses are very important in the troposphere. 

UV-VIS Aldehydes and ketones have two absorption bands in the ultraviolet region Both involve excitation of an electron to an antibonding tt orbital In one called a TT TT transition the electron is one of the tt electrons of the C=0 group In the other called an n ir transition it is one of the oxygen lone pair electrons Because the tt electrons are more strongly held than the lone parr electrons the transition is of... 

Most other studies have indicated considerably more complex behavior. The rate data for reaction of 3-methyl-l-phenylbutanone with 5-butyllithium or n-butyllithium in cyclohexane can be fit to a mechanism involving product formation both through a complex of the ketone with alkyllithium aggregate and by reaction with dissociated alkyllithium. Evidence for the initial formation of a complex can be observed in the form of a shift in the carbonyl absorption band in the IR spectrum. Complex formation presumably involves a Lewis acid-Lewis base interaction between the carbonyl oxygen and lithium ions in the alkyllithium cluster. 

The n—>7T absorption band of noneonjugated ketones (f region of about 2800 A, corresponding to 102 kcal/mole. 

UV-VIS Aldehydes and ketones have two absorption bands in the ultraviolet region. 

This section is completed with a brief review of the synthesis and properties of this epimer (20) of the precursor of thiazole in bacteria. This pentulose is conveniently accessible by an unconventional route (Scheme 19). Methyl 2,3 4,6-di-O-isopropylidene-a-D-mannopyranoside, readily available from methyl ot-D-mannopyranoside, is converted to the ketonic glycoside by butyllithium in 91% yield, following a method first published by Klemer and Rodemeyer43 and scaled up by Horton and Weckerle.44 This was converted by means of lithium hydroxide in a water-ether mixture into 3,5-0-benzylidene-l-deoxy-D-eryf/iro-2-pen-tulose in 55% yield. Hydrolysis to the free pentulose (20) proceeded in 73% yield in aqueous acetic acid. This product was obtained as a syrup with a characteristic absorption band at 1705 cm 1 as a film. Thus, there is a fair proportion of the open-chain ketone under these conditions, as with the D-threo epimer.45... 

Different molecules absorb radiation of different wavelengths. An absorption spectrum will show a number of absorption bands corresponding to structural groups within the molecule. For example, the absorption that is observed in the UV region for the carbonyl group in acetone is of the same wavelength as the absorption from the carbonyl group in diethyl ketone. 

As discussed before, a,P-unsaturated ketones are usually reduced to saturated ketones in isopropyl alcohol as solvent, the reaction proceeding via the enol after H-abstraction by the carbonyl group. When some selected a,p-enones are irradiated in their n-n absorption band (i.e. excitation... 

Figure 14.5. Infrared spectra of a ketone, upper spectrum, and aldehyde. Both have strong =0 absorption bands. Hexanal has a chain of -CH2 groups and -CH3 group, and thus the strong absorption just less than 3000 cm"1. 

Most UV absorption bands correspond to transitions of electrons from ra->7i, or n o molecular orbitals. Besides aromatic compounds, organic functional groups such as carbonyl, carboxylic, amido, azo, nitro, nitroso, and ketone groups have absorbance in the UV region. 

Figure 8.34 Strong, sharp absorption at 1700 cm 1 indicating a carbonyl group. No other significant patterns except the C-H pattern on the low side of 3000 cm . It is an aliphatic aldehyde or ketone. Figure 8.35 A benzene ring is indicated because of the band on the high side of 3000 cm-1 and the series of weak peaks between 1700 and 2000 cm . Aliphatic C-H bonds are also indicated (absorption bands on the low side of 3000 cm-1). Possibly ethylbenzene, or a similar structure.

Due to the side chains this poiymer is amorphous with a glass transition temperature of 185 °C. In contrast, unsubstituted poly(arylene ether ketone)s are crystalline and high melting (T > 300 °C).The iR spectrum shows absorption bands at 3300 cm" and 1710-1730 cm" for the acid group in the side chain and at 1650 cm" for the keto group. 

The diphenyl derivative 388, R = R = Ph, has an absorption band at 1495 cm S which has been assigned to the carbonyl group. This assignment could be questioned. The following chemical reactions of the compound 388, R = R = Ph, may be noted (a) Raney nickel desulfurization followed by oxidation with manganese dioxide yields dibenzyl ketone,(b) hydrazine yields 3,5-diphenyl-4-hydroxypyrazole (390), (c) phenylhydrazine yields PhCH2COCPh=NNHPh," (d)... 

Aldehydes and ketones Spectroscopic techniques have proven particularly useful for the smaller aldehydes, which have distinct infrared and UV-visible absorption bands. As seen in Table 11.2 and discussed earlier, HCHO has been measured by FTIR in polluted urban areas as well as by TDLS and matrix isolation spectroscopy. In addition, as seen in Table 11.3, DOAS has high sensitivity for HCHO due to its strongly banded absorption in the 300- to 400-nm region (see Chapter 4.M). 

All hydrazones show a significant absorption band due to the C —N double bond at 1600-1645 cm-1 for ketone hydrazones, and at 1600-1610 cm"1 for the aldehyde derivatives. Mass spectra are characterized by a typical fragmentation pattern and a base peak corresponding to the methoxymethyl fragment6. 

The ultraviolet absorption spectra of ketones are characterized by a low intensity n -n absorption band in the 270-300 nm region, and a high intensity... 

In general, forbidden transitions give rise to low-intensity (low s) absorption bands ( <10,000), but two important forbidden absorptions are seen quite commonly the transition of ketones at... 

A ketone possesses an absorption band with a peak centred around 1 710cm-1. From this information deduce a value for the force constant of the C=0 double bond. 

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