Aldehydes ultraviolet spectrum

In some cases spectroscopic evidence for such species exists. Thus the ultraviolet spectrum of acetophenone in aqueous sulfuric acid changes markedly as the concentration of acid is increased.278 In concentrated sulfuric acid simple aldehydes and ketones exists completely in the conjugate acid form. Unsaturated ketones give colored solutions in sulfuric acid and have -factors greater than two. The higher i-factors... 

Intense peaks also appear in the ultraviolet spectrum when a carbonyl function is conjugated with the imidazole ring as in the imidazole aldehydes,188-189 carboxylic acids,76,185 and acylimida-zoles.138 Thus, imidazole 2-aldehyde has Amax 285 m/x (emax = 12,500 in ethanol)189 imidazole-4-aldehyde (neutral molecule) has Amax 257 m/x (emax = 11,900), the corresponding cation has Amax 238 m/x (emax = 7300), and the anion has Amax 281 m/x(emax = 16,900),188and2-acetyl-4-methylimidazole has Amax 290 m/x (emax = 10,000 in methanol).35,138... 

The liquid phase photolysis of several alcohols was first systematically studied in the years between 1910 and 1913 by Berthelot and Gaudechon using a medium pressure mercury arc (21). They recognized that the main photolysis products of primary and secondary alcohols were H2 and an equivalent amount of aldehyde and ketone, respectively (21a,b). They also found (21d) that the part of the ultraviolet spectrum active on alcohols must lie below 250 nm, while carbonyl compounds are readily decomposed by light around 250 nm and above, yielding much CO. It was concluded (21d,22) that CO was a secondary product of the alcohol photolyses and due to the photolysis of the primarily produced carbonyl compound. In aqueous solutions of the alcohols the rate of decomposition was lower (21d) but led to the same products as those found in the neat photolyses. Similar to the liquid phase results Bates and Taylor (1927) found H2 as the major product in the methanol and ethanol gas phase photolysis (23). 

Let us now look at an ultraviolet spectrum, shown in Figure 4.1. The spectrum is that of a saturated ketone, butan-2-one, CH3COCH2CH3. The spectrum shows a single absorption peak at Amax 279 nm, max 16.6. This very low value of max shows us that this is a forbidden transition, an n jr transition, characteristic of an aldehyde or ketone group or a nitro group. These peaks all occur within the general range of 275-290 nm. Note that... 

Like the carbonyl groups of simple aldehydes and ketones, the carboxyl group shows only weak absorption in the ultraviolet spectrum unless it is conjugated with a carbon-carbon double bond or an aromatic ring. 

The product of the oxidation of 3-hydroxyanthranilic acid appears to be an unstable aliphatic compound having an ultraviolet spectrum with maxima at 275 and 360 xoft 324,329). There is also evidence that it contains an aldehyde group and a primary amino group 326-329). The half-life at room temperature of the intermediate is only a few hours, at low temperature in ethanol it can be kept for a few days. 

These structural problems are also insoluble by physical methods alone. The infrared spectrum often gives an unambiguous decision about the structure in the solid state the characteristic bands of the carbonyl or the hydroxyl group decided whether the compound in question is a carbinolamine or an amino-aldehyde. However, tautomeric equilibria occur only in solution or in the liquid or gaseous states. Neither infrared nor ultraviolet spectroscopy are sufficiently sensitive to investigate equilibria in which the concentration of one of the isomers is very small but is still not negligible with respect to the chemical reaction. 

The Pharmacopea and the Essential Oil Association of the United States require the following analyses as an index of oil purity refractive index, optical rotation, refractive index and optical rotation of a 10% distillate, specific gravity, aldehyde content, evaporation residue and ultraviolet absorption spectrum (Kesterson et al. 157 and Sale 29). 

RetinalS. The structure and photophysics of rhodopsins are intimately related to the spectroscopic properties of their retiny1-polyene chromophore in its protein-free forms, such as the aldehyde (retinal), the alcohol (retinol or vitamin A), and the corresponding Schiff bases. Since most of the available spectroscopic information refers to retinal isomers (48-55), we shall first center the discussion on the aldehyde derivatives. Three bands, a main one (I) around 380 nm and two weaker transitions at 280 nm and 250 nm (II and III), dominate the spectrum of retinals in the visible and near ultraviolet (Fig. 2). Assignments of these transitions are commonly made in terms of the lowest tt, tt excited states of linear polyenes, the spectroscopic theories of which have been extensively discussed in the past decade (56-60). In terms of the idealized C2h point group of, for example, all-trans butadiene, transitions are expected from the Ta ground state to B , A, and A" excited states... 

The solution frequently becomes warm,12 and its refractive index,1 viscosity,2 freezing point-composition curve,3 and ultraviolet absorption spectrum 36 are not those which would be expected if no reaction took place. Usually hydrates or hemiacetals of simple aldehydes are too unstable to be isolated, but a number of them are actually known and their physical properties have been determined.4 When the carbonyl group is attached to an electron-attracting group (making the carbonyl carbon atom abnormally positive), stable hydrates are frequently formed. Glyoxal, chloral, and ketomalonic acid are common examples. 

The constitution (XXI) was confirmed by a synthesis of dl-cus-pareine, which was identical with the alkaloid in boiling point, in ultraviolet absorption spectrum, and in its sensitivity to hot mineral acid which caused reddening and resinification. dl-Norcuspareine (XXIV) was formed by the catalytic reduction of the products obtained by the condensation of veratric aldehyde with quinalcfine or with o-nitrobenzal-acetone, or of o-nitrobenzaldehyde with veratralacetone, and on methyla-... 

Simple aldehydes and ketones show only weak absorption in the ultraviolet region of the spectrum owing to an n to tt electronic transition of the carbonyl group. If, however, the carbonyl group is conjugated with one or more carbon-carbon double bonds, intense absorption (g = 8,000 - 20,000 M cm ) occurs as a result of a 77 to tt transition as with polyenes, the position of absorption is shifted... 

Compounds that have a carbonyl moiety (group), C=0, on an end carbon (aldehydes) or middle carbon (ketones) are often the first species formed, other than unstable reaction intermediates, in the photochemical oxidation of atmospheric hydrocarbons. Aldehydes are important in atmospheric chemistry because they are second only to NO2 as atmospheric sources of free radicals produced by the absorption of light. This is because the carbonyl group is a chromophore, a molecular group that readily absorbs light and it absorbs well in the near-ultraviolet region of the spectrum to produce active species that can take part in atmospheric chemical processes. 

Measurements of the absorption spectrum of 2,2-dimethylpropanal were reported for 11 wavelengths in the ultraviolet by Zhu et al. (1999), shown in figure IX-B-12. It is surprising that the absorption by 2,2-dimethylpropanal is about 40% smaller than those observed with the other aldehydes of similar size. Zhu et al. (1999) rationalized this difference in terms of a possible effect on the n energy induced by the bulky (CH3)3C group present in 2,2-dimethylpropanal. If indeed the aldehyde sample used by Zhu et al. was 97% pure as stated, then the observed effect is most interesting, and reasons for diminution of its cross sections from that of aldehydes of similar size should be explored theoretically. 

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