Nitro ultraviolet spectra

Davies and Warren" found that when 1,4-dimethylnaphthalene was treated with nitric acid in acetic anhydride, and the mixture was quenched after 34 hr, a pale yellow solid with an ultraviolet spectrum similar to that of a-nitro-naphthalene was produced. However, if the mixture was allowed to stand for 5 days, the product was i-methyl-4 nitromethylnaphthalene, in agreement with earlier findings. Davies and Warren suggested that the intermediate was 1,4-dimethyl-5 nitronaphthalene, which underwent acid catalysed rearrangement to the final product. Robinson pointed out that this is improbable, and suggested an alternative structure (iv) for the intermediate, together with a scheme for its formation from an adduct (ill) (analogous to l above) and its subsequent decomposition to the observed product. 

The stabilizing influence in the hydrated cation is the amidinium resonance. If a solution of the cation is neutralized, a short-lived hydrated neutral molecule (4) (half-life 9 sec at pH 10) is obtained with an ultraviolet spectrum similar to that of the hydrated cation but shifted to longer wavelengths (5 m/ ). Supporting evidence can be derived from the anhydrous nature of the cation of 4-nitroiso-quinoline (pK 1.35), in which the nitro group has a similar electronic influence to that of the ring nitrogen atom N-I in quinazoline and where amidinium resonance is not possible. 

Tile ultraviolet spectrum of 3-iiitro-l,8-iiaphthyridiiie in methanol A ax [m/A](log e) = 206 (4.06), 238 (4.47), 311 (3.68), and 323 (3.65) showed bathochromic effect of the long-wavelength bands of 10 and 5 m/A with respect to the parent 1,8-naphthyridine (87MI2). Tlie bathochromic effect of 3-nitro group in 1,8-naphthyridine was compared with the effects of some other substituents. 

An example of the steric inOuence on the ultraviolet-spectrum of the nitro group in non-aromatic systems was given by T. Urbahski, Piotrowska and K d-zierski [18]. Absorption bands of the nitro group ( r— ir ) in S-nitro-l -dioxane were found to be 279 and 283 nm respectively. This was confirmed by EUel [19]. 

From structure CXCIX, it might be expected that haemanthidine exists in equilibrium with the open chain and hemiacetal forms (CCII, R = H and CCIII, R = H, respectively). It would appear that only a very small amount of CCII (R = H) can be present at any one time, since the alkaloid shows no carbonyl band in the infrared and a normal methylenedioxyphenyl ultraviolet spectrum. Haemanthidine, spotted on paper, gives no secondary amine test with alkaline sodium nitro-prusside and acetaldehyde. However, it seems likely that, under certain... 

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... 

To clarify the effect of substituents we will discuss the spectrum of 4-nitro-benzenol, even though the compound has no value as a dye. It is a pale yellow compound ( raax 320 nm) with an ultraviolet absorption band tailing into the visible, as in Figure 28-8. Its close relatives are benzene, benzenol, and nitrobenzene ... 

The nitro group is a chromophor. It produces an absorption band in the ultraviolet region of the spectrum. The position and the intensity of the band depend on several factors which will be discussed later. 

Benzene itself of course exhibits absorption in the near ultraviolet. Substituted benzenes, such as nitrobenzene, have more extensive absorption bands in about the same region of the spectrum. Now if we compare the absorption curves of three isomeric di-substituted benzenes, we should see whether deviations occurring in the orfAo-position manifest themselves in the benzene bands or e.g. for nitrobenzene in the nitro-band or in both. 

It should however be borne in mind that positions 9 and 10 in anthracene are not typically aromatic. They are manifested ]>y a higher reactivity than positions Ur and 0 as established by MO calculation [50]. In addition 9-nitroanthracene shows a non-planar structure with the nitro group out of plane [51] as pointed out by Cerfontain and Telder [48]. This is very similar to the position of the nitro group in o-dinitrobenzene and all derivatives of benzene with two ortho nitro groups. It is well known that the nitro groups in o-dinitrobenzene are not planar and there is no conjugation of double bonds in this compound. Tlie fact is also reflected in ultraviolet-absorption spectrum of o-dinitrobenzene which deviates from those of m- and p-dinitrobenzenes (Vol. I, p. 169, Table 20). 

The classical example of the spectrum of a nitro compound is that of nitro-methane. It consists of two broad bands a higli intensity band at X ax = 210 nm (log = 4.2) and a weak band at X ,ax = 270 nm (log e— 1.3) which probably arise from 7r2 —and rtg — rrj transitions respectively. Theoretically a third band n —from the transition ns —is also present at a very low wave-length in the vacuum ultraviolet and is of low intenaty. Ultraviolet-spectra of nitroalkanes including polynitro compounds were described in detail by Slovetskii [6] and reviewed by Novikov et al. in their monograph [7]. 

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