Carbonyl structure

It is not possible to draw unambiguous Lewis structures for excited states of the sort that are so useful in depicting ground-state chemistry. Instead, it is common to asterisk the normal carbonyl structure and provide information about the nature and multiplicity of the excited state ... 

The rormation of carbonyl structure was observed in the reaction of PECH at higher temperature but not for the reaction of P(ECH-EO). The polymer 2 is interesting because it crosslinked rapidly by the Lewis acid... 

In general, it may be expected that thiocar-bonyl compounds are easier to reduce ca-thodically than the corresponding carbonyl structures. There are a few examples dealing with the cathodic reduction of thiones. Let us quote the case of thiobenzophenone, which affords readily a rather stable radical anion at a potential that is -1-0.5 V more positive than that of benzophenone, when... 

The structure of the labile intermediate compound "Y" is uncertain. On the basis of its physical properties and chemical behaviour (which cannot be detailed here), possible formulations that were originally (485) considered included a six-membered bridged carbonyl-structure, as well as seven- or five-membered rings. The Italian group of workers (536) favoured the seven-membered ring-structure (CXVI) for the intermediate "Y", and 5-phenylguanamine structures (CXXI) for the final stable products. 

The amino groups are replaced with oxygen. Although here a biochemical reaction, the same can be achieved under acid-catalysed hydrolytic conditions, and resembles the nucleophilic substitution on pyrimidines (see Section 11.6.1). The first-formed hydroxy derivative would then tautomerize to the carbonyl structure. In the case of guanine, the product is xanthine, whereas adenine leads to hypoxanthine. The latter compound is also converted into xanthine by an oxidizing enzyme, xanthine oxidase. This enzyme also oxidizes xanthine at C-8, giving uric acid. 

TABLE 3. Effect of carbonyl structure on allenylzinc bromide additions... 

The observation of a bent Cr-H-Cr bond in the tetraethylammonium salt without an accompanying substantial deformation of the linear architecture of the nonhydrogen atoms in the [Cr2(CO)io(M2-H)] monoanion reflects the inherent flexibility of the bond. The deformability of the[M2(CO)io(M2-H)] monoanion species to adopt an appreciably bent, staggered carbonyl structure was first reported by Bau and co-workers (23) from neutron diffraction studies of two crystalline modifications of the electronically equivalent, neutral W2(CO)9(NO)(m2-H) molecule. Subsequent x-ray diffraction studies (15) of the analogous [W2(CO)io(m2-H)] monoanion found that the nonhydrogen backbone can have either an appreciably bent structure for the bis(triphenylphosphine)-iminium salt or a linear structure for the tetraethylammonium salt, with the W-W separation 0.11 A less in the bent form. Crystal packing forces probably were responsible (15) for the different molecular configurations of the monoanion in the two lattices. In solution, however, all known salts of the [W2(CO)io(m2-H)] monoanion exhibit the same three-band carbonyl ir absorption spectrum char-... 

The flavone. isoflavone. and flavonul-type dyes owe their importance 10 the presence of an o-hydroxy carbonyl structure within the molecule. Positions 4 and 5 can chelate with different metallic salts to give colored insoluble complexes. In other words, these dyes require a mordant in order lo lix them onto the fiber. 

Exercise 15-15 How can D-glucose, D-fructose, and D-ribose be considered products of the addition of an alcohol to the carbonyl group of an aldehyde or ketone Name each of the carbonyl compounds by the IUPAC system. For the ribose carbonyl structure, determine the configuration at each chiral center, using the D,L system. 

C—. It differs from the common acyl groups of the type R—C— in that it has the a//coxycarbonyl structure rather than an a//cy/carbonyl structure. The most used examples are ... 

The free radical initiated copolymerization of CH2CHOAc with CO has been reported 25>. Copolymers with up to 30 mol% CO were obtained. At 60 °C, the monomer reactivitry ratios were rVA = 0.24, rco = 0.33. The magnitude of rc0 indicated the possibility of the presence of vicinal CO groups in the polymer chain. Indeed, the results of a periodate oxidation of the copolymer showed that 30 % of the CO were present in 1,2-diketo structures. The acetate groups in the copolymer could be hydrolyzed in the presence of methanolic NaOH. However, the IR and the UV-vis spectra of the hydrolyzed copolymer showed the presence of significant amounts of a,P-unsaturated carbonyl structures, formed by the base induced dehydration. 

The ionization — Ae curve for untreated Bjorkman spruce milled wood lignin has a maximum at 357 m/z (Figure 5) which is attributed to phenolic a-carbonyl structures (31). Kraft-cooked Bjorkman lignin, however, has... 

Structure IX was unstable during the kraft cooking, but the presence of catechol, a-CO, or other highly conjugated carbonyl structures could... 

A test for reducing sugars, employing the same silver-ammonia complex used as a test for aldehydes. A positive test gives a silver precipitate, often in the form of a silver mirror. Tollens reagent is basic, and it promotes enediol rearrangements that interconvert ketoses and aldoses. Therefore, both aldoses and ketoses give positive Tollens tests if they are in their hemiacetal forms, in equilibrium with open-chain carbonyl structures, (p. 1118)... 

Fig. 8-11. Formation of carbanions and conjugated carbonyl structures (Gierer and Imsgard, 1977).

Gellerstedt and Pettersson (121) suggested that conjugated double bonds in lignin are susceptible to light-induced oxidation, yielding carbonyl structures that participate in the discoloration reaction. 

Halo substituents do not influence the general tautomeric patterns of the hydroxythiophenes <2000J(P2)1453>. The trichlorinated hydroxythiophenes are stable when kept under nitrogen. The hydroxythiophenes with a bromine substituent, however, started to polymerize quite rapidly, even when kept at temperatures below 0 °C under nitrogen. The carbonyl structure of 2,5-dihydrothiophen-2-one 198bis the only detectable tautomer of compound 198. 

For a metal with eight valence electrons, electron precise carbonyl structures see Electron Precise Compound) are M(C0)5 and [M(CO)4]3, which are known for all three metals of the iron group, albeit with quite different properties for the different metals. The next member, [M(CO)3]6, is not formed since a four-connected vertex requires more metal-metal bonding orbitals than can be arranged on the surface of a sphere. Bonding orbitals directed towards the inside of the cluster, however, form MOs that will accommodate more electrons than would be predicted by an 18 valence electron count for each metal see Eighteen Electron Compounds), that is, 86 instead of 84 for a six-vertex closo cluster see Closo Cluster). This situation is realized in the iron group by the anions [M6(CO)i8], M = Fe, Ru, Os (isolectronic to M6(CO)i6, M = Co, Rh, Ir). 

In diastereoselective asymmetric oxygenation of chiral enolates the introduction of new stereo-genic centers (chiral a-hydroxy carbonyl structural units) are induced by covalently bonded chiral units that are i) incorporated into the target molecule (substrate-induced diastereosclec-tivity) or ii) removed after the stereoinduction step (auxiliary-induced stereoselectivity). 

---