Quinone Molecule hydrogenation

The great affinity of quinone for hydrogen leads to the formation of anilinoquinone which can now, in the same way, add a further equivalent of aniline to the half of the molecule not yet involved. The dianilino-quinol is converted into dianilinoquinone in the same way as the first reaction product was changed. (Write the equation.)... 

A typical CIDEP initial polarization system is summarized in Fig. 11. For the quenching by a hydrogen donor (SH) of triplet quinone molecules, a plot of V-- - versus [SH] 1 will yield a straight line (see eq. 38), the ordinate intercept of which will give V"1 and from the slope an estimate of Tjt (or Tf) kq can be made. Because Vo can be obtained from the intercept and eliminated from the slope to obtain kq, it can be seen that only relative values of V are needed (i.e., the absolute are not required). (Absolute can be directly measured in a time-resolved CIDEP experiment, if needed.) Obviously, if the value of 3Tl is known, kq (absolute) can be calculated. Unfortunately, Tj values are usually not known. However, by studying a series of hydrogen donors, the relative quenching efficiencies can be estimated from the ratios of the slopes for plots of eq. 38. 

An alternative scheme was proposed by Moore and Waters 10°) to account for the high yield obtained in the reaction of benzaldehyde with phenanthrenequinone. They suggested that the initially formed radical R adds to a ground-state quinone molecule to give a new radical which then abstracts hydrogen from RH to regenerate the radical R ... 

On the other hand, in the case of quinone as hydrogen acceptor, an electrochemical mechanism may be instrumental. On platinum, hydrogen molecules may not only dissociate but also ionize anodically and quinone may be reduced cathodically by uptake of electrons and hydrogen ions in accord with Vetter 123). Thus, one has the following tentative scheme in acid solution ... 

It has been shown that in hydrogenation of triptycene quinones the more complicated structure of the quinone molecules leads to an increase of the rate, which changes with decreasing constant of formation of semiquinone. 

The sharp spectrum is probably that of the complex in the crystal-dine state in view of its sharpness and the concentration dependence of its relative intensity. The y-CD complex which shows the longer wavelength spectrum is probably the 1 2 complex [10] in which the orientation of the quinone molecule is axial, in consideration of the dimensions of AQ-2S and NQ in Fig. 1. On the other hand, the y-CD complex which shows the shorter wavelength spectrum is tentatively assigned to the 1 1 complex in which the orientation of the quinone molecule is equatorial [8], and consequently in which the quinone may form the hydrogen-bond with the solvent. 

As well as the cr-complexes discussed above, aromatic molecules combine with such compounds as quinones, polynitro-aromatics and tetra-cyanoethylene to give more loosely bound structures called charge-transfer complexes. Closely related to these, but usually known as Tt-complexes, are the associations formed by aromatic compounds and halogens, hydrogen halides, silver ions and other electrophiles. 

Hydrogen bromide adds to acetylene to form vinyl bromide or ethyHdene bromide, depending on stoichiometry. The acid cleaves acycHc and cycHc ethers. It adds to the cyclopropane group by ring-opening. Additions to quinones afford bromohydroquinones. Hydrobromic acid and aldehydes can be used to introduce bromoalkyl groups into various molecules. For example, reaction with formaldehyde and an alcohol produces a bromomethyl ether. Bromomethylation of aromatic nuclei can be carried out with formaldehyde and hydrobromic acid (6). 

A 6 positional state that is stabilized by the interaction of His 161 with a molecule of the inhibitor stigmatellin bound in the quinone binding pocket (41), which is supposed to mimic the hydrogen bonding pattern of the reaction intermediate, semiqui-none (43)... 

More generally, double bonds between two carbons or one carbon and a heteroatom, possibly conjugated with other unsaturated moieties in the molecule, are eligible for two-electron/two-proton reactions according to Scheme 2.20. Carbonyl compounds are typical examples of such two-electron/two-proton hydrogenation reactions. In the case of quinones, the reaction that converts the quinone into the corresponding hydroquinone is reversible. With other carbonyl compounds, the protonation of the initial ketyl anion radical compete with its dimerization, as discussed later. 

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