Aliphatic alcohols protonation

In addition, there is a large number of studies involving aromatic alcohols such as phenol [166] or naphthol, which have in part been reviewed before [21], These include time-resolved studies [21], proton transfer models [181], and intermolecular vibrations via dispersed fluorescence [182]. Such doubleresonance and more recently even triple-resonance studies [183] provide important frequency- and time-domain insights into the dynamics of aromatic alcohols, which are not yet possible for aliphatic alcohols. 

Rader, C. P. Hydroxyl Proton Magnetic Resonance Study of Aliphatic Alcohols. 

Electronic absorption spectra of alcohols in strong proton acids (H2SO4) were obtained by Rosenbaum and Symons (1059, 1960). They observed for a number of simple aliphatic alcohols absorption maxima... 

The intermediacy of ion-neutral complexes is neither restricted to even-electron fragmentations nor to complexes that consist of a neutral molecule and an ion. hi addition, radical-ion complexes and radical ion-neutral complexes occur that may dissociate to yield the respective fragments or can even reversibly interconvert by hydride, proton or hydrogen radical shifts. Many examples are known from aliphatic alcohols, [180-183] alkylphenylethers, [184-187] and thioethers. [188]... 

Addition of 0- to double bonds and to aromatic systems was found to be quite slow. Simic et al. (1973) found that O- reacts with unsaturated aliphatic alcohols, especially by H-atom abstraction. As compared to O, HO reacts more rapidly (by two to three times) with the same compounds. In the case of 1,4-benzoquinone, the reaction with O consists of the hydrogen double abstraction and leads to the 2,3-dehydrobenzoquinone anion-radical (Davico et al. 1999, references therein). Christensen et al. (1973) found that 0- reacts with toluene in aqueous solution to form benzyl radical through an H-atom transfer process from the methyl group. Generally, the O anion-radical is a very strong H-atom abstractor, which can withdraw a proton even from organic dianions (Vieira et al. 1997). 

Feuer and co-workers ° conducted extensive studies into alkaline nitration with nitrate esters, exploring the effect of base, time, stoichiometry, concentration, solvent, and temperature on yields and purity. Reactions are generally successful when the substrate a-proton acidity is in the 18-25 p A a range. Alkoxide bases derived from simple primary and secondary aliphatic alcohols are generally not considered compatible in reactions using alkyl nitrates. Optimum conditions for many of these reactions use potassium tert-butoxide and amyl nitrate in THF at —30 °C, although in many cases potassium amide in liquid ammonia at —33 °C works equally well. 

Class (3) reactions include proton-transfer reactions of solvent holes in cyclohexane and methylcyclohexane [71,74,75]. The corresponding rate constants are 10-30% of the fastest class (1) reactions. Class (4) reactions include proton-transfer reactions in trans-decalin and cis-trans decalin mixtures [77]. Proton transfer from the decalin hole to aliphatic alcohol results in the formation of a C-centered decalyl radical. The proton affinity of this radical is comparable to that of a single alcohol molecule. However, it is less than the proton affinity of an alcohol dimer. Consequently, a complex of the radical cation and alcohol monomer is relatively stable toward proton transfer when such a complex encounters a second alcohol molecule, the radical cation rapidly deprotonates. Metastable complexes with natural lifetimes between 24 nsec (2-propanol) and 90 nsec (tert-butanol) were observed in liquid cis- and tra 5-decalins at 25°C [77]. The rate of the complexation is one-half of that for class (1) reactions the overall decay rate is limited by slow proton transfer in the 1 1 complex. The rate constant of unimolecular decay is (5-10) x 10 sec for primary alcohols, bimolecular decay via proton transfer to the alcohol dimer prevails. Only for secondary and ternary alcohols is the equilibrium reached sufficiently slowly that it can be observed at 25 °C on a time scale of > 10 nsec. There is a striking similarity between the formation of alcohol complexes with the solvent holes (in decalins) and solvent anions (in sc CO2). 

Finch and Symons,87 on reinvestigation of the absorption of aliphatic alcohols and alkenes in sulfuric acid solution, showed that the condensation products formed with acetic acid (used as solvent for the precursor alcohols and alkenes) were responsible for the spectra and not the simple alkyl cations. Moreover, protonated mesityl oxide was identified as the absorbing species in the system of isobutylene, acetic acid, and sulfuric acid. 

Phenolic compounds are, in general, weak acids. Compared to the hydroxyl group of unsubstituted aliphatic alcohols, however, the phenolic OH-group is more acidic. The reason for this is that the anion formed after abstracting the proton from the hydroxyl group is relatively stable because of the existence of several mesomeric structures. The anion is referred to as the phenolate anion. Hence, phenol (2.5) is a weak acid, with a pKa value of 10. This places phenol in between carboxylic acids (pKa = 4-5) and aliphatic alcohols (pKa = 16-19). 

Fluorogenic labelling of pesticides. The subject has been reviewed earlier by Lawrence and Frei (27). Labelling consists in replacing a proton or other atom of a pesticide with a so-called labelling compound such as dansyl chloride (V) or fluores-camine (VI). The former reacts with primary and secondary amines, phenols, some thiols and aliphatic alcohols. The latter reacts very selectively with primary amines. 

The influence of the alcohol on the reaction was evaluated (Scheme 26). The results of a competition experiment between the alcohols are shown in Table 7. Both alcohols were treated with mono-alkoxysilane le using 10 % Pd/C as the catalyst. The silyl ketals of both alcohols were isolated as a mixture and the area under the methine protons, from the (+)-ethyl lactate moiety of both silyl ketals, was compared by NMR analysis. The difference in reactivity of primary, versus secondary, versus tertiary alcohol was small. The differences in reactivity range from 1.5 1 for 1° vs 2°, to 3 1 for 1° vs 3°. The reactivity of a benzyl alcohol is slower than the aliphatic alcohol as shown in entries 4 to 6. Entries 4 and 5 show an increase in the ratio of 1° 2° alcohol and a decrease in ratio for the 2° 3° for the secondary benzyl alcohol. Entries 6 and 7 confirm that benzyl alcohols are less reactive than aliphatic alcohols. The inductive electron withdrawing effect of the aryl group in the benzyl alcohol renders it less nucleophillic and this may affect the rate of reaction with the silane. Although the difference in reactivity is small, this trend may be informative. The influence of the alcohol s nucleophilicity on the reaction mechanism will be addressed in a later section. 

Wiberg determined the relative rates of addition of various alcohols and amines to silene 6, the results of which have been summarized above in Table 26. The fastest rates of addition were observed with aliphatic alcohols and amines, leading to the hypothesis that the first step of the reaction involves complexation of the neutral nucleophile at silicon, followed by proton transfer to the silenic carbon. Subsequent reports of the X-ray crystal... 

The reactivity of an organic compound toward eaq depends on its functional groups because the main hydrocarbon chain is non-reactive. Aliphatic alcohols, ethers, and amines are also nonreactive (k 106 M 1 s-1), although alkylammonium ions show a slight reactivity and can transfer a proton to the hydrated electron. Isolated double bonds are practically nonreactive, for ethylene k <2-5 X 106 M -1 s-1, but conjugated systems or double bonds with an electron withdrawing group attached to them are very reactive. For example, butadiene and acrylic acid react with practically diffusion controlled rates ( 10 0 M -1 s-1). 

The P-acetylenic alcohol, 3-hexyne-l-ol, also reacted with the P=N species via addition of the 0-H moiety. In this case, however, the next step is a simple proton migration from nitrogen to phosphorus to give the P-H bonded P isomer. Similar addition-migration processes are found in the reactions of the two-coordinate phosphinimine with simple aliphatic alcohols (5). 

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