Butyl alcohols reaction pathways

The authors actually detected the dibromomethylpropane and the r-butyl alcohol, but found no evidence for a-bromosulphides in their reaction products, which can be rationalized by the pathway outlined in equation (13). 

Dehydration of butyl alcohols over HZSM-5 and AAS acid catalysts provides unique opportunities to elucidate experimentally how confinement of the reagents, intermediates, and products inside the pores of the catalyst influences the reaction pathways. Indeed, in both HZSM-5 and AAS the dehydration reaction proceeds over the same active site, viz. 

When / -dicarbonyl enolates are allowed to react with alkynyliodonium salts, typically in ter/-butyl alcohol or THF, alkynyl- and/or cyclopentenyl- -dicarbonyl compounds are obtained. The product compositions are largely regulated by the migratory aptitude of R in the alkynyl moiety and the availability of alkyl side chains for the MC-insertion (MCI) pathway (equation 45). These divergent modes of reactivity are nicely illustrated by the reactions of the 2-phenyl-1,3-indandionate ion with ethynylfphenyl)- and 4-methyl-1-hexynyl(phenyl)iodonium tetrafluoroborates (equation 1 15)27 2. 

This is a two-step transformation. The first step is an addition-ehmination reaction of methyllithium with methyl acetate transiently forming acetone. The second step is a 1,2-addition of methyllithium to acetone forming the final tert-butyl alcohol. Hydrochloric acid is present only to quench the formed anions and liberate a neutral product. The steps of this transformation are illustrated below using arrow pushing. Please note that, for simplicity, association of the lithium cations with the anions of the illustrated mechanistic pathway is not shown. 

In the absence of added nucleophiles, nitrosation occurs virtually irreversibly by an acid-catalysed pathway, presumably by attack by HjNO or NO". The third order rate constant from the rate equation equivalent to (46) has a value of 840 dm moF s- at 31°C (c/. 456 and 6960 dm mol- s for cysteine and thiourea respectively at 25°C) which suggests that for this neutral substrate the reaction rate is somewhat less than that expected for an encounter-controlled process. There is a major difference between the nitrosation of alcohols and that of thiols in that, whilst the former reactions are reversible (with equilibrium constants around 1), the reactions of thiols are virtually irreversible. It is possible to effect denitrosation of thionitrites but only at high acidity and in the presence of a nitrous acid trap to ensure reversibility (Al-Kaabi et al., 1982). Direct comparisons are not possible, but it is likely that nitrosation at sulphur is much more favoured than reaction at oxygen (by comparison of the reactions of N-acetylpenicillamine and t-butyl alcohol). This is in line with the greater nucleophilicity expected of the sulphur atom in the thiol. For the reverse reaction of denitrosation [(52) and (53)], the acid catalysis observed suggests the intermediacy of the protonated forms... 

Illustrative of the dehydrohalogenation pathway, the formation of trans-l,2-di-t-butylcyclopropanone reported earlier by Greene and coworkers has now been described in detail. The reaction may be carried out heterogeneously in ether, or homogeneously in t-butyl alcohol, conditions which correspond to those of the well-known Favorskii reaction. In this reaction it is crucial to use exactly one equivalent of base, since even a small excess results in complete conversion to the ester (equation 4). 

As might be expected, tertiary alcohols undergo a fairly rapid acid-catalysed exchange of oxygen with water. There are, however, several pathways by which this can be achieved. In the case of t-butyl alcohol (13) these various reactions are ... 

Two mechanistic pathways will be discussed for the dissolving metal reduction of enones30. In both cases the first step is the reversible transfer of an electron from the metal to a vacant orbital of the substrate, yielding a radical anion. This can be protonated to the neutral radical which can dimerize or accept another electron and a proton, Alternatively, a second electron can be transferred reversibly to the radical anion, giving the dianion capable of accepting two protons. The sequence and timing of these steps depends on the substrate, the reduction potential of the reaction medium and the nature of the proton source, as well as on various other factors. In general, the thermodynamically more stable product is formed predominantly, as illustrated in the reduction of cxocyclic enone 134. The rrmw-substituted cyclohexane, with both substituents in an equatorial position, is formed preferentially if the reaction is carried out in the presence of /erf-butyl alcohol as proton source. 

For the other fuel oxygenates detailed studies on the reaction pathways are not as abundantly available in literature. However, the similarity in chemical structure implies similar reaction by-products. In the case of TAME, tert-amyl formate and tert-amyl alcohol were observed instead of fBF and fBA, and their subsequent degradation products however, acetone and methyl acetate were observed as well [117]. The attack on the methoxy group was observed to be the major pathway, corresponding to the MTBE ehmination. During the elimination of ETBE the same reaction by-products were observed as with MTBE, with the exception of fert-butyl acetate which was formed instead of fBF [30]. 

An important principle, called microscopic reversibility, connects the mechanisms of the forward and reverse reactions. It states that in any equilibrium, the sequence of intermediates and transition states encountered as reactants proceed to products in one direction must also be encountered, and in precisely the reverse order, in the opposite direction. Just as the reaction is reversible with respect to reactants and products, so each tiny increment of progress along the mechanistic pathway is reversible. Once we know the mechanism for the forward reaction, we also know the intermediates and transition states for its reverse. In particular, the three-step mechanism for the acid-catalyzed hydration of 2-methylpropene shown in Mechanism 6.3 is the reverse of that for the acid-catalyzed dehydration of tert-butyl alcohol in Mechanism 5.1. 

The formation of an alkanethiol by reaction of an alkyl halide or alkyl /Moluenesulfonatc with thiourea occurs with inversion of configuration in the step in which the carbon-sulfur bond is formed. Thus, the formation of (R)-2-butanethiol requires (.S Kvcc-butyl /Moluenesulfonatc, which then reacts with thiourea by an SN2 pathway. The /Moluenesulfonatc is formed from the corresponding alcohol by a reaction that does not involve any of the bonds to the stereogenic center. Therefore, begin with (.S )-2-bulanol. 

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