Reaction mechanisms aromatization steps

Systematic studies of the selectivity of electrophilic bromine addition to ethylenic bonds are almost inexistent whereas the selectivity of electrophilic bromination of aromatic compounds has been extensively investigated (ref. 1). This surprising difference arises probably from particular features of their reaction mechanisms. Aromatic substitution exhibits only regioselectivity, which is determined by the bromine attack itself, i.e. the selectivity- and rate-determining steps are identical. 

The second aromatization reaction is the dehydrocyclization of paraffins to aromatics. For example, if n-hexane represents this reaction, the first step would be to dehydrogenate the hexane molecule over the platinum surface, giving 1-hexene (2- or 3-hexenes are also possible isomers, but cyclization to a cyclohexane ring may occur through a different mechanism). Cyclohexane then dehydrogenates to benzene. 

The structural similarity of the catalytic domains of the enzymes of the AAH family, together with the identical reaction that they catalyze (i.e., hydroxylation of aromatic substrates) and the common dependency on BH4 and 02 (Section I), suggests that the mechanisms by which these enzymes operate are similar. It is assumed that the general AAH reaction mechanism follows a two-step reaction route in which a high-valent iron-oxo (FeIV=0) complex is formed in the first step, and that this intermediate is responsible for the hydroxylation of the aromatic amino acid substrate in the second step (15,26-28,50). The first step starts with 02 binding and activation and proceeds via a Fe-0-0-BH4 bridge and a subsequent heterolytic cleavage of the... 

Additional evidence to this scheme was reported applying temporal analysis of products. This technique allows the direct determination of the reaction mechanism over each catalyst. Aromatization of n-hexane was studied on Pt, Pt—Re, and Pd catalysts on various nonacidic supports, and a monofunctional aromatization pathway was established.312 Specifically, linear hydrocarbons undergo rapid dehydrogenation to unsaturated species, that is, alkenes and dienes, which is then followed by a slow 1,6-cyclization step. Cyclohexane was excluded as possible intermediate in the dehydrocyclization network. 

The mechanism for the displacement reactions has also received considerable attention (Eaborn, 1960). A two-step process is generally regarded as satisfying the facts. Reasoning by analogy with conventional aromatic substitution reactions, the first step is presumed to be... 

The accepted reaction mechanism for the electrophilic aromatic nitration was postulated by Ingold in 1969[3] and involves several steps (Scheme 5.1). Firstly, the nitric acid is protonated by a stronger acid (sulfuric). The protonated nitric acid gives water and the nitronium ion (N02+) which is the electrophilic active species for nitration of aromatics. Nitric acid heterolysis is considered to be accelerated by the polarity of the solvent, and solvation of nitronium ion in different media affects its reactivity and the selectivity of the reaction. Combination of nitronium ion and an aromatic molecule form an intermediate named the Wheland complex or er-complex. The loss of a proton from the er-complex gives the aromatic nitrocompound (Scheme 5.1). 

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