Electrophilic addition definition

The half-reactions formulated here in the masterlist, and used in the general sequence lists, have value for the synthetic chemist not only in clarifying our sense of definition of construction reactions but also in pointing out deficiencies in our practical collection of viable reactions. A check on the frequency of appearance of the various half-reactions in the general sequence lists can show which are most important and lead to reexamination of those with practical limitations. In particular substitutions and nucleophilic or electrophilic additions to simple, unactivated olefins, (respectively Ba, I2 andia) seem to need better im-lementation, perhaps through organometallic mediation. 

An especially interesting case of oxygen addition to quinonoid systems involves acidic treatment with acetic anhydride, which produces both addition and esterification (eq. 3). This Thiele-Winter acetoxylation has been used extensively for synthesis, stmcture proof, isolation, and purification (54). The kinetics and mechanism of acetoxylation have been described (55). Although the acetyhum ion is an electrophile, extensive studies of electronic effects show a definite relationship to nucleophilic addition chemistry (56). 

An attempt has been made to analyse whether the electrophilicity index is a reliable descriptor of the kinetic behaviour. Relative experimental rates of Friedel-Crafts benzylation, acetylation, and benzoylation reactions were found to correlate well with the corresponding calculated electrophilicity values. In the case of chlorination of various substituted ethylenes and nitration of toluene and chlorobenzene, the correlation was generally poor but somewhat better in the case of the experimental and the calculated activation energies for selected Markovnikov and anti-Markovnikov addition reactions. Reaction electrophilicity, local electrophilicity, and activation hardness were used together to provide a transparent picture of reaction rates and also the orientation of aromatic electrophilic substitution reactions. Ambiguity in the definition of the electrophilicity was highlighted.15... 

Consequently, we were faced with the task of formulating a widely acceptable and consistent definition of bond activation . Our research, discussions, and analyses led to a conclusion that bond activation should refer to a process of increasing the reactivity of a bond in question and as such encompasses an entire spectrum of possible mechanisms. Also, we argue that activation is not equivalent to reaction or, in other words, that activation of a bond is not the same as cleavage of a bond. For the latter process we proposed the general term bond transformation . It should be emphasized that both bond activation and bond transformation are general terms and, therefore, information about the reaction and mechanism category should be specified by additional descriptors (cf. C-H bond arylation via electrophilic metalation, C-H bond metalation via concerted metal insertion). 

Markovnikov orientation the 1,2-addition of an unsymmet-rical electrophilic reagent to an alkene with attachment of the more positive moiety of the reagent (H in the original definition) to the less substituted carbon Michael addition conjugate addition addition of a nucleophile to the /3-position of a carbon-carbon double or triple bond that is conjugated to a carbonyl or other electron-withdrawing group... 

Deschamps showed that the phosphorus atoms of 3,3, 4,4 -tetramethyl-l,r-diphosphaferrocene (DPF) are electrophilic. The reaction of DPF at - 80 C in THF with two equivalents of t-BuLi, followed by addition of three equivalents of CH3I, gives a stable, water-soluble, paramagnetic monocation (XXXII) which has a bis-(> -diene)iron structure as definitively proved by X-ray structural analysis. 

It seems appropriate to inquire whether or not it is possible to carry out other Michael reactions and, generally, other nucleophilic additions to unsaturated compounds as a sequence of kinetically independent steps using one s choice of nucleophiles and electrophiles The answer is definitely yes . A rationale similar to that used to describe the Robinson annulation provides us with the key to how this goal may be attained. First of all, the initial step of the reaction, addition of the nucleophilic component across a double (or triple) bond, needs to be carried out in the absence of the external electrophiles (preferably in aprotic solvents). Secondly, a carbanionic intermediate, incipiently formed at this step, requires sufficient stabilization to survive as a chemical entity under... 

This is the synthesis of a somewhat controversial food preservative called BHT. It is an antioxidant that has been put in packaging to keep foods from becoming rancid from air oxidation. A balanced reaction would require two molecules of the alkene for each of the starting aromatic. The aromatic ring is relatively electron rich with two donors attached to it. With sulfuric acid present, the medium is definitely acidic. The first step is to generate the excellent electrophile needed for electrophilic aromatic substitution. For simplicity, let s symbolize sulfuric acid as H-A. The Markovnikov addition of a proton to isobutylene gives the tert-butyl carbocation, an excellent electrophile. 

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