Competition from other reaction

In the literature many examples of attacks of nucleophiles upon carbonyl centres can be found and they have been covered adequately in a recent review (Bowie, 1984b). These attacks are rarely observed, however, without competition from other reaction channels. This is well demonstrated by the reactions of NH2 with methyl formate deuterated in the formyl position, where at least five different primary product ions are formed as summarized in eqns (15a)-(15f). Some of these product ions are consumed by reaction with the NH3 and DC02CH3 molecules present as can be seen from Fig. 5. This shows how the chemistry evolves as a function of reaction time in the FT-ICR instrument, and the results are in excellent agreement with the observations made for the same system in a FA apparatus (DePuy et at.,... 

The objective in selecting the reaction conditions for a preparative nucleophilic substitution is to enhance the mutual reactivity of the leaving group and nucleophile so that the desired substitution occurs at a convenient rate and with minimal competition from other possible reactions. The generalized order of leaving-group reactivity RSOj" I- > BF > CF pertains for most Sw2 processes. (See Section 4.2.3 of Part A for more complete data.) Mesylates, tosylates, iodides, and bromides are all widely used in synthesis. Chlorides usually react rather slowly, except in especially reactive systems, such as allyl and benzyl. 

The silene 42 itself is liberated from the vinylsilacyclobutanes 40 and undergoes, in competition with other reactions, an intramolecular [2 + 2] addition giving 44 (equation 181). 

The limitation to die widespread synthetic use of the thermal or photochemical cyclization of trienes are twofold the difficulty in preparing the starting, geometrically correct triene array and the periselec-dvity (i.e. the competition widi other reactions occurring from he same substrate). Much of the following discussion is limited to the latter topic although it must be recognized that an efficacious synthesis of... 

A detailed reaction scheme has been formulated to account for the laboratory data of Altshuller et al. (12). In Figure 3 the calculated con-centration-time behavior based on this scheme is compared with the experimental data for C3H6, NO, NO2, and O3. More than 150 elementary reactions were initially considered in constructing this model. About 50 of these were eliminated from the scheme because their rate constant and/or the relative concentrations of the reactants are too small to be competitive with other reactions. Another 20 reactions were discarded... 

A. B. Astra soon realized that Ugi s group had introduced this new 4CR, but had rather different interests and concepts. The company was mainly interested in avoiding competition from others, whereas Ugi proposed that, by this new 4CR, similar collections of compounds with the same reaction and thus backbone could be produced simultaneously. These compounds should be tested in order to find many new chemical entities with similar or even better biological properties. Hopefully, many local anesthetics can be discovered in future using this approach. 

Yields and reaction rates tend to increase with increasing alkyl substitution on the carbon bearing the halide-leaving group due to diminished competition from side reactions such as direct nucleophilic substitution by the base. Other competing reaction products that have been observed include epoxyethers, a-epoxyketals, and a,P-unsaturated ketones. 

Surface reaction and coverage can be strictly controlled. Interference from other compounding additives can be minimised. Surface reaction is very much at the mercy of processing conditions, including competition from other additives. Residual, unreacted, additive more likely to be present and may cause problems. 

Zahler and elaborated in a series of papers by Miller and co-workers and in Bunnett s publications, many of which are cited in Section I, D. It should be pointed out that the effects of substituents on nucleophilic substitutions show important differences from their effects on other reactions or on equilibria which involve competition for a lone-pair of electrons on another group or stabilization of negative charge on some atom of the reacting moiety. The cr-constants for nucleophilic substitutions differ from those determined in the latter work in that they show the response of the substituent to a strong demand for stabilization of negative charge on the substituent itself, especially by resonance. 

Here, a primary ion P+ formed by the radiation field reacts with a gas molecule M to give an intermediate complex [PM +] which can either dissociate to a secondary species S + and a neutral fragment N or react with another molecule to produce another complex [PM2 + ]. The latter then dissociates into a tertiary ion T+ or propagates the chain by forming a third intermediate [PM3 + ]. A quaternary ion Q+ may result from dissociation of [PM3 + ], or the chain may continue through reaction of [PM3 + ]. Wexler and Jesse (38), on the other hand, have suggested a model which states that reactive intermediate complexes are not involved in the propagation, but rather the polymerization proceeds by chains of simple consecutive and competitive ion-molecule reactions,... 

---