Carbanions bimolecular reactions

C olvents have different effects on polymerization processes. In radical polymerizations, their viscosity influences the diffusion-controlled bimolecular reactions of two radicals, such as the recombination of the initiator radicals (efficiency) or the deactivation of the radical chain ends (termination reaction). These phenomena are treated in the first section. In anionic polymerization processes, the different polarities of the solvents cause a more or less strong solvation of the counter ion. Depending on this effect, the carbanion exists in three different forms with very different propagation constants. These effects are treated in the second section. The final section shows that the kinetics of the... 

The observed first-order rate constant for carbanion formation may be controlled through the choice of the basic proton acceptor. Relatively strong carbon acids undergo detectable deprotonation by the weak base water in a pseudo-first-order reaction (Scheme I.IA), but stronger general bases (Scheme I.IB) or hydroxide ion (Scheme I.IC) are required to give detectable deprotonation of weaker carbon acids in bimolecular reactions. 

This statement is a generalization. Whether one observes racemization with either radicals or carbanions depends on the reaction conditions and the rates of inversion and of bimolecular reaction. 

Since the sensitivity towards water in many organic reactions lies in the order carbanion > carbonium ion > free radical, it appears likely that as water is progressively removed from a-methylstyrene—and, perhaps, other vinyl monomers—the free radical propagation is augmented or supplanted by a carbonium ion mechanism, which, in turn, is further enhanced at low water content, by a carbanion mechanism. Under the latter conditions, one would expect a termination mechanism which is bimolecular with regard to the total concentration of propagating species and hence a square-root dependence of the polymerization rate on the dose rate. This is the order dependence observed in a-methylstyrene at the highest polymerization rates and lowest water content. 

Table 11 presents one more result important for the chemistry of epoxy compounds, namely within the experimental error the rate constant of the free ion is the same for all counterions. This means that such strong nucleophilic particles as carbanions (and evidently alkoxy anions) are capable of opening the epoxy ring without additional electrophilic activation. This result explains the apparently contradictory results that, depending on the reaction conditions, either tri-140 144,166-I71) or bimolecular kinetics 175-I79> is observed. The bimolecular kinetics also can be explained in terms of the trimolecular mechanism, since proton-donor additives play a dual role. 

In comparison to carbanions, which maintain a full octet of valence electrons, carbenium ions are deficient by two electrons and are much less stable. Therefore, the controlled cationic polymerization requires specialized systems. The instability or high reactivity of the carbenium ions facilitates undesirable side reactions such as bimolecular chain transfer to monomer, /1-proton elimination, and carbenium ion rearrangement. All of that limits the control over the cationic polymerization. 

In Section 4.5 we discussed reactions in which electrophilic substitution of a metal ion takes place by a bimolecular pathway. The unimolecular substitution is less common, although there are some examples in cases where the carbanion is well stabilized.120 For our purposes here the most important SE1 reactions are those in which the leaving group is a proton or a neutral carbon molecule. 

In the bimolecular concerted beta elimination reaction, E2, heterolytic cleavage of the C—X and C—H bonds takes place within the same reaction step, without formation of an intermediate (see Vol. 9). It appears that the energy barrier for the concerted process is lower than each of the barriers for the separate steps involving either a carbanion or a carbonium ion intermediate. 

The autoxidation of other phosphonate carbanions derived from diethyl diphenylmethylphosphonate (9) and diethyl fluorenylphosphonate (10) showed that DBA (9,10-dibromoanthracene, a triplet energy aacceptor) enhanced the chemiluminescence in spite of the lower energy for the excited triplet benzophenone (68-69 kcal/mol) and fluorenone (53 kcal/mol) than that for singlet DBA (71 kcal/mol). The Stem-Volmer plot of the double reciprocal of the DBA concentration and the chemiluminescence quantum yields established a bimolecular process with the fluorophor and the excited species in these chemiluminescence reactions. The emission quantum yields at the infinitive DBA concentration were calculated to be... 

Carbanions, being strong bases and nucleophiles, react readily with acids and undergo the SN2 reactions considered in Chapter 1 and the bimolecular E2 eliminations considered in Chapter 3. Other important reactions of carbanions (and nucleophiles) include addition to unsaturated compounds, especially those containing carbonyl groups, fragmentation (the reverse of addition) and elimination to give carbenes. 

The rate-limiting step for the reaction of CH30 with CF2=CHC6H5 generates a carbanion intermediate, CH30CF2CHC6H5, and would result in bimolecular kinetics. This reaction will be discussed in Section 18.2. 

Recent studies have made it very probable that such decarboxylations are bimolecular and not unimolecular.38 SE2 reactions of this type occur preferably in acid solution, where the -carbon atom of, / -unsaturated acids are protonated before the original carbon-carbon bond between the carboxyl group and the -carbon atom is broken. Decarboxylations of this type thus involve, not carbanion intermediates, but carbonium intermediates having the... 

Despite the obvious differences in behaviour of the rates of carbanion and bimolecular mechanisms towards acidity functions, the latter have seldom been used to illustrate ElcB reactions in probable cases. One drawback is that... 

It is not entirely clear that the carbanion intermediate exists as such or whether protonation occurs very rapidly, so that the decarboxylation may really be a bimolecular (E2) elimination, during which C-4, C-5, the ring oxygen atom and the original hydrogen atom on C-5 all come to lie in a plane. The loss of CO2 makes the reaction irreversible. 

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