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Elucidation of Reaction Mechanisms - Introduction / Products and Intermediates

Identification of Products and Intermediates - "Chemical" Methods

Isolation of the intermediate products

If an intermediate product is stable enough, or its lifetime is sufficiently long, and, above all, its concentration is high enough, its isolation and subsequent structure elucidation may be possible. For this purpose, it may be necessary to arrest the reaction through rapid cooling to a very low temperature. At least, the reaction rate must be reduced so that the intermediate product is not noticeably converted into the products during the measuring process. At this point, evidence is needed that the isolated compound is actually an intermediate and not a side product.

Trapping of intermediates with interceptors

The intermediates are often not capable of being isolated, as they are not stable enough or they are otherwise too low in concentration. In such cases, an attempt may be made to trap the intermediate with a suitable interceptor. The trapping of an intermediate in the following is illustrated by a well-known example - the addition of bromine to alkenes.

The known mechanism of the addition of bromine to alkenes.

One question that arises in the elucidation of this mechanism is whether the reaction shows a concerted mechanism in which the two bromine atoms of a bromine molecule are added to the alkene in one step (an elementary reaction) without the formation of an intermediate, or whether an intermediate is formed. Such an intermediate could, for instance, be electrophilic and also carry a positive charge, when a heterolytic cleavage with subsequent diffusing away of the bromide anion precedes the addition of only one of the bromine atoms.

How could the existence of such an intermediate be shown? In this case, a suitable nucleophile, whose reaction with the assumed electrophilic intermediate yields a characteristic product, which can be isolated and unequivocally identified, is given to the reaction mixture. The trapping reaction must proceed at least as rapidly or actually more rapidly than the otherwise occuring addition of the bromide anion to the intermediate. This may be controlled by the type of the nucleophile, as well as its concentration.

Examples of trapping of bromonium ions with different nucleophiles.

In the case of the addition of bromine to alkenes, such an intermediate can actually be captured! For instance, vicinal bromochloroalkanes, among other compounds, are obtained by adding chloride anions to the reaction mixture. If the reaction is carried out in a nucleophilic solvent (very high concentration of the intercepting nucleophile), such as water or ethanol, only the bromohydroxyalkane (in water) or the "vicinal" bromoalkyl ethyl ether (in ethanol), is usually formed.

The formation of these products is an indication of an electrophilic intermediate in the addition of bromine to alkenes. However, this information is only a small mosaic piece in the complete elucidation of the mechanism. The trapping experiments with nucleophiles other than that of the bromide anion do not yield the exact structure of the intermediate. Nevertheless, the trapping experiments prove that an electrophilic intermediate monobromo compound is formed and that the addition of a nucleophile to this intermediate occurs in the vicinal position to the bromine substituent. According to these results, the intermediate could also be a bromocarbenium ion, instead of a bromonium ion. Therefore, further experiments are required for the elucidation of the reaction mechanism. In this case, above all, the examination of the stereochemical result of the reaction yields additional insights into the structure of the intermediate.

The bromonium ion is the intermediate in the addition of bromine to alkenes, not the bromocarbenium ion.

It usually helps to prove that a certain compound is not an intermediate of the mechanism that is investigated. As the reaction could have failed, the absence of an intermediate is not proven if the trapping reaction with an apparently suitable interceptor is unsuccessful. However, the suggested intermediate may possibly be synthesized through another reaction and then successfully captured by the inceptor under the same reaction conditions. Thus, its appearance in the reaction, whose mechanism is being investigated, can be excluded.

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