Electrophilic addition reactions unimolecular

The four-membered cyclic transition state is not allowed by orbital symmetry theory and parity rules. It requires inversion of configuration at the a-carbon and trans addition to the alkene by a conrotatory process, which is sterically impossible [261,263]. The six-membered transition state is allowed by parity rules, but the relative contributions of this pathway and that by unimolecular ionization depends on their relative rate constants and therefore their free energies of activation. Since the transition state of electrophilic addition to alkenes proceeds with a very late transition state requiring an electrophile with a highly developed charge, covalent species are not sufficiently polarized to react directly with alkenes. Thus, the reaction should occur in two steps rather than by a concerted addition [264],... 

Carbocations, however formed, are very electrophilic. They react readily with nucleophiles, as shown in reaction (5.18). These reactions are important as steps in electrophilic addition to double bonds and unimolecular nucleophilic substitution (SnI) reactions. 

The reaction of ozone with allyl alcohol is expected to involve, as for other unsaturated alcohols and olefins, electrophilic addition at the carbon-carbon double bond leading to the formation of a trioxide followed by its unimolecular decomposition into two carbonyls and two biradicals (Criegee intermediates) ... 

Self-Assessment Exercises 79a. Nucleophilic substitution corresponds to a substitution (either Sxjl or S[.j2) for aliphatic compounds. Electrophilic aromatic substitution is typical for aromatic compounds (an atom is replaced by an electrophile) 79b. An addition reaction is the opposite of an elimination reaction. In an addition reaction, two or more atoms (molecules) combine to form a larger one. 79c. S[,jl reaction involves the formation of carbocation. Sf.j2 reaction, on the other hand, is a one-step process in which bond breaking and bond making occur simultaneously at a carbon atom with a suitable leaving group. 79d. El reactions are unimolecular elimination reactions that proceed via carbocation intermediates. E2 reactions are bimolecular, one-step reactions that require an antiperiplanar conformation at the time of rr-bond formation and /3-bond breaking and do not involve carbocations. 

In contrast, in protic solvents and at low bromine concentration, the addition process is characterized by a second order rate law (first order in bromine), Scheme 2, path b. In this case, due to the ability of the solvent to provide a specific electrophilic solvation to the leaving bromide ion, the reaction occurs via an SN1 -like unimolecular ionization of the 1 1 it complex to form a bromonium or P-bromocarbenium bromide ion pair. It is worth noting that protic solvents can also give nucleophilic assistance, depending on their specific solvent properties. 

With ions or dipolar substrates, radical ions undergo nucleophilic or electrophilic capture. Nucleophilic capture is a general reaction for many alkene and strained-ring radical cations and may completely suppress (unimolecular) rearrangements or dimer formation. The regio- and stereochemistry of these additions are of major interest. The experimental evidence supports several guiding principles. 

A strongly solvent-dependent electrophilic reaction is the addition of halogens to alkenes [79-81] and alkynes [81a]. In a rapid equilibrium, a loose transitory EPD/EPA complex (1 1) between halogen and alkene is formed [512]. This is followed by the ratedetermining step, which involves an SNl-hke unimolecular ionization to form a halo-nium intermediate which can be either symmetrical or unsymmetrical. This then reacts with a nucleophile Nu to give the products cf. Eq. (5-29). 

It is convenient to categorize reactions with concise descriptive labels. For substitution reactions we often use the notation SxM, in which the letter S indicates a substitution reaction. The subscript x indicates something of the mechanism, such as N for nucleophilic or E for electrophilic. M usually indicates the molecularity of the reaction, the nature of the reacting species, or additional information. The most familiar terms for substitution reactions are SnI (for substitution nucleophilic unimolecular ), as shown in equation 8.4,... 

The rate-determining step in the reaction of Ph3SnCHaCH=CHR, where R = H, Me, or Ph, with iodine is thought to be unimolecular decomposition of a 7r-complex between the organotin compound and the iodine. Kinetic parameters and product distributions have been obtained for this reaction in a variety of solvents, in some cases with the addition of iodide ion to the reaction mixture. In acetone or acetonitrile the mechanism is said to involve attack at the carbon atom y to the tin, but in the more polar solvents methanol and dimethyl sulphoxide there is also some direct electrophilic attack by iodine or iodide ion at the tin. ... 

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