Unimolecular reaction organic reactions

New synthetic transformations are highly dependent on the dynamics of the contact ion pair, as well as reactivity of the individual radical ions. For example, the electron-transfer paradigm is most efficient with those organic donors yielding highly unstable cation radicals that undergo rapid unimolecular reactions. Thus, the hexamethyl(Dewar)benzene cation radical that is generated either via CT activation of the [D, A] complex with tropylium cation,74... 

Bowen, R.D. Potential Energy Profiles for Unimolecular Reactions of Isolated Organic Ions Some Isomers of [C4HioN] and [C5H12N]. J. Chem. Soc., Perkin Trans. 2 1980, 1219-1227. 

Unimolecular Reactions of Isolated Organic Ions Reactions of the Immonium Ions [CH2=N(CH3)CH(CH3)2]7... 

Bowen, R.D. Williams, D.H. Unimolecular Reactions of Isolated Organic Ions. The Importance of Ion-Dipole Interactions. J. Am. Chem. Soc. 1980, 102, 2752-2756. 

Bowen, R.D. Colburn, A.W. Denick, P.J. Unimolecular Reactions of Isolated Organic Ions Chemistry of the Unsaturated Oxonium Ion [CH2=CHCH=0CH3] 7 Org. Mass Spectrom. 1992, 27, 625-632. 

Bowen, R.D. Williams, D.H. Hvisten-dahl, G. Kalman, J.R. Potential Energy Profiles for Unimolecular Reactions of Organic Ions [CsHsN]-" and [CaHjOlt Org. Mass Spectrom. 1978,13, 721-728. 

Concerning their structure and reactions, organic radical cations have been the focus of much interest. Among bimolecular reactions, the addition to alkenes and their nucleophilic capture by alcohols, which lead to C—C and C—O bond formation, respectively have been investigated in detail. Unimolecular reactions like geometric isomerization and several other rearrangements have also attracted attention. 

These, and similar data for other systems, demonstrate the tremendous potential that the MICR technique has for the qualitative elucidation of potential energy surfaces of relatively complex organic reactions. Once implementation of the quadrupolar excitation technique has been effected to relax ions to the cell center, the technique will become even more powerful, in that the determination of highly accurate unimolecular decomposition lifetimes of chemically activated intermediates will also become possible. No other technique offers such a powerful array of capabilities for the study of unimolecular dissociation mechanisms and rates. 

Photochemical reactions are central to organic chemistry and are playing a key role in atmospheric aerosols [1], A variety of reactions occur, and the majority of them involve more than one molecule, mostly surrounded by a cluster (e.g., water). A common approach in modeling is to simplify the system and to treat only unimolecular reactions [2, 3], However, understanding the processes involved in cluster is of great interest itself and approaches for modeling those processes need to be developed. 

In the area of reaction energetics. Baker, Muir, and Andzehn have compared six levels of theory for the enthalpies of forward activation and reaction for 12 organic reactions the unimolecular rearrangements vinyl alcohol -> acetaldehyde, cyclobutene -> s-trans butadiene, s-cis butadiene s-trans butadiene, and cyclopropyl radical allyl radical the unimolecular decompositions tetrazine -> 2HCN -F N2 and trifluoromethanol -> carbonyl difluoride -F HF the bimolecular condensation reactions butadiene -F ethylene -> cyclohexene (the Diels-Alder reaction), methyl radical -F ethylene -> propyl radical, and methyl radical -F formaldehyde -> ethoxyl radical and the bimolecular exchange reactions FO -F H2 FOH -F H, HO -F H2 H2O -F H, and H -F acetylene H2 -F HC2. Their results are summarized in Table 8.3 (Reaction Set 1). One feature noted by these authors is... 

A typical unimolecular reaction is the decomposition of organic peroxides for which always positive activation volumes of up to 15 cm3/mol have been observed. The decomposition of di(t-butyl)peroxide, an effective initiator for the high pressure polymerisation of ethylene, into two t-butoxyradicals, exhibits a positive activation volume of 13 cm3/mol (Table 3.2-1, a). When new bonds are formed as in the association... 

In the case of closed-shell organic molecules M can be an excited singlet or triplet state. M can react on its own in unimolecular reactions (dissociations, isomerizations) or it can react with another (ground state) molecule N in bimolecular processes (e.g. additions, substitutions, etc.). 

Almost all organic peroxides are thermally and photolytically sensitive owing to the facile cleavage of the weak oxygen-oxygen bond. This cleavage is a unimolecular (first-order) reaction. The thermal decomposition rates are affected by the structure of the organic peroxide and the decomposition conditions. 

Every organic reaction can be classified into one of three, more or less exclusive categories—ionic, radical and pericyclic. Ionic reactions involve pairs of electrons moving in one direction. In a unimolecular reaction, like the ionization of a tertiary alkyl halide, the carbon-halogen bond cleaves with... 

One of the most common reasons for lowyields is an incomplete reaction. Rates of organic reactions can vary enormously, some are complete in a few seconds whereas rates of others are measured on a geological timescale. Consequently, to ensure that the problem of low yields is not simply due to low reactivity, reaction conditions should be such that some or all of the starting material does actually react. If none of the desired product is obtained, but similar reactions of related compounds are successful, the mechanistic implications should be considered. This situation has been referred to as Limitation of Reaction, and several examples have been given [32 ] the Hofmann rearrangement, for example, does not proceed for secondary amides (RCONHR ) because the intermediate anion 28 cannot form (Scheme 2.11). Sometimes, a substrate for a mechanistic investigation may be chosen deliberately to exclude particular reaction pathways for example, unimolecular substitution reactions of 1-adamantyl derivatives have been studied in detail in the knowledge that rear-side nucleophilic attack and elimination are not possible and hence not complications (see Section 2.7.1). 

Investigations of organic reactions in supercritical solvents are subject to several constraints, one attributable to supercritical fluid properties and others imposed for interpretive and experimental simplicity. Because supercritical fluid properties are affected by changes in temperature, a reaction should be selected which does not require heat for initiation and is not highly exothermic. Additionally, for experimental simplicity and clarity of interpretation, a clean, well-understood reaction should be chosen and one should expect an experimentally observable response to changes in pressure. Finally, a unimolecular reaction which produces a single product obviates the complication of controlling the concentrations of two reactants and simplifies product analysis. The photoisomerization of trans-stilbene meets these requirements. 

---