Driving Force 1 - Cation Stability

A carbocation will be even more stable if there is a neighbouring oxygen atom which can donate electron density from its lone pairs of 

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The mechanism involves a simple 1,2 shift. The ion (52, where all four R groups are Me) has been trapped by the addition of tetrahydrothiophene. It may seem odd that a migration takes place when the positive charge is already at a tertiary position, but carbocations stabilized by an oxygen atom are even more stable than tertiary alkyl cations (p. 323). There is also the driving force supplied by the fact that the new carbocation can immediately stabilize itself by losing a proton. 

The B state responds much less to changes in donor and acceptor properties than the TICT state, and Eq. (5.1) can often be easily fulfilled by increasing donor and/or acceptor strength. In addition to these two factors which deliver the decisive part of the reaction driving force, polar solvent stabilization SOiv and the mutual Coulombic attraction C of the linked donor and acceptor radical anion/cation also help to preferentially stabilize the TICT state with respect to the precursor B state. 

Easily ionizable anthracene forms the cation-radical as a result of sorption within Li-ZSM-5. In case of other alkali cations, anthracene was sorbed within M-ZSM-5 as an intact molecule without ionization (Marquis et al. 2005). Among the counterbalancing alkali cations, only Li+ can induce sufficient polarization energy to initiate spontaneous ionization during the anthracene sorption. The lithium cation has the smallest ion radius and its distance to the oxygen net is the shortest. The ejected electron appears to be delocalized in a restricted space around Li+ ion and Al and Si atoms in the zeolite framework. The anthracene cation-radical appears to be in proximity to the space where the electron is delocalized. This opens a possibility for the anthracene cation-radical to be stabilized by the electron s negative field. In other words, a special driving force for one-electron transfer is formed, in case of Li-ZSM-5. 

The activation energy for this separation is 38 kJ mol in the ground state, with no barrier in the first excited state (Nielsen et al. 2000). The driving force is the high proton affinity of the amino group. This leads to the formation of such stabilized distonic cation-radicals. 

Cationic species are also formed when sulfides (Meissner et al. 1967 Adams 1970 Bonifacic et al. 1975a Janata et al. 1980 Hiller et al. 1981 Davies et al. 1984 Ramakrishna Rao et al. 1984) or thioureas (Wang et al. 1999 Schuchmann et al. 2000) react with OH. Especially stable are the dimeric radical cations [reactions (48) - (50)]. In the case of thiourea, the high stability of the dimeric radical cation may contribute to the driving force which leads, in acid solution, to its forma-... 

The [l,2]-alkyl migration A —> B of Figure 14.7 converts a cation with a well-stabilized tertiary carbenium ion center into a cation with a less stable secondary carbenium ion center. This is possible only because of the driving force that is associated with the reduction of ring strain a cyclobutyl derivative A is converted into a cyclopentyl derivative B. 

A variety of 1,3-diselenolylium an(j, 3-thiaselenolylium ions has been generated by the reaction of protons with 1,3-diselenoles and 1,3-thiaselenoles, respectively. Protonation of an exocyclic double bond at the 2-position with trifluoroacetic acid has generated 1,3-diselenolylium cations (54) and (55) (79JHC1303). The driving force for such reactions is presumably the heteroatom stabilization of the positive charge or formation of the aromatic sextet. 

Proteins solubilized in aqueous solution interact more or less with hydrophilic groups of surfactants at the oil-water interface. Therefore, the type of hydrophilic group is strongly influenced by the protein extraction efficiency. Anionic and cationic surfactants interact with charged protein surfaces more strongly than non-ionic surfactants. This feature also means that the non-ionic surfactants are favourable for protein stabilization in water droplets because of the not-so-hard interaction between the protein and the surfactant. In protein extraction, such an electrostatic interaction between proteins and surfactants is the main driving force in protein transfer. 

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