Reactivity geometrical isomerization

Butenes or butylenes are hydrocarbon alkenes that exist as four different isomers. Each isomer is a flammable gas at normal room temperature and one atmosphere pressure, but their boiling points indicate that butenes can be condensed at low ambient temperatures and/or increase pressure similar to propane and butane. The 2 designation in the names indicates the position of the double bond. The cis and trans labels indicate geometric isomerism. Geometric isomers are molecules that have similar atoms and bonds but different spatial arrangement of atoms. The structures indicate that three of the butenes are normal butenes, n-butenes, but that methylpropene is branched. Methylpropene is also called isobutene or isobutylene. Isobutenes are more reactive than n-butenes, and reaction mechanisms involving isobutenes differ from those of normal butenes. 

The effect of association on photochemical reactivities of stilbenes and alkyl cinnamates having poor solubilities in water has been demonstrated recently (Schemes 37 and 38) [140, 205,206]. Even at concentrations of trans-stilbene in water as low as 10 6 M, dimerization occurs efficiently. The ratios of dimers were similar to those obtained in benzene when the initial stilbene concentration is high. However, in organic solvents, geometric isomerization is the only reaction observed at low stilbene concentrations. Similar behavior... 

Acyclic a,jS-unsaturated ketones do not provide much photochemistry, except for facile geometric isomerization about the double bond. Although some also isomerize to /3,y-unsaturated ketones, others, such as mesityl oxide, apparently undergo no net reaction upon absorption of irradiation. Their low reactivity might result from a rapid relaxation to an appreciably more twisted geometry in their triplet states than can be obtained in the cyclic enones. Below are some examples of typical photochemical behavior.426,427... 

In addition to these direct measurements, we shall soon see that two important aspects of alkene chemistry are consistent with the quantum mechanical picture of the double bond, and are most readily understood in terms of that picture. These are (a) the concept of hindered rotation and the accompanying phenomenon of geometric isomerism (Sec. 5.6), and (b) the kind of reactivity characteristic of the carbon-carbon double bond (Sec. 6.2). 

Extension of the linear carbon chain to five or six atoms opens up additional possible reaction paths. Higher 1,3-alkadienes are expected to behave in a similar fashion to 1,3-butadiene, except the two double bonds may differ in reactivity under the influence of the alkyl substituent, and their reduction naturally affords distinguishable products. Thus for example 1,3-hexadiene can give 1- and 3-hexenes by respectively 3,4- and 1,2-addition as well as 2-hexenes by 1,4-addition or isomerisation. A new feature in the higher 1,3- alkadienes is the existence of geometrical isomerism Z- and -l,3-pentadienes have been examined separately. ... 

Silver also has been demonstrated to be reactive in solution systems. Thus, silver perchlorate has been shown to influence the photochemical reactivity of stilbene in acetonitrile and methanol. The fluorescence of the stilbene is quenched on addition of the perchlorate and this is good evidence for the enhancement of the So-Ti crossing induced by the heavy ion Ag+. It seems likely that an Ag+/stilbene complex is formed. The perturbation of the system is better in methanol than in acetonitrile. However, cis.trans isomerism of the stilbene is reduced within the excited Ag+/stilbene complex since it is difficult for the geometrical isomerism to occur. Enhanced isomerism is observed with the Ag+/azobenzene system. In this complex there are steiic problems encountered in the nitrogen rehybridization process that is operative in the isomerism . Enhanced So-T crossing is also seen in the Ag+/1 1 complex with tryptophan where the fluorescence is quenched and there is a threefold increase in phosphorescence . Complexes between Ag+ and polynucleotides and DNA cause quenching of the fluorescence. Enhancement of phosphorescence and a 20-fold increase in the dimerization of thymine moieties has also been observed when silver ions are added to the reaction system . ... 

Odd-Odd intermolecular bicentric reactions. Potential energy surfaces for geometric isomerization and radical combinatioa - Odd-odd intramolecular multicentric reactions. - Even-even intramolecular multicentric reactions. -Mechanisms of electrocyclic reactions. - Triplet reactivity. -Photophysical processes. - The importance of low lying nonvalence orbitals. - Divertissements. - A contrast of accepted" concepts of organic reactivity and the present work. 

Then, how can one capture the global aspects of the phase-space geometry from the geometric structure of the phase space in that local region One can find an essential clue from an insightful classical theory for isomerization reactions composed of two DOFs, so-called reactive island theory (RIT) developed by De Leon, Marston, Mehta, and Ozorio De Almeida [34—37] (see also Ref. 55). 

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