Electronic Structure of Alkenes

We saw in Section 1.9 that the carbon atoms in a double bond are sp -hybridized and have three equivalent orbitals that lie in a plane at angles of 120 to one another. The fourth carbon orbital is an unhybridizedp orbital perpendicular to the sp plane. When two such carbon atoms approach each other, they form a a bond by head-on overlap of sp orbitals and a tt bond by sideways overlap of p orbitals. 

The w bond must break for rotation to take place around a carbon-carbon double bottd. 

Broken w bond after rotation p orbitals are perpendicular) 

An experimental determination of how much energy is required to break the 17 bond of ethylene gives an approximate value of268 kJ/mol (64 kcal/mol), J so it s clear why rotation does not occur. Recall that the barrier to bond rota-1 tion in ethane is only 12 kj/mol. 

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Figure 6.120 Change in the electronic structure of alkene activates chameleonic behavior of the OMe group (see Chapter 5 for additional discussion of stereoelectronic chameleons).

We should first recall the electronic structure of alkenes, with carbon atoms sp hybridized and a sigma framework of bonds at approximately 120 ° to each other. Above and below the plane of the molecule is a jt-orbital, derived from the two remaining p orbitals (11.1). 

Perhaps due to oxidizing quinoid type electronic structure of benzotriazol-2-yl derivatives, some of their properties are completely different from those of isomeric benzotriazol-l-yl derivatives. Thus, anions derived from 2-alkylben-zotriazoles 388 are rapidly converted to appropriate radicals that undergo coupling to form dimers as mixtures of racemic 289 and meso 390 forms <1996LA745>. When the reaction mixture is kept for an extended period of time at —78 °C, (Z)- 391 and (E)- 392 alkenes are formed. When benzophenone is added to the reaction mixture, alcohols 387 are obtained in good yields however, benzaldehyde does not react under these conditions (Scheme 63). 

When the 7r-systerns of two or more double bonds overlap, as in conjugated dienes and polyenes, the 7r-clccIrons will be delocalized. This has chemical consequences, which implies that the range of possible chemical reactions is vastly extended over that of the alkenes. Examples are various pericyclic reactions or charge transport in doped polyacetylenes. A detailed understanding of the electronic structure of polyenes is therefore of utmost importance for development within this field. We will first discuss the structure of dienes and polyenes based on theoretical studies. Thereafter the results from experimental studies are presented and discussed. 

Cis-trans isomerism in alkenes arises because the electronic structure of the carbon-carbon double bond makes bond rotation energetically unfavorable at normal temperatures. Were it to occur, rotation would break the pi part of the double bond by disrupting the sideways overlap of two parallel p orbitals (Figure 23.2). In fact, an energy input of 240 kj/mol is needed to cause bond rotation. 

Rate-controlling C-protonation is dominant for many of the enamines over most or all of the pH range surveyed. As a class, simple enamines are among the most reactive of all alkenes toward electrophiles. We can estimate (following Tidwell and coworkers103) that the kH+ values for 1-pyrrolidinoethene and 1-phenyl-1-pyrrolidinoethene are greater than 108 M 1 s 1. That is, simple enamines are protonated on carbon faster than all the common classes of nucleophilic olefins except for the simple enolates (see Tables 4 and 6). However, the carbon basicity of enamines, in both the thermodynamic and kinetic senses, is quite sensitive to aspects of amine moiety structure, of alkene moiety structure, and of interactions (both electronic and steric) between the two. 

The electronic structure of alkynes is related to that of alkenes, and the photochemistry of the two classes of compound reflects this similarity. Because the photochemistry of alkenes has received greater attention and has already been described in systematic form - it is not unexpected that the present account should point out the ways in which alkyne photochemistry parallels, or is markedly different from, that of alkenes. There is a considerable difference, however, in the range of compounds which has been studied in each class. Reports of photochemical reactions of alkynes very often refer to mono- or disubstituted acetylenes in which the substituents are alkyl, aryl or alkoxycarbonyl. There have been studies on diyne and enyne systems, but as yet there has emerged nothing in alkyne chemistry to match the wealth of photochemistry reported for dienes and polyenes. This reflects in part the greater tendency of the compounds containing the C=C bond to undergo photopolymerization rather than any other reaction on irradiation. Within this limitation there is a wide variety of reactions open to the excited states of alkynes, and quite a number of the processes have synthetic application or potential. 

The differences in selectivity between catalysts cannot be explained only in terms of the strength of reactant adsorption. A tentative explanation lies in the preference of platinum for concerted addition of protons to adsorbed alkenes with simultaneous electron transfer (25). The electronic structure of the surface intermediate of the concerted step appears to lead to halide cleavage. Palladium, on the other hand, can participate in insertion reactions (305) and promotes surface reaction between hydrogen atoms and adsorbed alkenes 4Sa. It is possible that palladium adsorbs vinyl halides on two different sites or at two different states, dependent on potential, one of which... 

The spectroscopy, electrochemistry, and magnetic properties of (18) indicate that its iron center is equivalent to that of Compound I of HRP. The spectroscopic and electrochemical properties of (19), and its reduced reactivity with alkenes, indicate that the electronic structure of its iron-oxygen center is analogous to that of Compound II of HRP. 

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