Alkene electron distribution

An effect that results when two or more atoms or groups interact so as to alter the electron distribution in a system is called an electronic effect. The greater stability of more highly substituted alkenes is an exanple of an electronic effect. 

The different electron distribution in the excited state also may lead to other types of reactions. As an example, alkenes and polyenes display a low intermole-cular reactivity, but undergo extremely fast rearrangements, since the tt bonding character dramatically diminishes in the excited state. Thus, free rotation becomes feasible and, where appropriate, electrocyclic and sigmatropic processes take place (Figure 3.3). 

In the preceding chapters we have seen how new bonds may be formed between nucleophilic reagents and various substrates that have electrophilic centres, the latter typically arising as a result of uneven electron distribution in the molecule. The nucleophile was considered to be the reactive species. In this chapter we shall consider reactions in which electrophilic reagents become bonded to substrates that are electron rich, especially those that contain multiple bonds, i.e. alkenes, alkynes, and aromatics. The jt electrons in these systems provide regions of high electron density, and electrophilic reactions feature as... 

The only electrons that might be useful in the kind of attraction we have discussed so far are the lone pair electrons on bromine. But we know from many experiments that electrons flow out of the alkene towards the bromine atom in this reaction—the reverse of what we should expect from electron distribution. The attraction between these molecules is not electrostatic. In fact, we know that reaction occurs because the bromine molecule has an empty orbital available to accept electrons. This is not a localized atomic orbital like that in the BF3 molecule. It is the antibonding orbital belonging to the Br-Br G bond the c orbital. There is therefore in this case an attractive interaction between a full orbital (the Jt bond) and an empty orbital (the o orbital of the Br-Br bond). The molecules are attracted to each other because this one interaction is between an empty and a full orbital and leads to bonding, unlike all the other repulsive interactions between filled orbitals. We shall develop this less obvious attraction as the chapter proceeds. 

The last three compounds obviously form a group with the same skeleton and only the alkene oester conjugation in all three hut this is the only kind in the last aa wund. The first is most conjugated with the lone pair on nitrogen delocalized into the carhonyl p. The middle compound just has the alkene and the ester conjugated. This time we have used arrow representations for all three compounds and a dotted-line electron distribution for the first. 

Bearing in mind what you already know about the electron distribution in alkynes and also about the nature of the intermediates in the reaction pathway, suggest three reasons to explain why alkenes react faster with bromine than do alkynes. 

Electron-rich alkenes also react with electrophilic carbene complexes, as equation 10.48 shows. Although reaction temperatures tend to be lower than necessary for cyclopropanation of electron-poor alkenes, the distribution of products is a function of CO pressure. In the absence of CO, alkene 62 forms predominantly.72 At 100 bar CO pressure, cyclopropane formation predominates. Presumably, at low CO pressure a metathesis pathway (Chapter 11) can occur. At high CO pressure, loss of CO is unlikely, so RE elimination to form cyclopropane predominates. 

Veillard [19] covers a similar range of molecules but from the Hartree-Fock and post-HF view. The discussion is organised more in terms of molecular properties. Thus, he deals with metal carbonyls, carbides, cyanides, C02 complexes, alkyls, carbenes, carbynes, alkenes, alkynes and metallocenes under the headings of electronic states, electronic spectra, optimised geometries, binding energies, Ionisation Potentials and Electron Affinities, nature of M-L bonding and other properties (e.g. vibrational spectra, dipole moments and electron distributions). 

A singlet carbene is inherently both an electrophile and a nucleophile, what is behaviorally decisive is whether, in the carbene/alkene addition transition state, it is the LUMO(carbene)/HOMO(alkene) or HOMO(carbene)/LUMO(alkene) interaction (cf., Fig. 5) which dominates and determines the electronic distribution. If the former interaction dominates, the carbene will exhibit electrophilic selectivity if the latter interaction is more important, nucleophilic selectivity will be observed. If both interactions are comparable, the carbene will display an ambiphilic selectivity pattern, in which it acts as an electrophile toward electron-rich alkenes, but as a nucleophile toward electron-poor alkenes. [8,69]... 

The regioselectivity of the AA reaction depends on the substitution pattern and, to some extent, the electron distribution of the alkene substrates. While many... 

The same sort of thing happens with alkenes. We ll concentrate on cyclohexene so as to make a good comparison with benzene. The six identical protons of benzene resonate at 7.27 ppm the two identical alkene protons of cyclohexene resonate at 5.68 ppm. A conjugating and eiectron-withdrawing group such as a ketone removes electrons from the double bond as expected—but unequally. The proton nearer the C=0 group is only slightly downfield from cyclohexene but the more distant one is over 1 ppm downfield. The curly arrows show the electron distribution, which we can deduce from the NMR spectrum. 

The first example of a stable pyridinium yhde of type 80 is the pyridinium phenacylide (80, A = CH2COPh), obtained from the N-phenacylpyridinium ion (79, A = CH2COPh) by deprotonation with Na2C03 [97]- The reactivity of the pyridinium betaines is determined by their electron distribution (80a-c). Thus, they can be smoothly alkylated or acylated at the N-substituent ( 81) as 1,3-dipoles, they undergo dipolar cycloadditions with activated alkynes or alkenes [98] for example, the sequence 80 —> 82 83 establishes an efficient principle of indolizine synthesis (cf. p. 154). 

The electrophile in oxymercuration reactions, " HgX or Hg, is a soft acid and strongly polarizing. It polarizes the -electron distribution of an alkene to the extent that a three-center two-electron bond is formed between mercury and the two carbons of the double bond. A three-center two-electron bond implies weaker bridging in the mercurinium ion than in the three-center four-electron bond of a bromonium ion. Bridging in the mercurinium ion is much more pronounced than is the case for bridging by hydrogen. 

This implies that Br2 is the electrophile. But how does Br2 function as an electrophile The bond between the two bromine atoms is a covalent bond, and we therefore expect the electron density to be equally distributed over both Br atoms. However, an interesting thing happens when a Br2 molecule approaches an alkene. The electron density of the pi bond repels the electron density in the Br2 molecule, creating a temporary dipole moment in Br2. 

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