Electrophiles empty orbitals

This solvent is called tetrahydrofuran, or THF for short. Even though it somewhat stabilizes the empty p orbital on the boron atom in BH3, nevertheless the boron atom is very eager to look for any other sources of electron density that it can find. It is an electrophile—it is scavenging for sites of high electron density to fill its empty orbital. A pi bond is a site of high electron density, and therefore, a pi bond can attack borane. In fact, this is the hrst step of our mechanism. A pi bond attacks the empty p orbital of boron, which triggers a simultaneous hydride shift ... 

It has since become increasingly clear that zwitterionic zirconates may be generated in many other reactions and may lead to unexpected and interesting chemical consequences, as suggested by the results and interpretations shown in Scheme 1.73. It should be noted that the empty orbital and electrophilicity of Zr must lead to zwitterionic species containing zirconates and carbocationic centers. Further systematic investigations in this area appear to be desirable. 

The second conclusion which may be drawn from Hoffmann s calculations so>56) is that carbenes having substituents with vacant orbitals interacting with the -orbital should be electrophilic, whereas the interaction with substituents with filled orbitals should result in nucleophilic behaviour. There is experimental proof 57) to the effect that singlet Ccirbenes such as dihalocarbenes with their empty -orbital react as electrophiles. 

In the gas phase, divalent mercury has been shown to be linear and therefore to be sp hybridized. However, in solution the X—R—X, R—Hg—X, or R—Hg—R bond angle in divalent mercury compounds varies from 130 to 180°. The variation in geometry is not yet entirely understood, so we shall follow Jensen s example and assume that, even in solution, divalent mercury is sp hybridized and that if a divalent mercury compound donates one empty orbital to coordinate with a Lewis base it rehybridizes to sp2 (F. R. Jensen and B. Rickborn, Electrophilic Substitution of Organomercurials, pp. 35, 36). 

Ionic reactions are simpler still the only important interaction involves the HOMO of the nucleophile and the LUMO of the electrophile (Figure 3.2b). This is because a nucleophile (or any electron-rich compound) readily donates electrons, so it will react through its HOMO, where the highest energy electrons are localized. Conversely, an electrophile (or any electron-poor compound) accepts electrons easily. These electrons can only be put into vacant orbitals. Obviously, the lower the energy of the empty orbital, the more easily it accepts electrons. Thus an electrophile generally reacts through its LUMO. 

More often, reaction occurs when electrons are transferred from a lone pair to an empty orbital as in the reaction between an amine and BF3. The amine is the nucleophile because of the lone pair of electrons on nitrogen and BF3 is the electrophile because of the empty p orbital on boron. 

Electrostatic forces provide a generalized attraction between molecules in chemical reactions. In the reaction between chloride anions and sodium cations described above, the way in which these two spherical species approached one another was unimportant because the charges attracted one another from any angle. In most organic reactions the orbitals of the nucleophile and electrophile are directional and so the molecular orbitals of the reacting molecules exert important control. If a new. bond is to be formed as the molecules collide, the orbitals of the two species must be correctly aligned in space. In our last example, only if the sp3 orbital of the lone pair on nitrogen points directly at the empty orbital of the BF3 can bond formation take place. Other collisions will not lead to reaction. In the first frame a successful collision takes place and a bond can be formed between the orbitals. In the second frame are three examples of unsuccessful collisions where no orbital overlap is possible. There are of course many more unproductive collisions but only one productive collision. Most collisions do not lead to reaction. 

Now we shall discuss a generalized example of a neutral nucleophile, Nu, with a lone pair donating its electrons to a cationic electrophile, E, with an empty orbital. Notice the difference between the curly arrow for electron movement and the straight reaction arrow. Notice also that the nucleophile has given away electrons so it has become positively charged and that the electrophile has accepted electrons so it has become neutral. 

We normally think of protons as acidic rather than electrophilic but an acid is just a special kind of electrophile. In the same way, Lewis acids such as BF3 or AICI3 are electrophiles too. They have empty orbitals that are usually metallic p orbitals. We saw above how BF3 reacted with Me3N. In that reaction BF3 was the electrophile and Me3N the nucleophile. Lewis acids such as AICI3 react violently with water and the first step in this process is nucleophilic attackby water on the empty p orbital of the aluminium atom. Eventually alumina (AI2O3) is formed. 

Nucleophiles may donate electrons (in order of preference) from a lone pair, a 7t bond, or even a o bond and electrophiles may accept electrons (again in order of preference) into an empty orbital or into the antibonding orbital of a Jt bond (it orbital) or even a o bond (o orbital). These antibonding orbitals are of low enough energy to react if the bond is very polarized by a large electronegativity difference between the atoms at its ends or, even for unpolarized bonds, if the bond is weak. 

A curly arrow represents the actual movement of a pair of electrons from a filled orbital into an empty orbital. You can think of the curly arrow as representing a pair of electrons thrown, like a climber s grappling hook, across from where he is standing to where he wants to go. In the simplest cases, the result of this movement is to form a bond between a nucleophile and an electrophile. Here are two examples we have already seen in which lone pair electrons are transferred to empty atomic orbitals. 

Draw a curly arrow from the nucleophile to the electrophile. It must start on the filled orbital or negative charge (show this clearly by just touching the bond or charge) and finish on the empty orbital (show this clearly by the position of the head). You may consider a push or a pull mechanism at this stage... 

With a-bromo carbonyl compounds, substitution leads to two electrophilic groups on neighbouring carbon atoms. Each has a low-energy empty orbital, Jt from C=0 and a from C-Br (this is what makes them electrophilic), and these can combine to form a molecular LUMO (Jt + a ) lower in energy than either. Nucleophilic attack will occur easily where this new orbital has its largest coefficients, shown in orange on the diagram. 

The general conclusion was that rather weak bonding was established between the cation and the carbon monoxide molecule. The scheme of the bonding was depicted as essentially a transfer of-the carbon lone pair to the cation empty orbitals with a variable extent back-donation from the cation filled orbitals to the antibonding II orbitals of the CO molecule. This bonding scheme generally resulted in an electron deficient carbonyl carbon especially in view of the weak back-donation to the II molecular orbitals of CO. This makes the carbon particularly suitable for nucleophilic attacks by electrophiles. 

Complexes involving a formal metal-carbon double bond are referred to as either carbene or alkylidene complexes. For simplicity we shall consider the orbitals of singlet methylene, which consist of a filled sp2 orbital ( lone pair ) and an empty and highly electrophilic p orbital (Figure 1.15). 

When the two alkynyl functions are separated by a sUyl or a phosphano group, an interesting rearrangement takes place giving rise to tricyclic species (Scheme 30). This skeletal rearrangement proceeds via intermediate zwitterionic zirconate (102). The empty orbital and electrophilicity of Zr are probably the reasons that lead to zwitterionic species containing zirconates and carbocationic centers. 

In one step of a reaction mechanism electrons flow from a site rich in electrons to an electron-deficient site. When you draw a mechanism you must make sure that the electrons flow in one direction only and neither meet at a point nor diverge from a point. One way to do this is to decide whether the mechanism is pushed by, say, a lone pair or an anion or whether it is pulled by, say, a cation, an empty orbital, or by the breaking of a reactive weak Jt bond or o bond. This is not just a device either. Extremely reactive molecules, such as fluorine gas, F2, react with almost anything—in this case because of the very electrophilic F-F o bond (low energy F-F o orbital). Reactions of F2 are pulled by the breaking of the F-F bond. The nearest thing in organic chemistry is probably the reactions of carbon cations such as those formed ... 

It has been theoretically and experimentally well established that silylenes have a singlet ground state [1]. Such species posses a free electron pair in a o-orbital and an empty orbital of Jt-symmetry therefore, they are a priori ambiphilic compounds, which can react either as an electrophile or as a nucleophile towards appropriate substrates. However, most silylenes have revealed a distinctive "electrophilic character". Dimethylsilyene, e g., adds to olefins and alkynes in the gas phase via a rate-controlling step that is accelerated by electron-donating substituents [2] these experimental results are in good agreement with a theoretical study of the reaction of SiH2 with ethylene, which shows that this cycloaddition proceeds via an initial electrophilic phase in which the silylene LUMO interacts with the 7t-electron system of the double bond [3]. Up to now, only some stable silylenes, such as recently described 1 [4] or silicocene 2 [5] have shown nucleophilic reactivity. 

---