LUMO lowest unoccupied reaction

Knowledge of molecular orbitals, particularly of the HOMO Highest Occupied Molecular Orbital) and the LUMO Lowest Unoccupied Molecular Orbital), imparts a better understanding of reactions Figure 2-125b). Different colors e.g., red and blue) are used to distinguish between the parts of the orbital that have opposite signs of the wavefunction. 

Another aspect of qualitative application of MO theory is the analysis of interactions of the orbitals in reacting molecules. As molecules approach one another and reaction proceeds, there is a mutual perturbation of the orbitals. This process continues until the reaction is complete and the new product (or intermediate in a multistep reaction) is formed. PMO theory incorporates the concept of frontier orbital control. This concept proposes that the most important interactions will be between a particular pair of orbitals. These orbitals are the highest filled oihital of one reactant (the HOMO, highest occupied molecular oihital) and the lowest unfilled (LUMO, lowest unoccupied molecular oihital) orbital of the other reactant. The basis for concentrating attention on these two orbitals is that they will be the closest in energy of the interacting orbitals. A basic postulate of PMO... 

The HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied MO) levels for hydrogen donors used in coal liquefaction are not yet well known, but the principles involved can be illustrated with the group transfer reaction between molecular hydrogen, a (4n+2)e donor with n=0, and naphthalene, a (4m)e acceptor with m=l ... 

The presence of an aroyl fragment in azomethine ylides obtained from opening of three-membered rings in the case of dipolarophiles with high LUMO (lowest unoccupied molecular orbital) energy or in the absence of an external dipolarophile can lead to the possibility of such unusual reactions as intramolecular 1,3-dipolar cycloaddition [80]. Examples of such reactions are the thermal isomerization of aroyl aziridines 63 into a pyrrole derivative 64 [81, 82] or into 2,5-diphenyloxazole 65 (in the presence of diphenyliodonium iodide) [83] (Scheme 1.16). 

The Woodward-Hoffmann rules can be rationalized by examining the properties of the frontier MOs of the reactants, i.e., the HOMOs (highest occupied molecular orbitals) and LUMOs (lowest unoccupied molecular orbitals) of the reactants. In order to understand pericyclic reactions, then, you need to be able to construct the MOs of a polyene system from the constituent p orbitals. 

Dienes that contain electron-donating groups (activated dienes) are more reactive in Diels-Alder reactions than unsubstituted or electron-deficient dienes. In molecular orbital formalism, the substituents on the diene perturb the tT-electron density to cause an increase in the energy of the highest occupied molecular orbital (HOMO Figure 1). In a normal-demand Diels-Alder reaction this results in an increase in the interaction between the HOMO of the diene and the LUMO (lowest unoccupied molecular orbital) of the dienophile. This interaction, in turn, lowers the transition state energy of the reaction. Similar arguments have also been used to explain the increased reactivity of activated dienes towards heterodienophiles such as aldehydes. 

The two new a bonds that are formed in a Diels-Alder reaction result from a transfer of electron density between the reactants. Molecular orbtial theory provides an insight into this process. When electrons are transferred between molecules, we must use the HOMO (highest occupied molecular orbital) of one reactant and the LUMO (lowest unoccupied molecular orbital) of the other because only an empty orbital can accept electrons. It doesn t matter whether we use the HOMO of the dienophile and the LUMO of the diene or the HOMO of the diene and the LUMO of the dienophile. We just need to use the HOMO of one and the LUMO of the other. 

Carbocations are electron-deficient species that are the most important intermediates in several kinds of reactions. A common model for carbocation stmcture is a planar species exhibiting sp hybridization, as shown in Figure 2.4 for methyl cation. The p-orbital that is not utilized in the hybrids is empty and is often shown bearing the positive charge since it represents the orbital available to accept electrons. There is a vacant p orbital perpendicular to the plane of the molecule this is the LUMO (lowest unoccupied molecular orbital). In all reactions of carbocations there is an interaction between this LUMO and the HOMO (highest occupied molecular orbital) of another molecule. A structure with an empty p orbital should be more stable than a structure in which an orbital with s character is empty. In general, a carbocation is a purely ionic species. 

Relative rates of reaction with toluene (A ) and benzene (Ag) LUMO Lowest unoccupied molecular orbital... 

According to the frontier orbital theory, the orbitals that control these reactions are the aforementioned HOMO of one reactant and the LUMO (lowest unoccupied molecular orbital) of the other reactant. So, for this reaction, we have two possible scenarios interaction between HOMO ij/2 of butadiene and LUMO (p of ethylene or that between HOMO ti of ethylene and LUMO of butadiene. As the following illustration indicates, both possibilities lead to bonding overlap between the interacting orbitals of the reactants so the reaction is allowed, as we aU know. In addition, as fotmd by theory, between these two possible scenarios, we favor the interaction between the HOMO of the electron-rich reactant (butadiene in this case) with the LUMO of the electron-poor reactant (ethylene). 

In Chapter 3 we first mentioned the importance of the interaction of a HOMO (highest occupied molecular orbital) of one molecule with the LUMO (lowest unoccupied molecular orbital) of another when two molecules react with each other (see The Chemistry of... box, Section 3.3A). These ideas carry forth into our understanding of addition reactions between alkenes and electrophiles. Open the molecular models at the book s website for ethene and BH3 and view the HOMO and LUMO for each reactant. Which reactant is likely to have its HOMO involved in the hydroboration of ethene Which molecule s LUMO will be involved As you view the models, can you envision favorable overlap of these orbitals as the reaction occurs ... 

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