Sigma orbitals reactions

The H3 and H4 systems were discussed above. Another type of sigma bonds involves a p orbital lying along the reaction coordinate, as, for example, in reaction (15) (Fig. 8). 

Hiickel-type systems (such as Hilcfcel pericyclic reactions and suprafacial sigmatropic shifts) obey the same rules as for sigma electron. The rationale for this observation is clear If the overlap between adjacent p-electron orbitals is positive along the reaction coordinate, only the peraiutational mechanism can... 

The carbon—carbon double bond is the distinguishing feature of the butylenes and as such, controls their chemistry. This bond is formed by sp orbitals (a sigma bond and a weaker pi bond). The two carbon atoms plus the four atoms ia the alpha positions therefore He ia a plane. The pi bond which ties over the plane of the atoms acts as a source of electrons ia addition reactions at the double bond. The carbon—carbon bond, acting as a substitute, affects the reactivity of the carbon atoms at the alpha positions through the formation of the aHyUc resonance stmcture. This stmcture can stabilize both positive and... 

Sigma (a) bonds Sigma bonds have the orbital overlap on a line drawn between the two nuclei, simple cubic unit cell The simple cubic unit cell has particles located at the corners of a simple cube, single displacement (replacement) reactions Single displacement reactions are reactions in which atoms of an element replace the atoms of another element in a compound, solid A solid is a state of matter that has both a definite shape and a definite volume, solubility product constant (/ p) The solubility product constant is the equilibrium constant associated with sparingly soluble salts and is the product of the ionic concentrations, each one raised to the power of the coefficient in the balanced chemical equation, solute The solute is the component of the solution that is there in smallest amount, solution A solution is defined as a homogeneous mixture composed of solvent and one or more solutes. 

When a cyclic polyene is large enough, it can exist in both cis- and iraws-forms. Our approach to polyene cyclization has tacitly assumed an all cis -n chain in the form of a band or ribbon that would slip smoothly on to the surface of a cylinder of appropriate diameter. Should the orbitals of the two polyenes in (36) have a mismatch in their orbital symmetries, a single twist in the tt band of one of them could remedy this (Fig. 10c). Cycloaddition would now be allowed and the reaction would proceed, provided other factors were favorable. Such cases of Mobius (Zimmerman, 1966), anti (Fukui and Fujimoto, 1966b) or axisymmetric (Lemal and McGregor, 1966), as opposed to Hiickel, syn, or sigma-symmetric ring closure are unknown (or, at least, rare). A Mobius form has, however, been proposed as the key intermediate in the photochemical transformations of benzene (Farenhorst, 1966) in (48) in place of the disrotatory cyclization proposed by van Tamelen (1965). 

Let s carefully compare the movements of the atoms that must occur in these reactions to cause such different stereochemical results. As shown in Figure 22.1, the process is quite simple. The two end carbons of the conjugated pi system need only rotate so that their p orbitals begin to overlap to form the new sigma bond of the cyclobutene. In the case of the thermal reaction, both carbons rotate in the same direction, a process that is... 

In electrocyclic reactions the end carbons of the conjugated system must rotate for the p orbitals on these carbons to begin to overlap to form the new carbon-carbon sigma bond. The preference for the stereochemistry of the rotation in these reactions can be understood by examination of the new orbital overlap in the HOMO as the rotation occurs. For the formation of the new sigma bond to be favorable, rotation must occur so that the overlap of the orbitals forming this bond is bonding in the HOMO. 

The requirement for a favorable bonding overlap of the orbitals forming the new sigma bond in the HOMO allows the preferred rotation to be predicted for any electro-cyclic reaction. Let s consider the photochemical conversion of a butadiene to a cyclobutene. Because the reaction occurs through the excited state, the HOMO is it. As can be seen, disrotation is necessary here for the new overlap to be bonding ... 

Cycloaddition reactions can be viewed as involving the flow of electrons from the HOMO of one reactant to the LUMO of the other. Therefore, examination of the interaction of these MOs is used to determine whether the reaction is favorable. For a cycloaddition to be allowed, the overlap of the orbitals of the HOMO of one component and the LUMO of the other must be bonding where the new sigma bonds are to be formed. Some examples will help make the application of this rule clear. 

In this reaction a hydrogen that is sigma bonded to the carbon that is adjacent to the pi bond (C-l) migrates to the end carbon of the pi bond (C-3). As this occurs, the pi bond migrates from C-2-C-3 to C-l-C-2. A picture of the orbitals involved in this reaction may help make the process clearer ... 

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