Alkenes orbital overlap model

The structure of ethylene and the orbital hybridization model for its double bond were presented m Section 2 20 and are briefly reviewed m Figure 5 1 Ethylene is planar each carbon is sp hybridized and the double bond is considered to have a a component and a TT component The ct component arises from overlap of sp hybrid orbitals along a line connecting the two carbons the tt component via a side by side overlap of two p orbitals Regions of high electron density attributed to the tt electrons appear above and below the plane of the molecule and are clearly evident m the electrostatic potential map Most of the reactions of ethylene and other alkenes involve these electrons... 

Bonding m alkenes is described according to an sp orbital hybridization model The double bond unites two sp hybridized carbon atoms and is made of a ct component and a rr component The ct bond arises by over lap of an sp hybrid orbital on each carbon The rr bond is weaker than the CT bond and results from a side by side overlap of p orbitals... 

In Chapter 3, Section 3.5, molecular orbitals described the bonding in alkanes using the hybridization model. Specifically, sp hybrid orbitals overlap to form a sigma-covalent bond (a o-bond). It is possible to have two covalent bonds between adjacent carbon atoms, a carbon-carbon double bond. One of the two bonds is the usual o-bond, but the other is called a 7i-bond. Hydrocarbons that contain one 7t-bond are called alkenes. In other words, an alkene will have a C=C unit. Each carbon atom of the C=C unit will have four bonds, but only three of the bonds are o-bonds, and the fourth bond is a 7i-bond. 

Alkenes contain a C=C double bond. The C=C double bond can be described with two different models. According to the most commonly used model, a C=C double bond consists of a <7- and a tr-bond. The bond energy of the a-bond is 83 kcal/mol, about 20 kcal/mol higher than the tr-bond (63 kcal/mol). The higher stability of <7 bonds in comparison to n bonds is due to the difference in the overlap between the atomic orbitals (AOs) that form these bonds. Sigma bonds are produced by the overlap of two spn atomic orbitals (n 1,2,3), which is quite effective because it is frontal. Pi bonds are based on the overlap of 2,pz atomic orbitals, which is not as good because it is lateral or parallel. 

Whereas transition metal complexes of alkenes and their chemistry have been well explored, comparatively little is known about the structure and reactivity of n complexes obtained from strained olefins. The stability of transition metal complexes of alkenes in general is preferably discussed in terms of the Dewar-Chatt-Duncanson model (171). A mutual er-type donor-acceptor interaction accounts for the bonding overlap of the bonding 71-MO of the olefin with vacant orbitals of the metal together with interaction of filled d orbitals with the 7r -MO of the double bond (back bonding) leads to a partial transfer of. electron density in both directions (172). The major contribution to the stabilizing interaction is due to back-bonding. 

However, they do provide an explanation for the preferred endo mode of cycloaddition for those alkenes giving meta adducts. Most c/.v-disubstitutcd alkenes like cyclopentene1159 give substantially more endo adduct 8.134 than exo 8.135. Because of the good HOMO/ HOMO and LUMO/ LUMO match with simple alkenes, there will be two sets of secondary orbital interactions to consider. One 8.136 will be between the HOMO of the doubly X-substituted alkene, crudely modelled by i >3 of butadiene, and ip 2 of benzene, and the other 8.137 will be between the LUMO of the alkene, modelled by ip4 of butadiene, and ip5 of benzene. In both cases the secondary overlap is in favour of the endo mode of cycloaddition.1157... 

When a diene has a substituent at Cl and/or C2 and reacts with an alkene bearing a substituent, the product will have one or more stereogenic centers. If there are two or more stereogenic centers in the product, it may be formed as a mixture of diastereomers. In the reaction of 18 and 19, for example, the cycloadduct has two stereogenic carbons and the two diastereomers are 20, where the two groups have a syn relationship, and 21, where they have an anti relationship. Section 24.3.2 discussed the model of overlap of orbital coefficients to predict the 1,2 product from the reaction. Because acrylonitrile has Tt-bonds in the nitrile unit, secondary orbital interactions lead to a preferred endo transition state represented in Figure 24.4, although one exo transition state is also shown. 

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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