Alkenes hybrid orbitals

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... 

The carhon-carbon double bond in alkenes is more reactive than carbon-carbon single bonds and gives alkenes their characteristic properties. As we saw in Section 3.4, a double bond consists of a a-bond and a 7r-bond. Each carbon atom in a double bond is sp2 hybridized and uses the three hybrid orbitals to form three cr-bonds. The unhvbridized p-orbitals on each carbon atom overlap each other and form a Tr-bond. As we saw in Section 3.7, the carbon-carbon 7r-bond is relatively weak because the overlap responsible for the formation of the 7r-bond is less extensive than that responsible for the formation of the a-bond and the enhanced electron density does not lie directly between the two nuclei. A consequence of this weakness is the reaction most characteristic of alkenes, the replacement of the 77-bond by two new a-bonds, which is discussed in Section 18.6. 

Figure 6-13. Synergic back-bonding in a platinum alkene complex. In (a), the interaction of a (filled) platinum 5d orbital with the tf molecular orbital of the alkene is shown, whilst in (b), the interaction of a dsp hybrid orbital with the n molecular orbital of the alkene is shown. Note that the two interactions result in electron density moving in opposite directions.

The double bond in ethylene contains one a bond and one 7r bond. The a bond forms from the end-on overlap of two hybrid orbitals, and the 7i bond forms from the side-by-side overlap of two atomic p orbitals. Figure 10-21 shows the complete orbital picture of the bonding in ethylene. Ethylene is the simplest of a class of molecules, the alkenes, all of which contain CDC double bonds. The alkenes are the subject of our Box on page 404. 

All alkenes contain a carbon-to-carbon double bond. We can use ethene as a typical example to explain the bonding in alkenes. On each carbon atom of the double bond, the 2s orbital mixes with two of the 2p orbitals to form three degenerate sp hybrid orbitals. The remaining 2p orbital is left unhybridised. 

Exercise 21-24 a. Sulfur dioxide is an angular molecule that can be represented as having a nonbonding electron pair in an sp2 hybrid orbital and one vacant p orbital on sulfur. Use this formulation to derive a thermally allowed transition state for the reversible 1,4-cycloaddition of S02 to 1,3-butadiene (Section 13-3C). b. The three-membered ring sulfone, shown below, is very unstable and rapidly dissociates to S02 and ethene. This process is used for the synthesis of alkenes by the dissociation of cyclic sulfones (Ramberg-Backlund reaction). Determine whether the transition state for the thermally favorable reaction is conrotatory or disrotatory. 

So effective was the collinear three-center transition state in clarifying the Walden inversion problem, that concerted backside attacks were proposed or considered for the Beckman rearrangement, isomerization of alkenes, etc. (Olson, 1933 Marvel, 1943), as well as substitution in alkenes (Gold, 1951 Ross et al., 1952). Since an appropriate arrangement of hybrid orbitals is available, some of the transition states did not... 

The overlap between the trigonal sp hybridized orbitals of the two carbon atoms of an alkene leads to the planar n-bonded system of the alkene and leaves an electron in a p orbital on each of the carbons. Overlap between these p orbitals leads to the tt-system of the alkene. There is a region of increased electron density above and below the plane of the... 

R groups increase the stability of an alkene because R groups are sp hybridized, whereas the carbon atoms of the double bond are sp hybridized. Recall from Sections 1.1 OB and 2.5D that the percent -character of a hybrid orbital increases from 25% to 33% in going from sp to sp. The higher the percent s-character, the more readily an atom accepts electron density. Thus, sp hybridized carbon atoms are more able to accept electron density and hybridized carbon atoms are more able to donate electron density. 

Hunsdiecker reaction, 341 Hybridization, 17 Hybrid orbital number, 17, 18, 32 Hybrid, resonance, 24 Hydration of cyclohexane derivatives, 191 Hydrazine, 4 Hydride shift, 93 Hydroboration, 95 Hydroboration-oxidation, 258, 270 Hydrocarbons, cyclic, 162 unsaturated, 87 Hydrogenation of alkenes, 57 Hydrogen bond, 22 Hydroperoxides in ethers, 284 Hydroquinone, 430 Hydroxy acids, 344... 

In CH4, the electron density of the four sp orbitals of C each overlap with an s orbital of H to form four o bonds. In C2H4 (an alkene), two sp orbitals on each C overlap with H s orbitals, the remaining sp orbitals overlap with each other in a CT bond, and the p orbitals (drawn as shaded shapes) overlap with each other above and beneath the carbon atoms in a tt bond (also drawn as shaded shapes). In CO2. the C atom has two sp hybrid orbitals and two p orbitals. These form one ct bond and one tt bond with the two unfilled p orbitals on each 0 atom. In C2H2 (an alkyne), a triple bond forms with one crand two tt bonds. 

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