Bonding in ethylene

We ai e free to pick a tefei ence poitit of energy once, but otily otice, for each system, l,et us choose the reference point t.. We have obtained the energy eigenvalues of the x bond in ethylene as one [f greater than y. 011)11 bunding) and one p lower than y ( bunding) (Fig, 6-3),... 

These absorptions are ascribed to n-n transitions, that is, transitions of an electron from the highest occupied n molecular orbital (HOMO) to the lowest unoccupied n molecular orbital (LUMO). One can decide which orbitals are the HOMO and LUMO by filling electrons into the molecular energy level diagram from the bottom up, two electrons to each molecular orbital. The number of electrons is the number of sp carbon atoms contributing to the n system of a neuhal polyalkene, two for each double bond. In ethylene, there is only one occupied MO and one unoccupied MO. The occupied orbital in ethylene is p below the energy level represented by ot, and the unoccupied orbital is p above it. The separation between the only possibilities for the HOMO and LUMO is 2.00p. 

FIGURE 2 17 The carbon-carbon double bond in ethylene has a cr component and a tt compo nent The cr component arises from overlap of sp hybridized orbitals along the internuclear axis The tt component results from a side by side overlap of 2p orbitals... 

The TT bond in ethylene generated by overlap of p orbitals of adjacent carbons... 

FIGURE 5 1 (a) The planar framework of u bonds in ethylene showing bond distances and angles (b) and (c) The p orbitals of two sp hybridized carbons overlap to produce a tt bond (d) The electrostatic potential map shows a region of high negative potential due to the tt elec trons above and below the plane of the atoms... 

The double bond in ethylene is stronger than the C—C single bond in ethane but It IS not twice as strong Chemists do not agree on exactly how to apportion the total C=C bond energy between its ct and rr components but all agree that the rr bond is weaker than the ct bond... 

The hybridization concept can also be applied to molecules containing double and triple bonds. The descriptive valence bond approach to the bonding in ethylene and... 

We conclude this introduction to hydrocarbons by describing the orbital hybridization model of bonding in ethylene and acetylene, parents of the alkene and alkyne families, respectively. 

It s easy to identify the carbons involved in the double bond in ethylene atoms 1 and 2. In propene, things are slightly more complicated. In this case, the carbon which has three hydrogens attached to it is not involved in the double bond. We can identify this carbon atom as center number 3 by noting that it along with the final three hydrogen atoms all lie in the third quadrant (-X and -Y). Therefore the two carbons of interest are again atoms 1 and 2. 

Figure 1.18 A molecular orbital description of the C=C tt bond in ethylene. The lower-energy, tt bonding MO results from a combination of p orbital lobes with the same algebraic sign and is filled. The higher-energy, -tt antibonding MO results from a combination of p orbital lobes with the opposite algebraic signs and is unfilled.

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. 

Fig. 4. a) Trimethylene + ethylene level diagram, b) The same for trimethylene + butadiene. S and A classification with respect to reflection in a plane bisecting angle a in trimethylene and passing through the midpoint of the double bond in ethylene and of the single bond in butadiene. 

In ethylene, there are two types of bonds. Sigma (tr) bonds have the overlap of the orbitals on a line between the two atoms involved in the covalent bond. In ethylene, the C-H bonds and one of the C-C bonds are sigma bonds. Pi (ir) bonds have the overlap of orbitals above and below a line through the two nuclei of the atoms involved in the bond. A double bond is always composed of one sigma and one pi bond. A carbon-to-carbon triple bond results from the... 

Mulliken draws on the equivalence between two descriptions of a localized double bond, as (a) a tt and a bond superposed and (b) two bent a bonds, as applied, for example, to the case of bonding in ethylene (Hall and Lennard-Jones, 1951). In the present case a pseudo-double bond is visualized involving the tt orbital 4>r oii fhe atom r under attack and a pseudo-77 orbital formed by combining the valence orbitals of the... 

The bonding in ethylene is based initially on one C-C CT bond together with four C-H a bonds, much as we have seen in ethane. We are then left with a p orbital for each carbon, each carrying one electron, and these interact by side-to-side overlap to produce a IX bond (Figure 2.15). This makes the ethylene molecule planar, with bond angles of 120°, and the TX bond has its electron density above and below this plane. The combination of the C-C ct bond and the C-C Jt bond is what we refer to as a double bond note that we cannot have Jt bond formation... 

Energies of 7t-Bonds in Ethylene and Its Analogs H2CXH2 (X = C,Si,Ge,Sn) Determined by Different Methods (in kcal/mol)... 

Use of this wave function with Eq. (19) then yields a theoretical value for Ahh in CH4 of 12.5 cps which is to be compared with the experimental value of 12.3 0.6 cps. Valence bond calculations of this nature have successfully accounted for the variation with H—C—H angle of the proton-proton coupling constants in substituted methanes (45) (Fig. 3), for the difference between AHwcls and AHwran across double bonds in ethylenes, and for the difference between AHH [Pg.241]

The same basic diagram provides a detailed description of the twisting of a -bond (Salem, 1973 Salem and Rowland, 1971). This is illustrated in Fig. 10. The -bond in ethylene may exist in a number of predetermined states, each of which may be approximated by the appropriate MO configuration. The possible states are S0, the ground state, ( 2) Tj, the first... 

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