Bond angle in ethylene

Actually the HCH bond angle in ethylene is rather larger than the tetrahedral value. According to the equivalent-orbital picture, this can be attributed to the closing up of one pair of bonds leading to the opening of the other pair. 

The bond angles in ethylene are 120°, and the hybridization on carbon is sp2, as in BF3. Each carbon atom forms two bonds to the two hydrogen atoms and one bond to the other carbon atom, and so uses all three sp2 orbitals. There remains on each carbon atom one p orbital perpendicular to the plane of the C—H and C—G bonds. The second bond between the carbon atoms can be formed with these two orbitals and the remaining two electrons. The overlap between the /7-orbitals is different in the second bond because it will have to occur sideways. ... 

Problem 8.5 Predict the differences in bond angles in ethylene and in chloro-methane with respect to 120° and 109.5°, respectively. 

Using valence-shell electron-pair repulsion (VSEPR Section 1.3), we predict a value of 120° for the bond angles about each carbon in a double bond. The observed H — C — C bond angle in ethylene is 121.7°, a value close to that predicted by VSEPR. In other alkenes, deviations from the predicted angle of 120° may be somewhat larger as a result of strain between groups bonded to one or both carbons of the double bond. The C — C — C bond angle in propene, for example, is 124.7°. 

To see how Ihese ideas are used, consider ethylene (C2H4), which possesses a C=C double bond. The bond angles in ethylene are all approximately 120° (Figure 9.23 ), suggesting that each carbon atom uses sp hybrid orbitals (Figure 9.18) to form m bonds with the other carbon and wifli two hydrogens. Because carbon has four valence electrons, after sp hybridization one electron remains in Ihe unhybridized 2p orbital ... 

Three regions of electron density about an atom are farthest apart when they are coplanar (in the same plane) and make angles of 120° with each other. Thus, the predicted H—C—H and H—C—O bond angles in formaldehyde and the predicted H—C—H and H—C—C bond angles in ethylene are all 120° and the atoms are coplanar. The experimentally measured angles are quite close to this prediction, as shown in Figure 1.5. 

Fij ure4-8 The TI C C II Torsional Bond Angle in Deformed Ethylene. I he normal torsional angle is fj) — (E. 

Ethylene, bond angles in, 16 bond lengths in, 16 bond strengths in, 16 electrostatic potential map of, 74, 147... 

To illustrate this rule, consider the ethylene (C2H4) and acetylene (C2H2) molecules. You will recall that the bond angles in these molecules are 120° for ethylene and 180° for acetylene. This implies sp2 hybridization in C2H4 and sp hybridization in C2H2 (see Table 7.4). Using blue lines to represent hybridized electron pairs,... 

Table I. Bond lengths (in A) and bond angles (in degrees) for the ethylene molecule...

The units by which crystallographers describe interatomic distances are Angstrom units (A = 10 8 cm.). Normal values for carbon-carbon interatomic distances are 1.34 A for a double bond (as in ethylene) and 1.54 A (as for-diamond) for a single bond. In a truly aromatic compound (such as benzene) the C-C bond length, as mentioned above, is 1.39 A. C-C-C angles are 109.5° for a tetrahedral carbon atom (sp3) and 120.0° for a trigonal carbon atom (sp2). 

But union between two carbon atoms by a double bond, as in ethylene results in considerable displacement of valency bonds, which become parallel and therefore the bond angle is reduced to zero. 

FIGURE 42. Calculated geometries at MP2 and relative energies at MP4 of the stationary points of the reaction path for the addition of ( A ) EH2 to ethylene. Precursor complexes Comp-EH, transition states TS-EH and products Pro-EH. Bond distances are in A, bond angles in deg. The energy values are A //" values and they are in kcalmol-1. The numbers in parentheses give the A//298 va ues Reproduced by permission of John Wiley Sons, Inc. from Reference 87... 

Moreover, the carbon-fluorine bonds in tetrafluoroethylene are sp3 hybridized, and the bond angle FCF in tetrafluoroethylene in the doubly bonded difluoromethylene group is 110° compared with the bond angle in HCH in ethylene (117° 12 ). The more fluorine atoms around the double bond in fluorinated alkenes, the easier it is to form four-membered rings. 

They have used model of lone pairs for water. It would be interesting to check the result CC bond length in ethylene and acetylene, and O-H bond length and H-O-H angle for water. The results have been collected in Table 19. 

Stretched and twisted ethylene again may serve as examples. As the H—H separation decreases, and as the twist angle in ethylene decreases, the covalent perturbation y increases in absolute magnitude. In both cases, this results in a stabilization of the Sg state (bond formation) relative to the antibonding T state. It also results in the stabilization of the S, and S2 states of H2, although in the usual description the first of these states has as many electrons in the antibonding as in the bonding orbital, and the second only... 

Several series of mono-aryloxo and bis-aryloxo titanium derivatives have been prepared by the routes outlined in Scheme 349. Activated by MAO the compounds are used as efficient catalysts for the syndiospecific polymerization of styrene and the co-polymerization of ethylene and styrene. The molecular structures of some of these compounds have been determined by X-ray diffraction. The Ti-O-C bond angle in the structure of Cp TiCl2(0-2,6-Pr12C6H3) differs significantly from those that are observed for the other structures. The effect of the substituents, both on the Cp ring and the alkoxo group, plays an essential role for the catalytic activity and the properties of the polymer obtained.578,779,837-847... 

FIGURE 5.1 (a) The framework of bond distances in picometers and bond angles in degrees. All six atoms are coplanar. The carbon-carbon bond is a double bond made up of the hybridized carbons overlap to produce a tt bond. An electron pair in the tt bond is shared by the two carbons. 

FIGURE 16. 6-31G transition state structures for the addition of H20 to silaethylene and to ethylene. Bond distances are in A, bond angles in deg. Reproduced by permission of Kluwer Academic Publishers from Ref. 175b. 

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