Ethane, bond angles

Practice working with your Learning By Modeling software Construct molecular models of ethane ethylene and acetylene and compare them with respect to their geometry bond angles and C—H and C—C bond distances... 

The structural features of methane ethane and propane are summarrzed rn Ergure 2 7 All of the carbon atoms have four bonds all of the bonds are srngle bonds and the bond angles are close to tetrahedral In the next sectron we 11 see how to adapt the valence bond model to accommodate the observed structures... 

FIGURE 2 7 Structures of methane ethane and propane showing bond distances and bond angles... 

Bonding m n butane and isobutane continues the theme begun with methane ethane and propane All of the carbon atoms are sp hybridized all of the bonds are ct bonds and the bond angles at carbon are close to tetrahedral This generalization holds for all alkanes regardless of the number of carbons they have... 

Bond Distances, Bond Angles, and Bond Energies in Ethane, Ethene, and Ethyne (Table 9 1, p 342) Stmctures of a-Ammo Acids (Table 27 1, pp 1054-1055)... 

Structure. Ethylene is a planar molecule with a carbon—carbon bond distance of 0.134 nm, which is shorter than the C—C bond length of 0.153 nm found in ethane. The C—H bond distance is 0.110 nm, and the bond angles are [Pg.432]

Next come the dihedral angles (or torsions), and the contribution that each makes to the total intramolecular potential energy depends on the local symmetry. We distinguish between torsion where full internal rotation is chemically possible, and torsion where we would not normally expect full rotation. Full rotation about the C-C bond in ethane is normal behaviour at room temperature (although 1 have yet to tell you why), and the two CH3 groups would clearly need a threefold potential, such as... 

The three simplest alkanes. The bond angles in methane, ethane, and propane are all close to 109.5°, the tetrahedral angle. 

The bond angles are also satisfactorily reproduced in most of the cases. This is also true for dihedral angles which are sometimes more accurately predicted than by the use of the larger, double-zeta sets. For instance, experimental evidence favors the gauche structure for 1,2 difluoro ethane. STO-3G calculations lead to a gauche structure, while 4-31G calculations predict a trans structure. 

Figure 1.30 Bond angles and bond lengths of ethyne, ethene, and ethane.

If this carbon holds in different atoms, the bond angles are somewhat (a little) changed and the tetrahedron ceases to be regular. But the real foundation for conformational study was laid in 1935 when it was observed that there was discrepancy between the entropy of ethane as found from the heat capacity measurements and as calculated from spectral data. From this the physical chemists concluded that there must be hindrance to rotation about the carbon bond in ethane. Later it was found that there was tortional barrier to free rotation to the extent of about 2.8 K cals per mole. 

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... 

For example, where the reaction is rotation about the carbon-carbon bond in ethane, the reaction coordinate may be thought of as simply the HCCH torsion angle, and the structure may be thought of in terms of this angle alone. Thus, staggered ethane (both the reactant and the product) is a molecule for which this angle is 60° and eclipsed ethane is a molecule for which this angle is 0°. 

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]

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