Double bond rotation about

Because of the weight of 43, the system 41/43 undergoes an easier rotation about its formally Ge = C double bond than about its formally C-C single bond. The diradical character of 41 should favor any addition with radical intermediates and polymerization reactions.45... 

With a double bond, rotation would destroy the tt bond that arises from overlap of p orbitals consequently, there is a very large barrier to rotation. It is of the order of 263 kJmol , which is very much higher than any of the barriers to rotation about single bonds that we have seen for conformational isomerism. Accordingly, cis and trans isomers do not interconvert under normal conditions. Ring systems can also lead to geometric isomerism, and cis and trans isomers... 

In 77, N has maintained an sp3 pyramidal configuration rather than an sp2 planar configuration. The morpholino group is rotated out of the plane of the double bond by about 33°. In 83 the three morpholino groups are not equivalently bonded to the ethene moiety. The two C1—N distances with 1.398 and 1.415 A are significantly shorter than the C2—N3 distance of 1.442 A. This indicates conjugative delocalization of... 

Figure 5.5. Hindered rotation about carbon-carbon double bond. Rotation would prevent overlap of p orbitals and would break n bond.

The C-N bond length to the carbonyl group is closer to that of a standard C-N double bond (127 pm) than to that of a single bond (149 pm). This partial double bond character is responsible for the restricted rotation about this C-N bond. We must supply 88 k) moH if we want to rotate the C-N bond in DMF (remember a full C-C double bond takes about 260 k) moC ). This amount of energy is not available at room temperature and so, for all intents and purposes, the amide C-N bond is locked at room tern- o... 

Scheme 5 illustrates the conformational complexity which is built into symmetrical IV compounds that have unsymmetrical M groups attached to symmetrical bridges by single bonds. Rotation about these bonds interconverts syn and anti conformations of the M groups. For these compounds, double nitrogen inversion also interconverts syn and anti ferf-butyl group conformations, so there are four diastereomeric conformations of 18. All are present in equal amounts for neutral 18,... 

The two isomers of but-2-ene are stereoisomers because they have exactly the same constitution but a different spatial arrangement of their atoms in space. As we learned in Section 11, a double bond between C atoms consists of the overlap of hybrid orbitals to form a a bond and the sideways overlap of p orbitals to form a tt bond. Because of the ir bond, rotation about a double bond is severely restricted. Molecule (a) cannot be converted into molecule (b) simply by twisting one end of the molecule through 180°, so the two molecules are distinctly different above. To differentiate these two molecules, we call molecule (a) ds-but-2-ene and we call molecule (b) trans-but-2-ene. Because of differences in their molecular structures, the compoimds have different physical properties. For example, the melting points are —139 °C for ds-but-2-ene and —106 °C for tr ns-but-2-ene the boiling points are 3.7 °C for cis-but-2-ene and 0.9 °C for trans-but-2-ene. 

Figure B2.4.1. Proton NMR spectra of the -dimethyl groups in 3-dimethylamino-7-methyl-l,2,4-benzotriazine, as a fiinction of temperature. Because of partial double-bond character, there is restricted rotation about the bond between the dunethylammo group and the ring. As the temperature is raised, the rate of rotation around the bond increases and the NMR signals of the two methyl groups broaden and coalesce.

The carbon atoms of the double bond have a trigonal planar configuration and free rotation about the C—C bond is prevented by the n bond. The inability to rotate means that geometrical isomers can be produced, with substituents a and b, thus ... 

To conclude this computer project, we shall lirst search the potential surface for rotation of u-butane about its 23 C C bond, for which we think we know the answei, then seai ch the potential sutface foi I-butene, foi which we do not. In I -butene, the double bond establishes a rigid plane but the methyl group can take up several d i ffe ren t positions re I at i ve to i t by rotation ab ou t th e 2 - 3 s i n g I e bo n d,... 

In principle cis 2 butene and trans 2 butene may be mterconverted by rotation about the C 2=C 3 double bond However unlike rotation about the C 2—C 3 single bond in butane which is quite fast mterconversion of the stereoisomeric 2 butenes does not occur under normal circumstances It is sometimes said that rotation about a carbon-carbon double bond is restricted but this is an understatement Conventional lab oratory sources of heat do not provide enough energy for rotation about the double bond m alkenes As shown m Figure 5 2 rotation about a double bond requires the p orbitals of C 2 and C 3 to be twisted from their stable parallel alignment—m effect the tt com ponent of the double bond must be broken at the transition state... 

Isomeric alkenes may be either constitutional isomers or stereoisomers There is a sizable barrier to rotation about a carbon-carbon double bond which corresponds to the energy required to break the rr component of the double bond Stereoisomeric alkenes are configurationally stable under normal conditions The configurations of stereoisomeric alkenes are described according to two notational systems One system adds the prefix CIS to the name of the alkene when similar substituents are on the same side of the double bond and the prefix trans when they are on opposite sides The other ranks substituents according to a system of rules based on atomic number The prefix Z is used for alkenes that have higher ranked substituents on the same side of the double bond the prefix E is used when higher ranked substituents are on opposite sides... 

Geometrical Isomerism. Rotation about a carbon-carbon double bond is restricted because of interaction between the p orbitals which make up the pi bond. Isomerism due to such restricted rotation about a bond is known as geometric isomerism. Parallel overlap of the p orbitals of each carbon atom of the double bond forms the molecular orbital of the pi bond. The relatively large barrier to rotation about the pi bond is estimated to be nearly 63 kcal mol (263 kJ mol-i). 

In Eq. (2), the dihedral tenn includes parameters for the force constant, Ky, the periodicity or multiplicity, n and the phase, 8. The magnimde of Ky dictates the height of the barrier to rotation, such that Ky associated with a double bond would be significantly larger that that for a single bond. The periodicity, n, indicates the number of cycles per 360° rotation about the dihedral. In the case of an bond, as in ethane, n would... 

The activation energy for rotation about a typical carbon-carbon double bond is very high—on the order of 250 kj/mol (about 60 kcal/ mol). This quantity may be taken as a measure of the tt bond contribution to the total C=C bond strength of 605 kJ/mol (144.5 kcal/mol) in ethylene and compares closely with the value estimated by manipulation of thermochemical data on page 191. 

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