Ch_322a_1.016 - localized p-orbitals 90 o 90 o C C"...

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Restricted Rotation Around the Double Bond A rotation of one carbon relative to the other uncouples (loss of orbital overlap) the p-orbitals . When the p-orbitals are 90 o apart, there is no overlap and the electronic energy is the same as for an electron residing in an isolated p-orbital. C C C C !" bond or 90 o C C rotation 90 o view along the C-C axis C C localized p-orbitals loss of orbital overlap The picture of the carbon-carbon double bond as separate ! and " bonds is consistent with the observed large barrier (approx. 60 kcal/mol) to rotation around the C-C axis. While !# bonds are cylindrically symmetrical around the axis of the bond (meaning that rotation around the bond axis does not cause loss of orbital overlap), " -bonds are not.
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C C ! -electron potential energy kcal/mol 60 0 degree of rotation 45 90 135 180 !" bond The Energy Barrier to Rotation
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Unformatted text preview: localized p-orbitals 90 o 90 o C C !" bond An energy barrier of close to 60 kcal/mol is estimated during rotation around the carbon-carbon double bond. Cis-Trans Isomerism in Alkenes The two compounds below with the same formula of C 2 H 2 Cl 2 illustrate this isomerism. C=C Cl Cl H H C=C Cl Cl H H cis-1,2-dichloroethene trans-1,2-dichloroethene " cis " ( Latin : "on this side") " trans " ( Latin : "across") MP -80 o C-50 o C BP 60 o C 48 o C dipole moment 1.90 D 0 D These isomers are sometimes called geometric isomers . The barrier of close to 60 kcal/mol means that rotation around the carbon-carbon double bond does not occur under ordinary conditions . This restricted rotation leads to a type of isomerism : the existence of different compounds with the same formula....
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