CHEM 281 Chapter Notes - Chapter 4: Unsaturated Hydrocarbon, Pi Bond, Chemical Formula
30 Aug 2018
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Chapter 4 notes part 2: isomers: the arrangement of atoms in space. Rotation about a double bond occurs only if the pi bond breaks that is, only if the p orbitals are no longer parallel (figure 4. 1). Consequently, the energy barrier to rotation about a carbon carbon double bond is much greater than the energy barrier to rotation about a carbon carbon single bond: 62 kcal/mol versus 2. 9 kcal/mol (section 3. 11). The compound with the hydrogens on the same side of the double bond is called the cis isomer. The compound with the hydrogens on opposite sides of the double bond is called the trans isomer. Notice that the cis and trans isomers have the same molecular formula and the same bonds but have different configurations they differ in the way their atoms are oriented in space. If one of the sp2 carbons is attached to two identical substituents, then the compound cannot have cis and trans isomers.
