Geometric Cis-Trans Isomerism in Alkenes

A major difference between a single bond and a double bond is the degree to which rotation occurs about the bond. As discussed in Section 10.7, rotation abont a donble bond is highly restricted dne to the overlap between unhybridized p orbitals on the adjacent carbon atoms. Consider the difference between 1,2-dichloroethane and 1,2-dichloroethene  

The hybridization of the carbon atoms in 1,2-dichloroethane is sp, resulting in relatively free rotation about the sigma single bond. Consequently, the two structures are identical at room temperature because they quickly interconvert. 

In contrast, rotation about the double bond (sigma -I- pi) in 1,2-dichloroethene is restricted, so at room temperature, 1,2-dichloroethene exists in two isomeric forms. 

These two forms of 1,2-dichloroethene are different compounds with different properties as shown in Table 20.9. This kind of isomerism is a type of stereoisomerism (see Section 20.3) called geometric (or cis-trans) isomerism. We distinguish between the two isomers with the designations cis (meaning same side ) and trans (meaning opposite sides ). Cis-trans isomerism is common in alkenes. As another example, 

Name Structure Space-filling Model Density (g/mL) Melting Point (°C) Bolling Point (°C) 

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