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This overview focuses on cis-trans isomerism, particularly within organic molecules and transition metal (TM) complexes. We discuss the characteristics of cis-trans isomers, emphasizing that they occur in specific alkene structures where rotation around the double bond is restricted. In the context of TM complexes, we explore how ligands can be positioned 90° (cis) or 180° (trans) apart. For an alkene to exhibit cis-trans isomerism, it must have unique substituents on both sides of the double bond. This foundational concept is key in organic chemistry.
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Geometric Isomers in Organic Molecules • The only type of geometric isomer “type” we will discuss is cis-trans isomerism • With TM complexes, we had cis-trans isomerism in square planar and octahedral complexes • Ligands were either 90° (cis) or 180° (trans) apart from one another • Cis = “same side”; trans = “across from one another” • In organic molecules, cis-trans isomers can occur in certain alkenes • Alkene a double bond is necessary (but not sufficient
Example of a cis-trans isomer pair Trans = “opposite sides of double bond” (across the double bond) Cis = “same side of double bond”
These two compounds are only different because rotation about a db is restricted—a p bond would need to break
When is cis-trans isomerism possible? D A C C B E • Consider the following “generalized” alkene with substituents • To have cis-trans isomers, bothof the following must be true: • A B AND • D E
