OC307-5-11-Ch5 - Organic Chemistry Chapter 5 Stereoisomers...

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Unformatted text preview: Organic Chemistry Chapter 5 Stereoisomers H. D. Roth 1 LECTURE POSTING V Stereoisomerism A type of isomerism; two compounds are stereoisomers when they differ only in the spatial relationship of their components. In order to discuss isomerism we use the following terms (with which you are familiar): Composition : the type and number of atoms in a molecule; Constitution : the way in which these atoms are connected (we also call this connectivity ); Configuration : the arrangement of the atoms in three-dimensional space; The term Conformation : also describes the arrangement of atoms in 3D space, when several arrangementsare possible; A. Constitutional isomerism Organic Chemistry Chapter 5 Stereoisomers H. D. Roth 2 B. Conformation al isomerism The conformation can be changed by free rotation about one or two single bonds; Br Br axial Br equatorial Br C. Configurational isomerism I: cis-trans or geometric isomerism The configuration cannot be changed; it is fixed, by restricting free rotation. 1) Rotation can be restricted if two carbons of an alkane chain are tied up forming a ring. You have seen examples in the cycloalkanes: substituents can be on the same side (cis) or on opposite sides (trans) of the ring plane; two examples are shown below. CH 3 H CH 3 H H CH 3 CH 3 H 2) Rotation can be restricted also when two adjacent carbon atoms are connected by a bond in addition a bond (see chapter 1); the bond holds the substituents in one plane; they can be on the same side (cis) or on opposite sides (trans) of the double bond (more in chapter 11). cis-2-butene H 3 C CH 3 H 3 C CH 3 H H H H trans-2-butene Geometric isomers could be interconverted by breaking a bond and then remaking it; a bond of a cycloalkane could be broken or bond of an alkene. These reactions do occur, but they require high energies therefore the geometric isomers of cycloalkanes and alkenes are stable at room temperature (unlike conformers, which are readily interconverted). Organic Chemistry Chapter 5 Stereoisomers H. D. Roth 2 B. Conformation al isomerism The conformation can be changed by free rotation about one or two single bonds; Br Br axial Br equatorial Br C. Configurational isomerism I: cis-trans or geometric isomerism The configuration cannot be changed; it is fixed, by restricting free rotation. 1) Rotation can be restricted if two carbons of an alkane chain are tied up forming a ring. You have seen examples in the cycloalkanes: substituents can be on the same side (cis) or on opposite sides (trans) of the ring plane; two examples are shown below. CH 3 H CH 3 H H CH 3 CH 3 H 2) Rotation can be restricted also when two adjacent carbon atoms are connected by a bond in addition a bond (see chapter 1); the bond holds the substituents in one plane; they can be on the same side (cis) or on opposite sides (trans) of the double bond (more in chapter 11)....
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OC307-5-11-Ch5 - Organic Chemistry Chapter 5 Stereoisomers...

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