Chapter 5

Chapter 5 - CHAPTER 5 Stereoisomers We have already covered...

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HAPTER 5 CHAPTER 5 Stereoisomers
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We have already covered two kinds of isomerism: onstitutional Isomers (structural isomers) •Constitutional Isomers (structural isomers) •Stereoisomers
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Examples of Constitutional Isomers: Examples of Stereoisomers:
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Another type of stereoisomerism is called mirror-image stereoisomerism . Mirror-image related stereoisomers are said to be left-handed and right-handed and occur when a molecule and its mirror image are non-superimposable.
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Chiral Molecules 5-1 he radical bromination of butane to form 2 romobutane The radical bromination of butane to form 2-bromobutane appears to yield a single product:
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Chiral molecules cannot be superimposed on their irror images mirror images. he two 2- romobutane The two 2 bromobutane molecules formed by the radical bromination of butane are actually non- superimposable and are therefore not identical. molecule that is not A molecule that is not superimposible on its mirror image is said to be chiral . this case each isomer is In this case each isomer is called an enantiomer .
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Compounds whose mirror images are superimposable are called chiral achiral . Examples of chiral and achiral molecules: bove all the chiral examples contain an atom that is connected Above all, the chiral examples contain an atom that is connected to 4 different substituent groups. This atom is called an asymmetric atom or a stereocenter . Often, asymmetric atoms are marked with an asterisk. Molecules having one stereocenter are always chiral.
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The symmetry in molecules helps to distinguish hiral structures from achiral ones. chiral structures from achiral ones. For most organic molecules, a sufficient test for chirality is absence of a plane of symmetry (mirror plane). A mirror plane is one that bisects the molecule in such a way that the half of the molecule on one side of the plane is the mirror image of the half on the other side of the plane. Methane has 6 planes of symmetry, chloromethane has 3, dichloromethane 2, bromochloromethane 1, and romochlorofluoromethane none: bromochlorofluoromethane none:
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Optical Activity 5-2 nantiomers cannot be distinguished on the basis of their physical Enantiomers cannot be distinguished on the basis of their physical properties, such as boiling points, melting points, and densities. Enantiomers can be distinguished by the way they interact with plane-polarized light. When plane-polarized light is passed through a sample of one of the enantiomers, the plane of polarization is rotated either clockwise or counterclockwise. When the experiment is repeated with the other enantiomer, the plane-polarized light is rotated an equal amount, but in the opposite direction. If facing the light source: •Clockwise rotation: enantiomer is dextrorotary (+) •Counterclockwise rotation: enantiomer is levorotary (-) This interaction with light is called optical activity and nantiomers are often called ptical isomers enantiomers are often called optical isomers .
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Optical rotation is measured with a polarimeter.
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Chapter 5 - CHAPTER 5 Stereoisomers We have already covered...

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