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Understanding Stereoisomers and Optical Isomers in Chemistry

Learn about different isomers, including structural and optical isomers, stereochemistry, chiral centers, and optical activity. Dive into drawing techniques like dash-wedge formulas and mirror images to enhance your understanding. Explore the properties and vocabulary related to optical isomers. Engage with interactive animations and experiments to grasp these concepts effectively.

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Understanding Stereoisomers and Optical Isomers in Chemistry

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  1. Optical Isomers

  2. Constitutional Isomers • Have same molecular form but different structural formula • Both isomers to the right are C4H10 • These have different chemical and physical properties B.P = -0.5C B.P = -11.7 C

  3. Stereoisomers • Appear to have same structure in 2D • In 3D have different structures • Are non-superimposable mirror images of each other • With modeling kit build a model: carbon atom with four different colors attached (use same size bonds!) • Look at your model in a mirror, build what you see in the mirror. • Are the two superimposable? Animation (watch Chirality)

  4. How to draw 3D: Dash-Wedge Formula • Draw two straight lines about 110  from each other • Bond angle is 109.5 • These represent the bonds in the plane of the page • Draw dashed line for the bond that extends behind the plane of the page • Draw wedge for bond that extends in front of the plane of the page

  5. Drawing Enantiomers • Draw the molecule you built, using the dash-wedge formula and colored pens • Use a mirror to see the enantiomer • Sketch the mirror image, using the dash-wedge formula • Draw the two stereoisomers of 1-chloro-1-bromo-ethane. • There is no convention for which atom is attached to a wedge.

  6. Chiral centers • Carbon attached to 4 different substituent groups • Even if two substituent groups start with C, they could still be different • C=O, never chiral (C only attached to 3 things) • CH3, CH2 groups, never chiral because they are symmetric:

  7. Vocabulary • Stereoisomers have chiral centers. • Carbon atoms can be • chiral or achiral • Asymmetric or symmetric • If two molecules are stereoisomers, they are also called enantiomers • Chiral molecules are optically active • Enantiomers are optical isomers

  8. Properties of Optical Isomers • same physical and chemical properties • rotate plane polarized light • Each isomer rotates it in a different direction • What is plane polarized light? • Animation (watch Optical Activity) • Try polarizers

  9. Optical Activity • Stereoisomers are said to be optically active if the rotate plane polarized light • Each type of enantiomer rotates light the same amount, but in different directions. • Amount and direction of rotation must be experimentally determined using a polarimeter

  10. Optical Activity • The amount of rotation depends on • Length of sample tube • Concentration of enantiomers • Both isomers present, rotation cancels out called: racemic mixture

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