Optical Isomers
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 = -0.5C B.P = -11.7 C
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)
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
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.
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:
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
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
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
Optical Activity The amount of rotation depends on Length of sample tube Concentration of enantiomers Both isomers present, rotation cancels out called: racemic mixture