CHEM 2211 1st Edition Lecture 10 Outline of Last Lecture I II III IV V Rotation about the Double Bond Conformation vs Configuration Chiral Molecules Isomer with one Asymmetric Center Drawing Enantiomers Outline of Current Lecture I II III IV Isomers are Optically Active Measuring Rotation Enantiomeric Excess Compounds with more than One Asymmetric Center Lecture Notes I Isomers are Optically Active A Though enantiomers have similar properties they interact with plane polarized light differently B Achiral molecules do not rotate plane polarized light C Chiral molecule rotate plane polarized light either clockwise right or counterclockwise left i A in front of the name of a molecule indicates the molecule rotates light to the right and is dextrorotary ii A in front of the name of a molecule indicates the molecule rotates light to the right and is levorotatory REMEMBER and are not the same as R and S configurations One R molecule can be levorotay while another can be dextrorotary R and S refer to the arrangement of substituents around an asymmetric carbon II Measuring rotation A Specific rotation is the measure of the amount of rotation caused by the pure liquid or by 1 00g of materials dissolved in 100mL B The following equation is used to determine the amount of rotation light has done T III lxc i is specific rotation ii T is the temperature in C iii wavelength of incident light is indicated as D iv is observed rotation v l is the length of the sample tube vi c is the concentration of sample in g 100mL C If on enantiomer has a specific rotation of 5 75 the specific rotation of the other one is 5 75 D A racemic mixture is the mixture of equal parts of both enantiomers When tested with a lane polarized light these mixtures yield a rotation of zero Enantiomeric Excess A Observed specific rotation is the specific rotation of the sample B Enatiomerically pure compounds have only one type of enantiomer C Enantiomeric Excess ee aka optical purity tells us how pure a mixture is enatiomeric excess observed specific rotation x 100 specif ic rotation of the pure enantiomer Example 2 bromobutane has a specific rotation of 23 1 An experiment yielded that a 2bromobutane mixture has an observed specific rotation of 9 2 enatiomeric excess 9 2 x 100 40 23 1 D This means that 40 of the mixture is excess S enantiomers and 60 is a racemic mixture meaning a total of 70 40 30 is S enantiomer and the IV remaining 30 is R enantiomer Compounds with more than one asymmetric center A Diastereomers are stereoisomers that are neither identical nor mirror images B Unlike enantiomers diastereomers have different physical and chemical properties from each other C Enantiomers with hydrogens on the same side are called erythro enantiomers Erythro Enantiomer of 3 chloro 2 butanol Staggered D Enantiomers with the hydrogens on the opposite side are called threo enantiomers Figure 1 Threo Enantiomers of 3 chloro 2 butanol Staggered
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