CHEM 2211 1st Edition Lecture 10 Outline of Last LectureI. Rotation about the Double BondII. Conformation vs. ConfigurationIII. Chiral MoleculesIV. Isomer with one Asymmetric CenterV. Drawing EnantiomersOutline of Current LectureI. Isomers are Optically ActiveII. Measuring RotationIII. Enantiomeric ExcessIV. Compounds with more than One Asymmetric CenterLecture NotesI. Isomers are Optically ActiveA. Though enantiomers have similar properties, they interact with plane polarized light differentlyB. Achiral molecules do not rotate plane polarized lightC. 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 dextrorotaryii. A (-) in front of the name of a molecule indicates the molecule rotates light to the right and is levorotatoryREMEMBER: (+) 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 rotationA. Specific rotation is the measure of the amount of rotation caused by the pure liquid or by 1.00g of materials dissolved in 100mLB. The following equation is used to determine the amount of rotation light has done[∝]λT=∝l x ci. [α] is specific rotationii. T is the temperature in °Ciii. λ wavelength of incident light is indicated as Div. α is observed rotationv. l is the length of the sample tubevi. c is the concentration of sample in g/100mLC. If on enantiomer has a specific rotation of -5.75, the specific rotation of the otherone is +5.75D. 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 zeroIII. Enantiomeric ExcessA. Observed specific rotation is the specific rotation of the sampleB. Enatiomerically pure compounds have only one type of enantiomerC. Enantiomeric Excess (ee) aka optical purity tells us how pure a mixture isenatiomeric excess=observed specific rotationspecif ic rotation of the pure enantiomerx 100 %Example: 2-bromobutane has a specific rotation of +23.1. An experiment yielded that a 2-bromobutane mixture has an observed specific rotation of +9.2.enatiomeric excess=+9.2+23.1x 100 %=40 %D. This means that 40% of the mixture is excess S enantiomers and 60% is a racemicmixture meaning a total of 70% [40% + ½ (30%)] is S enantiomer and the remaining 30% is R enantiomer.IV. Compounds with more than one asymmetric centerA. 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 enantiomersErythro Enantiomer of 3-chloro-2-butanol (Staggered)D. Enantiomers with the hydrogens on the opposite side are called threo enantiomersFigure 1 Threo Enantiomers of 3-chloro-2-butanol
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