CHM2210 Exam 3 Study Guide Sourav Saha Spring 2013 Chapter 7 REMINDER this is a conceptual study guide you should not rely solely on this to study for the test Make sure to practice problems as well to prepare for the test 7 1 Chirality By definition a molecule is chiral if it is not superimposable on its mirror image o Ex your left and right hands are chiral You not never arrange the two in the exact way Ex these two molecules are chiral Both of these molecules have the same physical properties chemical formula molecular weight boiling point etc however they have a different 3D shape o This causes different biological and 3D reactivity WHY SO IMPORTANT A compound called Thalidomide was used as an anti nausea medicine for pregnant women o The R stereoisomer was successful at relieving nausea o The S stereoisomer however was found to inhibit the formation of blood vessels causing many birth defects This goes to show that while they may be the exact some compound their 3D arrangement can cause different effects 7 2 The Chirality Center Here s a quick review of isomers Rem ember an isomer is 2 separate molecules with the same molecular formula A Structural isomers same formula but different connectivity a ex butane vs isobutene right B Stereoisomers same connectivity but different 3D shape These are the isomers we will be dealing with hence the name stereochemistry The most common way to determine whether or not a molecule is chiral is to find a chirality center o Put simply a chiral center is a molecule usually a carbon that is bonded to 4 separate atoms or substituent groups For example the carbon that is bonded to the OH group in 2 butanol below is chiral because it is bonded to CH2CH3 an OH a CH3 and a Hydrogen implied o A carbon atom in a ring is a chirality center if it bears two different groups and the path trace around the ring from that carbon in one direction is different from that traced in the other Ex The carbon bearing the hydroxyl group in 2 cyclopentenol is a chirality center Any molecule with a chirality center will have an enantiomer o Two separate molecules that are mirror images of each other are called enantiomers It is a distinct mirror image isomer o If a molecule has more than one chirality center then multiple stereoisomers are possible If there a n chirality centers then there are 2n possible isomers Can be enantiomers or diastereomers o Diastereomers a stereoisomer that is NOT an enantiomer The molecules have the same connectivity but has different configurations at one or more chiral centers in the molecule the picture on the right is an example of enantiomers vs diastereomers 7 3 Symmetry in Achiral Structures REVIEW a plane of symmetry is any line that bisects a molecule so that one half of the molecule is the mirror image of the other half I hope to god you know this or you shouldn t be in orgo IF AN OBJECT IS SYMMETRIC THEN IT WILL BE IDENTICAL TO ITS MIRROR IMAGE AND IT WILL BE ACHIRAL IF AN OBJECT IS NOT SYMMETRIC IT DOES NOT NECESSARILY MEAN ITS CHIRAL HOWEVER MOST NON SYMMETRIC MOLECULES WILL BE DIFFERENT FROM THEIR REFLECTION MAKING THEM CHIRAL o Since symmetry is not a sure way to determine chirality the best way to do so is to look for a chirality center o 7 4 Optical Activity specific rotation 100 c x l Definition measured using a polarimeter the ability of a chiral substance to rotate the plane of plane polarized light and is o this is a physical property of all chiral substances and this ability is measured by its o where is the specific rotation is the measured rotation c is the concentration in g mL and l is the length of the sample tube usually 1 dm To be optically active a sample must contain a chiral substance and one of its enantiomers must be present in excess of the other o Mixtures containing the same amount of two different enantiomers are called racemic mixtures o A sample containing only one enantiomer is said to be optically pure 7 5 Absolute Configuration We mentioned earlier that two molecules that are not superimposable on each other are chiral and that two molecules that are mirror images of each other are called enantiomers o BUT HOW DO WE DETERMINE WHETHER THE ENATIOMER IS RIGHT HANDED OR LEFT HANDED ANSWER We need to find the molecule s absolute configuration or its exact 3 dimensional spatial arrangement at a chirality center o To do this there are a few requirements STEPS TO DETERMINE ABSOLUTE CONFIGURATION 1 The molecule you are using must have a chiral center or else this is useless 2 Rank the substituent groups at the chirality center by atomic weight 1 4 with one being the biggest and 4 being the smallest Make sure the smallest group is going away from the plane of the paper 3 Rotate around the molecule starting from the highest group and ending at the lowest group If you rotate clockwise it is an R enantiomer If you rotate counterclockwise it is an S 4 enantiomer a NOTE if the lowest ranked group isn t going away from the plane of the paper then you switch its designation i Ex If you rotate counterclockwise but lowest group is facing you then the molecule is an R enantiomer instead of an S The molecule on the left is an R enantiomer while the molecule on the right is an S enantiomer 7 7 Fischer Projections Fischer projections are a simplified way to draw chiral molecules Here are the steps to drawing a Fischer projection 1 First arrange the molecule so that its horizontal bonds at chirality center point towards you and the vertical bonds away from you 2 Draw a cross on the page for each chiral center 3 The projection of the molecule on the page implies that horizontal lines are bonds coming towards you while vertical bonds are bonds going away from you In molecules that have more than one chiral center above there is more than one cross o In order to draw a molecule with more than one chiral center as a Fischer projection it must be in its eclipsed conformation Fischer projections make it easier to determine the absolute configuration of a chiral center o If there is a center in a Fischer that is not chiral you can still determine its absolute configuration in the same way as taught However denote these centers with a lowercase r or s remember that the molecule must be achiral overall to mark a non chiral center with a lowercase letter 7 9 Reactions that produce Chirality Centers Optically inactive achiral reactants cannot give optically active products Remember for a product to be optically active it must be