Chem 227 SantanderExam # 2 Study Guide Lectures: 6-10 Chapter 5Stereochemistry- Some objects are not perfectly equivalent to their mirror imageso A right-handed is different from a left-hand. The property is commonly called “handedness”- Organic molecules (including many drugs) have handedness that results from substitution patterns on sp3 hybridized carbon- This phenomenon of handedness will normally occur in carbon atoms that contain 4 different substituent groupsExamples of molecules with handedness:1. 4 substituent groups: H, F, Cl, Br2. 4 substituent groups: H, COOH, CH3, rest of molecule- This matters, for example, in medicines. The right handed version of the molecule/medicine can be beneficial to your health while the left handed version can be detrimental… not the same molecule and not the same chemistryEnantiomers are molecules that are not the same as their mirror image- There are two forms of lactic acid- All of the following are the same, they do not have enantiomers because you can superimpose the mirror image on top of the original- Enantiomers are NON-superimposable mirror images5.2 The reason for Handedness in molecules: Chirality- Molecules that are not superimposable with their mirror images are chiral (have handedness)- A plane of symmetry is a plane that divides an entire molecule into two pieces that are exact mirror images- A molecule with a plane of symmetry is the same as its mirror image and is said to be achiralWhich molecules are chiral? Resonance hybrid has plane of symmetry: not chiral Plane of symmetry No plane of symmetry; 4 different substituents on carbon molecule; even with rotating the mirror image, the mirror image is not the same as the original molecule… CHIRALPlane of symmetry No chirality here because propyl groups are the same… 4 substituentsare not all differentYES CHIRAL… 4 different substituentsChirality Centers- A point in a molecule where four different groups (or atoms) are attached to carbon is called a chirality centerEX: Identify chirality centers in these molecules NONE NONE YES 1 chirality center: 4 groups around one carbonYES 2 different chirality centers5.3 Optical Activity- Plane-polarized light that passes through solutions of achiral compounds retains its original plane of polarization- Solutions of chiral compounds rotate plane-polarized light and the molecules are said to be optically activeo Light passes through a polarizero Plane polarized light is rotated in solutions of optically active compoundso Magnitude of rotation is measured with a polarimeter rotation, in degrees, is [α]o Clockwise rotation is called dextrorotatoryo Anti-clockwise is levorotatory- If you get a α of 180º, how do you know if dextrorotatory or levorotatory?o Add water and dilute it and it should rotate a little less A polarimeter measures the rotation of plane-polarized light that has passed through a solution- The angle between the entrance and exit planes is the optical rotationSpecific Rotation [α]D for an optically active compound - [α]D = observed/(pathlength x concentration)- = α/(I x C)- = degrees/(dm x g/mL)- (Specific rotation is that observed for 1 g/mL in solution in cell wit a 10 cm path)- The specific rotation of the enantiomer is equal in magnitude but opposite in sign5.4 Pasteur’s discovery of enantiomers- Optically active forms of tartaric acid salts, racemic acid.5.5 Sequence rules for specification of configuration- A general method applies to the configuration at each chirality center- The configuration is specified by the relative positions of all the groups with respect to eachother at the chirality center- The groups are ranked in established priority sequence and comparedo Rule 1 Arrange them according to atomic number Set the lowest priority group pointing away Look for sequence of the other three: clockwise is designated R, counterclockwise is designated So Rule 2 If a decision cannot be reached by ranking the first atoms in the substituents, look at the second, third, or fourth atoms until the difference is foundo Rule 3: Multiple-bonded atoms are equivalent to the same number of single bonded atoms When the hydrogen is pointing away from you, the clockwise/counterclockwise orientation should be exactly as it is on the paper, but if the hydrogen is pointing toward you, you have to mentally turn the molecule, and the R/S switches (because you need to look at it from the other side (because when assigning R or S, the hydrogen (or lowest ranking group) should be pointing away from you))o PRACTICE THIS5.6 Diastereomers- Molecules with more than one chirality center usually have mirror image stereoisomers that are enantiomers- When you can have more than one place in the molecule that can be R or S, you have many different combinations of the molecule that could existo If you have 2 chirality centers, you could have R, R or R, S, or S, S or S, Ro If you have 3 chirality centers you could have R, R, R or R, R, S or R, S, S, etc. There are manyyyyy- In addition they can have stereoisomeric forms that are not mirror images, called diastereomers- MAXIMUM Number of Stereoisomers = 2n o Where n = number of chirality centerso Example: 2 chirality centers- Number of stereoisomers = 22 = 4 3 chirality centers- Number of stereoisomers = 23 = 8Here is an example of a molecule with 2 chirality centers (the 4 (22) stereoisomers):- 2R,3R and 2S,3S are enantiomers because they are mirror images of each other- 2R,3S and 2S,3R are enantiomers because they are mirror images of each other- The only difference between the mirror images of each other is their optical rotationo One will be dextrorotatoryo One will be levorotatory- But they are not all mirror images of each othero The molecules that are not mirror images of each other are called Diastereomerso When one chirality center is the same but one is differento Ex: 2R,3R and 2R,3S are diastereomers of each other5.7 Meso compounds- Tartaric acid has two chirality centers and two diastereomeric forms.- One form is chiral and the other is achiral, but both have two chirality centers- An achiral compound with chirality centers is called a meso compound- Consider the figure belowo The two structures on the left are chiral and mirror images of each othero The two structures on the right in the figure are identical so the compound (2R, 3S) is achiral- When the two chirality centers have the same attachmentso When there is a plane