Chem 227 1st Edition Lecture 7 Outline of Last Lecture I. Cycloalkanesa. Naming cycloalkanesII. Cis-Trans Isomerism in Cycloalkanesa. Cis vs. TransIII. Stability of cycloalkanesa. StrainIV. Conformations of cycloalkanesV. Conformations of cyclohexanea. Chair, boat, etc.VI. Chair conformationsa. Equatorial vs. Axialb. Ring flipsc. 1,3-diaxial interactionsVII. Conformations of disubstituted cyclohexanesOutline of Current Lecture I. StereochemistryII. Chiralitya. What is chirality?b. Chirality CentersIII. Optical ActivityIV. Pasteur’s DiscoveryV. Sequence rules for specification of configurationCurrent LectureStereochemistry- 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 groupsThese notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.Examples 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… CHIRAL Plane 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 carbon YES 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 2If 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