CHEM 227 1st Edition Lecture 8 Outline of Last Lecture I. Rules for assigning priority/R and S (CONT.)II. 5.6 DiastereomersIII. 5.7 Meso compoundsIV. 5.8 Racemic Mixtures and the Resolution of Enantiomers (Separation)V. 5.9 Review of IsomerismOutline of Current Lecture I. 5.10 Chirality at Nitrogen, Phosphorus, and SulfurII. 5.11 ProchiralityIII. 5.12 Chirality in Nature and other Chiral environmentsIV. 6.1 Kinds of Organic ReactionsV. 6.2 How organic reactions occur: mechanisms and stepsCurrent Lecture5.10 Chirality at Nitrogen, Phosphorus, and Sulfur- N, P, S commonly found in organic compounds and can have chirality centers- Trivalent nitrogen is tetrahedral because of the lone pair - Does not form a chirality center since it rapidly flips- Individual enantiomers cannot be isolatedo No point in defining chirality in Nitrogen- However, phosphorous flips more slowly- You can isolate the different enantiomers of this phosphorous compoundThese 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.5.11 Prochirality - A molecule that is achiral but can become chiral by a single alteration is a prochiral molecule- This is a reduction reaction (because the double O bond is partially broken and there is LESS bonding to an electronegative atomreduction)- The starting material can receive the hydrogen from the front or from the back – this makes a difference when assigning R and S.- Prochiral distinctions: Faceso Planar faces that can become tetrahedral are different from the top or bottomo A center at the planar face at a carbon atom is designated re if three groups in priority sequence are clockwise and si if they are counterclockwise- Biological reactions often involve making decisions between prochiral faces or groups- Chiral entitites (such as enzymes) can always make such a distinctiono Ex: addition of water to fumarate- An sp3 carbon with two groups that are the same is a prochirality center- The two identiacal groups are distinguished by considering either and seeing if it were increased in priority in comparison with the other center- If the center becomes R then the group is pro-R and pro-S if the center becomes S5.12 Chirality in Nature and Chiral Environments- Stereoisomers are readily distinguished by chiral receptors in nature- Properties of Drugs depend on stereochemistry6.1 Kinds of Organic Reactions4 major categories of Reactions- Additonso Adding some pieces to original molecule- Eliminationso Taking away some pieces from original molecule- Substitutionso Substituting some pieces for others on original molecule- Rearrangementso Connecting the pieces in a different way on original molecule- Addition Reactions- Easy way to identify: when a pi bond is gone- Elimination Reactions- Easy way to identify: pi bond is created- Substitution- Easy way to identify: one group takes place of another- Rearrangement- Easy way to identify: same pieces, different connectivity6.2 How Organic Reactions occur: Mechanisms and Steps- In an organic reaction, we see the overall transformation that has occurred. The mechanism describes the steps behind the changes that we can observe.- Reactions occur in defined steps that lead from reactant to product- A step involves either the formation or breaking of a covalent bondo Steps can occur individually or in combination with other stepso When several steps occur at the same time, they are said to be concerted- Example: How does a substitution take place?o Simultaneous bond making and bond breaking?o A group leaves and the new group comes?o A new group comes and later the original group departs?- Types of Steps in Reaction mechanismso Bond formation or breakage can be symmetrical or unsymmetrical Symmetrical – homolytic Unsymmetrical – heterolyticIndicating steps in mechanisms- Curved arrows indicate breaking and forming of bonds- Arrowheads with “half” head (“fish-hook”) indicate a single electron movement (a radical reaction)- Arrowheads with a complete head inticate heterolytic and heterogenic steps (called polar processes)Radical Reactions- Initiationo Breaking of bond and creation of 2 radicals- Propagationo One radical is being consumed and Receiving of another radical- Terminationo Taking 2 radicals and making a