Understanding Isomers and Stereochemistry PDF

🌈 9/8/24, 6:05 PM Understanding Isomers and Stereochemistry Isomers Isomers are molecules with the same chemical formula, but different structures. What are Enantiomers? 🤔 Enantiomers are...

🌈 9/8/24, 6:05 PM Understanding Isomers and Stereochemistry Isomers Isomers are molecules with the same chemical formula, but different structures. What are Enantiomers? 🤔 Enantiomers are non-superimposable mirror-image molecules. This almost always involves having a carbon atom bonded to four different groups in a tetrahedral geometry. The R,S Naming System 📚 The R,S naming system is used to distinguish one enantiomer from another. The steps to determine the R,S configuration are: Step 1: Find your stereocenter atom. Step 2: Prioritize the four appendages coming off of the stereocenter atom: Highest priority = highest atomic number. If there's a tie, keep going out, one bond at a time, until you break the tie. Step 3: Number your substituents 1, 2, 3, and 4 (1 = highest priority, 2 = second highest priority, etc.) Step 4: Point the lowest-priority substituent (priority #4) three-dimensionally away from you. Step 5: Make a circle from substituent 1 to substituent 2 to substituent 3: Clockwise = R Counterclockwise = S Multiple-Bonded Atoms 🌐 Multiple-bonded atoms are counted as the same number of single-bonded atoms. An Overview of Isomers 📊 Same Chemical Formula Same Bonding Connections Non-Superimp Isomers Yes No Constitutional Isomers Yes No Stereoisomers Yes Yes Enantiomers Yes Yes What are Diastereomers? 🤝 Diastereomers are stereoisomers that are not enantiomers. There are three kinds of diastereomers: 1. Cis/Trans Isomers of ringed compounds 2. Cis/Trans or E/Z Isomers of alkenes 3. Stereoisomers with multiple stereocenters that do NOT have exactly-opposite R,S configurations How Many Stereoisomers? 🤔 The number of possible stereoisomers a molecule can have is determined by the number of stereocenters: https://www.turbolearn.ai/content/f5527ac6-7869-49f5-a143-1b88ba57ccd3 1/3 9/8/24, 6:05 PM Understanding Isomers and Stereochemistry of possible stereoisomers = 2^n (n = # of stereocenters) Chirality (Optical Activity) 🔍 Molecules that have the ability to rotate plane-polarized light are said to be chiral or optically active. Molecules with stereoisomers are often chiral. Physical Properties 📊 Enantiomers Physical Properties Same Separation Difficult Meso Compounds 🌈 A meso compound is a molecule that has two or more stereocenters AND can be bisected by a line of symmetry. Meso compounds are achiral. Meso Compounds Ena Enantiomers Same as the original molecule Different from Achirality Achiral Fischer projections are a flat way of drawing molecules with stereocenters. Instead of using wedges or dashes, we draw a flat depiction. A Fischer projection is a two-dimensional representation of a molecule that depicts the horizontal bonds as coming towards the viewer and the vertical bonds as going away from the viewer. Converting Between Fischer Projections and Wedge-and-Dash Representations To convert a wedge-and-dash representation to a Fischer projection: Draw the molecule as a flat, two-dimensional structure Assume that the horizontal bonds are coming towards the viewer and the vertical bonds are going away from the viewer Example: CH₃CH₂CH₃ → CH₃CH₂CH₃ (Fischer projection) To convert a Fischer projection to a wedge-and-dash representation: Identify the horizontal and vertical bonds Use wedges and dashes to indicate the direction of the bonds in three-dimensional space Example: COH₂H → COH₂H (wedge-and-dash representation) 🔄 Stereochemistry: D vs. L Sugars 🔄 What are D and L Molecules? Glyceraldehyde is one of the structurally simplest sugars on earth, with two enantiomers: D-Glyceraldehyde L-G Configuration R https://www.turbolearn.ai/content/f5527ac6-7869-49f5-a143-1b88ba57ccd3 2/3 9/8/24, 6:05 PM Understanding Isomers and Stereochemistry D-Glyceraldehyde L-G Optical Rotation Dextrorotatory (clockwise) Levorot The D,L naming system was invented before the R,S naming system, so there is no direct connection between D vs. L and R vs. S. The D,L Naming System The D,L naming system is used to classify sugars based on the direction of the bottommost OH group along the spine of the sugar. D Sugar Bottommost OH Group Points to the right Examples: D-glucose L-fructose Categorizing Molecules as D or L Molecule COH₂H COH₂H Threose CO₂+H₃N Amino Acids S-alanine is an L amino acid. All naturally occurring amino acids are L amino acids. https://www.turbolearn.ai/content/f5527ac6-7869-49f5-a143-1b88ba57ccd3 3/3