Kaplan MCAT Organic Chemistry Chapter 2: Isomers – Flashcards
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Structural isomers
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also called constitutional isomers. Least similar of the isomers. The only thing they share is their molecular formula.
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Stereoisomers
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have same molecular formula. Share the same atomic connectivity. Stereoisomers differ in how the atoms are arranged in space. Any isomer that is not a structural isomer falls under this category.
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Two classes of stereoisomers
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Conformational and configurational
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Conformational isomers
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differ in rotation around a single bond . Same molecule but at a different place in their free rotation.
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Anti-staggered conformation in Newman projection
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the two largest groups are antiperiplanar, in the same plane but on opposite sides. lowest energy - preferred
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Gauche comformation
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two largest substituents are 60 degrees apart
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Eclipsed conformation
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two largest substituents are 120 degrees apart
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Totally eclipsed conformation
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largest substituents are 0˚ apart
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What happens in a chair flip?
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all axial groups become equitorial and all equitorial groups become axial. All dashes remain dashes and all wedges remain wedges large, bulky groups prefer equatorial position
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What position in a chair do bulky groups favor?
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equitorial positions
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Configurational Isomers
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can only change from one form to another by breaking and reforming bonds. Two categories: Enantiomers and Diasteriomers
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Enantiomers
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configurational isomers that are nonsuperimposable mirror images of one another. Have opposite configurations at every chiral center. have same chemical and physical properties different optical activity and reactions in chiral environments
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Diasteriomers
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configurational isomers that are are not mirror images of each other. Differ at some but not all chiral centers for configuration. different chemical and physical prope
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Racemic mixture
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when both enantiomers are present in equal concentrations, the rotations cancel each other out and no optical activity is observed
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Priority in regards to E/Z
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higher atomic number means higher priority. If atomic numbers are equal, priority is determined by the next atoms outward.
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Describe a fischer-projection
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horizontal lines indicate bonds that project outwards (wedges) and vertical lines inidicate bonds going into the plane (dashes)
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angle strain
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bond angles deviate from their ideal values by being stretched or compressed
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torsional strain
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cyclic molecules must assume conformations that have eclipsed or gauche interactions
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nonbonded strain (van der Waals repulsion)
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nonadjacent atoms or groups compete for same space
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What is the most stable conformation for cyclohexane?
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chair
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Chirality
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if its mirror image can't be superimposed lacks internal plan of symmetry
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Number of possible stereoisomers
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for n chiral centers, there are 2^n possible stereoisomers
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Physical properties
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no change in composition of matter ex: boiling point, melting point, solubility, odor, color, density
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Chemical properties
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reactivity of molecule, resulting in change in composition usually because of functional groups
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meso compounds
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molecule with chiral centers that has an internal plane of symmetry not optically active because there is a plane of symmetry
