Lecture 11Outline of Last Lecture:I. Cyclohexane Conformation continuedOutline of Current Lecture:I. Chair Flip reviewII. Converting 2D to 3D images (or vice versa)III. Disubstituted CyclohexanesIV. Polycyclic MoleculesV. Stereochemistry and Tetrahedral Centers introChair Flip Review:-chair flip: not the whole molecule! The flip is INTERNAL1-3, diaxial interactions:-torsion: due to substituents on adjacent carbons (twist)-steric: due to substituents on non-adjacent carbons-When determining conformation of substituents, the bigger substituent “wins” axial vs. equatorial dilemma (want to be equatorialConverting 2D to 3D images (or vice versa): Chem 261 1st Edition-On the left hand of the above image, a cyclohexane with cis-face substituents is drawn in two different ways (2D on the top, 3D on the bottom). Note how the cis-methyl groups are drawn as substituents in the 3D image. -On the right hand side of the above image, a cyclohexane with trans-face substituents is drawn in two different ways, 2D on the top and 3D on the bottom. Note how the trans-methyl groups are oriented as substituents in the 3D image. Disubstituted Cyclohexanes:-Monosubstituted cyclohexanes are always most stable with their substituent in the equatorial position-But, disubstituted cyclohexanes are more complex than this; the steric effects of the substituents must be accounted for when determining the ideal conformation of the molecule.Book example: Compare the two isomer conformations of cis-1,2-dimethylcyclohexane and trans-1,2-dimethylcyclohexane -To determine the preferable conformation, the energy costs of the interactions (gauche, 1,3-diaxial, etc.) must be added together. Polycyclic Molecules:-Polycyclic molecules= less flexible. Flips and rotations are less frequent. Have a “fused ring” structure. Chem 261 1st EditionStereochemistry and Tetrahedral Centers Intro:Types of isomers…Constitutional isomer: same formula, different connectionsStereoisomers: same connections, different spatial arrangementsDiastereoisomers: non-mirror images, not super-imposableEnantiomer: mirror images, non-superimposable Chem 261 1st
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