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Chem 266: Stereochemistry Review Notes

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University of Waterloo
CHEM 266
Steve Forsey

 Conformations (rotamers) – temporary molecular shapes resulting from rotation around single bonds  Dihedral angle = angle between 2 groups can range anywhere from 0° to 180°  Staggered conformation – lowest in energy o Everything is furthest from each other (little steric interaction)  Eclipsed conformation – highest in energy o Steric interaction  Torsional strain – difference in energy between staggered and eclipsed conformations o Caused by eclipsing C-H bonds on adjacent carbons  there is rotation around single bonds, however they may be hindered due to repulsion of atoms (steric interaction) and torsional strain  anti conformation = dihedral angle of 180° o lowest energy conformation o maximum separation of methyl groups (furthest apart)  gauche conformation = dihedral angle of 60° o methyl groups are closer to each other o their electron clouds are repelling each other, causing an increase in energy o gauche interaction – a form of steric interaction (NOT the same as torsional strain)  angle strain = in cycloalkanes, having bond angle different than 109.5° Conformations of cyclohexane  chair o no angle strain – all bonds are ~109.5° o no torsional strain (all H’s are staggered; none eclipsed)  boat o no angle strain – all bonds are ~109.5° o less stable than chair conformation by 30 kJ/mol o 2 sources of torsional strain o 8 eclipsing H’s on bottom of the boat o H’s on either side of the ring experience flagpole interactions o Twist boat – can alleviate torsional strain by twisting  Boat conformation is much higher in energy than the chair conformation of cyclohexane 1 Axial-Equitorial Positions  Axial = bonds that go straight up and straight down  Equatorial = bonds at obtuse angles (>90°) from axial substituent o If axial bond is up, the equatorial bond will be below the horizontal plane o If axial bond is down, the equatorial bond will be above the horizontal plane o SUMMARY: each carbon will have 1 substituent above and below the horizontal plane  When splitting the chair structure with an imaginary vertical line down the middle, all substituents on the right of the structure point right, and all on the left point left ← axial ← equitorial > 90°  Larger groups (such as methyl) prefer to be equatorial rather than axial o As substituent gets larger, more likely to see it in equatorial form o This results in less steric (1,3-diaxial steric interaction / gauche (60°)) interaction, and is therefore lower in energy o When substituent equatorial, it is equivalent to an anti interaction (180°)  If bond is above the horizontal plane, it is indicated by a wedge on a flat 2D structure  If bond is below the horizontal plane, it is indicated by a dash on a flat 2D structure  Ring flip = conformational change where all C-C bonds rotate o Carbon 1 goes down from top right to bottom right o Carbon 4 goes up from bottom left to top left o KEY: all axial substituents in original structure become equatorial in flip chair! o KEY: all substituents retain their UP / DOWN orientation! 2 Isomers (different compounds with the same molecular formula) Structural Isomers Stereoisomers (constitutional isomers) (Isomers that have the same connectivity but (Isomers whose atoms have a different that differ in the arrangementof their atoms in connectivity) space) Enantiomers Diastereomers (Stereoisomers that are mirror reflections of each other) (a pair of stereoiomers which are not (an equal mixture of enantiomers is called a racemic mirror images of each other / are not mixture) enantiomers) Structural isomers  Alkanes with 3 or less carbons have no isomers (ex. methane, ethane, propane)  As the number of carbons increase, the number of isomers increase Stereoisomers  Ex. geometric isomers  cis- (Z) and trans- (E) molecules  wedge and dash (E) and (Z) System of Nomenclature  atoms with higher atomic number (greater atomic weight) receive higher priority  if atoms are the same, explore outward until the first point of difference o if there is no point of difference (atoms are the same), then there is no stereoisomerism  if a double bond is reached (for example, C=O), draw a pneumo
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