Study Guide for Exam II
● Converting Dash/Wedge cyclohexane into a chair conformation:
○ If dashed substituents, the substituents should be pointing down in a chair conformation ○ If wedged substituents, the substituents should be pointing up in a chair conformation ● Chirality
○ Two molecules are identical (conformers) if models of the two can be superimposed without breaking any bonds
○ Mirror images (enantiomers): look like reflections of one another
■ Ex. change from R configuration to a S configuration
○ Chiral not identical to its mirror image (nonsuperimposable on its mirror image) ○ Achiral superimposable on its mirror image and has a plane of symmetry (meso compound; reviewed later in this study guide)
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● Steps for assigning configurations:
○ Assigning priority 14, 4= lowest priority
○ Usually assigned by atomic number (higher atomic number=higher priority) ○ If 13 is going clockwise, the configuration is R. If 13 is going counterclockwise, the configuration is S.
● Reminder: A stereocenter is bonded to 4 DIFFERENT groups.
● When the lowest group, (#4) is not dashed, the configuration is switched. For example, if the configuration from 13 is R but #4 is wedged, the configuration will be S instead. ● For IUPAC naming for simple molecules, R/S in parentheses appear first in the name. If the molecule has two different stereocenters with different configurations, the R/S appear in parentheses next to their specific orientation.
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○ Ex: (S)1Bromo1chloroethane; (R)3Bromo(S)chloropentane
● Double and triple bonded substituents are treated differently
○ C=C is treated as two CC bonds
○ Carbon triple bonded to Carbon is treated as two CCC bonds.
■ Not superimposable
■ A mirror image
■ Not superimposable
■ Not a mirror image (usually a change to one stereocenter instead of all) ● Meek compound
○ Has chiral centers that is achiral
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○ Meso compound only has a mirror plane if it is in eclipsed conformation ● Fischer Projections
○ Drawing a molecule with straight horizontal and vertical lines. Horizontal lines are wedged bonds (coming towards you) and vertical lines are dashed bonds (going away from you)
● To calculate how many stereocenters a compound will have= 2 to the nth power (n= number of chiral centers)
● To calculate how many pairs of enantiomers the compound will have= 2 to the nth power divided by 2.
● Conformers can be interconverted by a single bond without breaking that bond. (Alternative of the same molecule)
● Constitutional Isomers have the same molecular formula but different atom connectivity ● Stereoisomers have the same molecular formula and the same connectivity but cannot be superimposed or interconverted via a single bond rotation
● For enantiomers, both chiral centers will have opposite configuration names and for diastereomers, only one chiral center will have opposite configuration names. Resonance Structures (Review)
● Two or more forms of the same molecule, but changing the placement of electrons ● Curved arrow always shows the flow of electrons We also discuss several other topics like What did muhammad wanted to remove out of the kaaba?
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● Three basic moves that are allowed for resonance structures=
○ Double bond to a lone pair
○ Lone pair to a double bond
○ Moving the double bond
● The structure becomes less important when it contains too much charges ● Don’t forget the OCTET RULE!
● More stable when the negative charge is on the more electronegative atom, and the more polar atom.