Ch. 5 - Isomerism 2

what are enantiomers?

configurational isomers that are non-superimposable mirror images

what are diastereomers?

configurational isomers that are not mirror images

what are two ways that diastereomers can arise?

1) different configurations about a double bond

2) different about one or more chiral centers

what makes a molecule chiral?

if it has an enantiomer, meaning it has four different substituents about its stereocenter (if it has one)

how can we predict if a molecule is chiral?

if it doesn’t have a plane of symmetry (in ANY conformer (remember that single bonds can rotate!!))

what is a chiral/stereocenter?

a tetrahedral atom bonded to 4 different substituents

true or false: the presence of a chiral center guarantees chirality

false; it can be achiral if it has a plane of symmetry

why do we have R and S configurations?

because stereocenter configuration changes depending on the orientation of the bonds (AKA whether they are dash or wedge and which atoms are dash or wedge)

when you would reverse the configuration you first obtained?

when the lowest priority subst. is in the front of the plane (typically H)

what should you do if the lowest subst. is in the plane?

freeze the other planar bond in place and rotate the subst. until the lowest priority is in the back (dash bond)

what configuration is R? S?

clockwise

counterclockwise

what to do to find the enantiomer?

take the mirror image OR exchange substituents

what to do to find diasteromers?

exchange one chiral center

what can you do to produce the opposite stereochemical configuration with a Fisher projection?

exchange any 2 substituents, take a mirror image, or rotate the projection 90 degrees

the left of a zigzag corresponds to what part of the Fisher projection?

top

for the peak of a zigzag, which side of the Fisher projection is the wedge bond on?

the left

for the valley of a zigzag, which side of the Fischer projection is a wedge bond on?

right

what is the only difference between enantiomers

their optical activity - optical rotation of plane polarized light

what is levorotatory?

light rotated left

what is dextrorotatory?

light rotated to the right

what is a racemic mixture?

50/50 mixture of enantiomers that is optically inactive

what is specific rotation

a standard that represents the degree to which light will rotate for a pure enantiomer

what is the formula for specific rotation?

what is optical purity (enantiomeric excess)

fraction of a mixture that is not racemic and contributes to plane polarized light rotation

how to calculate ee?