chem231 - ch.5 - stereochemistry

constitutional isomers

same molecular formula, different atom connectivity

isomer

different compounds of the same molecular formula

stereoisomers

same molecular formula, different orientation in space

enantiomers

non-superimposable mirror images (mirror images)

• exist in pairs

• is chiral

  • ex. hands

• can’t move which C it’s on

chiral

a chiral object (molecule) is chiral if it has an enantiomer

• has:

  • chiral center

  • no plane of symmetry

  • axial chirality, planar chirality, inherent chirality

achiral

not chiral

• identical, superimposable

• no chiral center

chirality

the phenomenon where objects are non-superimposable mirror images

diastereomers

stereoisomers that are not enantiomers

• may/may not be chiral

• not mirror images

stereocenter

a point where switching 2 groups would lead to a different molecule

• many diff types, we focus on chiral centers

chiral center

an atom (usually C) bearing four different groups

• chiral centers are stereocenters

• sp² carbons NEVER chiral centers

• if any 2 substituents have switched → opposite configurations

enantiomer methods

1. same molecule, switched dash & wedge

2. mirror images

3. R/S configuration

• when a bond is dash/wedge, assume hydrogen (if there) is the opposite wedge/dash

enantiomers: method 1

molecules are exact same, but all wedges & dashes switched

enantiomers: method 2

molecule are mirror images

• not flipped

  • if flipped, wedge → dash, vice versa

enantiomers: method 3

all chiral centers have opposite R/S configuration

R/S steps

• assign Cahn-Ingold-Prelog priorities of all atoms directly bonded to chiral center according to atom #

  • 1 high → 4 low

• lowest priority substituent (H) in back & connect 1 → 2 → 3

  • if in front, do it normal but switch R & S

  • if in plane, consider 3D orientation

• label chiral center R or S

  • R = clockwise

  • S - counterclockwise

R/S 4th sub in plane

consider 3D orientation

• look down bond from C w/ H behind it (NP)

• rotate bond

• “switch” method

R/S: method 1

look down bond from C w/ H directly behind chiral center

• like Newman projection

R/S: method 2

rotate bond so 4th priority subst. (H) isn’t in plane anymore

R/S: method 3

“switch” method

• assign priorities

• switch 2 substituents & switch other 2 substituents

  • any 2 groups

  • want H in back (dash)

• assign R/S as usual

racemic mixture

when there are no wedges or dashes drawn, molecule represents racemic mixture (50:50) of enantiomers

• half R

• half S

R/S 2 same atom

continue along the chain until you get a point of difference

• double bonds count twice

• triple bonds count thrice

diastereomers

a stereoisomer that is not an enantiomer

• must have multiple chiral centers OR

• an alkene

diastereomers: multiple chiral centers

• identical → same configurations

• enantiomer → opposite at chiral centers (dash/wedge)

• diastereomer → at least 1, but not all chiral centers opposite (dash/wedge)

diastereomers: alkene

designate as E, Z or neither

• Z → same “ze same”

• E → opposite “e-opposite”

meso compound

has a superimposable mirror image & are achiral

• has

  • chiral center (but not chiral)

  • plane of symmetry

• 2 chiral centers, but molecule itself is not chiral b/c internal plane of symmetry

enantiomer properties

• identical physical & chemical properties except they rotate plane polarized light in opposite directions (optical rotation)

• behave differently in chiral environments (enzymes)

diastereomer properties

• different physical properties (similar / drastically different)

• utilize these diff physical properties to separate diastereomers

  • boiling point

  • melting point

  • solubility

  • etc

Fischer projections

Emil Fischer (1902)

• a way to look at carbohydrates/sugars in the highest E conformation

  • skeletal → lowest E conformation

reading FP

• | horizontal → wedge

• — vertical → dash

converting FP

• turn counterclockwise

converting FP to skeletal

to skeletal

• vertical axis → plane (solid line)

• 1 side is dashed, the other is wedged

• “tip” on its side

• want carbons in zig zag formation

converting NP to FP

• make everything eclipsed w/ FP

• push from/back carbon to top/bottom so sides point towards you

  • point ↑ → push ↑

  • point ↓ → push ↓

FP R/S

stereochemistry flow chart