Stereoisomerism and Chirality

conformations

different 3D shapes a molecule can adapt by rotating around single (sigma) bonds—-no bonds are broken

newman projections

look. atconformation sby looking straight down the bond axis

staggered conformation

• H atoms are as far apart as possible

• minimal electron repulsion

• most stable conformation

• all negihtboring H atoms are at 60 degrees

eclipsed conformation

• hydrogen overlap when viewed down the bond

• increased electron repulsion

• less stable

• all neighboring Hydrogen atoms are aligned

What conformation (staggered and eclipsed) is high energy vs. low energy?

Spaced (staggered)
Lies (low)
Ease (energy)

Eavesdropping (eclipsed)

Heats (high)

Everything (energy)

anti-conformation

• form of staggered conformation

• groups are opposite (180 degrees apart)

• most stable

• THINK: “distance makes the heart grow fonder”

gauche conformation

• form of staggered conformation

• groups are next to each other

• less stable because of steric hindrance

  • steric hindrance: slowing down/prevention of a chemical reaction because of bulky or crowded molecules involved

• groups are 60 degrees apart

eclipsed

• groups align

• very high energy

• 0 degrees apart

chair conformation

• most stable in molecules like cyclohexane

• no angle strain or torsional strain

  • angle strain: increase in potential energy in a molecule because of bond angles deviating from ideal values

  • torsional strain:: increase in energy in a molecule because of repulsion between electron clouds which are separated by 3 bonds

    • occurs when in eclipsed conformation

• substituents are axial (up and down) or equatorial (around the belt)

• THINK: if you’re in a chair, you have no discomfort and are just chilling

boat conformation

• hydrogens clash and there are flagpole interactions

• less stable in molecules like cyclohexane

• THINK: if you were squeezed in a boat, you’d be rocking back and forth and uncomfortable

constitutional isomers

same molecular formula but are connected differently

• THINK: fraternal twins who wear same outfits but go to different clubs (chess vs. nightclub)

stereoisomers

same formula and same connections but different arrangements in 3D space

• THINK: identical twins everyone mixes up—until they talk. One’s bubbly and the other is emo

what are the 2 main types of stereoisomers?

enantiomers and diastereomers

enantiomers

a pair of molecules that are mirror images of each other but can’t be placed on top of each other

• same everything but opposite vibes

• R spins right

• S spins left

• have identical physical properties (melting point and boiling point)

• differ in ways they interact with chiral things (enzymes, receptors, light)

  • THINK: can’t tell them apart until they enter a chiral club—one stands in line and other is VIP

diastereomers

same molecular formula and connectivity but differ in spatial arrangements

• have different properties

meso compounds

has a stereocenter but isn’t chiral because of internal symmetry

what is the family tree of isomers and how are they all related?

chiral center

(aka a stereocenter) a carbon bonded to 4 different groups

• makes 2 non-superimposable mirror images

checklist for knowing if it is a chiral center

• Is it bonded to 4 other atoms/groups?

• Are all 4 groups different?

• Is it sp³ hybridized? (aka only has single bonds?)

• Is there no symmetry surrounding it?

If all are true, it is a chiral center and is asymmetric

What is not a chiral center?

• Carbon in double bond or triply bond (sp² or sp)

• Carbon bonded to 2 of same group

• Carbon In a symmetrical environment