stereochem Flashcards
stereoisomers
molecules with the same connectivity of atoms but different spatial arrangements
types of stereoisomers
- Enantiomers: Non-superimposable mirror images; created by inverting all stereocenters. Always chiral.
- Diastereomers: Stereoisomers that are not mirror images; includes any stereoisomer that isn’t an enantiomer.
non superimposable mirror images
Mirror-image molecules that cannot be aligned perfectly; enantiomers fall into this category.
elements of stereochemistry
- Stereocenters: Usually sp³ hybridized carbons with four different groups attached.
- Alkenes: C=C double bonds in oppositely substituted even-sized rings; restricted rotation leads to cis/trans (E/Z) isomerism.
stereocenter
Typically an sp³ hybridized atom (often carbon) attached to four different groups.
what is the purpose of a stereocenter
They are the main contributors to chirality; inverting a stereocenter changes its spatial arrangement.
how do double bonds affect rotation in molecules?
C=C double bonds do not rotate freely like single bonds, leading to geometric isomers (E/Z or cis/trans).
what are oppositely substituted even sized rings
Rings with double bonds and substituents opposite to each other; important for identifying cis/trans isomerism.
how do you calculate the number of stereoisomers?
Use the formula 2^n, where 𝑛
n is the number of stereocenters or stereogenic elements (e.g., chiral centers or double bonds). DON’T FORGET TO SUBTRACT NUMBER BY 1, BECAUSE ORIGINAL MOLECULES DOESN’T COUNT
What is chirality
The property of a molecule lacking a mirror plane (asymmetrical), making it non-superimposable on its mirror image.
what are optically active molcules
chiral molecules that rotate plane-polarized light.
what are enantiomers
Always chiral molecules that have a chiral counterpart that is a mirror image.
what are meso compounds
Compounds that are achiral despite having stereocenters due to internal symmetry (mirror plane).
what are the rules for assigning R/S configuration using CIP rules?
- Prioritize Substituents: Based on atomic number; higher atomic number = higher priority.
- Multiple Bonds: Count as if bonded to additional atoms.
- Identical Atoms: Move one bond away to find the first point of difference.
NBE: Receives the lowest priority (0).
how do you determine the R/S configuration?
- Orient the molecule so that the lowest-priority group points backward.
- Trace from the highest priority (1) to third priority (3):
Clockwise (CW) = R
Counterclockwise (CCW) = S
what if the lowest prioirty isn’t in the back?
Newman Projections: View the molecule from different perspectives.
Break and Switch Method: Swap the lowest-priority substituent with one in the back, determine R or S, then switch the label.
Double Displacement: If a simple switch isn’t feasible, perform two swaps.
what are structural isomers?
Isomers with different connectivity but the same molecular formula.
what are stereoisomers
Isomers with the same molecular formula and connectivity, differing in spatial orientation (R/S, E/Z).
what are rigid structures in relation to double bonds?
Structures with rigid double bonds or rings that restrict rotation.
how do even sized rings exhibit stereoisomerism?
Even-sized rings can show cis/trans isomerism, especially with opposite substitutions.
what are the E/Z rules for alkenes?
E (entgegen): High-priority groups on opposite sides of the double bond.
Z (zusammen): High-priority groups on the same side.
how do you apply priority assignment for E/Z?
Use the same priority rules as R/S; determine if highest priority groups are on the same (Z) or opposite (E) sides.
what are the special cases for rings with hydrogen substituents?
Cis/trans naming applies when comparing H atoms across double bonds in rigid rings.
what is unique about meso compounds
Meso compounds could have multiple stereocenters but are achiral due to an internal mirror plane; they don’t have enantiomers
how do oppositely substituted rings behave?
They show stereoisomerism based on substituent position but lack true stereocenters due to symmetry.
what is a chiral center
A carbon with four distinct groups; it is the main cause of chirality.
what are useful methods for visualizing configurations?
Newman Projections: Visualize group arrangements on stereocenters.
Break and Switch: For quick orientation swaps, especially for identifying enantiomers.
Double Displacement Method: Use when the lowest-priority group is hard to position in the back.
do double bonds in small rings count as elements of stereochemistry?
NO!
is formation of a carbocation high or low energy?
forming a CC is an unstable process, so highest energy
delocalization contributes to
resonance stability, which makes for an easier and faster reaction
what do you need for something to be optically active?
stereocenters!
how to calculate the number of diastereomers
2^n - 2
units of unsaturation formula
(2n + 2) - # Hs - # halogens + # Ns
units of unsaturation combos
1 double bond = 1 UU
1 triple bond = 2 UU
1 ring = 1 UU
