Isomerism: Chirality, Enantiomers, and Diasteriomers

exam 2, chem 233, WVU fall '26

Chiral center: definition

atom bonded to four different groups

Achiral: definition

atom bonded to at least two equivalent groups

Enantiomers: definition

mirror images; can’t line up on top of each other on a 2D plane

Stereoisomers include [ ] and [ ]

enantiomers, diasteriomers

Stereoisomers: definition

same connectivity but different arrangement in space (ex. one is a wedge and the other is a dash)

If molecules only differ in their R/S, E/Z, or cis/trans orientation, then they are stereoisomers

true

Cis/trans isomers and double bond isomers are always diastereomers

true

Same connectivity but opposite R/S assignment

enantiomers

Diastereomers: definition

molecules share one, but not all, chiral centers with identical R/S assignment

Meso compound: definition

enantiomer with a plane of symmetry

E: definition

trans

Z: definition

cis

Number of possible stereoisomers: equation

2ⁿ, n = number of chiral centers

When assigning R and S, the lowest priority group should be a __

dash

To make the lowest priority group a dash,

rotate about the axis

Clockwise

R

Counterclockwise

S

If you swap two groups, the enantiomer will change from R to S (or vice versa)

true

In meso compounds, you can’t rotate a sigma bond to make molecules symmetrical

false

Meso compounds must be 50% R and 50% S

true

To assign E and Z,

divide the molecule in half, cutting perpendicular to the C=C bond

Priority groups on the same side

Z

Priority groups on opposite sides

E

Stereocenter: definition

an atom where swapping two attached groups produces a different stereoisomer

Stereocenters may contain [ ], while chiral centers can only contain [ ]

double bonds, single bonds

Cahn–Ingold–Prelog convention

highest atomic number gets highest priority