Isomers CHEM 352

isomers

different structure, same molecular formula

stereoisomers

different orientations, same connectivity, similar properties

(differ in spatial arrangement of atoms, rather than order of atomic connectivity)

constitutional isomers

same molecular formula, different connectivity

chirality

molecule cannot be superimposed on its mirror-image

enantiomer

non-superimposable mirror-image molecules, similar physical properties

all chiral centers are inverted

diastereomer

non-superimposable, non-mirror-image

cis/trans

only some chiral centers are inverted

optical activity

ability to rotate plane polarized light

chiral center

sp³ hybridized → tetrahedral

4 electron groups, rankable by atomic number

Finding R or S

1. Prioritize groups by higher atomic number

2. Put “4” in back (dash)

3. Count 1,2,3 R (clockwise) and S (counter clockwise)

Fisher Projections

most useful for drawing molecules with multiple chirality centers such as sugars and can quickly access stereoisomeric relationships

skeleton with a bow tie

horizonal lines (coming out of page) and vertical (back into page)

E/Z

used for alkenes

each sp² hybridized C has two diff groups

“z” same side

• occurs cus double C=C bonds can’t rotate

cis/trans

cis (same) vs trans (opposite)

used for alkenes

each sp² has only 1 H and a diff. group

• used when there are common substituents on adj. C’s

plane of symmetry 

if a molecule has this, it is achiral 

%ee

(observed/specific) * 100% = more R/S than R/S, depending on the sign matching

observed = mixture specific rotation

specific = of either the R or S compound

• will never be more than 100%

• the specific rotation of R and S will be the same number but different signs

racemic mixture

50:50 mixture of two enantiomers

cancel each other out and the mixture is optically inactive