Chem 334 - Summer 1998 - Organic Chemistry I Dr. Carl C. Wamser

Chapter 5 - Stereoisomers

Stereochemistry

chemistry in three dimensions
includes both structure and reactivity effects

Enantiomers

mirror-image stereoisomers
like left and right hands
observed when a carbon atom has four different groups attached to it
e.g., CHXYZ or CX1X2X3X4

Chirality

property of having "handedness"(different from its mirror image)
a molecule with any element of symmetry (e.g., a mirror plane) must be achiral

Stereogenic Centers

chiral centers or stereocenters
a molecule with a stereogenic center (e.g., CX1X2X3X4) will be chiral

a stereogenic center cannot be:

sp- or sp2-hybridized (must be sp3)
an atom with 2 identical substituents
(e.g., any -CH2- group)

Properties of Enantiomers

enantiomers have identical physical and chemical properties, EXCEPT they

1) interact with another chiral molecule differently (like trying on left- or right-handed gloves - left and right hands react differently)

2) rotate the plane of plane-polarized light by equal amounts but in opposite directions

Optical Activity

chiral compounds rotate the plane of plane-polarized light
rotation measured in degrees clockwise (dextrorotatory or +)
or counterclockwise (levorotatory or -)

polarimeter - instrument for measuring optical activity

Specific Rotation

standard amount of optical rotation by 1 g/mL of compound in a standard 1 decimeter (10 cm) cell

[alpha] = alpha / l C

where [alpha] is specific rotation

alpha= observed rotation in degrees
l = path length in dm
C = concentration in g/mL

Absolute Configuration

nomenclature method for designating the specific arrangement of groups about a stereogenic center
differentiates between enantiomers using sequence rules and designations of priorities

Sequence Rules

assign priority based on the atoms directly attached to the carbons of the double bond

higher priority goes to:

higher atomic number
break ties by considering the next bonded atoms

R and S Designations

assign priorities 1-4 (or a-d) to the four different groups on the stereogenic center

align the lowest priority group (4 or d) behind the stereogenic carbon

if the direction of a-b-c is clockwise, it is R

if a-b-c is counterclockwise, it is S

Right- and Left-Hand Views

analogy - steering wheel

alternative analogy - your hands
assign priorities to your fingers in order of height
a = middle finger, b = pointer finger, c = thumb, d = wrist

R - this works for your right hand
S - this works for your left hand

Multiple Stereogenic Centers

compounds with more than 2 stereocenters have more than 2 stereoisomers
e.g., 2-bromo-3-chlorobutane

(2R,3R) and (2S,3S) are enantiomers
(2R,3S) and (2S,3R) are enantiomers

in general, n stereocenters give 2^n stereoisomers

Diastereomers

stereoisomers that are not enantiomers
e.g., (2R,3R) and (2R,3S)
(they are not mirror images, but they are not the same either)

diastereomers may have different chemical and physical properties

Meso Compounds

compounds with stereogenic centers but which are not chiral

e.g., (2R,3S)-2,3-dibromobutane
(same as its mirror image)

Identifying Meso Compounds

mirror plane of symmetry
one stereocenter is the mirror image of the other

cis-1,2-disubstituted cycloalkanes are meso if the two substituents are identical

Racemic Mixtures

an equal mix of both enantiomers (also called a racemate)
a common form in the laboratory (but not in nature)

optical resolution -

separating enantiomers from a mix (typically difficult)

Isomerism - Summary

isomers - same molecular formula (same collection of atoms used)

constitutional isomers - differ in the connections between atoms

different carbon skeletons
different functional groups
different locations of a functional group

Stereoisomers - Summary

stereoisomers - same connections but in different 3D arrangement

enantiomers - mirror-image stereoisomers

diastereomers - non-mirror-image stereoisomers, including cis-trans isomers

Stereochemistry in Reactions

achiral reactants always give optically inactive products

if chiral products are formed, they will be racemic

butane + Br2 --> 2-bromobutane (racemic)

intermediate 2-butyl radical can add Br to either side

but chiral reactants can give chiral products (the usual case in nature)