Chemistry Reference
In-Depth Information
Diastereomers
are stereoisomers that are not mirror images of one another. They
may differ in all types of properties, and may be chiral or achiral.
Compounds with
n
different stereogenic centers may exist in a maximum of 2
n
forms.
Of these, there will be 2
n
/2 pairs of enantiomers. Compounds from different enantiomeric
pairs are diastereomers. If two (or more) of the stereogenic centers are identical, certain
isomers will be achiral. A
meso
form
is an optically inactive, achiral form of a compound
with stereogenic centers.
Tartaric
acid
, which has two identical stereogenic centers, exists
in three forms: the
R,R
and
S,S
forms (a pair of enantiomers) and the achiral
meso
form.
Stereoisomers may be classified as conformational or configurational, chiral or
achiral, and enantiomers or diastereoisomers.
The stereochemistry of organic reactions depends on the nature of the reactants.
Achiral molecules can react to give a chiral product. In such reactions, both enantiomers of
the product will always be formed in equal amounts. When chiral molecules react with
achiral reagents to create a new stereogenic center, diastereoisomers are formed in
unequal amounts.
A
racemic
form
is 50:50 mixture of enantiomers. It is optically inactive. A racemic
mixture of configurational isomers cannot be separated (resolved) by ordinary chemical
means (distillation, crystallization, chromatography) unless the reagent is chiral. One way to
separate a pair of enantiomers is to first convert them to diastereomers by reaction with a
chiral reagent, then separate the diastereomers and regenerate the (now separate)
enantiomers.
Learning Objectives
1.
Know the meaning of: chiral, achiral, enantiomers, plane of symmetry,
superimposable and nonsuperimposable mirror images, racemic mixture.
2.
Know the meaning of: stereogenic carbon atom, stereogenic center,
R-S
convention, priority order,
E-Z
convention, Fischer projection.
3.
Know the meaning of: diastereomer,
meso
compound, lactic acid, tartaric acid,
resolution.
4.
Know the meaning of: plane-polarized light, polarimeter, optically active or optically
inactive, observed rotation, specific rotation, dextrorotatory, levorotatory.
5.
Given the concentration of an optically active compound, length of the polarimeter
tube, and observed rotation, calculate the specific rotation. Given any three of the
four quantities mentioned, calculate the fourth.
6.
Given a structural formula, draw it in three dimensions and locate any plane of
symmetry.
7.
Given the structure of a compound, determine if any stereogenic centers are
present.
8.
Given the structure or name of a compound, tell whether it is capable of optical
activity.
9.
Know the rules for establishing priority orders of groups in the
R-S
convention.
10.
Given a compound with a stereogenic center, assign the priority order of groups
attached to it.
