Chapter 5

Chiral Molecules
 Stereosiomers are molecules
 Same formula and connectivity
 Differ by arrangement in space only
 Molecules are non-superimposable
Not identical
Achiral and Chiral Carbon
 Tetracarbon with 4 different atoms/groups attached
 chrial center or chiral carbon
Test for Chirality: Planes of
 mirror plane: an imaginary plane that bisects a
molecule in such a way that two halves of the
molecule are mirror images of each other
 Molecules with plan of symmetry are achiral
Chiral objects
Chiral Molecules
 Has no plane of symmetry
 Have non-superimposable mirror image
Chiral Molecules
 Molecules that have no plane of symmetry
 Molecules that have non-superimposable mirror image
 Your left hand and right hand
Subdivision of Isomers
 Enantiomers: stereoisomers whose molecules are
nonsuperimposable mirror image of each other
 Exact MW, BP, MP ect …except how it rotates in polarized
 Diastereoisomers: stereoisomers whose molecules are not
mirror image of each other
Nomenclature of Enantiomers:
The R,S-System
 Each chiral carbon in a chiral molecule is designated a “R”
(Rectus: right) or “S” (Sinitrus: left) configuration
 Rules: Assign priorities to atoms directly attached to chiral
carbon based on Atomic number
 # 1 = highest atomic #
 # 4 = lowest atomic #
Nomenclature of Enantiomers:
The R,S-System
 If there’s a tie (e.g two carbons) move to the next carbon atom
until tie is broken
 If there’s a double bond = 2 single bonds
 With the lowest priority group (#4) point away from you,
rotate 14
 Clockwise  R configuration
 Counterclockwise  S configuration
Nomenclature of Enantiomers:
The R,S-System
 Vinyl group,
isopropyl group
is of higher priority than the
 Assign (R) or (S) designation to each of the following
Molecules with Multiples
Chiral Centers
 The maximum number of stereiosomers can be
 2n
n = number of chirality centers
Naming compounds with multiple
chirality center
 Followed same rules for naming R,S
 Designate position of chiral carbon
Molecules with multiple chirality
 Draw alll possible stereoisomers and provide an appropriate
name for each stereoisomer
Fisher Projection Formulas
 Show three dimension structure
 Compound with several chiral centers
 Used carelessly, these projection formulas can easily lead to
incorrect conclusion
 Draw all stereoisomers of
Meso compounds
 A structure with two chirality centers does not always have
four possible stereoisomers.
 achiral occurred within chiral molecules when carbon is
Look for internal plane of symmetry
Properties of Enantiomers
 Recall: the molecules of enatiomers are not superposable
one on the other
 Enantiomers have identical M.P, B.P
 Different physical properties and direction which they rotate
plane-polarized light
Same amount in rotation but opposition direction
 Different reaction rate when interact with another chiral
Plane-Polarized light
 When regular light beam is
passed through a polarizer, all of
the light waves, except those
whose electromagnetic fields
ossicllate in a single direction,
are filter out
  plane-polarized light
  optically active
Plane-Polarized light
 A substance that is rotated in a
clockwise rotation
 α (measured of degree) is
positive (+)
 Dextrorotatory
 A substance that is rotated in a
counterclockwise rotation
 α is negative (-)
 levorotatory
Specific Rotation
Specific Rotation
 Depends on the temperature and the wavelength of light
that is employed
Specific Rotation
 The direction of rotation of plane-polarized light is often
incorporated into the names of optically active compounds
 No obvious correlation exists between the (R) and (S)
configurations of enantiomers and the direction ([(+) or (-)] in
which they rotate plane-polarized light
Racemic Mixtures
 An equimolar mixture of two enantiomers is called a
racemic mixture (racemate, racemic form)
 No net rotation of plane-polarized light
 50:50 mixture
 Resulted from chemical reaction of achiral molecule
Enantiomeric excess
 A sample of an optically active substance that consists of a
single enantiomers is said to be enantiomerically pure or to
have an enantiomeric excess of 100%
Resolution: Separating
 A common way of separation uses the conversion into
diastereomers, that are not mirror images of each other
 Recrystallization or chromatography
Resolution: Separating
Stereoisomerism of cyclic
 Is trans-1,2-dimethylcyclopentane superposable on its mirror image
 Is cis-1,2-dimethylcyclopentane superposable on its mirror image?
 Is cis-1,2-dimethylcyclopentane a chiral molecule?
 Would cis-1,2-dimethylcylcopentane show optical activity?
 What is the stereoisomeric relationship between 1 and 3
Cyclohexane Derivative
Compounds with Chirality
Centers Other than Carbons