Stereochemistry
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Specific attention has been paid to nomenclature to state the exact spatial relationship
between substituents on a carbon – spatial relationship lends different chemical and
physical properties to a molecule.
The chemistry concerned with the 3D structural arrangement of the atoms in a molecule
is called Stereochemistry
Stereochemistry and Carbon
- think about the relationship between your two hands – they're made up of the same
components (thumb and fingers, palm), but they are not identical – they're mirror images
of one another.
- Mirror images cannot be rotated to be super-imposable upon one another.
- Molecules that have the same composition (isomers) but are not super-imposable upon its
mirror image are enantiomers.
- In our chemistry, results when you have a tetrahedral carbon bound to 4 separate
substituents:
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This property is known as chirality:
o A chiral molecule forms when the molecule and mirror image cannot be superimposed: they do not contain a plane of symmetry:
o Plane of symmetry is a line that transects the object so one half is same as other.
o Flask has plane, hand, screw, shoe do not
A chiral atom most commonly C with 4 substiturents, and a carbon bound this way is
known as a chiral center, or stereocenter.
Chirality is a molecular property, a stereocenter is the cause of chirality:
Optical Activity
- Plane polarized light
o Light rotates in an infinite # of planes perpendicular to the direction of travel
o when passed through a polarizer, only light of a single plane passes through: plane
polarized: example
- Example of something of everyday use that is polarized? (sunglasses – vertical slits don't
allow horizontal light to pass – therefore glare reduced) HTML page
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By rotation of 2 polarizers, we can determine the extent of rotation that has occurred:
using a polarizer (1st filter) and an analyzer (2nd filter).
- First is  – a degree measurement – determines the extent of rotation of the light, looking
from observer to analyzer filter to point where light passes through 2nd filter – if the
polarizer is turned to the right (clockwise) – dextrorotary (D); to the left (counter
clockwise) – levorotary
- By convention, Levorotary(L) is negative (-) and Dextrorotary is positive (+)
MOVIE
Rotation Data
- what can  tell us?
- The extent of rotation depends on more than just the structure of the molecule – it
depends upon the number of molecules that the light will interact with – so more
molecules either by increasing concentration or increasing the depth of the sample, the
larger the rotation ie. Increase concentration 2X, a increases 2X
- So set of standard conditions are therefore set by convention : known as the specific
rotation []D
- []D = observed (rotation)/ l (dm) c (g/mL) = /lc when wavelength of 598.6 nm
(sodium line) is used.
- This is a physical constant characteristic of the compound
Enantiomers
- optical isomer
- discovered by Pasteur (1849) when noticed 2 types of crystals formed in tartaric acid,
from an optically inactive solution; separate the crystals; redissolved; and the 2 resultant
solutions were both active
- Identical physical properties, different optical properties
How do we name chiral centers?
- Uses similar set of rules as for E-Z nomenclature
1. priority ot atomic number
2. look at 2nd , 3rd and 4th atoms outward to find first difference
3. multiple bonds equalivalent to same number of single atoms
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number them from highest to lowest (1-4)
Put bond four into background (into page), and result in a three spoke wheel; place
position 1 at top (12 o'clock)
If 1 to 2 to 3 turns right, it is R (rectus latin right)
If 1 to 2 to 3 turns left, it is S (sinister latin left)
(+) and (-) optical rotation and R, S are NOT RELATED
Diasteromers
- examples shown previously have single chral centers – what happens when you have
more than one chiral center?
- 2 amino 3 hydroxy butanoic acid (tartaric acid)
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results in two paris of enantiomers (2 sets of mirror images): how do we describe the 2
pairs which are not mirror images? These are diasteromers
Enantiomers are opposite at ALL stereocenters; Diasteromers are opposite at only some
sterocenters
What happens is the steroisomer has a plane of symmetery?
- not an optically active compound as it is achiral
- called meso compounds – sterocenters exist, but are not optically active
Biological compounds are specific:
Cholesterol:
Racemic mixtures
- mixture which is a 50:50 mix of both steroisomers.
- Denoted by (+/-) or d,l
- Most racemic mixtures resolved by generating an acid base reaction to form a salt.
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Steroisomers have different properties: Table 6.3 text
Relationship between Types of Isomerism: