Stereochemistry
-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:
-This property is known as chirality:
- A chiral molecule forms when the molecule and mirror image cannot be super-imposed: they do not contain a plane of symmetry:
- Plane of symmetry is a line that transects the object so one half is same as other.
- 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
- Light rotates in an infinite # of planes perpendicular to the direction of travel
- 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
-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
- priority ot atomic number
- look at 2nd , 3rd and 4th atoms outward to find first difference
- multiple bonds equalivalent to same number of single atoms
-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)
-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.
-Steroisomers have different properties: Table 6.3 text
Relationship between Types of Isomerism: