Stereochemistry - Lecture 1

isomer

molecules with the same molecular formulas but a different arrangement of atoms

two classes of isomers

structural isomers and stereoisomers

structural isomers

molecules with the same molecular formula but different atom connections

name the two types of structural isomers

chain isomers

position isomers

Chain isomers

different arrangement of a molecules carbon skeleton

position isomers

The differing position of the same functional group in the molecule.

stereoisomers

Compounds with the same structural formula but with a different arrangement of the atoms in space.

two types of stereoisomers

conformational isomers and configurational isomers

conformational isomers

Stereoisomers that differ by rotation about one or more single bonds, usually represented using Newman projections.

they rapidly interconvert by rotation around a C-C single bond

configurational isomers

cannot be interconverted without breaking bonds

two types of configurational isomers

geometric isomers (E/Z isomers)

Optical isomers (isomers with chiral centres)

geometric isomers

have different substituents around a double bond

optical isomers

differ by the placement of different substituents around one or more atoms in a molecule

they are either enantiomers or diastereomers

what is a chiral object? what does it result in?

an object which cannot be superimposed on its mirror image by any translations or rotations

results in two versions of the same molecule

what are the two versions of the same molecule know as

enantiomers

biological importance, give an example

they may have the same physical properties but different biological properties

e.g. (+) limonene and (-) limonene

different smells because they bind differently to nasal receptors

pharmacological importance

2 enantiomers can have completely different pharmacological properties/ effects

it can even happen that one is biologically active and one is toxic

there can also be difference in pharmacological activity caused by the binding of the molecule - different spacial arrangement = different binding

what does a wedged dash indicate

the substituent is coming out towards the the viewer

what does a dashed wedge indicate

the substituent is facing away from viewer

optical activity

a compound is optically active when in solution it is capable to rotate the plane of polarized light either to the right or left

what is it called when it is rotated to the right in optical activity

dextrorotary

what is it called when it is rotated to the left in optical activity

levorotary

how is the optical activity of a compound measured

by the determination of its specific optical rotation [a]20D using a polarimeter and applying the following equation

[a]20D = 1000a/ l . c

what does a stand for in the equation

optical rotation - whats measured

what does l stand for in the equation

length of tube (light path) in decimeter

what does c stand for in the equation

conc g/l (whats prepared)

what does 20 stand for in the equation

operating temp in degrees C

what does D stand for in the equation

the spectrum of sodium light (589.3nm)

what is a chiral carbon

a carbon atom which is attached to 4 different types of atoms or groups of atoms

it is a SATURATED carbon - i.e. NO double bonds