ORGANIC CHEMISTRY - stereochemistry Flashcards
isomer definition
Molecules that have the same molecular formula but different structural formulae are called isomers.
what is a structural isomer
Structural isomers occur when the atoms are bonded together in a different order in each isomer
what are stereoisomers and what types are there
Stereoisomers occur when the order of the bonding in the atoms is the same but the** spatial arrangement of the atoms is different** in each isomer.
There are two types of stereoisomer, geometric and optical.
geometric isomers
Geometric isomers:
♦ can occur when there is** restricted rotation around a carbon-carbon double bond** or a carbon-carbon single bond in a cyclic compound
♦ must have** two different groups attached to each of the carbon atoms** that make up the
bond with restricted rotation
♦ can be labelled cis or trans according to whether the substituent groups are on the
same side (cis) or on **different sides (trans) **of the bond with restricted rotation
♦ have differences in physical properties, such as melting point and boiling point
♦ can have differences in chemical properties
optical isomers
Optical isomers:
♦ occur in compounds in which four different groups are arranged tetrahedrally around a central carbon atom (chiral carbon or chiral centre)
♦ are asymmetric
♦ are non-superimposable mirror images of each other
♦ can be described as enantiomers
♦ have** identical physical properties**, except for their effect on plane-polarised light
♦ have identical chemical properties, except when in a chiral environment such as that
found in biological systems (only one optical isomer is usually present)
♦ rotate plane-polarised light by the same amount but in opposite directions and so are
optically active
♦ when mixed in equal amounts are optically inactive because the rotational effect of the
plane-polarised light cancels out — this is called a racemic mixture
