3.3 - Stereo chemistry

Question 1

what are isomers

Answer 1

Molecules that have the same molecular formula but different structural formulae

Question 2

what are structural isomers

Answer 2

when the atoms are bonded together in a different order in each isomer

Question 3

what are stereoisomers

Answer 3

when the order of the bonding in the atoms is the same but the spatial arrangement of the atoms is different in each isomer

Question 4

what are the 2 types of stereoisomer

Answer 4

geometric and optical

Question 5

Geometric isomers:

Answer 5

 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

Question 6

Optical isomers:

Answer 6

 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