stereochemistry

Question 1

define isomer

Answer 1

molecules with the same molecular formula but a different structural formula

Question 2

when do structural isomers occur

Answer 2

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

Question 3

when do stereo-isomers occur

Answer 3

stereo-isomers occur when the order of the bonding is the same but the spatial arrangement of the atoms is different in each isomer

Question 4

what are the 2 types of stereo-isomer

Answer 4

geometric and optical

Question 5

when can geometric isomers occur

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

Question 6

some properties of geometric isomers are

Answer 6

♦ 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 7

what are some properties of optical isomers

Answer 7

♦ 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

Question 8

how do optical isomers occur

Answer 8

♦ occur in compounds in which four different groups are arranged tetrahedrally around a
central carbon atom (chiral carbon or chiral centre)

Question 9

what is a chiral carbon?

Answer 9

a carbon with 4 different groups attached to it

Question 10

what is an enantiomer?

Answer 10

a pair of optical isomers

Question 11

what does the term racemic mixture mean

Answer 11

if a mixture contains an equal amount of both enantiomers no rotation is observed