[13.2] stereoisomerism

Question 1

what is stereoisomerism?

Answer 1

molecules with the same molecular and structural formulas but a different arrangement of the atoms in space

Question 2

what type of stereoisomerism do alkenes exhibit and how is this possible?

Answer 2

• alkenes exhibit a type of stereoisomerism called E/Z isomerism
• this arises because rotation about the double bond is restricted

Question 3

what is needed for stereoisomerism?

Answer 3

2 identical groups on both sides of the C=C

Question 4

what is E isomerism?

Answer 4

when two groups are on opposite horizontal sides ie. one is on top half, other is on bottom

• E means entgegen (opposite)
• eg. E 1,2 - dichloroethene
• draw an example

Question 5

what is Z isomerism?

Answer 5

when two groups are on the same horizontal side eg. both are below the C=C

• Z means zusammen (together)
• eg. Z 1,2-dichloroethene
• draw an example

Question 6

what does cis mean?

Answer 6

if there are two hydrogens on the same horizontal side, write cis- at the start of the name

• draw an example (cis-1,2-dichloroethene)

Question 7

what does trans mean?

Answer 7

if there are two hydrogens on different horizontal sides, write trans- at the start of the name

• draw an example (trans-1,2-dichlorothene)

Question 8

how do you decide if it is E/Z isomerism if both sides are not identical?

Answer 8

• draw a vertical line through the C=C
• on each side, the one with the higher atomic number has higher priority
• determine if it is E or Z isomerism from this