E-Z Notation

Question 1

what is the problem that comes with using the cis-trans notation

Answer 1

it only works for some compounds

Question 2

why doesnt the cis-trans notation work with:
H Cl H Br
| | | |
C=C C=C
| | | |
F Br F Cl

Answer 2

• there are 4 different groups
• so the idea of two identical groups being in a cis-trans relationship doesnt work
• as there are not two groups that are the same

Question 3

what system would we use instead rather then cis-trans notation

Answer 3

the E-Z system

Question 4

what is the first step when using the E-Z system

Answer 4

• work out the part of the name that can be used for both isomers
• by using the normal nomenclature rules

Question 5

what would the common name between these two isomers

Answer 5

1-bromo-1-chloro-2-floroethene

Question 6

what is the second step in the system

Answer 6

using the priority rules to decide which of the two toms on the left hand side of the double bond has higher priority

Question 7

how is priority decided in this case

Answer 7

• it is decided by the atom with the higher atomic number
• H=1 and F=9
• so F has the higher priority

Question 8

what is step 3

Answer 8

• do the same thing for the atoms on the right hand side of the double bond
• Cl = 17 and Br = 35
• so Br has higher priority

Question 9

what is the final step of the E-Z system

Answer 9

decide whether the two higher priority atoms are at opposite ends if the molecule (across and up / down) or not

Question 10

what kind of isomer do you have if the higher priority atoms are at opposite ends of each other

Answer 10

an E-isomer (equivalent of trans)

Question 11

what kind of isomer do you have if the higher priority atoms are at opposite sides but no opposite ends of each other

Answer 11

a Z-isomer (equivalent of cis)

Question 12

what would be the isomer of the first and second molecule previously shown

Answer 12

• the first one would be a Z-isomer
• the second would be an E-isomer
• as in the first the F and Br are only at opposite sides
• whereas in the second they are are opposite ends

Question 13

what is a way of remembering the difference between E and Z isomer

Answer 13

• the E in E-isomer stands for enemy
• as the high priority atoms are enemies with each other
• ans therefore want to stay as far away from each other as possible
• so they would be at opposite ends
• Z-isomers are just the other option

Question 14

where do the letters E and Z come from in the first place

Answer 14

• the letter E used comes from the german word entegen
• which means opposite
• the letter Z used comes from the german word zusammen
• which means together (opposite sides but shoulders touching kinda thing)

Question 15

what is the definition of structural isomer

Answer 15

• compounds with the same molecular formula

- but with different structural formulae

Question 16

what is the definition of a stereoisomer

Answer 16

• compounds with the same molecular and structural formula

- but with a different arrangement of atoms or groups in a 3 dimensional field

Question 17

what is the definition of a geometric isomer

Answer 17

• compounds containing a C=C bond

- with atoms or groups attached at different positions

Question 18

what is the definition of restricted rotation

Answer 18

when the atoms or groups bonded to a C=C bond have their postions fixed