Stereoisomerism (pages 97- 99) Flashcards
Can double Bonds rotate?
No, doulbe bonds can’t rotate.
Carbon atoms in a C=C doulbe bond and the atoms bonded to these carbons all lie in the same plane (they’re planar), why?
because of the way they’re arranged. They’re actually said to be trigonal planar - the atoms attached to each double-bonded carbon are at the corners of an imaginary equilateral triangle. (see diagram 1 on page 97 - you will see the bond angles in the planar unit are all 120°).
look at the diagram 1 on page 97, and you will see Ethene, C2H4 is completely planar but in larger alkenes (in diagram 2), only the >C=C< unit is planar.
Another important thing about C=C double bonds is that atoms can’t rotate around them like they can around single bonds, why?
because of the way the p orbitals overlap to form an π bond (see page 96). In fact, double bonds are fairly rigid - they don’t bend much either.
Even though atoms can’t rotate about the double bond, things can still rotate about any single bonds in the molecule.
The restricted rotation around the C=C double bond is what causes alkenes to form stereoisomers. (see diagram 3 on page 97) - you will see that both the molecules have the structural formula CH3CHCHCH3. The restricted rotation around the double bond means you can’t turn on into the other so they are isomers).
What are E/Z isomerisum?
it is a type of Stereoisomerism.
1) Stereoisomers have the same structural formula but a different arrangement in space.
2) because the lack of rotation around the double bond, some alkenes can have stereoisomers.
3) stereoisomers occur when the two double-bonded carbon atoms each have two different atoms or groups attached to them.
4) one of these isomers is called the ‘E-isomer’ and the other is called the ‘Z-isomer’, (hence the name E/Z isomerism).
5) The Z-isomer has the same groups either both above or both below the double bond, whilst the E-isomer has the same groups positioned across the double bond.
(remember when you’re naming steroisomers, you need to put ‘E’ or ‘Z’ at the beginning of the name)
Look at diagram 4 on page 97 of Example: But-2-ene.
What is an easy way to work out which isomer is which regarding E/Z isomerism?
remember that in the Z isomer, the groups are on ‘ze zame zide’,
but in the E isomer they are ‘enemies’
What do the E/Z isomerism stand for?
Z stands for ‘Zusammen’ - the german word meaning ‘together’.
E stands for ‘Entgegen’ - the german word meaning ‘opposite’
The E/Z system works even when all the groups are different, How?
When the carbons on either end of a double bond both have the same groups attached, then it’s easy to work out which is the E-isomer and which is the Z-isomer (like the example on page 97).
It only starts to get problematic if the carbon atoms have 3 or 4 different groups attached.
Using the Cahn-Ingold-Prelog (CIP) rules
Who are the three clever people who created the Cahn-Ingold-Prelog (CIP) rules?
Mr (Robert Sidney) Cahn,
Mr (Christopher Kelk) Ingold
Mr (Vladimr) Prelog
they came up with a solution to work out which is the E-isomer and which is the Z-isomer for any alkene.
What are the rules of the Cahn-Ingold-Prelog (CIP)
1) Look at the atoms directly bonded to each of the C=C carbon atoms. the atom with the high atomic number on each carbon is given the higher priority.
Example (diagram 1 on page 98).
one of the stereoisomers of 1-bromo-1chloro-2-gluoro-ethene:
The atoms directly attached to carbon- are bromine and chlorine. Bromine has an atomic number of 35 and chlorine has and atomic number of 17. So bromine is higher priority group.
The atoms directly attached to carbon-2 are fluorine and hydrogen. Fluorint has an atomic number of 9 and hydrogen has an atomic number of 1. So fluorine is the higher priority group.
2) Now you can assign the isomers as E- and Z- as before, just by looking at how the groups of the same priority are arranged.
(see diagram 2 on page 98 for example).
In the diagram, this steroisomer of 1-bromo-1chloro-2 fluoroethene, the higher priority groups (bromine and fluorine) are positioned across the double bond from one another. So it’s the E-isomer. see diagram for Z-isomer).
3) Be careful if you’re doing this for an alkene with only 3 different groups. The E/Z system gives the positions of the highest priority group on each carbon, which aren’t always the matching groups. (see diagram 3 on page 98 of E-2-chlorobut-2-ene Z-2-chlorobut-2-ene).
If the atoms directly bonded to the carbon are the same then you have to look at the next atom in the groups to work out which has the higher priority with the CIP rules, explain how?
The carbon (diagram 4 on page 98) is directly bonded to two carbon atoms, so you need to go further along the chain to work out the ordering.
The methyl carbon is only attached to hydrogen atoms, but the ethyl carbon is attached to another carbon atom. So the ethyl group is higher priority.
E/Z Isomers can sometimes be called Cis-Trans Isomers why?
If the carbon atoms have at least one group in common (like in but-2-ene), then you can call the isomers ‘cis’ or trans’ (as well as E- or Z-
What do ‘cis’ mean in Cis-Trans Isomers?
cis means the same groups are on the same side of the double bond.
what do ‘trans’ mean in Cis-Trans Isomers?
trans means the same groups are on opposite sides of the double bond.
e.g.
E-but-2-ene can be called trans-but-2-ene, and Z-but-2-ene can be called cis-but-2-ene
look at diagram 1 on page 99. The H atoms are on opposite sides of the double bond, so this is trans-1-bromopropene.
remember: cis means on the side side while trans means across.
If the carbon atoms both have totally different groups attached to them, the cis-trans naming system cant cope, why?
the cis/trans naming system doesn’t work because the carbon atoms have different groups attached so there’s no way of deciding which isomer is cis and which isomer is trans.
look at diagram 2 on page 99.
The E/Z system keeps on working through - in the E/Z syste, Br has a higher priority the F, so the names depend on where the Br atom is in relation to the CH3 group. see diagram 3 on page 99.
Why is an ehtene molecule said to be planar?
because there is no free rotation around the carbon-carbon double bond.
