11.3 Isomerism Flashcards
What is an isomer
Molecules that have the same molecular formula but whose atoms are arranged differently
What are the two types of isomers
Structural and stereoisomerism
What are structural isomers
What are the 3 types
They have the same molecular formula but a different structural formulae
There are 3 types of structural isomers:
Chain isomerism
Positional isomerism
Functional isomerism
What is positional isomerism
Where the same functional groups are attached to the main chain at different points.
For example
1-Chloropropane and 2-chloropropane
So the chlorine functional group is attached to the main chain at different points.
C3H7Cl
What is functional group isomerism
Where the functional groups are different between two isomers but they have the same molecular formula
Eg Cyclopentane and 2-pentene
Because the formula is both C5H10
Also, ethanol and methoxymethane (an ether) An ether is two alkyl groups bonded by an oxygen
Or with an aldehyde to a ketone, when the double bonded oxygen goes from the end to the middle
What is chain isomerism
Different arrangement of hydrocarbon chain (eg branching)
So the length of the longest chain may change so there will be a completely different name
Eg butan-1-ol and 1-methylpropan-1-ol
What is stereoisomerism
What are the two types
Where 2 or more compounds have the same structural formula but have a different arrangement of bonds in space
Eg EZ isomerism
And optical isomerism
What is EZ isomerism
It tells us about the positions of substituents at either side of a carbon=carbon double bond
If the substituents are at the same side of the bond, it is Z
If the substituents are opposite each other it is E
Why are there no isomers of a single carbon bond
Why are there isomers around a double carbon bond
. Substituent groups joined by a single carbon bond can rotate around the single bond so there are no isomers
. There is no rotation around a double carbon bond, so z and e isomers are separate compounds and aren’t easily converted into the other.
How do you know if molecule shows cis or trans isomerism if all 4 molecules are different
Use CIPR rules
So look on one side of the double bond, and look at the atomic number of the atoms directly bonded to the carbon
So eg if the carbon bonds to CH3, and H look at the C on it which has atomic number 6, and H has atomic number 1
The atom directly bonded to it that has the highest atomic number is the priority group.
So on the other side of the double bond, repeat, so you find both priority groups
If the priority groups are opposite each other across the double bond, it is E isomerism
If the priority groups are both at the bottom or both at the top, it is Z isomerism
What is an enantiomer
Enantiomers are a pair of molecules that exist in two forms that are mirror images of one another but cannot be superimposed one upon the other.
What is an asymmetric carbon atom
A carbon that has 4 different atoms bonded to it
