Introduction To Organic: Isomerism

Question 1

What are stereoisomers?

Answer 1

Stereoisomers have the same structural formula, but their atoms are arranged differently in space. One type of stereoisomerism is E/Z isomerism, which you see in molecules with C=C double bonds.

Question 2

How do carbon double bonds contribute to rigidity?

Answer 2

An important thing about C=C double bonds is that atoms can’t rotate around them like they can around single bonds. In fact, double bonds are fairly rigid — they don’t bend much. Things can still rotate about any single bonds in the molecule though.

Question 3

How are stereoisomers caused?

Answer 3

The restricted rotation around the C=C double bond in alkenes causes a type of stereoisomerism called E/Z isomerism. You can have an E or a Z isomer

Question 4

How do the CIP rules work?

Answer 4

A molecule that has a C=C bond surrounded by four different groups still has an E- and a Z-isomer — it’s just harder to work out which is which.
Fortunately, you can solve this problem using the Cahn-Ingold-Prelog (CIP) priority rules:
• Start by assigning a priority to the two atoms attached to each side of the double bond. To do this, you look at the atoms that are directly bonded to each of the C=C carbon atoms.
• The atom with the higher atomic number on each carbon is given the higher priority.
• If the atoms directly bonded to each carbon are the same, then you look at the next atom in the groups to work out which has the higher priority.
• To work out which isomer you have, just look at how the two highest priority groups are arranged. If they’re positioned across the double bond from each other, you have the E-isomer. If they’re both above or below the double bond, you have the Z-isomer.

Question 5

What are structural isomers?

Answer 5

Structural isomers have the same molecular formula, but a different structural formula. There are 3 types of structural isomers — chain isomers, position isomers and functional group formula (i.e the atoms are connected in different ways).

Question 6

What are chain isomers?

Answer 6

Chain isomers have the same functional groups but different arrangements of the carbon skeleton. Some are straight chains and others are branched

Question 7

What are functional group isomers?

Answer 7

These have the same atoms formed into different functional groups i.e a carbon double bonds is converted into a cycle molecule

Question 8

What is a positional isomer?

Answer 8

The functional group is attached to the main chain at different points