Stereoisomerism

Question 1

What are stereoisomers?

Answer 1

Stereoisomers have the same structural formula but a different arrangement of the atoms in space.

Question 2

What are the two types of stereoisomerism covered in this content?

Answer 2

• E/Z isomerism
• Optical isomerism

Question 3

Under what conditions does E/Z isomerism occur?

Answer 3

• There is a C=C double bond
• Different groups attached to each carbon atom of the double bond

Question 4

Why does stereoisomerism occur around double bonds?

Answer 4

Rotation about the double bond is restricted, fixing the groups attached to each carbon atom relative to each other.

Question 5

Which compound can form E/Z isomers: but-1-ene or but-2-ene?

Answer 5

But-2-ene can form E/Z isomers.

Question 6

What are the structures of the E and Z isomers of but-2-ene?

Answer 6

• (E)-but-2-ene
• (Z)-but-2-ene

Question 7

What is cis-trans isomerism?

Answer 7

Cis-trans isomerism is a special case of E/Z isomerism where one of the attached groups on each carbon atom of the double bond must be hydrogen.

Question 8

How are the cis and trans isomers defined in but-2-ene?

Answer 8

• The cis isomer is the Z isomer
• The trans isomer is the E isomer

Question 9

When can the cis-trans system of naming be used?

Answer 9

The cis-trans system can only be used when each carbon atom in the double bond is attached to a single hydrogen atom.

Question 10

What is the Cahn—Ingold—Prelog priority rule?

Answer 10

In this system, atoms attached to each carbon in a double bond are given priority based on their atomic number.

Question 11

What does it indicate if the groups of higher priority are on the same side of the double bond?

Answer 11

The compound is the Z isomer.

Question 12

What does it indicate if the groups of higher priority are diagonally placed across the double bond?

Answer 12

The compound is the E isomer.

Question 13

What is the first step in assigning priority according to the Cahn—Ingold—Prelog rules?

Answer 13

Examine the atoms attached directly to the carbon atoms of the double bond and decide which has the highest priority based on atomic number.

Question 14

What should be done if the two atoms attached to a carbon atom in the double bond are the same?

Answer 14

Find the first point of difference; the group with the higher atomic number at this point is given higher priority.