Introduction to Organic Chemistry: Isomerism - E/Z Isomers

Question 1

What is stereoisomerism?

Answer 1

• Stereoisomers have the same structural formula, but their atoms are arranged differently in place

Question 2

What is E/Z isomerism?

Answer 2

• E–Z isomerism is a form of stereoisomerism and occurs as a result of restricted rotation about the planar carbon– carbon double bond

Question 3

Explain why molecules with a C=C double bond are planar and have restricted rotation

Answer 3

• Carbon atoms in a C=C double bond and the atoms bonding to these carbons all lie in the same plane
• Described to be trigonal planar
• Atoms around C=C double bonds cannot rotate like they can around single bonds

Question 4

When do E/Z isomers occur in alkenes?

Answer 4

• E/Z stereoisomers are caused by the restricted rotation about a C=C double bond
• If both double-bond carbons have two different groups or atoms attached to them, you get E and Z isomers

Question 5

What are E isomers?

Answer 5

• E (entgegen) means opposite
• If the two substituents with the highest molecular mass are on different sides of the double bond (top/bottom), it is the E isomer

Question 6

What are Z isomers?

Answer 6

• Z (zusammen) means together
• If the two substituents with the highest molecular mass are on the same side of the double bond (top/bottom), it is the Z isomer

Question 7

What are the Cahn-Ingold-Prelog priority rules?

Answer 7

• Assign a priority to the two atoms attached to each side of the double bond (left/right)

• Look at atoms directly bonded to each of the C=C carbon atoms
• Atom with higher atomic number is given higher priority
• If the same, look at next atom in groups

• Work out if they are E or Z

• Look at how two highest priority groups are arranged
• If they’re both on same side of C=C bond (top/bottom), it is Z
• If they’re on different sides, it is E