Isomerism

Question 1

Explain why E/Z isomerism is shown in some alkenes.

Answer 1

-Restricted rotation about C=C bond
-if there are different groups attatched to each carbon, the molecules will have a different arrangement of atoms in space

Question 2

State and explain the cause of the bond angle around a C=C bond

Answer 2

120*
There are three bonding regions which repel each other equally so they are as far apart as possible

Question 3

Why do some straight chain alkenes not show E/Z isomerism?

Answer 3

Because groups on one (or more) carbons on each C=C bond are identical

Question 4

What is meant by ‘stereoisomers’?

Answer 4

Molecules with the same structural formula but different arrangements of atoms in space.

Question 5

What is meant by the term ‘structural isomers’?

Answer 5

Molecules with the same molecular formula but different structural formulae.

Question 6

What determines Cahn-Ingoldt priority of a group?

Answer 6

1. Atomic number
2. If atomic numbers equal, then length of carbon chain.

Question 7

Explain why an E -stereoisomer is an E -stereoisomer in terms of Cahn-Ingoldt priority of groups.

Answer 7

• On the left Carbon on the C=C bond, there will be 2 attached groups. If one has a higher atomic number than the other, it will take priority, otherwise, priority will be given to the group with the longest carbon chain.
• If high priority groups are on opposite sides compared to eachother, it will be an E- stereoisomer.

Question 8

Why is cyclohexene only found as a Z- stereoisomer?

Answer 8

If cyclohexene was a Z-stereoisomer, it would break the cyclic structure of the molecule.