Stereoisomerism

Question 1

What is a sigma bond?

https://testbook.com/learn/chemistry-sigma-and-pi-bond/

Answer 1

Direct overlapping of orbitals and is one of the most strongest bonds

Has free rotation

Represented by ‘σ’

Question 2

What is a pi bond?

https://testbook.com/learn/chemistry-sigma-and-pi-bond/

Answer 2

Lateral overlapping of orbitals of 2 lobes of an atom to a different atom

Represented by ‘π’

Question 3

What is meant by a stereoisomer?

Answer 3

Compounds with the same molecular formula AND structural formula, but different arrangements of atoms in space

Question 4

Do sigma bonds and pi bonds have free rotation?

Answer 4

Sigma bonds have free rotation

Pi bonds do not have free rotation

Question 5

Why do alkenes exhibit E-Z stereoisomerism? [2 marks] MS

Answer 5

Because there is NO FREE rotation around a C=C…

…therefore the atoms attached to each carbon are in a FIXED position

Question 6

E/Z isomerism can occur when…

Answer 6

Restricted rotation around the C=C

Each C atoms of the C=C have 2 different groups

Question 7

How can you tell if a molecule is a Z isomer?

Answer 7

When the higher PRIORITY atoms attached to each C on the C=C is on the SAME side.

Or when the same functional group are on the same side

Z isomer
‘zame zide’

Question 8

What is the name for when the functional group is on the same side?

Answer 8

Cis-isomer

Question 9

What is the name for when the functional group is on the opposite side?

Answer 9

Trans-isomer

Question 10

How can you tell if a molecule is an E isomer?

Answer 10

When the higher PRIORITY atoms attached to each C on the C=C is on the OPPOSITE side - crosslinks.

Or when the same functional group is on the opposite side.

‘E’ for Enemy - opposite side.

Question 11

How would you work out which side chain has a higher priority?

If confused - https://www.youtube.com/watch?v=nx_Il1uFpKc

Answer 11

1) Work out the atomic number

Ar for A = 58
Ar for B = 27

2) The side chain with the atom with the highest Ar takes priority on that side of the atom.

A > B, therefore A has a higher priority

If each carbon in the double bond is bonded to (e.g. O) you cannot use that when working out priorities. Instead you look at the NEXT atom after O and find the Ar.

Question 12

What does the Cahn-Ingold-Prelog (CIG) Priority Rules suggest?

Answer 12

PRIORITY: H < CH₃ < CH₂CH₃

Highest value opposites: E “Enemies”
Highest value together: Z “zame zide”

Question 13

When would you know you have a stereoisomer?

Answer 13

If there is a C=C bond

Alkenes