Org 2 - Stereochemistry

Question 1

Define Stereochemistry

Answer 1

The study of the arrangement of atoms in a molecule, in 3 dimensions

Question 2

What are Isomers ?

Answer 2

Two different molecules with the same number and type of atoms (e.g. the same molecular formula)

Question 3

Explain the two main categories of Isomers

Answer 3

1. Structural (constitutional) isomers differ by the way their atoms are connected
2. Stereoisomers (spatial isomers) differ in the way their atoms are arranged in space

Question 4

Name 3 types of structural Isomers

Answer 4

1. Structural Isomers e.g. difference in the chain/skeleton
2. Functional Isomers - same molecular formula but different functional groups present e.g. Alcohol and Ether
3. Positional or RegioIsomers are structural Isomers where the functional group changes position on the parent structure

Question 5

What are the two classifications of Spatial/Stereo Isomers ?

Answer 5

1. Enantiomers - two non superimposable molecules, which are mirror images of each other
2. Diastereomers - any pair of stereoisomers that are not enantiomers

Question 6

Why do Cis/Trans Isomers occur ?

Answer 6

Carbons that are in a ring or double bond structure are not able to freely rotate resulting in cis and trans compounds.

Question 7

Explain Cis and Trans Isomers, and describe which is more stable

Answer 7

• When the substituents are on the same side of the ring or double bond, it is CIS. When they are on opposite sides, it is TRANS
• In TRANS isomers, the substituents are further apart, electron shell repulsion is minimised and the molecule is more stable

Question 8

Explain Alkene E-Z notations for geometric stereoisomers

Answer 8

Particularly useful for Isomeric compounds with 4 different substituent groups bonded to the double bond, and more complex molecules

Question 9

Explain how to assign E-Z notation to geometric/spatial stereoisomers

Answer 9

1. Assign priorities to each substituent at the double bond
2. If the two groups of higher priority are on the Epposite sides = E (Entgegen)
3. If the two groups of higher priority are on the Zame side = Z (Zusammen)

Question 10

What are StereoIsomers ?

Answer 10

• Different compounds with the same structure, but differing in the spatial orientation or configuration of the atoms
• They can be Enantiomers or Diastereomers

Question 11

What is a Chiral molecule ?

Answer 11

A molecule that is non-superimposable on it’s mirror image

Question 12

What is a Chiral atom ?

Answer 12

An atom that is bonded to four different substituent groups

Question 13

What are Enantiomers ?

Answer 13

• Enantiomers come in pairs, they are two non superimposable mirror images of each other.
• To have an Enantiomer, a molecule must be chiral

Question 14

What are the physical properties of Enantiomers ?

Answer 14

Enantiomers have the same chemical and physical properties, the only difference is their interaction with other chiral molecules, and the rotation of PPL.

Question 15

What is a Fischer projection ?

Answer 15

• Used mainly for Carbohydrates
• A 2-D way of observing 3-D structures

Question 16

What is a racemic mixture ?

Answer 16

A 50:50 mixture and L and D Enantiomers, it will show no rotation of Plane Polarized Light.

Question 17

What are Meso compounds ?

Answer 17

Achiral (optically inactive) diastereomers of chiral stereoisomers.

An internal plane of symmetry exists by drawing a line that will cut the molecule in half (MeSo = Mirror of Symmetry)

Question 18

What are Ligands ?

Answer 18

Atoms or groups attached to a central carbon

Question 19

What are Conformational Isomers ?

Answer 19

Isomers which differ by rotation about single bonds, they are not true Isomers as they are different spatial orientations of the same molecule.

Question 20

Name 3 types of Conformational Isomer ?

Answer 20

1. Eclipsed - maximally close
2. Anti/staggered - maximally apart
3. Gauche Formation - anywhere in between !

Question 21

Explain which is the most stable type of Conformational Isomer, and why

Answer 21

Anti formation is the most stable, as it minimises electron shell repulsion.