Chirality and Molecules

Question 1

What determines a molecules properties?

Answer 1

The structure
- types and arrangement of atoms
- electron density between surrounding atoms
- shape

Question 2

What is a popular qualitative explanation of molecular structure based on geometry?

Answer 2

VSEPR

Question 3

Hybridization of single, double, triple bonds in carbon

Answer 3

Single - sp3 (mixing AO’s, four identical hybrid orbitals)
Double - sp2 (1 s and 2 p)
Triple - sp (1 s and 1 p)

Question 4

Isomers

Answer 4

Compounds with same molecular formula

Question 5

Constitutional (structural) isomers

Answer 5

Have same formula but very different atomic connectivity
(different physical/chem. properties)

Question 6

Stereoisomers

Answer 6

Have same molecular formula and same connectivity but their 3D (spatial) arrangement of atoms differs

Question 7

Two categories of stereoisomers

Answer 7

Conformational stereoisomers: isomers that can be interconverted without breaking any bonds (eg rotating, sometimes called rotamers)

Enantiomers (mirror images that are not identical, or non-superimposable)

Diastereomers (not mirror images and are non-superimposable)

Question 8

Staggered (conformational isomer)
Eclipsed (conformational isomer)

Answer 8

Staggered = lower in energy
Eclipsed = higher in energy

Question 9

What do tetrahedral presentations (wedges and dashes) provide information on?

Answer 9

Stereochemistry and conformation

Question 10

Enantiomers

Answer 10

Pair of molecules that are non-superimposable mirror images of each other

Any molecule not superimposable is chiral, any molecule that is superimposable is achiral

Have identical chem/phys properties

Question 11

What is the proper term if a source of chirality is at a tetrahedrally coordinated atom?

What if it is not a tetrahedrally coordinated atom?

Answer 11

Tetrahedrally coordinated atom = stereogenic centre or stereocentre (chirality is property of the whole molecule)

If not a tetrahedrally coordinated atom, then source of chirality is called a stereogenic element

Question 12

What stereochemistry are all natural animal sugars?

What stereochemistry are all animal amino acids?

Answer 12

D- sugars
L- amino acids

Question 13

How are enantiomers labelled with plane polarized light?

Answer 13

If enantiomerically pure sample rotates light:
clockwise = (+) = d
counter-clockwise = (-) = l

Question 14

When do enantiomers have different chemical properties?

Answer 14

When reacting with another achiral species or in the presence of a chiral additive/solvent/catalyst

Ingesting chiral molecules can lead to different biological effects from the two enantiomers

Question 15

What are the two theories of how specific enantiomers come to dominate in living things on earth

Answer 15

1) Pure chance, just happened that the first replicating molecule had a stereocentre

2) Rotating magnetic field (earth) can interact with high energy particles from the sun to create circularly polarized light which will destroy one enantiomer more quickly than another (causing small ee)

Question 16

Stereogenic axis (axial chirality)

Answer 16

Orthogonal pi-systems with the two groups on the same side being different in each case (can replace one, or both, pi bonds with rings)

Question 17

Biaryls

Answer 17

Two ortho substituted rings connected by a C-C bond (no longer co-planar)
Called atropisomers when the two enantiomers cannot convert at room temperature because of the magnitude of Ea

Question 18

Stereoplane (planar chirality)

Answer 18

Restricted rotation around a plane
Atropisomers where enantiomers are conformational (not configurational)

Question 19

Helicity (helical chirality)

Answer 19

Helix is non-superimposable
Clockwise = P
Counterclockwise = M

Question 20

How many stereoisomers can a compound have (relation to stereocentres/stereogenic elements)

Answer 20

Max of 2^n stereoisomers

Question 21

Molecule with the same relative stereochemistry will have the same ____

Answer 21

Properties

Question 22

Are meso forms chiral?

Answer 22

Nope
Have an internal plane of symmetry

Question 23

Pseudoasymmetric centres

Answer 23

Only stereocentres if the configurations on the surrounding atoms are opposite
eg. C3 is only a stereocentre (denoted s or r) if C2 and C4 have opposite configurations