Stereochemistry

Question 0

What is a constitutional isomer?

Answer 0

Same molecular formula but different patterns of covalent bonding. They have different IUPAC names, and have a cis or trans relationship.

Question 1

What is an isomer?

Answer 1

Same molecular formula but different chemical structures

Question 2

What is a stereoisomer?

Answer 2

Same patterns of covalent bonding but different spatial arrangements of their atoms/functional groups.

Question 3

What is an enantiomer?

Answer 3

Optical isomers (non-superimposable) that are mirror images of each other.

Question 4

What are diastereomers?

Answer 4

Optical isomers (non superimposable) that are not mirror images of each other.

Question 5

Enantiomers rotate plane polarised light. If rotated anti-clockwise, is it L or D?

Answer 5

L and can also be notated with a “-“ sign

Question 6

Enantiomers rotate plane polarised light. If rotated clockwise, is it L or D?

Answer 6

D, can also be noted with a “+” sign.

Question 7

If you have equal amounts of both D and L enantiomers in a mixture, what is it called?

Answer 7

Racemic mixture.

Question 8

What is a meso isomer?

Answer 8

A compound that has two or more chiral centres, however it has a plane of symmetry making it an achiral compound.

Question 9

How do you assign priority groups?

Answer 9

Highest priority is the highest atomic number. Lowest priority is the lowest atomic number.
ONLY INCLUDE THE ATOM DIRECTLY ATTACHED TO THE CARBON, NOT THE WHOLE GROUP.

Question 10

What if any of the atoms attached the carbon are the same?

Answer 10

Include the next atom along as well.

Question 11

When assigning priority groups, what do you do with multiple bonds, such as C=C?

Answer 11

They are treated as having the equivalent number of single bonded species, so C=C would be C-C-C

Question 12

If the priority groups go in a clockwise order, is it R or S?

Answer 12

R

Question 13

If the priority groups go in an anti-clockwise order, is it R or S?

Answer 13

S

Question 14

When assigning R or S, how do you look at the compound?

Answer 14

The lowest priority group, usually hydrogen, is projected backwards away from you.

Question 15

What are conformational isomers?

Answer 15

Stereoisomers that have different spatial arrangements of their atoms, that are interconverted by rotation around single bonds.

Question 16

Why is the rotation around a single bond restricted?

Answer 16

There is an energy barrier associated with bond strain to convert from one conformer to another.

Question 17

What are energetically preferred conformers called?

Answer 17

Totamers.

Question 18

In open chain molecules, what are the most energetically favourable and unfavourable conformers and why?

Answer 18

Favourable: Those with staggered bonds. Atoms are far apart as possible, repulsive forces are minimised and there is maximised stability.
Unfavourable: Those with eclipsed bonds. Atoms are closer, repulsive forces increased and there is reduced stability.

Question 19

In non-aromatic cyclic molecules, what is the most energetically favourable and unfavourable forms, and why?

Answer 19

Favourable: Chair: Staggered bonds and minimal torsional strain.
Unfavourable: Boat: Eclipsed bonds, increased torsional strain and is less stable as steric interaction is being produced.

Question 20

How do geometric isomers arise? What are the two types of geometric isomers?

Answer 20

When a group of atoms are arranged asymmetrically around a double bond.
E- isomer (trans)
Z- isomer (cis)

Question 21

Why are E and Z isomers not readily interconverted?

Answer 21

This required the pi bond to be broken so that the groups at either side of the now sigma bond can rotate. Then, the pi bond is reformed. The whole process is energetically unfavourable.

Question 22

Give examples of topological isomers.

Answer 22

Nucleic acids, proteins and polysaccharides
Alpha-helices
These biomolecules are polymers involving chiral molecules . Their macromolecular forms can also be chiral and form helices.

Question 23

When naming molecules, what is the difference between alpha and beta?

Answer 23

Alpha: Going backwards. Bonds below the plane.
Beta: Towards you. Bonds above the plane.

Question 24

How do you see if a compound is optically active?

Answer 24

Should have a chiral carbon and shouldn’t have a plane of symmetry.

Question 25

What is torsional energy?

Answer 25

Energy required to rotate C-C bond. Strain is encountered when a conformer rotates away from most stable form due to repulsive forces between electrons in C-H bonds as they pass each other during rotation.

Question 26

What does equatorial mean?

Answer 26

Hydrogens approximately in the same plane as the ring.

Question 27

What does axial mean?

Answer 27

Hydrogens out of the plane of the ring.

Question 28

What is chair interconversion?

Answer 28

One chair can adopt another chair conformation by undergoing a ring flip. Hydrogens interconvert between axial and equatorial orientations.

Question 29

If a compound has two stereocentres, how many isomers will it have?

Answer 29

4.

Square the number of stereocentres.

Question 30

What is the difference between erythro and threo?

Answer 30

Erythro: Compounds that have similar groups on the same side of the molecule.
Threo: Compounds that have similar groups on opposite sides.

Question 31

When naming R and S enantiomers, what happens if there are two chiral carbons in the compound?

Answer 31

Look at them separately and decide if each one is R or S.

Then it will have the configuration of SS, RR, RS or SR.