Stereochemistry

Question 1

What are isomers?

Answer 1

Molecules with the same molecular formula but a different shape

Question 2

What are stereoisomers?

Answer 2

Molecules with the same molecular formula but a different arrangement of atoms in space

Question 3

What is sp2 hybridisation?

Answer 3

Where the s and p orbitals in a molecule overlap to form 3 combined sp2 orbitals and a half filled p orbital, this enables a C=C bond to be formed, the half filled p orbital combines with a half filled p orbital from the other carbon to form the pi bond that is above and below the molecule

Question 4

How do you determine the priority of a group that is attached to a carbon atom?

Answer 4

The atom that is attached to the carbon that has the highest atomic mass, if the first atom attached is the same then you look at the next atom attached and see which has the highest atomic mass

Question 5

How do you determine whether a molecule is an E or Z isomer?

Answer 5

You determine which is the highest priority group on each of the carbons, circle each one. If these are on the same side of the double bond then its a Z (cis) isomer, if they’re on opposite sides then it’s an E (trans) isomer

Question 6

When isotopes of the same element are attached to the same carbon, which isotope is higher priority?

Answer 6

The isotope with the larger atomic mass

Question 7

How can sp3 carbon atoms give stereoisomers?

Answer 7

4 different groups must be attached to the central carbon atom, the two stereoisomers will be non superimposable mirror images of each other

Question 8

How do you determine whether enantiomers are R or S?

Answer 8

• The lowest priority group must be drawn at the back of the molecule
• Ensure that the molecule that you have drawn with the lowest priority group drawn at the back has the other groups arranged in the same order as the original drawing
• Assign priority to the other groups with 1 being the highest
• If the groups are 1,2,3 clockwise its an R enantiomer
• If the groups are 1,2,3 anticlockwise its an S enantiomer

Question 9

How is optical activity measured?

Answer 9

• Normal light is passed through a polariser
• Plane-polarised light that only vibrates in one plane is passed through a solution of the enantiomer
• The enantiomer will either rotate the light clockwise or anticlockwise
• A polarimeter is used to measure the optical activity

Question 10

What is specific rotation?

Answer 10

• Specific rotation = Observed rotation ( ° ) / concentration (g/ml) x path length (dm)
• [ α ] = α / c x l

Question 11

What is the specific rotation of a racemic mixture?

Answer 11

0

Question 12

How is a racemic mixture formed?

Answer 12

A racemic mixture is formed when a chiral compound is formed from an achiral starting molecule, both enantiomers are formed in equal amounts

Question 13

How do you calculate the amount of stereoisomers a compound will have based on how many chiral centres it has?

Answer 13

2^n , n= no. of chiral centres

Question 14

What is a diastereoisomer?

Answer 14

Stereoisomers that are not enantiomers

Question 15

What are meso-isomers?

Answer 15

Where the mirror image of the molecule is identical to the original molecule, this means a meso-isomer is a form of a diastereoisomer and they only occur when the original molecule has 2 identical chiral centres

Question 16

What is a staggered isomer?

Answer 16

An isomer with 2 chiral centres that when you look down the C-C central bond, you can see the hydrogens on the back carbon
- Hydrogen atoms on adjacent carbons are as far away from each other as possible

Question 17

What is an eclipsed isomer?

Answer 17

• An isomer with 2 chiral centres that when you look down the C-C central bond you can’t see the hydrogens on the back carbon as the hydrogens on the front carbon eclipse them
• Hydrogen atoms on adjacent carbons are as close together as possible

Question 18

What is the angle of torsion for a staggered isomer?

Answer 18

60°, 180°

Question 19

What is the angle of torsion for an eclipsed isomer?

Answer 19

0°, 120°

Question 20

When drawing a Newman projection for butane what is the name of the eclipsed isomer with an angle of 0° between the two CH3 groups?

Answer 20

Synperiplanar

Question 21

When drawing a Newman projection for butane what is the name of the staggered isomer with an angle of 60° between the two CH3 groups?

Answer 21

Synclinical

Question 22

When drawing a Newman projection for butane what is the name of the eclipsed isomer with an angle of 120° between the two CH3 groups?

Answer 22

Anticlinical

Question 23

When drawing a Newman projection for butane what is the name of the staggered isomer with an angle of 180° between the two CH3 groups?

Answer 23

Antiperiplanar

Question 24

Which isomer of butane has the lowest energy?

Answer 24

Antiperiplanar as the CH3 groups are furthest away and therefore have the least repulsion so the antiperiplanar isomer is the most stable

Question 25

Which isomer of butane has the highest energy?

Answer 25

Synperiplanar as the CH3 groups are closest together and therefore have the most repulsion

Question 26

Do staggered or eclipsed isomers have a higher energy?

Answer 26

Eclipsed as the C-H bonds are closest together and the electrons in these bonds repel each other meaning that eclipsed isomers have a higher energy

Question 27

What is torsional strain?

Answer 27

The energy difference between staggered and eclipsed isomers, it is approximately 12kJmol-1 in ethane and approximately 21kJmol-1 in butane

Question 28

What happens when the energy barrier to rotation increases above 80kJmol-1?

Answer 28

The molecule can’t rotate about that bond at room temperature anymore, this enables you to separate conformers

Question 29

What is an atropisomer?

Answer 29

Enantiomers without chiral centres, this is possible as the energy barrier to rotation of a bond is too large so the molecule can’t rotate about that bond and therefore you can have enantiomers of this molecule

Question 30

Why isn’t cyclobutane a planar molecule?

Answer 30

If cyclobutane was planar, there would be eight pairs of eclipsing C-H bonds from the 4 carbons, this causes a large amount of torsional strain. Instead one of the carbon atoms moves out of the plane and the molecule adopts a butterfly conformation

Question 31

What happens to the angle strain of cyclobutane in the butterfly conformation?

Answer 31

The angle strain is increased as the bond angle decreases from 90° to 88°, however the torsional strain decreases massively and therefore the butterfly conformation is far more stable

Question 32

Does the boat or chair conformation of cyclohexane have a higher energy?

Answer 32

• The boat conformation has a higher energy as it has steric crowding of the two hydrogens at the top of the boat, it also has an eclipsing strain from the two parallel bonds in the diagram
• The chair conformation has a small steric strain, no angle strain and no eclipsing strain. This causes it to be 30kJmol-1 more stable

Question 33

When cyclohexane has a substituent group attached, which position of the substituent makes cyclohexane more stable?

Answer 33

When the substituent is in the equatorial plane the chair conformer is more stable as there is less torsional strain due to the substituent not being close enough to interact with the hydrogens on carbons 3 and 5. This means that cyclohexane has a lower energy when substituent is in equatorial position

Question 34

What is the other name for eclipsing strain?

Answer 34

Torsional strain

Question 35

What happens to the position of equilibrium between the substituent being in the axial or equatorial position as the substituent gets larger?

Answer 35

The equilibrium shifts towards the substituent being in the equatorial position as the larger the substituent is he closer it is to the hydrogens on carbons 3 and 5 and therefore the diaxial interactions are stronger and therefore the molecule has more energy and is less stable. This means that the molecule mainly exists with the substituent in the equatorial plane.

Question 36

Is it more stable to have substituents in the equatorial or axial position?

Answer 36

Equatorial as the substituents can’t interact with each other when in equatorial position