Stereochemistry

Question 1

What are the 3 ways that isomers can be different?

Answer 1

Constitution, configuration and conformation

Question 2

What are constitutional isomers?

Answer 2

Molecules with the same molecular formula but a different structure

Question 3

What are configurational isomers? Name 2 types and where they occur

Answer 3

Stereoisomers which have a different 3D spacial arrangement which cannot be interconverted to the other form rapidally. Geometric(E/Z)(sp²) and optical(sp³)

Question 4

What are conformational isomers? Name 2 molecular systems where this occurs

Answer 4

Stereoisomers that can rapidally interconvert but exist in different spacial arrangements that comes from free rotation of a single bond. A simple carbon chain and a cyclic carbon chain.

Question 5

What are tautomers?

Answer 5

2 or more structural isomers that are in equilibrium by the movement of an atom or group

Question 6

How do you use cis/trans notation?

Answer 6

Cis means same so the 2 identical groups are on the same side on a ring or double bond. Trans means opposite so the identical groups will be on opposite sides of the bond.

Question 7

When can’t cis/trans notation be used?

Answer 7

When you have a bond with 4 different groups on the 2 carbons

Question 8

How can E/Z isomers be seperated and distinguished?

Answer 8

They have different boiling points and will rotate plane polarised light in opposite directions

Question 9

Using Cahn-Ingold-Prelog priority sequencing rules, which group takes priority out of [Cl, Cl, Cl] and [H, H, Br]?

Answer 9

[Cl, Cl, Cl]

Do not sum atomic masses

Question 10

Using Cahn-Ingold-Prelog priority sequencing rules, how do you account for double bonds, such as for -CHO?

Answer 10

You treat them as individual bonds to the same atom, therefore -CHO will be [H, O, O]

Question 11

Using Cahn-Ingold-Prelog priority sequencing rules, what susbstituents would a carboxylic acid have?

Answer 11

[O, O, O]

Question 12

How many stereoisomers are there for a molecule with one chiral centre? Describe and name them

Answer 12

2 (2¹), non-superimposable mirror image isomers called enantiomers

Question 13

How do you assign the 2 enantiomers?

Answer 13

Using Cahn-Ingold-Prelog rules assign the 4 groups 1-4. Locate the lowest priority backwards and join up the remaining 3 in order. If the arrow rotates clockwise the molecule is R (right from 12), if the arrow points anticlockwise the molecule is S (sinister/left from 12).

Question 14

When is D/L notation used and what does it refer to?

Answer 14

Alpha amino acids and sugars, it refers to which enantiomer of glyceraldehyde the molecule is a derivative of. D refers to (+)-glyceraldehyde, L refers to (-)-glyceraldehyde.

Question 15

What is the equation for specific rotation?

Answer 15

Observed angle of rotation(°)/pathlength(dm) x density (gcm^-3)

Question 16

What do the supercript and subscript refer to in [α]_D²⁵?

Answer 16

25 is the temperature in °C. The subscript refers to the wavelength of light used in the polarimeter, D refers to the wavelength of the D line in a sodium lamp(589nm).

Question 17

What do the (+) or (-) in the name of chiral compounds refer to?

Answer 17

Which direction the isomer rotates plane polarised light, + exhibit dextrorotatory behavior and rotate light clockwise, - exhibit laevorotatory behavior and rotate light anticlockwise.

Question 18

What is enantiomeric excess(ee) and how can we work it out?

Answer 18

It is a measure of enantiomer purity. We work it out by using [+% - -%]. The percentages are worked out by using the observed rotation/rotation of pure compound to find the % excess of one enantiomer, e.g +6°(observed)/12°(pure)=+50%(ee), so there must be a 75:25 proportion of +:- enantiomers.

Question 19

What will the enantiomeric excess be for an optically active product from an achiral reactant?

Answer 19

0%, a racemate/racemic mixture will be produced as both enantiomers are produced at the same rate

Question 20

How many stereoisomers will there be for a molecule with n chiral centres?

Answer 20

2ⁿ

Question 21

What 3 ways do we represent molecules with 2 chiral centres on paper?

Answer 21

Flying wedge, sawhorse and Newman

Question 22

In a molecule with 2 chiral centres, how many stereoisomers are there and how are they related?

Answer 22

4, 2 pairs of enantiomers and 4 pairs of diastereoisomers

Question 23

What is a diasteriomer/diastereomer?

Answer 23

Diastereoisomers are stereoisomers that are not mirror images but are not superimposable

Question 24

How many stereoisomers will a molecule with 2 identical chiral centres have? How are they related?

Answer 24

4-1=3 due to one meso-isomer, 1 pair of enantiomers and 2 diastereoisomers

Question 25

What phrase explains meso-isomers?

Answer 25

Internal compensation, where the R-R and S-S enantiomers are identical.

Question 26

What are the 2 conformations of ethane? What is the energy barrier between these conformers? What interaction is the cause of this energy barrier in ethane?

Answer 26

Eclipsed and staggered. 12kJmol^-1. The tortional strain between hydrogen atoms in the eclipsed form

Question 27

What are the 4 conformers for butane in order of stability? What is the energy barrier between the highest and lowest energy form?

Answer 27

Antiperiplanar>synclinal(gauche)>anticlinal>synperiplanar. 21kJmol^-1

Question 28

How does the speed of rotation between conformers compare in ethane and butane?

Answer 28

Both have very fast rotation due to relatively low energy barriers however a greater proportion of butane molecules will be in the lowest energy form at any given moment.

Question 29

What situation would lead to the synclinal conformer beng favoured?

Answer 29

When there is attraction between 2 groups on adjacent carbon atoms, e.g ethan-1,2-diol has internal hydrogen bonding only in synclinal conformer so that will be the favoured conformer

Question 30

What is the rotation energy for 2,2’-Diiodobiphenyl and how fast is its rotation?

Answer 30

88kJmol^-1, slow rotation

Question 31

At what energy barrier do we say rotation about a single bond is slow?

Answer 31

>80kJmol^-1

Question 32

What does the slow rotation of a compound mean for its conformers? What is the name we give to this?

Answer 32

They are seperable, as they are seperate isomers they are called atropisomers/atroisomers

Question 33

What is the lowest energy shape of cyclobutane? Which type of repulsion is most prominent in the square, planar shape?

Answer 33

V shape, some angle strain(90°) but mostly eclipsing strain

Question 34

What are the 2 types of repulsion in conformers and the third that occurs in cyclic compounds?

Answer 34

1. Tortional/eclipsing strain (repulsion between aligned groups)
2. Steric hinderance (large groups blocking one another)
3. Angle strain

Question 35

What is the lowest energy shape of cyclopentane? Which type of repulsion is most prominent in the pentagon, planar shape?

Answer 35

Envelope shape with one carbon out of the plane with the others. Tortional strain as the bond angle is not very strained (108°)

Question 36

What are the names and structures of the 2 major conformers of cyclohexane? What is the energy difference between these conformers?

Answer 36

Boat and chair, boat is around 30kJmol^-1 less stable than chair.

Question 37

How and why does a monosubstituted cyclohexane perform a flip?

Answer 37

It converts to a boat before completing the flip to make all previously up carbons down. This is done to put substituents in the equitorial position as this is more stable than axial positons because there is fewer flagpole interactions in this position which lowers tortional strain.

Question 38

How does the proportions of equitorial to axial substituent chair conformations change from a methyl group to a -C(CH₃)₃ group?

Answer 38

95:5→98:2→99.9:0.1

Question 39

When presented with a 6 membered ring system, how do you know where to draw the substituents in a chair conformation?

Answer 39

Take all bonds coming out of the plane to be going up and all bonds going into the plane to be going down. Draw the structure in a chair conformation with all bonds going up in the axial position for up carbons and in the equitorial position for down carbons so they stay relatively pointing up. Draw the same for the fliped chair and decide which will be favoured.