Stereochemistry

Question 1

What does 3d skeletal formula look like? Which way should the bonds go?

Answer 1

The wedge bond always on the left

Question 2

What is a conformation?

Answer 2

Any spatial arrangement of atoms of a molecules than can be achieved by rotation about single bonds

Question 3

What do the two extremes of conformation look like in terms of wedges?

Answer 3

1) Staggered, both wedges are on the right
2) eclipsed, wedges are next to each other right and left

Question 4

What does the energy/torsion angle graph look like for butane?

Answer 4

Question 5

What are isomers? What are constitutional isomers? What are stereoisomers?

Answer 5

Isomer: non-identical molecules with the same molecular formula
Constitutional isomers: different connectivity of bonds e.g chain-isomers, functional group isomers
Stereoisomers: same connectivity but a different arrangement of atoms in space

Question 6

What are the types of stereoisomers and their definitions?

Answer 6

Enantiomers: stereoisomers related as non-superimposable mirror images
Diastereomers: stereoisomers which are not related as non-superimposable mirror images

Question 7

What is a stereogenic centre?

Answer 7

A carbon (or other atom) with 4 different substituents, so can be chiral

Question 11

What are the properties of enantiomers relative to each other?

Answer 11

Same physical and chemical properties in a non-chiral environments
Different properties in a chiral environment such as an enzyme

Question 12

How can plane polarised light be used to determine enantiomer ratios ?

Answer 12

Enantiomers will rotate plane polarised light in opposite direction - /+ specific rotation
Using a polarimeter, light passes through a polarising quartz, through the solution and detected, at a specific temp

specific rotation = measured rotation / length of cell x concentration
So if known conc are used to make a calibration curve of measured rotation, the observed rotation can be used to determine the ratio of enantiomers

Question 13

What is a racemic mixture?

Answer 13

Contains a 1:1 mixture of enantiomers for optically inactive

Question 14

When may a molecule have chiral centres but be achiral?

Answer 14

When there is a plane of symmetry in the molecule
This does not mean the molecule does not have diastereomers, just not enantiomers, see example
This is a meso compound

Question 15

How can phosphine and sulfoxide compounds be chiral? Why is nitrogen not considered chiral?

Answer 15

The lone pairs act as a fourth substituent, draw the mirror image as normal with the lone pair

Amines typically undergo pyramidal inversion, reverting to an sp2 transition state, and so switch between enantiomers rapidly

Question 16

How do the properties of diastereomers and enantiomers of a compound vary?

Answer 16

Different physical and chemical properties e.g NMR IR melting point

Question 17

How do you name a chiral compound?

Answer 17

Place the substituent with the lowest atomic number at the back, normally hydrogen
Start at the substituent with the highest atomic number, move to the next highest
- clockwise=R
- anticlockwise=S

Treat double bonds as 2 x C-C
First point of difference may be needed

Question 18

How do you name alkenes?

Answer 18

Assign higher priority based on atomic number based on first point of difference
Same side= Z
Opposite sides= E

Question 19

What does it mean if a molecule has a plane of symmetry?

Answer 19

Symmetrical when cut in half, including the same stereochemistry
Different wedges are not the same
See below

Question 20

What is a meso compound?

Answer 20

A compound which has at least 2 stereogenic centres but itself is achiral due to a plane of symmetry

Question 21

What should you check for when seeing if a compound if chiral?

Answer 21

• multiple chiral centres
  -plane of symmetry
• can a plane of symmetry exist at a different conformation
• check by drawing mirror image

Question 22

Apart from those with a stereogenic centre, what types of molecules may be chiral?

Answer 22

Allenes, consecutive pi bonds
Atropisomers- arising from restricted rotation around single bonds due to high steric repulsions
Check with the mirror images

Question 23

What is the resolution of racemates? What is a conglomerate?

Answer 23

Seperation of a 1:1 racemic mixture into its separate enantiomers
Conglomerates are compounds which crystallise and separate the enantiomers, though this is very uncommon

Question 24

How can enantiomers more generally be separated?

Answer 24

React them with a reagent that will not alter that chiral centre of interest, but is itself an enantiomer and pure
This means the product is a diastereomer as now possesses an additional stereocenter
Diastereomers can be separated via chromatography as they have different physical properties
The products can then be hydrolysed/ reacted to reform the original product, but enantiomerically pure
Even more efficient if the reagent produces salts as diastereomers

Question 25

What is absolute and relative configuration?

Answer 25

Absolute: know that this molecule has R,R stereochemistry Relative: only know how the stereocenters are related to each other, but not the actual e.g know they have the same config, but not if R,R or S,S

Question 26

What is an epimer?

Answer 26

DIastereomers related by a different in only one chiral centre

Question 27

What is the maximum number of isomers possible for n stereocentres? Other tips?

Answer 27

2^n no more than this, but if plane of symmetry, inversion (reflection and rotation same mol) then less than this

Question 28

When does an non-stereogenic centre matter with stereochem?

Answer 28

If a part of a ring, so 4 groups attached not always identical, then the orientation of this non-chiral carbon adds an enantiomer, see example

Question 29

Would a C=C=C=C be chiral?

Answer 29

No, as it is actually planar due to the additional double bond

Question 30

How would you draw the chirality of spiro compounds and allenes?

Answer 30

Draw them out, draw the mirror image and rotate