Stereochemistry

Question 1

Structural Isomers

Answer 1

Compounds with the same molecular formula, but different connectivities between atoms

e.g.chain isomers, functional group isomers, position isomers

Question 2

Tautomers

Answer 2

Interchangeable structural isomers.
Readily interconvert via chemical reaction.
e.g. neutral and zwitterion form of amino acid.

Question 3

Stereoisomers

Answer 3

Compounds with the same constitutional structure, but different arrangements of their atoms in 3D space.

Question 4

Stereoisomers at SP2 carbons

Answer 4

E/Z stereoisomers

Question 5

Which stereoisomers can be isolated from oneanother and why?

Answer 5

E/Z stereoisomers can be separated due to differences in their properties e.g. boiling point.

Diastereomers can also be separated due to differences in properties - e.g. boiling point, spectra

Atropisomers

Question 6

Which stereoisomers cannot be easily isolated from oneanother and why?

Answer 6

Enantionmers cannot be separated through simple means due to their identical physical properties.

Conformers cannot be separated.

When rotation is very slow (energy barrier >80kjmol) conformers may be separated. - atropisomers!
e.g. with very bulky substituents such as phenyl groups.

Question 7

Stereoisomers at SP3 carbons

Answer 7

R/S isomers

Optical isomers

Question 8

Enantiomers

Answer 8

Stereoisomers that are non-superimposable mirror images of one another

Question 9

What makes a carbon ‘chiral’?

Answer 9

Four different groups attached.

Question 10

Which is the -R isomer?

Answer 10

With the lowest priority group facing away from you, the remaining groups have 1-2-3 priority in the clockwise direction

Question 11

Equation for specific rotation

Measuring the optical activity of enantiomers

Answer 11

specific rotation (at 23°C under sodium light source) = observed angle of rotation(˚)/[path length (dm) x density (gcm⁻³)]

pay attention to the units!!

Question 12

What is the specific rotation of a racemic mixture ?

Answer 12

Zero

Question 13

Enantiomeric Excess

Answer 13

A measure of enantiomer purity.
EE(%) = % major enantiomer - % minor enantiomer

specific rotation of mixture = EE x specific rotation of major enantiomer

Question 14

For a compound with n chiral centres, what is the maximum number of possible stereoisomers?

Answer 14

2^n

Question 15

Diastereomers

Answer 15

Non-mirror image, non-superimposable stereoisomers of the same compound

Question 16

Mesoisomer

Answer 16

Optically inactive stereoisomer with internal symmetry

Question 17

Conformational Isomers

Answer 17

“Conformers”
Different spacial arrangements of a molecule that arise by rotation around a single bond.

Question 18

Conformers of ethane and their relative energies

Answer 18

Eclipsed -> energy maxima (torsional strain between eclipsing H groups due to repulsion of electron clouds)
Staggered-> energy minima

At any moment, most molecules will be in the staggered - lower energy - form.

Question 19

Rotation around the sigma bond of ethane is…

Answer 19

… very fast due to small H group substituents.

Rotation from one staggered - low energy - form to another.

Question 20

Conformers of butane and their relative energies

Answer 20

Synperiplanar (E) <- energy maxima
Anticlinal (E)
Synclinal (S)
Antiperiplanar (S) <-energy minima

LEARN THESE ON PAPER

Question 21

In what circumstance may synclinal be preferred over antiperiplanar?

Answer 21

Where there are substituents that can participate in hydrogen bonding (e.g. -OH)
Internal hydrogen bonding between groups in the synclinal arrrangement is more stable than the antiperiplanar arrangement where hydrogeon bonding is not permitted.

Question 22

Atropisomers

Answer 22

Enantiomers that result from restricted rotation around a single bond.

Can be interconverted by heating.

E.g. bi-phenyl systems

Question 23

What are three kinds of strain present in cycloalkane rings?

Answer 23

Torsional strain (repulsion due to interaction between electrons in adjacent orbitals)
Steric strain (interaction between substituent that are too close together in space)
Angle strain (when c-c-c bonds have a non ideal angle)

Remember, the ideal angle for a c-c-c bond is 109.5

Question 24

Conformations of cyclobutane

Answer 24

Planar - bond angle = 90
Puckered butterfly - bond angle = 88

Question 25

Conformations of cyclopentane

Answer 25

Planar - bond angle = 108
Envelope - bond angle = 105

Question 26

Conformations of cyclohexane

Answer 26

Planar - bond angle = 120
Boat
Twist
Chair - bond angle = 111

Boat conformation has steric clashes: flagpole-flagpole interactions

Question 27

Axial and Equatorial C-H bonds

Answer 27

Axial bonds - point up and down from the cyclohexane ring
Equatorial bonds - parallel to the C-C bonds of the ring

Question 28

In which position (axial or equatorial) will a substituent have higher energy? Why?

Answer 28

Generally, the substituent in the axial position has the higher energy.
Greater steric and torsional strain.
1,3-diaxial interactions

Question 29

What happens when monosubstituted cycloalkanes undergo a ring flip?

Answer 29

One chair form can flip to the other chair form via an intermediary boat conformation.
Axial susbtituents become equatorial and vice versa.

Question 30

Equilibrium between chair conformations of monosubstituted cycloalkanes is shifted to the right when…

Answer 30

… substituted group is larger.
(the larger the substituted group, the greater the steric strain in the higher energy conformation, so the greater the energy difference between the two conformations. The lower energy form is preferred to a greater degree than if the substituent was smaller)