stereochemistry

Question 1

what are constitutional isomers?

Answer 1

isomers with different connectivity

Question 2

what are conformational isomers

Answer 2

isomers that can be interconverted by free rotation about single bonds e.g. staggered to eclipded

Question 3

configurational isomers

Answer 3

differ in the spatial arrangement of their atoms, can only be interconverted by breaking bonds

Question 4

facts about chair conformation

Answer 4

1) C-C-C bond angles = 111 degrees (no angle strain)

2) all bonds staggered (no tortional strain)

Question 5

STERIC STRAIN

Answer 5

Repulsion between atoms trying

to occupy the same space

Question 6

facts about boat conformation

Answer 6

1) high steric strain (short 1,4 distance)
2) there is tortional strain from eclipsing C-H bonds
3) 27kj mol more stable than chair

Question 7

facts about twist boat conformation

Answer 7

1) less steric strain than boat (big 1,4 distance)
2) less tortional strain than boat (less eclipsing)
3) 6kj mol more stable than Boat

Question 8

Why subsituents prefer to be equatorial

Answer 8

1,3-Diaxial interactions (steric strain) minimised
Ring-flip is relatively easy — allows equilibration
The larger the group, the greater the preference

Question 9

enatiomers

Answer 9

stereoisomers which are non superimosable images of one another

Question 10

diastereoisomers

Answer 10

• stereoisomers which are not enantiomers
• have different physical properties e.g. mp
• only possibe when there is more than 1 stereogenic centre in a molecule

Question 11

stereospecific reactions

Answer 11

get 100% of one enantiomer product, mechanism related

Question 12

stereoselective reactions

Answer 12

get tendancy to form 1 enantiomer but dont get 100% of it, structure related

Question 13

Fischer convention

Answer 13

• Horizontal bonds are emerging

- vertical bonds are receding

Question 14

assigning R/S to biaryls

Answer 14

assign a and b priority to the top aryl and (a = down b=up) assign c and d to the bottom and then check as normal

Question 15

Epimers

Answer 15

diastereoisomers which differ in configuration at only 1 chiral centre

Question 16

meso compound

Answer 16

contains chiral centres but is achiral as it has a plane of symmetry

Question 17

enantiomeric excess

Answer 17

% more abundant isomer - %less abundant isomer

(X-Y)/100

Question 18

what is torsional strain

Answer 18

Torsional strain or eclipsing strain is the increase in potential energy of a molecule due to repulsion between electrons in bonds that do not share an atom i.e. more than 1 bond away

Question 19

what is syn addition

Answer 19

the addition of two substituents to the same side (or face) of a double bond or triple bond, resulting in a decrease in bond order but an increase in number of substituents. Generally the substrate will be an alkene or alkyne.

Question 20

what is anti addition

Answer 20

An addition reaction in which two substituents are added to opposite sides (or faces) of a double bond or triple bond. Electrophilic addition of Br2 to 1,2-dimethylcyclohexene is an anti addition reaction, resulting in trans-1,2-dibromocyclohexane

Question 21

methods of determining enantiomeric excess (e.e.)

Answer 21

1) polarimetry

2) sample derivistation

Question 22

methods of separating enantiomers

Answer 22

1) salt formation (easiest)
2) covalent bond formation (need to be able to break covalent bond after)
3) kinetic resolution
4) chiral chromatography

Question 23

separating enantiomers by salt formation

Answer 23

;used to resolve racemic acids, amines and bases

• separate by a crystalising salt
• get rid of resolving agent with acid/base extraction

Question 24

separating enantiomers by covalent bond formation

Answer 24

• esterificaltion most important

- separate diastereoisomers by chromatography

Question 25

separating enantiomers by kinetic resolution

Answer 25

• racemic substrate + homochiral reagent
• each enantiomer has a different transition state and different activation energies therefore different rates of reaction
• if rates are different enough the enantiomers can be separated using enzymes

Question 26

homotopic

Answer 26

if replacing 2 hydrogens gives identical products both times

Question 27

enantiotopic

Answer 27

if replacing a hydrogen on a carbon creates a chiral centre both times and the molecules formed are enantiomers

Question 28

diastereotopic

Answer 28

if replacing a hydrogen on a carbon creates a chiral centre both times and the molecules formed are diastereoisomers