Stereoselectivity: Fundamentals

Question 1

torsional strain

Answer 1

π overlap of 2 filled σ bonds
= destabilising

antibonding MO is more destabilised than bonding MO is stabilised = net repulsion

Question 2

hyperconjugation

Answer 2

σ to σ* / σ to π*
= stabilising

Question 3

chirality

Answer 3

non-superimposable mirror image

no Sn axes - no mirror planes/centres of inversion

Question 4

enantiomers

Answer 4

non-superimposable stereoisomers of chiral molecules

identical scalar physical properties

Question 5

achiral

Answer 5

can be superimposed on its mirror image

Question 6

configuration

Answer 6

breaking bonds

= different molecules

Question 7

conformation

Answer 7

rotating bonds

= same molecules

Question 8

diastereomers

Answer 8

stereoisomers that are NOT mirror images

Question 9

formula for number of stereogenic centres

Answer 9

stereoisomers = 2n

diastereoisomers = 2n-1

^ symmetry can cancel stereoisomers (degenerate)

Question 10

resolution

Answer 10

separation of 2 enantiomers

Question 11

how do large reagents approach a molecule?

Answer 11

equatorially

Question 12

how do small reagents approach a molecule?

Answer 12

axially

Question 13

assigning R + S

Answer 13

1. ensure lowest priority atom (usually H) is pointing away
2. Rank other sub.
3. CW = R
   ACW = S

Question 14

optical activity

Answer 14

enantiomers rotate plane-polarised light in equal but opposite directions

left (ACW) = laevorotatory
right (CW) = dextrorotatory

Question 15

what measures amount of rotation?

Answer 15

polarimeter

Question 16

enantiomerically pure

Answer 16

only has 1 enantiomer

Question 17

enantiomerically enriched

Answer 17

has both enantiomers but 1 predominates

Question 18

D (dextro)

Answer 18

FG on RHS

Question 19

L (levo)

Answer 19

FG on LHS

Question 20

how to slow down pyramidal inversion?

Answer 20

[increase s character of N l.p.]

1. increase electronegativity of ligands
2. ring strain

Question 21

stereoselective

Answer 21

preferential formation of one product over another

Question 22

stereospecific

Answer 22

2 different stereoisomeric starting materials to give 2 different stereoisomeric products

Question 23

stereoconvergent

Answer 23

2 different stereoisomeric starting materials react to give same stereoisomeric products

Question 24

diastereoselective

Answer 24

preferential formation of one product diastereoisomer over another

Question 25

types of diastereoselectivity

Answer 25

1. simple (= relative configuration between 2 newly-formed stereocentres)
2. induced (= relative configuration between newly-formed stereocentre + pre-existing stereocentre on substrate/reactant)