Chirality 3 & 4

Question 1

For a compound with n asymmetric C atoms, how many possible stereoisomers is there?

Answer 1

2*n

Question 2

What do molecules with 2 identical chiral carbons have?

Answer 2

2*2 theoretically possible stereoisomers BUT only 3 actual different isomers

Question 3

What are meso compounds?

Answer 3

2 or more asymmetric C + plane of symmetry

Question 4

What do chiral compounds not have?

Answer 4

Plane of symmetry

Question 5

So what are meso compounds?

Answer 5

Achiral

Question 6

What are meso stereoisomers?

Answer 6

Molecules with 2 identical chiral Cs that have 3 stereoisomers

Question 7

What do meso compounds form?

Answer 7

R,S or S,R

Question 8

Why is meso-DMSA important?

Answer 8

Used medically to chelate toxic metals in cases of metal poisoning

Question 9

How do you identify asymmetric Cs in cyclic compounds?

Answer 9

Non-superimposable mirror image BUT equal energy + interconvertible

Question 10

What is a stereoisomer of cyclic compounds + describe it?

Answer 10

1,4-dimethylcyclohexane
= neither cis or trans isomers optically active
= each has plane of symmetry

Question 11

Describe 1,3dimethylcyclohexane

Answer 11

Trans + cis compounds each have 2 stereogenic centres
Cis compound = plane of symmetry + meso
Trans = pair of enantiomers

Question 12

What the properties of diastereoisomers?

Answer 12

Most have same chemical reactivity

Different physical properties

Question 13

How are diastereoisomers separated?

Answer 13

By different physical characteristics

eg. different solubility in solvents

Question 14

How are diastereoisomers separated using solubility properties?

Answer 14

Use solvent in which one diastereoisomer is very soluble, + other not so
Both dissolve
Less soluble crystallises + filtered out
Evaporate solvent = pure other diastereoisomer

Question 15

What is the resolution method?

Answer 15

Separation of enantiomers from either racemic mixture or enantiomerically enriched mixture

Question 16

What is chiral chromatography method?

Answer 16

Normally HPLC or GC where racemic solution is passed over chiral stationary phase giving rapid + reversible diastereotopic interaction

Question 17

How do we separate enantiomers?

Answer 17

Use difference in physical properties

Question 18

Describe classical resolution: separation of diastereoisomers

Answer 18

R- acid + S-acid = enantiomers
---->
R,S-salt + S,S-salt diastereoisomers
----> crystallise + separate 
R,S-salt + HCl ----> S-baseH+ + R-acid
S,S-salt + HCl ----> S-baseH+ + S-acid

Question 19

Describe resolution by esterification

Answer 19

R- acid + S-acid = enantiomers
—-> S-alcohol + catalytic acid
R,S-ester + S,S-ester diastereoisomers
R,S-ester (ester hydrolysis) —-> S-alcohol + R-acid
S,S-ester (ester hydrolysis) —-> S-alcohol + S-acid

Question 20

How do you recover enantiomers?

Answer 20

Aqueous acid/ reflux (ester hydrolysis)

Question 21

If there is 2 identical chiral centres with internal mirror plane in one, what is produced?

Answer 21

Achiral-meso form

9only 3 stereoisomers)

Question 22

What are the pharmacological effects of chirality?

Answer 22

Appropriate recognition by targets
Strong binding to targets
Correct pharmacological activity
BUT if only 1 enantiomer active = inactive form can cause adverse effects sometimes

Question 23

What are the biochemical effects of chirality?

Answer 23

Effects reactions involving chiral Cs

Question 24

What is a stereoselective reaction?

Answer 24

Preferential formation of 1 stereoisomer over another

Question 25

What is a stereospecific reaction?

Answer 25

Each stereoisomeric reactant produces a different stereoisomeric product or a different set of products

Question 26

What happens chiral reactant +/or product can show no stereoselectivity?

Answer 26

Result in racemic mixture

Question 27

What happens chiral reactant +/or product can show stereoselectivity?

Answer 27

Invert stereochemistry or converse stereochemistry

Question 28

What are stereochemical outcomes important in?

Answer 28

Metabolism of drugs
Drug action at receptors
Toxicity through undesirable activity of stereoisomers

Question 29

What is a concerted reaction?

Answer 29

Bond making + breaking happen in same step

Stereochemistry either inverted or conserved

Question 30

What is an example of a concerted reaction?

Answer 30

Biological systems, such as enzymes

Question 31

What is a non-concerted reaction?

Answer 31

Bond making + breaking happens in different steps
Intermediates are formed
Racemisation if intermediate formed

Question 32

What do intermediates allow?

Answer 32

Changes to stereochemistry

Question 33

What are happens in concerted SN2 reactions?

Answer 33

Happen at sp3 C
Polarised bond between C + heteroatom = makes it E+
Nu- attacks electrophilic C = breaks bond as new bond is made = 1 step
No intermediate formed
1 product

Question 34

What happens in concerted reactions involving 2 chiral centre formation?

Answer 34

Only cis isomers obtained with alkenes containing fewer than 8 ring atoms

Question 35

What happens in non-concerted SN1 reactions?

Answer 35

Happen at sp3 C
Polarised bond between C + heteroatom breaks
Intermediate carbocation formed
Stereochemical definition lost
Reactants form new bond to C
Attack planar intermediate from above + below the plane
1:1 mix of enantiomers 
NO stereoselectivity

Question 36

Describe non-concerted electrophilic addition - alkenes

Answer 36

Occur at sp2 carbocations
More than 1 step
Intermediate carbocation or radical formed = planar

Question 37

What is syn?

Answer 37

Two substituents added to same side of double bond

Question 38

What is anti?

Answer 38

Two substituents added to opposite sides of double bond