s&r topic 5&6

Question 1

your doing great

Answer 1

sweetie

Question 2

define structural/constitutional isomer

Answer 2

same molecular formula but different connectivity

Question 3

define stereoisomers

Answer 3

same structural formula but different spatial arrangement

Question 4

name 2 types of stereoisomer

Answer 4

enantiomer

diastereomers

Question 5

how do diastereomers arise

Answer 5

a molecule with two or more chiral centres

pair of enantiomers + pair of enantiomers = four diastereomers

can have different combination of R and S configurations

a pair of enantiomers has another pair of enantiomers with different S and R configurations

• the two enantiomers in each pair are enantiomers of each other
• the total four stereoisomers are diastereomers of each other

see topic5 vid1 for diol examples

Question 6

how do you calculate the number of stereoisomers a molecule will have

in which cases does this not work

what are these cases called

Answer 6

for n number of chiral centres, there are 2ⁿ stereoisomers

doesn’t always work when molecules are chiral (superimposable)

• molecule has an internal plane of symmetry
• one pair of enantiomers are not actually a pair because they are superimposable

these molecules are called meso compounds = achiral molecules that have chiral centres
- meso compounds have opposite R/S labels for chiral centres either side of the internal plane of symmetry

see topic5 vid1 for cyclohexane-diol example

Question 7

give 2 properties that differ between diastereomers

Answer 7

different melting point

different solubility in water

Question 8

define trans isomer

Answer 8

similar priority groups on each C in a carbon-carbon double bond are on opposite sides of the double bond

Question 9

define cis isomer

Answer 9

similar priority groups on each C in the carbon-carbon double bond are on the same side of the double bond

they are both above or both below

Question 10

what are geometric isomers and how do their properties differ from one another

Answer 10

geometric isomers are cis-trans isomers
they have different physical properties 
- diff density 
- diff mp
- diff IR and NMR spectrum

Question 11

what are E/Z isomers

Answer 11

aka imine isomers

E = high priority groups on opposite sides of double bond
Z = high priority groups are on the same side of the double bond

lone pairs always take priority

Question 12

in square-planar complexes, how does cis-trans isomerism arise

Answer 12

transition metals take on the square-planar complexes with four bonds to a central atom

in monodentate ligands if there are two different types of substituent, cis-trans isomerism arises

cis isomer = similar ligands at 90° to one another
trans isomer = similar ligands at 180° to one another

Question 13

define monodentate ligand

Answer 13

substituent binds to metal with only one of its atoms

Question 14

define bidentate ligand

Answer 14

substituent binds to metal with two of it’s atoms

see topic5 vid2 for examples

Question 15

in octahedral complexes, how does cis-trans isomerism arise

Answer 15

in an MX₂Y₄ structure where M is the central metal and X and Y are substituents cis-trans isomerism an occur

isomerism is determined by the 2 X substituents

• cis = X substituents are 90° from one another
• trans = X substituents are 180° from one another

rest of the atoms fit around these two X atoms

Question 16

in octahedral complexes, how does fac-mer isomerism occur

what is meant by fac isomers and mer isomers

Answer 16

in an MX₃Y₃ complex it can occur

fac (facial) isomers = ligands occupy one face of the 3D octahedral

mer (meridional) isomers = identical set of ligands span around meridian of the octahedral structure

Question 17

what is a racemic mixture

why do S and R enantiomer form a racemic mixture

Answer 17

a 50:50 mixture of enantiomers

activation energy to form R and S enantiomers is the same so they both equally likely to form

Question 18

what is an enantiopure substance

where are they found

Answer 18

substance composed of only one enantiomer

usually found in biology or drugs to make them more effective

Question 19

name and describe two ways of separating chiral compounds/enantiomers

Answer 19

chiral resolution - reacting a racemic mixture with a single enantiomer (R or S) of another compound

• this forms two distereomers with different physical properties
• these physical properties can be used to separate them
• reverse the original reaction to get the desired enantiomer from the original racemic mixture

chiral stationary phase chromatography

• enantiomer with greater affinity for the stationary phase with elute from the column more slowly
• using HPLC