Covalent Molecules 2: Isomers

Question 1

What are isomers?

Answer 1

Groups of compounds that comprise the same atoms but have different chemical properties, physical properties, and/or structures

Question 2

What are the different types of isomers?

Answer 2

Structural isomers:
Chain, functional group, position

Stereoisomers:
Optical isomers (enantiomers), diastereomers (configurational isomers, cis-trans isomers, E/Z isomerism

Question 3

What are stereoisomers?

Answer 3

Molecules composed of the same atoms with the same structural formula but different spatial arrangement

Question 4

What are tautomers?

Answer 4

Pairs of structural isomers which differ in the position of one hydrogen atom
They can interconvert between these isomers via tautomerism

Question 5

What is keto-enol tautomerism?

Answer 5

Form of isomerism in which the isomers only differ by the attachment of one hydrogen atom and the position of a double bond

Question 6

What is an enol?

Answer 6

A compound containing an -OH group directly attached to a carbon that is involved in a C=C double bond

Question 7

What are cis-trans isomers?

Answer 7

Isomers that share the same atoms joined in the same way but have different configurations

Question 8

What are the two characteristics of molecules locked into specific configurations (i.e. no bond rotation)?

Answer 8

1) Possess a double bond
2) Are cyclic

Question 9

What is the difference between cis and trans isomers?

Answer 9

In the cis conformation, two pairs of the same group are on the same side of the double bond
In the trans conformation, two pairs of the same group are on oppostie sides of the double bond

Question 10

Why doesn’t methylbut-2-ene exhibit cis-trans isomerism?

Answer 10

Because there are two methyl groups on one of the carbons (there must be two different groups)

Question 11

What is E/Z isomerism?

Answer 11

Used when there are four different groups attached to a C=C double bond
Each group is given a different priority ranking depending on their atomic number (Cahn-Ingold-Prelog convention)

Question 12

What is the difference between E and Z isomers?

Answer 12

E-isomers exist where the highest priority groups on each side of the double bond are on opposite sides
Z-isomers exist where the highest proority groups on each side of the double bond are on the same side

Question 13

Why does cis-trans isomerism exist in cyclic structures?

Answer 13

Because the bonds in cyclic structures are unable to rotate freely
If two groups are both oriented in the same way above the plane of the ring, this is the cis isomer
If two groups are both oriented in opposite ways on either side of the ring, this is the trans isomer

Question 14

What are enantiomers?

Answer 14

A type of stereoisomer that exists when a compound is chiral and therefore there are two possible enantiomers, each of which are non-superimposable mirror images of the other
Characterised by their ability to rotate plane-polarised light in opposite directions

Question 15

What are diastereomers?

Answer 15

A type of stereoisomers that are not mirror images of each other
Occur when molecules have multiple chiral centres or multiple double bonds and differ in spatial arrangement around one of these centres but NOT ALL

Question 16

What are stereogenic atoms?

Answer 16

Atoms that give rise to stereoisomerism, creating different spatial arrangements
I.e. an asymmetric carbon atom

Question 17

What is chirality?

Answer 17

A chiral object is asymmetrical
A chiral molecule is one that has four different groups attached to a central atom (the chiral centre)

Question 18

How many pairs of enantiomers would a molecule with one chiral centre generate?

Answer 18

One pair of enantiomers

Question 19

How many pairs of enantiomers would a molecule with two chiral centres generate?

Answer 19

Two pairs of enantiomers

Question 20

How many pairs of enantiomers would a molecule with three chiral centres generate?

Answer 20

Four pairs of enantiomers

Question 21

What is (+)/(-) notation used for?

Answer 21

To describe the optical rotation of enantiomers
Optical isomers cause plane-polarised light to rotate either clockwise (+) or anticlockwise (-) depending on which isomer it is

Question 22

What is R/S nomenclature used for?

Answer 22

To describe the 3D arrangement of groups around a chiral centre

Question 23

Describe the steps involved in assigning R/S nomenclature.

Answer 23

1) Prioritise the four groups that are attached to the chiral centre based on their atomic number
2) Rotate the molecule so that the lowest ranking group is pointing behind the plane (on the dash)
3) Consider the direction needed to get from the highest to lowest priority group
4) If you need to go clockwise, it is the R enantiomer, if you need to go anti-clockwise, it is the S enantiomer

Question 24

What are Fischer projections?

Answer 24

A representation used to show molecules with one or more chiral centres
Used to draw 3D molecules in 2D
The horizontal bonds denote bonds coming out of the page (towards you)
The vertical bonds denote bonds that are pointing into the page (away from you)

Question 25

How do you assign R/S nomenclature based on the Fischer projection?

Answer 25

Lowest priority group on horizontal, and clockwise, S enantiomer
Lowest priority group on horizontal and anti-clockwise, R enantiomer
Lowest priority group on vertical, clockwise, R enantiomer
Lowest priority group on vertical, anti-clockwise, S enantiomer

Question 26

What is D/L notation used for?

Answer 26

To classify chiral molecules in relation to the reference molecule of glyceraldehyde
Based on the orientation of the hydrogen and hydroxyl group on the penultimate carbon atom in the molecule
In the D enantiomer, the OH is on the right
In the L enantiomer, the OH is on the left
Commonly used in context of biological molecules

Question 27

How do you assign D/L nomenclature to amino acids?

Answer 27

Draw the amino acid in Fischer projection
If the amino group lies to the left, it is the L enantiomer
If the amino group lies to the right, it is the D enantiomer

Question 28

Why is it advantageous for D-alanine to be present in bacterial cell walls?

Answer 28

Because animals contain enzymes capable of only digesting proteins containing the L-amino acids, so they are unable to digest D-alanine in bacterial cell walls
Allowing the bacterium to survive and multiply, facilitating infection of the host

Question 29

Which stereoisomer of lactic acid is found in human muscles?

Answer 29

(S)-lactic acid

Question 30

What does the term ‘enantiomerically pure’ mean?

Answer 30

Describes a system in which only one of a pair of enantiomers is present

Question 31

Why is it useful that enzymes are chiral?

Answer 31

Gives them a lot of selectivity in terms of the molecules it can interact with

Question 32

How do enantiomers differ from each other?

Answer 32

They have identical chemical and physical properties in vitro
In biological systems, they exhibit different properties due to the inherent chirality of biological systems

Question 33

How do you calculate observed angle of rotation of enantiomers?

Answer 33

Observed angle of rotation = observed value / conc of solution x path length

*If calculated value is the same as polarimeter/observed value, the product has good enantiomeric purity

Question 34

How do you meausre rotation of enantiomers?

Answer 34

Using a polarimeter

Question 35

Why would the calculated value of rotation be less than the observed value?

Answer 35

If you have a mixture of two enantiomers
If the product is impure

Question 36

What would the optical rotation of a pure racemic mixture be?

Answer 36

Zero
Because the R enantiomer rotates exactly the same amount as the S enantiomer

Question 37

What are meso compounds?

Answer 37

Compounds with chiral centres that are achiral
E.g. tartaric acid - has a plane of symmetry