Covalent Molecules 2: Isomers Flashcards
What are isomers?
Groups of compounds that comprise the same atoms but have different chemical properties, physical properties, and/or structures
What are the different types of isomers?
Structural isomers:
Chain, functional group, position
Stereoisomers:
Optical isomers (enantiomers), diastereomers (configurational isomers, cis-trans isomers, E/Z isomerism
What are stereoisomers?
Molecules composed of the same atoms with the same structural formula but different spatial arrangement
What are tautomers?
Pairs of structural isomers which differ in the position of one hydrogen atom
They can interconvert between these isomers via tautomerism
What is keto-enol tautomerism?
Form of isomerism in which the isomers only differ by the attachment of one hydrogen atom and the position of a double bond
What is an enol?
A compound containing an -OH group directly attached to a carbon that is involved in a C=C double bond
What are cis-trans isomers?
Isomers that share the same atoms joined in the same way but have different configurations
What are the two characteristics of molecules locked into specific configurations (i.e. no bond rotation)?
1) Possess a double bond
2) Are cyclic
What is the difference between cis and trans isomers?
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
Why doesn’t methylbut-2-ene exhibit cis-trans isomerism?
Because there are two methyl groups on one of the carbons (there must be two different groups)
What is E/Z isomerism?
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)
What is the difference between E and Z isomers?
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
Why does cis-trans isomerism exist in cyclic structures?
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
What are enantiomers?
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
What are diastereomers?
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
What are stereogenic atoms?
Atoms that give rise to stereoisomerism, creating different spatial arrangements
I.e. an asymmetric carbon atom
What is chirality?
A chiral object is asymmetrical
A chiral molecule is one that has four different groups attached to a central atom (the chiral centre)
How many pairs of enantiomers would a molecule with one chiral centre generate?
One pair of enantiomers
How many pairs of enantiomers would a molecule with two chiral centres generate?
Two pairs of enantiomers
How many pairs of enantiomers would a molecule with three chiral centres generate?
Four pairs of enantiomers
What is (+)/(-) notation used for?
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
What is R/S nomenclature used for?
To describe the 3D arrangement of groups around a chiral centre
Describe the steps involved in assigning R/S nomenclature.
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
What are Fischer projections?
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)
How do you assign R/S nomenclature based on the Fischer projection?
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
What is D/L notation used for?
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
How do you assign D/L nomenclature to amino acids?
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
Why is it advantageous for D-alanine to be present in bacterial cell walls?
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
Which stereoisomer of lactic acid is found in human muscles?
(S)-lactic acid
What does the term ‘enantiomerically pure’ mean?
Describes a system in which only one of a pair of enantiomers is present
Why is it useful that enzymes are chiral?
Gives them a lot of selectivity in terms of the molecules it can interact with
How do enantiomers differ from each other?
They have identical chemical and physical properties in vitro
In biological systems, they exhibit different properties due to the inherent chirality of biological systems
How do you calculate observed angle of rotation of enantiomers?
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
How do you meausre rotation of enantiomers?
Using a polarimeter
Why would the calculated value of rotation be less than the observed value?
If you have a mixture of two enantiomers
If the product is impure
What would the optical rotation of a pure racemic mixture be?
Zero
Because the R enantiomer rotates exactly the same amount as the S enantiomer
What are meso compounds?
Compounds with chiral centres that are achiral
E.g. tartaric acid - has a plane of symmetry
