L13 - Organic Chemistry (3) Flashcards
Define Isomer.
Molecules which have the same molecular formula, but different arrangement of atoms.
Describe the different types of isomers.
- Structural (Constitutional) Isomers: Differ in their bonding sequence / molecular skeleton
- Stereoisomers: Differ only in the arrangement of atoms in space
Define Structural Isomer.
Usually different compounds with different physical and chemical properties that differ in their bonding sequence / molecular skeleton.
Describe the different types of structural isomers.
- Chain / Skeletal Isomerism: Carbon skeleton
- Position Isomerism: Position of the functional group
- Functional Isomerism: Identity of the functional group
- Tautomerism: Movement of bonds and a proton
Define Stereoisomer.
Same structural formula but different 3D arrangement of atoms in space.
Describe the different types of stereoisomers.
- Geometric (cis-trans, E/Z) Isomerism
- Optical Isomerism
What causes Geometric Isomerism?
Restricted rotation around a double bond
Describe the possibility of rotation around a double (pi) bond.
- Rotation breaks the double (pi) bond
- Not allowed without an energy input
Describe the possibility of rotation around a sigma bond.
Free rotation
What does Geometric Isomerism require?
Different groups at each end of the bond
What does Geometric Isomerism result in?
Different physical and chemical properties
What is a Cis Isomer?
When two alkyl groups are on the same side of the double bond.
What is a Trans Isomer?
When two alkyl groups are on opposite sides of the double bond.
Describe Cis and Trans Isomers with reference to steric repulsion.
- Cis Isomer: Greater steric repulsion
- Trans Isomer: Less steric repulsion
Define Optical Isomers.
Non-superimposable mirror images.
What do Optical Isomers have the ability to do?
To rotate plane-polarised light in opposite directions
Describe the different types of isomers and the direction in which they rotate plane-polarised light.
- Dextrorotatory: d-isomer / + isomer rotates plane polarised light to the right, clockwise
(bold wedge) - Levorotatory: l-isomer / - isomer rotates plane polarised light to the left, anti-clockwise
(dashed wedge)
Define Chiral Molecule.
A molecule which has no plane of symmetry.
Define Chiral Centre / Asymmetric Carbon Atom.
A carbon atom bonded to four different substituent groups which lacks a plane of symmetry.
Compounds with one chiral centre will show _______ ________.
Compounds with one chiral centre will show optical activity.
(Optical isomers of these compounds are chemically identical).
Compounds with more than one chiral centre may or may not show _______ ________ depending on…
Compounds with more than one chiral centre may or may not show optical activity depending on whether or not they are non-superimposable on their mirror image (chiral) or superimposable (achiral).
What is another word for Optical Isomers?
Enantiomers
What is a meso compound?
A meso compound is a molecule with multiple stereocenters that is superimposable on its mirror image.
Has an internal mirror plane / plane of symmetry.
A molecule which contains chiral carbon atoms but is not chiral (no plane of symmetry) overall.
Describe the different lines/wedges used when drawing optical isomers.
- Normal Lines: In the plane of the page
- Bold Wedges: Come out of the plane of the page
- Dashed Lines: Go into the plane of the page
Describe the systematic nomenclature for chiral centres.
- R: Right-handed / Clockwise
- S: Left-handed / Anti-Clockwise
How is the chirality of amino acids and sugars often described?
Using d and l
What are Fischer Projections?
- Flat drawing that represents a 3D molecule
- Commonly used for sugars
- Carbon chain is vertical
- Horizontal lines are forward, out of the plane
- Vertical lines are behind, into the plane
What are Enantiomers?
Non-superimposable mirror image forms of a chiral molecule.
- Represent 2 optical isomers: (+) and (-)
- Their opposite rotatory powers are due to the opposite arrangements of groups around each asymmetric carbon atom
Describe the features of Enantiomers.
- Enantiomers have identical physical properties in all respects except in their interaction with plane of polarised light
- Enantiomers interact differently with other chiral molecules
- Generally, enantiomers are not interconverted under ordinary conditions
Define a Racemic Mixture.
A 1:1 mixture of the two enantiomers of a compound.
Describe the features of Racemic Mixtures.
- Equal quantities of d- and l- enantiomers
- Notation: (d, l) or (+, -)
- No optical activity
- The mixture may have different b.p. and m.p. from the enantiomers
Define Diastereomer.
Molecules with 2 or more chiral centres where at least one but not all differ.
(A structure with n chiral centres, which has 2n possible isomers).
- Stereoisomers that are not mirror images of each other
Describe the relationship between two Diastereomers.
- Two diastereomers will have different physical properties e.g. melting point, boiling point, stability
- They have different chemical reactivity with both chiral and achiral reagents
Diastereomers which differ at only one carbon atom are called _______.
Diastereomers which differ at only one carbon atom are called epimers.
Describe the importance of Enantiomers.
Proteins / receptors / enzymes made up of L-amino acids = chiral environment, differentiates between isomers
What is D-alanine?
- A component of bacterial cell wall not found in mammalian proteins
- Protects bacteria from proteases
What is (S)-Tirofiban?
- Anti-platelet drug
What is (R)-Tirofiban?
- Anti-platelet drug that is a thousand-fold less active than (S)-Tirofiban
What is S-thalidomide?
- Anti-angiogenic
- Teratogen
What is R-thalidomide?
- Sedative
