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
