Isomers Flashcards
Isomer
Have same molecular formula but different structure
Structural isomer
Aka constitutional isomer - all they share is molecular formula/weight
Least similar of all isomers
Physical properties
Processes that don’t change the composition of matter
Eg. melting point, boiling point, solubility, odour, colour, density
Chemical properties
Reactivity of molecule with other molecules and as a result changes in chemical composition -functional groups
Stereoisomer
Same chemical formula as well as same atomic connectivity but differ in spatial arrangements - can be conformational or configurational
Conformational isomer
Aka conformer - most similar - are the same molecule just at different points in their natural rotation around alpha bond
-single bonds free to rotate
Staggered (anti) conformation
Two largest groups are antiperiplanar - minimal steric repulsion - 180 apart - most energetically favourable
Gauche
Type of staggered conformation where two largest molecules are 60 apart
Eclipsed
Two largest groups are 120 apart - must pass through eclipsed when going from anti to gauche
Totally eclipsed
Two largest groups directly overlap with 0 degrees between them - highest energy state and least favourable - synperiplanar
Ring strain
Seen in cyclic groups - arises from angle strain, torsional strain and non-bonded/steric strain
Angle strain
Results when bond angles deviate from their ideal values by being stretched or compressed
Torsional strain
Results when cyclic molecules must assume confirmations that are eclipsed or gauche
Non-bonded strain
Van der waals repulsion - when non adjacent atoms or groups compete for the same space - dominant source of steric strain in flagpole interactions of cyclohexane boat
Three conformations of cyclohexane
Chair, boat, twist(skew boat)
Most stable form of cyclohexane
Chair conformation - eliminates all 3 types of strain
Axial
Up and down - bonds perpendicular to plane of ring
Equatorial
Parallel / sticking out horizontal
Chair flip
One chair form is converted to another - axial becomes equatorial and equatorial becomes axial - conversion slowed by bulky groups such as tert-butyl
Where do bulky groups sit in chair conformation
Will favour equatorial to avoid non-bonded strain/flagpole interactions with axial groups
Configurational isomers
Can only change from one to another by breaking and reforming covalent bonds
Consists of enantiomers and diastereomers
Optical isomer
Have different special arrangements that affects the rotation of plane polarized light
Included enantiomers and diastereomers
Chiral
Mirror image cannot be superimposed over original object - lacks an internal plane of symmetry
Eg. hands
Achiral
Object has mirror image that can be superimposed
Has a plane of symmetry
What is a carbon with four different substituents
A chiral center - is an asymmetrical core of optical activity
Enantiomers
A type of configurational isomer that has non-superimposable mirror images
Have the same connectivity but opposite configurations at every chiral centre
Have identical physical and chemical properties except for optical activity and reactions in chiral environments
Diastereomers
A type of configurational isomer that are chiral and share the same connectivity but are not mirror images because they differ in some of their multiple chiral centres
Optical activity
Rotation of plane polarized light by a chiral molecule
Polarizer
Allows light waves oscillating only in a particular direction to pass through producing plane polarized light
Describe how enantiomers rotate plane polarized light
They will rotate it in opposite directions but with the same magnitude
Dextrorotary
Compound that rotates plane polarized light to the right or clockwise d- / (+)
Levorotary
Rotates plane polarized light to the left or counterclockwise l- or (-)
How is direction of rotation determined
Has to be experimental - cannot determine from structure
Specific rotation
Rotation determine for specific conc of an optically active molecule and tube length compared to standard
[alpha]=alpha(obs)/(cxl)
[alpha] is specific rotation in degrees
C is concentration in g/ml
L is path length in dm
Standard conc is 1g/ml and length is 1dm(10cm)
Racemic mixture
Contains equal conc of + and - enantiomers so it is not optically active as rotations cancel each other out
Diastereomers
Non mirror image configurational isomer
Has 2 or more stereogenic centres and differs at some but not all of these
Must have multiple chiral centres
How many possible stereoisomers in a molecule with n chiral centres
2 to the power of n possible stereoisomers
Describe the physical and chemical properties relationship between diastereomers
They will have different chemical and physical properties
Will rotate plane polarized light
Geometric isomer
Aka cis-trans isomer
Type of diastereomer where substituents differ in position around immobile multiple bond or ring structure
Meso compound
A molecule with chiral centres that has an internal plane of symmetry so it is not optically active
Relative configuration
Configuration of a chiral molecule in relation to another molecule through inter conversion
Absolute conformation
The exact spatial arrangement of atoms in a chiral molecule independent of other molecules
E and Z forms
Looking at polysubstituteed double bonds - find highest priority substituents attached to carbon based on molecular weight - if both on the same side of double bond it is z - if on opposite sides it is E
R and S forms
Used for chiral/sterogenic centres. Assign priority based on atomic weight. Switch lowest priority group with the group pointing into page. Draw a circle from 1 to 3 - ignoring 4. If it is clockwise it is R and counterclockwise is S BUT THEN HAVE TO PICK OPPOSITE BECAUSE LOWEST PRIORITY GROUP WAS MOVED
Describe the features of a fischer projection
Horizontal lines project out of plane (wedges)
Vertical lines project into page (dashes)
Lowest priority must go into pages so can be top or bottom
What will happen if we:
- rotate Fischer projection 90? 180?
- switching a pair of substituents ? Both pairs of substituents
Rotating 90 or switching one pair of substituents will cause inversion
Rotating 180 or switching both pairs will retain stereochemistry
