Stereochemistry Flashcards
Structural Isomers
Compounds with the same molecular formula, but different connectivities between atoms
e.g.chain isomers, functional group isomers, position isomers
Tautomers
Interchangeable structural isomers.
Readily interconvert via chemical reaction.
e.g. neutral and zwitterion form of amino acid.
Stereoisomers
Compounds with the same constitutional structure, but different arrangements of their atoms in 3D space.
Stereoisomers at SP2 carbons
E/Z stereoisomers
Which stereoisomers can be isolated from oneanother and why?
E/Z stereoisomers can be separated due to differences in their properties e.g. boiling point.
Diastereomers can also be separated due to differences in properties - e.g. boiling point, spectra
Atropisomers
Which stereoisomers cannot be easily isolated from oneanother and why?
Enantionmers cannot be separated through simple means due to their identical physical properties.
Conformers cannot be separated.
When rotation is very slow (energy barrier >80kjmol) conformers may be separated. - atropisomers!
e.g. with very bulky substituents such as phenyl groups.
Stereoisomers at SP3 carbons
R/S isomers
Optical isomers
Enantiomers
Stereoisomers that are non-superimposable mirror images of one another
What makes a carbon ‘chiral’?
Four different groups attached.
Which is the -R isomer?
With the lowest priority group facing away from you, the remaining groups have 1-2-3 priority in the clockwise direction
Equation for specific rotation
Measuring the optical activity of enantiomers
specific rotation (at 23°C under sodium light source) = observed angle of rotation(˚)/[path length (dm) x density (gcm⁻³)]
pay attention to the units!!
What is the specific rotation of a racemic mixture ?
Zero
Enantiomeric Excess
A measure of enantiomer purity.
EE(%) = % major enantiomer - % minor enantiomer
specific rotation of mixture = EE x specific rotation of major enantiomer
For a compound with n chiral centres, what is the maximum number of possible stereoisomers?
2^n
Diastereomers
Non-mirror image, non-superimposable stereoisomers of the same compound
Mesoisomer
Optically inactive stereoisomer with internal symmetry
Conformational Isomers
“Conformers”
Different spacial arrangements of a molecule that arise by rotation around a single bond.
Conformers of ethane and their relative energies
Eclipsed -> energy maxima (torsional strain between eclipsing H groups due to repulsion of electron clouds)
Staggered-> energy minima
At any moment, most molecules will be in the staggered - lower energy - form.
Rotation around the sigma bond of ethane is…
… very fast due to small H group substituents.
Rotation from one staggered - low energy - form to another.
Conformers of butane and their relative energies
Synperiplanar (E) <- energy maxima
Anticlinal (E)
Synclinal (S)
Antiperiplanar (S) <-energy minima
LEARN THESE ON PAPER
In what circumstance may synclinal be preferred over antiperiplanar?
Where there are substituents that can participate in hydrogen bonding (e.g. -OH)
Internal hydrogen bonding between groups in the synclinal arrrangement is more stable than the antiperiplanar arrangement where hydrogeon bonding is not permitted.
Atropisomers
Enantiomers that result from restricted rotation around a single bond.
Can be interconverted by heating.
E.g. bi-phenyl systems
What are three kinds of strain present in cycloalkane rings?
Torsional strain (repulsion due to interaction between electrons in adjacent orbitals)
Steric strain (interaction between substituent that are too close together in space)
Angle strain (when c-c-c bonds have a non ideal angle)
Remember, the ideal angle for a c-c-c bond is 109.5
Conformations of cyclobutane
Planar - bond angle = 90
Puckered butterfly - bond angle = 88
Conformations of cyclopentane
Planar - bond angle = 108
Envelope - bond angle = 105
Conformations of cyclohexane
Planar - bond angle = 120
Boat
Twist
Chair - bond angle = 111
Boat conformation has steric clashes: flagpole-flagpole interactions
Axial and Equatorial C-H bonds
Axial bonds - point up and down from the cyclohexane ring
Equatorial bonds - parallel to the C-C bonds of the ring
In which position (axial or equatorial) will a substituent have higher energy? Why?
Generally, the substituent in the axial position has the higher energy.
Greater steric and torsional strain.
1,3-diaxial interactions
What happens when monosubstituted cycloalkanes undergo a ring flip?
One chair form can flip to the other chair form via an intermediary boat conformation.
Axial susbtituents become equatorial and vice versa.
Equilibrium between chair conformations of monosubstituted cycloalkanes is shifted to the right when…
… substituted group is larger.
(the larger the substituted group, the greater the steric strain in the higher energy conformation, so the greater the energy difference between the two conformations. The lower energy form is preferred to a greater degree than if the substituent was smaller)
