2. Isomerism Flashcards
What are constitutional isomers?
constitutional isomers are two molecules with the same molecular formula, but different connectivity (C5H12)
What are conformational isomers?
conformational isomers are two molecules with the same formula and connectivity, but different 3D orientation due to rotation around a sigma bond.
Conformers are actually the same molecule, but may exist slightly differently in space.
Staggered / eclipsed conformational isomers, which ism more stable?
Viewed as a Newman projection, staggered conformers are more stable as they have less steric hindrance and less electron repulsion.
Explain anti, syn, and gauche conformations of conformational isomers.
When a sigma bond has substituents on either side, the location of those substituents matters.
Anti –> staggered conformation where the bulky groups are in opposite directions (180 apart) .
Syn - eclipsed conformation where the bulky groups are in symmetrical positions
Gauche - staggered conformation where the bulky groups are 60 degrees apart.
Anti = most stable Syn = least stable
In what situation may a gauche position be more stable than an anti position, for conformers?
If the substituent groups can form an intramolecular hydrogen bond, the gauche conformer will be more stable.
Cyclohexane conformational isomers: What is the chair and boat?
cyclohexane can also exist as different conformers.
The chair conformer is lowest energy whereas the boat conformer is the less stable isomer.
t or f, two equally energetic chair conformations exist for cyclohexane.
true, this is the concept of a ring flip, which can occur if there is sufficient thermal energy (at 298 K there is)
What occurs during a ring flip?
All equatorial hydrogen’s become axial
All axial hydrogen’s become equatorial
This applies to any substituent
However, they do not change their position relative to the plane. If an axial H is out of plane, then it becomes an out of plane equatorial H.
What is a 1,3-diaxial interaction? What chair conformation is most stable for cyclohexane?
Substituents in an axial position can undergo unfavorable 1,3-diaxial interactions. Therefore, to increase stability, we want subs to be in the equatorial position.
t or f, to perform a ring flip, number the carbon atoms and then push up or down on the far right carbon.
True, this is helps with visualization.
What are stereoisomers?
stereoisomers are molecules with the same formula and connectivity, but differ in their spatial arrangement around around a chiral center (a center with 4 different substituents around it)
t or f, any molecule that cannot be superimposed onto itself is said to be chiral.
true
a molecule that CAN be superimposed onto itself is achiral.
what is a stereocenter?
A stereocenter or stereogenic center is simply a chiral carbon (an sp3 hybridized carbon with 4 different subs)
what is the maximum number of stereoisomers a molecule can have with n chiral carbons.
the maximum number of stereoisomer conformations a molecule can have is 2^n where n is the number of chiral carbons.
A molecule that is symmetrical but has two chiral carbons (or n chiral carbons), has how many stereoisomers?
When a molecule is symmetrical, it will have 1 less isomer since, if the constituents are in the same plane, the molecule is identical.
E.g. 1,3-dibromohexane has 3 stereoisomers (draw them).
What id deuterium (D) and Tritium (T)?
D is 2H (hydrogen with 2 neutrons).
T is 3H (hydrogen with 3 neutrons).
used to radio-label in chemistry
Absolute configuration (R or S): explain the 4 steps in finding a molecules absolute configuration.
- priority
- bonds
- configure
- name
- assign priority based on atomic number (Z)
- tie, we go down the line of connectivity
- isotope goes to higher atomic mass (A) T > D > H - multiple bonds are counted as 2 single bonds. E.g. C=O is considered two separate bonds to 2 oxygen atoms
- place the lowest priority sub in the back, facing away
- count the other subs from Highest to Lowest.
clockwise = R (i.e. to the right) counterclockwise = S
t or f, when assigning absolute configuration, if the lowest priority group is pointing towards you, then you can just trace the priory path as is, and then flip the absolute configuration sign.
True!
you can also switch two constituents, draw the path, flip the sign.
What are enantiomers?
These are stereoisomers (i.e. cannot be superimposed onto each other) that are mirror images of each other. think of holding your hand to a mirror
t or f, enantiomers will always have the opposite absolute configurations.
True. At every stereocenter, enantiomers will have opposite configurations
How do two enantiomers differ in terms of their physical properties?
They essentially have the same physical properties (BP, MP, etc) except for their optical activity. Enantiomers will bend plane-polarized light to the same magnitude in opposite directions.
What are dextrorotatory (d / +) and levorotatory (l / -) ?
This refers to how enantiomers rotate plane-polarized light.
clockwise = dextrorotatory (d or +)
counter-clockwise = levorotatory (l or -) (to the left)
What is specific rotation?
the magnitude of rotation of plane-polarized light for any compound is its specific rotation.
What is a racemic mixture?
A racemic mixture contains 50/50 d and l enantiomers. this mixture will have a specific rotation of 0 since the enantiomers cancel each other out. A racemic mixture is NOT optically active.
t or f, the R/S configuration is related to the +/- optically activity of enantiomers.
FALSE, no correlation.
For every stereocenter of a compound, we can determine whether its stereochemistry is R or S. Explain diastereomers.
diastereomers are stereoisomers (non-superimposeable) that are NOT mirror images (which would be enantiomers).
In diastereomers, the R / S configurations will differ at at least one chiral center, but not all.
If a molecule has three chiral centers all of which are R configuration, the same molecule with all three centers in the S configuration would be related to the first molecule how?
If you were to flip the first molecule over a mirror plane, you would get the second molecule. Thus, if all stereocenters are the same configuration the opposite is its enantiomer.
A molecule that is R,S is related to a molecule that is S,R in what way?
enantiomers! Since if we flip one molecule, we get the other (mirror images).
t or f, unlike enantiomers, diastereomers can have very different physical properties (MP, BP, etc).
True and this also applies to their optical activity, which has no relationship between each other (whereas enantiomers is equal and opposite).
What is enantiomer resolution? How is it done?
Since enantiomers have identical physical properties, separating R and S configurations is difficult.
enantiomer resolution involves adding a resolving agent (known as a chiral probe) which binds the two steromers and changes some physical property, such as solubility.
What are epimers?
Epimers are a subclass of diastereomers where only one chiral center is different between two molecules.
RRSR –> RRRR
these are epimers
How do you name D and L sugars?
- Place the CHO group at the top
- look at the orientation of the OH on the highest numbered carbon (at the bottom)
- pointing to the RIGHT = D
- pointing to the left = L
What is the epimeric carbon?
Recall, epimers only differ at one stereocenter. This stereocenter is called the epimeric carbon
What are anomers?
When epimers form upon a ring closure, they form anomers.
e.g. sugars: the C5 OH group attacks the C1 carbonyl group to create a ring system
At the anomeric carbon (stereocenter of anomers), how do you determine if it is an alpha or beta compound?
on the anomeric carbon…
OH pointing down = alpha
OH pointing up = beta
What is a meso-compound?
A meso-compound is a molecule that contains more than one chiral center, but has an internal line of symmetry (ie. one half of the molecule replicates the other half of the molecule).
t or f, since meso-compounds contain chiral centers, they have optical activity.
False, since meso-compounds have an internal line of symmetry, their optical activities cancel to zero. Thus, the molecule as a whole is achiral.
A meso-compound with two chiral centers has x amount of isomers.
Typically 2^n = x # of isomers.
However, since a meso-compound has its own symmetry, its isomer is actually identical to itself.
so instead of 4 isomers, its 3.
What are geometric isomers? How do you name them?
Geometric isomers are diasteromers that differ in orientation around a ring or double bond.
two highest priority groups on the same side = Cis (Z)
two highest priority groups on opposite sides = Trans (e)
When do we use cis / trans and when do we use Z / E?
Cis / Trans = ring geometric isomers
Z / E = double bond geometric isomers
not strict.
