Organic Chemistry- Isomers Flashcards
What is an important way to distinguish between molecules?
By identifying isomers of the same compound
What are isomers?
Those that have the same molecular formula but different structures
Can a molecule be an isomer by itself?
No
Structural isomers are what?
The least similar of all the isomers
What’s another anme for structural isomers?
Constitutional isomers
Structural isomers have different what?
Chemical and physical properties
What are physical properties?
Characteristics of processes that don’t change the composition of matter, such as melting point, boiling point, solubility, odor, color and density
What are Chemical properties?
Do with the reactivity of the molecule with other molecules and result in changes in chemical composition.
In organic chemicstry, the chemical properties are dicatated by what?
the functional groups in the molecule
How small are the conformational interconversion barriers?
19 kcal/mol between anti staggered butane and totally ecliped butane
Are conformational interconversion barriers easy to overcome?
Yes at room temperature.
However, at very low temperatures, conformational interconversions will dramatically slow. if the molecules do not possess sufficient energy to cross the energy barrier, they may not rotate at all (as happens to all molecules at absolute zero).
Are cycloalkanes fairly stable compounds or fairly unstable?
Both depending on the ring strain
What are the three factors that ring strain arises from?
Angle strain
Torsional Strain
Nonbonded strain
When does angle strain results?
When bond angles deviate from their ideal values by being stretched or compressed
When does torsional strain resullt?
When cyclic molecules must assume conformations that have eclipsed or gauche interactions.
When does nonbonded strain result?
(Van der Waals repulsion) results when nonadjacent atoms or groups compete for the same space
Nonbonded strain is the dominant source of ______ in the _______.
Steric strain in the flagpole interactions of the cyclohexane boat conformation.
To alleviate strain, cycloalkanes attempt to adopt what?
Various nonplanar conformations.
What are the different shapes that cyclobutane, cyclopentane, and cyclohexane adopt when under strain?
Cyclobutane: puckers into a slight V
Cyclopentane: Adopts an envelope conformation
Cyclohexane: goes into three- chair, boat, twist or ske-boat.
What is the most common cyclokane seen on the MCAT?
Cyclohexane
What is the most stable conformation of cyclohexane? Why?
The chair, which minimizes all three types of strain
What are axial hydrogen atoms?
Hydrogen atoms that are perpendicular to the plane of the ring (sticking up or down)
What ar eequatorial hydrogen atoms?
Those parallel (sticking out)
In a chain, what do the axial and equatorial orientations do?
Alternate around the ring, that is, if the wedge on C-1 is an axial group, the dash on C-2 will also be axial, the wedge on C-3 will be axial and so on.
Cyclohexane can undergo what?
A chair flip, in which one chair form is converted to the other
During chair flip, cyclohexane passes through which conformation?
Half-chair
After chair flip, what happens?
All axial groups become equatorial and all equatorial groups become axial. All dashes remain dashes, and all wedges remain wedges.
The interconversion of cyclohexane from one chair to the next can be slowed how?
If a bulky group is attached to the ring
Tert-butyl groups are classic examples
For substituted rings, the bulkiest group will fabor what position?
The equatorial position to reduce nonbonded strain with the axial groups in the molecule
In rings with more than one substituent, how will the formation be determined?
The preferred chair form is determined by the larger group, which will prefer the equatorial position.
When is a ring called cis?
If both groups are located on the same side of the ring
When is a ring called trans?
If both groups are on opposite sides of the ring
The words cis and trans are also used to classify what molecules?
Molecules with double bonds
What is the difference between conformational isomers and configurational isomers?
Conformational isomers interconvert by simple bond rotation
Configurational isomers can only change from one form to another by breaking and reforming covalent bonds.
What are the two categories of configurational isomers?
Enantiomers and diastereomers.
What is another name for the two categories of configurational isomers? Why?
Optical isomers because the different spatial arrangement of groups in these molecules affects the rotation of plane-polarized light.
What is chiral?
If an objects mirror image cannot be superimposed on the original; this implies that the molecules lacks an internal plane of symmetry.
What is the best example of chiralty?
Handedness. Think of your own hands.
What are achiral objects? Example?
Objects that have mirror images that can be superimposed:
Example: Form
How will chiral and achiral objects be tested on the MCAT?
With a carbon atom with four different substituents. This carbon will be an asymetrical core of optical activity and is known as a chiral center.
What is an example of a chiral centered carbon?
The c-1 carbon atom in 1-bromo-1-chloroethane
What are enantiomers?
Two molecules that are nonsuperimposable mirror images of each other.
If a carbon atom has only 3 ifferent subtituents, is it chiral or achiral? Why?
It has a plane of symetry and is achiral, because a simple 180 degree rotation around the vertical axis will allow the compound to be superimposed upon its mirror image.
What are Enantiomers with more detail?
(nonsuperimposable mirror images) have the same connectivity but opposite configurations at every chiral center in the molecule.
What are the two notable exceptions of enantiomers?
optical activity and reaction in chiral environments.
When is a compound optically active?
if it has the ability to rotate plane-polarized light.
Is oridnary light polarized?
No it is unpolarized
What does it mean if light is unpolarized?
It consists of waves vibrating in all possible planes perpendicular to its direction of propagation.
What does a polarizer allow?
Allows light waves oscillating only in a particular direction to pass through, producing plane-polarized light
What is the definition of optical activity?
The rotation of this plane-polarized light by a chiral molecule
At the molecular level, what does one enantiomer do to plane-polarized light?
Will rotate plane-polarized light to the same magnitude but in the opposite direction of its mirror image (assuming concentration and path lengths are equal)
What is dextrortatory?
A compound that rotates the plane of light to the right, or clockwise (d-) and is labeled (+)
What is levorotatory?
A compound that rotates light toward the left, or counterclockwise (l-) and is labeled (-)
Can the direction of rotation be determined by the structure of the molecule?
No, it must be determined experimentally, that is , it is not related to the absolute configureation of the molecule.
The amount of light rotation is dependent on what?
The number of molecules that a light wave encounters
What two factors determine the number of molecules that a light wave encounters?
The concentration of the optically active compound
Length of the tube through which the light pases
What are the standard conditions to compare the optical activities of different compounds?
1g/ml for concentration of 1dm (10cm) for length
What equation can be used to determine the specific rotation after measuring the rotation at different concentrations and tube lengths?
[alpha] = alpha obs/ c x l
What do the letters represent in the specific rotation equation?
alpha obs: the observed rotation in degrees
c: the concentration in g/ml
l: the path length in dm
What is a racemic mixture?
When both (+) and (-) enantiomers are present in equal concentrations, they form a racemix mixture
In racemic mixtures, what happens?
The rotations cancel each other out, and no optical activity is observed
If enantiomerism is analogous to handedness, then what is racemix mixtures?
The equivalent of ambidexterity, these solutions possess no handedness overall and will not rotate plane-polarized light.
What happens when you react two enantiomers with a single enantimoer of another compound?
Lead to two diastereomers
Explain the process of the reaction of enantiomers with another enantiomer of a different compound?
Two enantiomers that contain only one chiral carbon; these compound could be labeled (+) and (-). If each is reacted with only the (+) enantiomer of another comound, two products would result: (+,+) and (-,+). Because these two products differ at some chiral centers, they are necessarily diastereomers.
Do diastereomers have different physical properties?
Yes
The different physical properties in diastereomers allows one to what?
Separate these products by common laboratory techniques such as crystallization, filtration, distillation, and others.
Once diastereomers are separated, what can happen?
They can be reaccted to regenerate the original enantiomers.
What is a Diastereomer?
Non-mirror-image configureational isomers.
When do diastereomers occur?
A molecule has two or more sterogenic centers and fiffers at some but not all of these centers.
For any molecule with _______ chiral centers, there and ________ possible stereoisomers.
n chiral centers, there are 2^n possible stereoisomers
If a carbon has two chiral carbon atoms, what is the max possible stereoisomers?
Four possible stereoisomers
Even though diastereoners have different chemical properties, what might happen? Why?
They might behave similarly in particular reactions because they have the same functional groups
What is consitently different in diastereomers? Why?
Physical properties
Do diastereomers rotate plane-polarized light?
Yes
What is the difference between diasteromers rotating light and enantiomers?
Knowing the specific rotation of one diastereomer gives no indication of the specific rotation of antoher diastereomer.
Opposite with enantiomers, which will always have equal-magnitude rotations in opposite directions.
What are cis-trans isomers?
A specific subtype of diastereomers in which substituents differe in their position around an immovable bond, such as a double bond, or around a ring structure, such as a cycloalkane in which the rotation bonds is greatly restricted.
What is the former name of cis-trans isomers?
geometric isomers
If two substituents are on the same side of the immovable bond, the molecule is considered ______.
Cis
If two substituent bonds are opposite sides, the immoveable bond is considered ______.
Trans
For compounds that are more complicated with poly substituted double bonds, what is used instead of cis and trans?
(E)/(Z) nomenclature is used
For a molecule to have optical activity, it must what?
not only have chiral centers within it, but must also lack a plane of symmetry
If a plane of symmetry exists, the molecule is not what?
Optically active even if it possesses chiral centers.
What is a meso compound?
A molecule wtih chiral centers that has an internal plane of symmetry
Meso compounds are the molecular equivalent of what?
Racemic mixtures
The configureation of a stereoisomer refers to what?
The spatial arrangement of the atoms or groups in the molecule
What is the relative configureation of a chiral molecule?
Its configuration in relation to another chiral molecule (often through chemical interconversion)
Relative configuration can be used to determine what?
To determine whether molecules are enantiomers, diastereomers, or the same molecule
What is the absolute configureation of a chiral molecule?
The exact spatial arrangement of these atoms or groups, independent of other molecules.
When is (E) and (Z) nomenclature used?
For compounds with polysubstituted double bonds.
How do you determine the (E)/(Z) designation?
Start by identifying the highest-priority substituent attached to each double-bonded carbon. Using the Cahn-Inglold-Prelog priority rules.
What is the Cahn-Ingold-Prelog priority rule?
Priority is assigned based on the atom bonded to the double-bonded carbons: The higher the atomic number, the higher the priority. If the atomic numbers are equal, priority is determined by the next atoms outward; again, whichever group contain the atom with the highest atomic number is given top priority. If a tie remains, the atoms in this group are compared one-by-one in descending atomic number order until the tie is broken.
When is the alkene named (Z)?
(German: Zusammen, Together) if the two highest-priority substituents on each carbon are on the same side as the double bond.
When is the alkene named (E)?
(Entgenen, opposite) If the two highest-priority substituents are on opposite sides.
When are (R) and (S) nomenclature used?
For chiral centers in molecules
What is the sequence to determine absolute configuration?
- Assign priority
- (Classic Version) Arrange in space
- (Modified Version) Invert the stereochemistry
- Draw a circle
- Write the name
How do you assign priority?
Using the Cahn-Ingold-Prelog priority rules described earlier, assign priority to the four substituents, looking only at the atoms directly attached to the chiral center. Higher atomic number takes priority over lower atomic number. If the atomic numbers are equal, priority is determined by the combination of the atoms attached to these atoms; if there is a double bond, it is counted as two individual bonds to that atom. If a tie is encountered work outward from the stereocenter until the tie is broken.
What is the classic version of step two: Arrange in space?
Orient the molecule in three-dimensional space so that the atom with the lowest priority (usually a hydrogen atom) is at the back of the molecule. Another way to think of this is to arrange the pount of view so that the line of sight proceeds down the bond from the asymmetrical carbon atom (the ciral center) To the substituent with lower priority. the three substituents with higher priority should then radiate out from the central carbon
What is invert the stereochemistry?
Any time two groups are switched on a chiral carbon, the stereochemistry is incerted. By this logic, we can simply switche the lowest-priority group with the group at the back of the molecules.
What must be kept in mind if using the modified version of step two?
We have now changed the molecule to the opposite configuration. THerefore, if we use this modified step, we need to remember to switch our final answer (Either (R) to (S), or (S) to (R).
What is a common strategy used on Fischer diagrams?
The modified version of step two
What is involved in step 3: Draw a circle?
Drawing a circle connecting the substituents from number 1 to 2 to 3. Pay no attention to the lowest-priority group, it can be skipped because it projects directly into the page.
If the circle is drawn counterclockwise, the asymetric atom is called what?
(S) (latin: sinister, left)
If the circle is drawn clockwise, the asymetric atom is called what?
(R) (rectus, right)
How do you perform step 4: write the name?
Once the (R)/(S) designation has been determined, the name can be written out. (R) and (S) are put in parenthese and separated from the rest of the name by a hyphen. IF we have a compound with more than one chiral center, location is specified by a number preceding the R or S within the parentheses and without hyphen
What is one way to represent 3D molecules?
Fischer projection
What is Fischer projection?
Horizontal lines indicate bonds that project out from the plane of the page (wedges), wheras vertical lines indicate bonds going into the plane of the page (dashes). The point of intersection of the lines represents a carbon atom.
What are the benefits of fisher projections?
The lowest-priority group can be on the top or bottom of the molecule and still project into the page
What are the two ways that will invert the stereochemistry (R) and (s)?
Switching two substituents around a chiral carbon
Rotating a Fishcer projection in the plane of the page by 90 dgrees.
What will interchanging any two pairs of substituents do to the stereochemistry (R) and (S)? What about rotating the Fischer projection in the plane of the page by 180 degrees?
Switching any two pairs of substituents will revert the compound back to its original stereochemistry and rotating a Fischer projection in the plane of the page by 180 degrees will retain the stereochemistry of the molecule.
What are the three different option to determine the (R) and (S) designation for Fischer projections?
Make 0 switches
Make 1 switch
Make 2 switches
What is involved in option 1: make 0 switches for Figuring out (R) and (S) designation in Fischer Projecctions?
Determine the order of substituents as normal, drawing a circle from 1 to 2 to 3. Then obtain the (R)(S) designation , The true designation will be the oppsoite of what you just obtained.
What is involved in option 2: make 1 switch for Figuring out (R) and (S) designation in Fischer Projecctions?
Swap the lowest-priority group with one of the groups on the vertical axis. Obtain the (R)/(S) designation and, once again, the true designation will be the opposite. of what you just found.
What is involved in the 3rd option: Make 2 switches for Figuring out (R) and (S) designation in Fischer Projecctions?
Start with option 2, moving the lowest-priority group into the correct position. Then, switch the other two groups as well. Because we made two swtiches, this molecule will have the same designation as the initial molecule. This is the same as holding one substituent in place and rotating the other three in order.
