Ch. 2: Isomers Flashcards
char (2) + aka: structural isomers
aka: constitutional isomers
the least similar of all isomers
the only thing that structural isomers share is their molecular formula, meaning that their molecular weights must be the same
defn: physical properties
characteristics of process that don’t change the composition of matter, such as melting point, boiling point, solubility, odor, color, and density
defn: chemical properties
have to do with the reactivity of the molecule with other molecules and result in changes in chemical composition
generally dictated by the functional groups in the molecule
char (2): stereoisomers
- same chemical formula
- same atomic connectivity (same structural backbone)
how do stereoisomers differ from each other?
how these atoms are arranged in space (their wedge and dash pattern)
what are the two types of stereoisomers? defn them.
conformational isomers (conformers) = differ in rotation around single (sigma) bonds
configurational isomers = can be interconverted only by breaking bonds
why are conformational isomers the most similar of all isomers?
they are the same molecule, only at different points in their natural rotations around single bonds
what is the basis behind how conformational isomers work?
varying degrees of rotation around single bonds can create different levels of strain
defn + use + example with butane: Newman projection
molecule visualized along a line extending through a carbon-carbon bond axis
helpful for seeing conformational isomers
defn: anti conformation
the most energetically favored type of staggered conformation
the two largest groups are antiperiplanar (in the same plane, but on opposite sides) to each other
explain butane’s anti conformation and why this is its most stable conformation
butane is most stable when its two methyl groups (C1 and C4) are oriented 180 from each other –> minimal steric repulsion between the atoms’ electron clouds bc as far apart as possible –> atoms are happiest, in their lowest energy state
defn + 2 types: staggered conformation
there is no overlap of the atoms along the line of sight
types: anti, gauche
defn: gauche conformation
the two largest groups are 60 deg apart
how does a molecule convert from the anti to the gauche conformation?
must pass through an eclipsed conformation (the two methyl groups are 120 apart, overlap with the hydrogen atoms on the adjacent carbon)
defn + char: totally eclipsed molecule
the two methyl groups directly overlap each other with 0 deg separation
in its highest-energy state
why are totally eclipsed conformations the least energetically favorable?
because the two largest groups are synperiplanar (in the same plane, on the same side)
mnemonic: gauche vs. eclipsed
GAUCHE = unsophisticated or awkward = its gauche for one methyl group to stand too close to another group
ECLIPSED = the groups are completely in line with one another (like a solar or lunar eclipse)
how do conformational interconversion barriers operate at room temperature? at very low temperatures? why?
- these barriers are small and are easily overcome at room temperature
- interconversions are very, very slow at very low temperatures (if molecules do not have enough energy to cross the energy barrier, they may not rotate at all)
what determines the stability of cycloalkanes?
ring strain
what are the three factors of ring strain?
- angle strain
- torsional strain
- nonbonded/steric strain
defn: angle strain
results when bond angles deviate from their ideal values by being stretched or compressed
defn: torsional strain
results when cyclic molecules must assume conformations that have eclipsed or gauche interactions
defn: nonbonded strain (van der Waals repulsion)
results when nonadjacent atoms or groups compete for the same space
what is the dominant source of steric strain in flagpole interactions of the cyclohexane boat conformation? how do cycloalkanes attempt to alleviate this strain?
nonbonded strain
they attempt to adopt various nonplanar conformations
- cyclobutane puckers into a slight “V”
- cyclopentane adopts an envelope conformation
- cyclohexane (most common on MCAT) exist in: chair, boat, and twist/skew-boat conformations
what is the most stable conformation of cyclohexane? why?
the chair conformation because it minimizes angle, torsional, and nonbonded strain
defn: axial vs. equatorial hydrogens
AXIAL = hydrogen atoms perpendicular to the ring’s plane (sticking up or down)
EQUATORIAL = hydrogen atoms parallel to the ring’s plane (sticking out)
how do axial-equatorial orientations work around a ring?
axial-equatorial orientations alternate around the ring (if wedge on C-1 is axial, dash on C-2 will be axial, wedge on C-3 will be axial)
defn: chair flip (cyclohexane)
one chair form is converted to the other
process: chair flip (cyclohexane) (2)
- the cyclohexane molecule briefly passes through a fourth conformation called the half-chair conformation
- after the chair flip: all axial groups become equatorial and vice versa; dashes remain dashes, wedges remain wedges
what might slow down a chair flip interconversion?
if a bulky group is attached to the ring (such as tert-butyl)
what are the preferred conformations of substituted rings? rings with more than one substituent? why?
for substituted rings: bulkiest group favors the equatorial position to reduce nonbonded strain (flagpole interactions) with axial groups in the molecule
for rings with more than one substituent: preferred chair form is determined by the larger group which prefer the equatorial position
defn: cis vs. trans rings
CIS = both groups are located on the same side of the ring (i.e. wedge wedge)
TRANS = the groups are on opposite sides of the ring (i.e. wedge dash)
defn + 2 types: configurational isomers
can only change from one form to another by breaking and reforming covalent bonds
enantiomers, diastereomers
defn + 2 types: optical isomers
the different spatial arrangement of groups affects the rotation of plane-polarized light
enantiomers, diastereomers
defn + aka + example: chiral
its mirror image cannot be superimposed on the original object (lacks internal plane of symmetry)
= handedness (although identical, the left hand cannot fit into a right-handed glove)
defn + example: achiral
objects have mirror images that can be superimposed (i.e. a fork; a carbon atom with only 3 different substituents)
defn + char: chiral center
most common: carbon atom with 4 different substituents
an asymmetrical core of optical activity
defn + char (2): enantiomers
nonsuperimposable mirror images
- the same connectivity but opposite configurations at every chiral center in the molecule
- identical physical and chemical properties with 2 notable exceptions: optical activity and reactions in chiral environments
defn: diastereomers
chiral, share the same connectivity, but are not mirror images of each other because they differ at some (but not all) of their mulitple chiral centers
defn: optically active
it has the ability to rotate plane-polarized light
defn: unpolarized (ordinary) light
it consists of waves vibrating in all possible planes perpendicular to its direction of propagation
func: polarizer
allows light waves oscillating only in a particular direction to pass through, producing plane polarized light
defn: optical activity
the rotation of this plane-polarized light by a chiral molecule
how do enantiomers differ in terms of optical activity?
one enantiomer 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)
enantiomers and optical activity: d vs. l compounds
d/(+) = a compound that rotates the plane of polarized light right (clockwise) = dextrorotary
l/(-) = a compound that rotates the plane of polarized light toward the left (ccw) = levorotatory
how is the direction of rotation (d vs. l) determined? how is it not?
experimentally
NOT related to the absolute configuration of the molecule
what does the amount of rotation in optical activity depend on? (3)
- the number of molecules a light wave encounters. what does this depend on?
- the concentration of the optically active compound
- the length of the tube through which the light passes
eqn + func of eqn: standardized specific rotation
rotations measured at different concentrations and tube lengths can be converted to a standardized rotation using this equation
defn + char (2): racemic mixture
when both (+) and (-) enantiomers are present in equal concentrations
- the rotations cancel each other out, no optical activity is observed
- will not rotate plane-polarized light
if enantiomerism = handedness, what does racemic mixture =?
ambidexterity
how can one separate a racemic mixture into its two constituent isomers? (3)
- reacting 2 enantiomers with a single enantiomer of another compound leads to 2 diastereomers
- diastereomers have different physical properties which enable us to separate them by common lab techniques
- once separated, they can be reacted to regenerate the original enantiomers
defn (3): diastereomers
non-mirror-image configurational isomers
occur when a molecule has two or more stereogenic centers and differs at some, but not all, of these centers
ANY STEREOISOMER THAT IS NOT AN ENANTIOMER
char + why (3): diastereomers
- have different chemical properties, but might behave similarly in particular reactions because they have the same functional groups
- different physical properties bc they have different arrangements in space
- rotate plane-polarized light, but knowing the specific rotation of one diastereomer gives no indication of the specific rotation of another diastereomer (stark difference from enantiomers which will always have equal-magnitude rotations in opposite directions)
how many possible stereoisomers are there for any molecule with n chiral centers?
2^n
defn: cis-trans isomers (aka: geometric isomers)
a specific type of diastereomer in which substituents differ in their position around an immovable bond (a double bond, a ring structure)
cis = substituents on the same side of the immovable bond
trans = substituents on opposite sides of the immovable bond
when do we use cis-trans vs E/Z?
for single substituted double bonds: cis/trans
for polysubstituted double bonds: (E)/(Z)
what two things must be true for a molecule to have optical activity?
- must have chiral centers within it
- must lack a plane of symmetry (can occur through the chiral center or between chiral centers)
defn + char (2): meso compound
a molecule with chiral centers that has an internal plane of symmetry
- does not display optical activity
- the molar equivalent of a racemic mixture
defn: configuration of a stereoisomer
the spatial arrangement of the atoms or groups in the molecule
defn: relative configuration of a chiral molecule
its configuration in relation to another chiral molecule (often through chemical interconversion)
what is the function of the relative configuration?
to determine whether molecules are enantiomers, diastereomers, or the same molecule
defn: absolute conformation of a chiral molecule
the exact spatial arrangement of these atoms or groups, independent of other molecules
what are the Cahn-Ingold-Prelog priority rules used for?
determining the E/Z designation
what is E/Z nomenclature used for?
compounds with polysubstituted double bonds
explain the Cahn-Ingold-Prelog priority rules (3)
- 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, still based on the higher atomic number
- if a tie remains, the atoms in this group are compared one b one in descending atomic number order until the tie is broken
ultimately, what do E/Z mean from the Cahn-Ingold-Prelog priority rules?
Z = zusammen = together = if the two highest priority substituents on each carbon are on the same side of the double bond
E = entgegen = opposite = if they are on opposite sides
mnemonic: E/Z
Z = ‘Z’ame side
E = ‘E’pposite side
func: R/S nomenclature
used for chiral (stereogenic) centers in molecules
summarize the 4 steps it takes to determine the absolute configuration R/S of a chiral center?
- Assign priority by atomic number
- Arrange the molecule with the lowest-priority substituent in the back (classic) or invert the stereochemistry by switching two substituents (modified)
- Draw a circle around the molecule from highest to lowest priority (1-3)
- Write the name (clockwise = R, counterclockwise = S)
explain step 1 of determining absolute configuration R/S: assign priority (4)
- use the cahn-ingold-prelog priority rules described earlier to assign priority to the 4 substituents, looking only at the atoms directly attached to the chiral center
- 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
explain classic step 2 of determining absolute configuration R/S: arrange in space (2)
- orient the molecule in 3-D space so that the atom with lowest priority (usually H) is at the back of the molecule (arrange the POV so that the line of sight proceeds down the bond from the asymmetrical carbon atom (the chiral center0 to the substituent with the lowest priority
- the 3 substituents with higher priority should then radiate out from the central carbon
explain modified step 2 of determining absolute configuration R/S: invert the stereochemistry (3)
main simple rule: any time two groups are switched on a chiral carbon, the stereochemistry is inverted
- thus, we can simply switch the lowest-priority group with the group at the back of the molecule (the substituent projecting into the page)
- when we proceed to step 3, keep in mind that we have changed the molecule to the opposite configuration, so we need to remember to switch our final answer (R to S, or S to R)
explain step 3 of determining absolute configuration R/S: draw a circle (3)
- imagine drawing a circle connecting the substituents from number 1 to 2 to 3 (don’t pay attention to the lowest priority group)
- IF the circle is ccw, the atom is called S (sinister, left)
- IF the circle is cw, it is called R (rectus, right)
explain step 4 of determining absolute configuration R/S: write the name
(R) and (S) are put in parentheses and separated from the rest of the name by a hyphen
char + func: Fischer projection
one way to represent 3-D molecules
- horizontal lines indicate bonds that project out from the plane of the page (wedges)
- vertical lines indicate bonds going into the plane of the page (dashes)
- the point of intersection of the lines represents a carbon atom
how do we determine configurations using Fischer projections? what are two benefit to using Fischer projections for this?
we follow the same rules as we did earlier
- the lowest-priority group can be on the top or bottom of the molecule and still project into the page
- we can manipulate Fischer projections without changing the compound
we know that switching two substituents around a chiral carbon will invert the stereochemistry, what is the equivalent for a Fischer projection?
rotating a Fischer projection in the plane of the page by 90 degrees
how do we revert the stereochemistry back to the original in both forms?
- interchanging ANY two pairs of substituents
- rotating a Fischer projection in the plane of the page by 180 degrees
what do we do with a Fischer projection if our lowest-priority group is pointing to the side (and as such, out of the page)? (3 tricks)
- Make 0 switches
Determine the order of substituents as normal, drawing a circle from 1 to 2 to 3. Then obtain the R/S designation. The true is the opposite of that - Make 1 Switch
Swap the lowest-priority group with one of the groups on the vertical axis. Obtain the R/S designation, the true is the opposite of that - Make 2 swtiches
Move the lowest-priority group into the correct position. Switch the other two groups as well. Correct designation.
