Chapter 4 Flashcards
What are conformational isomers and configurational isomers, and how do they differ?
Conformational isomers (conformers): stereoisomers that rapidly interconvert at room temperature and cannot be separated due to this interconversion. (cannot isolate different conformers)
Configurational isomers: stereoisomers that cannot interconvert unless covalent bonds are broken. As a result, they can be separated.
What are the two types of conformational isomers, and what causes their differences?
Conformational isomers can arise from two main causes: 1. rotation about carbon-carbon single bonds
2. amine inversion
(chap 3): Rotation about carbon-carbon single bonds results in conformational isomers such as staggered, eclipsed, chair, boat, etc.
What are the two types of configurational isomers?
cis-trans isomers and isomers that contain asymmetric centers.
What is another name for cis-trans isomers?
geometric isomers
From what are cis-trans isomers resulted from?
restricted rotation
What causes restricted rotation in cis-trans isomers?
cyclic structure or double bond
What would need to happen for a double bond to start rotating?
if the π bond breaks and unhybridized p orbitals are not parallel anymore orbitals
Is energy barrier to rotation stronger in singke bond or double bonds?
double bonds
What are the two different configurations on a double bond?
■ The compound with the hydrogens on the same side of the double bond is called the cis isomer.
■ The compound with the hydrogens on opposite sides of the double bond is called the trans isomer.
cis and trans isomers have the same molecular formula and the same bonds but have different configurations
What happens if the one of the sp2 carbons is attached to two identical substituents on a double bond?
the compound cannot have cis and trans isomers.
Why can cis and trans isomers be separated from each other?
because they are different compounds with differ- ent physical properties
for example, they have different boiling points and different dipole moments.
In what compounds are E/Z used instead of cis/trans?
for alkenes that do not have a hydrogen attached to each of the sp2 carbons.*
*The IUPAC prefers the E and Z designations because they can be used for all alkene isomers. Many chemists, however, continue to use the “cis” and “trans” designations for simple molecules.
How are the substituents placed on E/Z isomers?
How do you determine priority in a E/Z isomer on a double bond?
relative priorities depend on the atomic numbers of the atoms bonded directly to the sp2 carbon.
The greater the atomic number, the higher the priority.
Notice that you use the atomic number of C, not the mass of the CH3 group, because the priorities are based on the atomic numbers of atoms, not on the masses of groups.)
How do you determine relative priority if both atoms attached to the carbon are the same (there is a tie)?
consider the atomic numbers of the atoms that are attached to the “tied” atoms.
The C of the CH2Cl group is bonded to Cl, H, H, and the C of the CH2CH2Cl group is bonded to
C, H, H. Cl has a greater atomic number than C, so the CH2Cl group has the higher priority
How do you determine priority when the first atom of the substituent is bonded to the rest of the atoms with a double or triple bond?
treats it as if it were singly bonded to two of those atoms. If an atom is triply bonded to another atom, it is treated as if it were singly bonded to three of those atoms.
example: Because the atoms bonded to the sp2 carbon are both carbons, there is a tie. Each of the carbons is bonded to C, H, H, so there is another tie.
We turn our attention to the groups attached to the CH2 groups to break the tie. One of these groups is CH2OH, and the other is C‚CH; the C of the CH2OH group is bonded to H, H, O; the triple-bonded C is considered to be bonded to C, C, C. Of the six atoms, O has the greatest atomic number, so CH2OH has the higher priority.
How do you determine priority if two isotopes (atoms with the same atomic number but different mass numbers) are being compared
the mass number is used to determine the relative priorities
For example, in the isomer shown next on the left, the sp2 carbon on the left is bonded to a deuterium (D) and to a hydrogen (H): D and H have the same atomic number, but D has a greater mass number, so D has the higher priority.
How do you call objects with a left-handed and a right-handed form?
chiral objects
What does its mean when an object is chiral?
(that has a left-handed and a right-handed form)
that it has a nonsuperimposable mirror image
What is an nonsuperimposable image?
an image that is different from the object
What is an achiral object?
The mirror image is the same as the object.
the image and the object are superimposable.
What causes chirality in a molecule?
asymmetric center
What is an asymmetric center?
also called a chiral center) is an atom bonded to four different groups.
define enantiomers
Molecules that are nonsuperimposable mirror images of each other
What differs between the terms asymmetric center and stereocenter?
stereocenter:
atom at which the interchange of two groups produces a stereoisomer.
include:
(1) asymmetric centers, where the interchange of two groups produces an enantiomer
(2) the sp2 carbons of an alkene or the sp3 carbons of a cyclic compound, where the interchange of two groups converts a cis isomer to a trans isomer or vice versa.
asymmetric centers are stereocenters, not all stereocenters are asymmetric centers.
What does interchanging any two groups on an asymmetric center do? And if changed any two other?
changed the first time: makes an enantiomer
changed a second time (no necessarily the two same groups): back to original compound
can be verified with R/S
To which priority system is R/S the same for?
priority for E/Z isomers
What are some properties that enantiomers share?
same boiling points, the same melting points, and the same solubilities (physical properties except those that stem from how groups bonded to the asymmetric center are arranged in space)
What is one property that enantiomers don’t share?
the way they interact with plane-polarized light
What is plane-polarized light?
light in which rays in the beam oscillate in a single plane in opposed to all directions like normal light
produced by passing normal light through a polarizer
What happens when plane-polarized light passes through a solution of achiral molecules?
the light emerges from the solution with its plane of polarization unchanged
What happens when plane-polarized light passes through a solution of chiral molecules?
the light emerges with its plane of polarization rotated either clockwise or counterclockwise
If one enantiomer rotates it clockwise, its mirror image will rotate it exactly the same amount counterclockwise.
How do you call a compound that rotates the plane of polarization of plane-polarized light?
the compound is optically active
Are chiral compound optically active or inactive?
optically active
Are achiral compound optically active or inactive?
optically inactive
What a dextrorotatory compound? how can it be indicated in its name?
an optically active compound that rotates the plane of polarization clockwise
by the prefix (+)
What a levorotatory compound? how can it be indicated in its name?
an optically active compound that rotates the plane of polarization counterclockwise
by the prefix ( - )
What can also be used as prefixes for dextrorotatory and levorotatory other than (+) and (-)?
d and l (lowercase L)
How do you write a compound name with R/S and (+)/(-)?
What is observed rotation?
The amount the analyzer in a polarizer is rotated (in degrees)
What is the formula for specific rotation?
How do you tell the specific rotation of a compound based on the specific rotation of its enantiomer?
the mirror-image molecule rotates the plane of polarization the same amount but in the opposite direction.
ex: If one enantiomer has a specific rotation of + 5.75, the specific rotation of the other enantiomer must be -5.75
What does this symbol mean?
used to specify a racemic mixture.
(±)-2-bromobutane indicates a mixture of 50% (+)-2-bromobutane and 50% (-)-2-bromobutane.
What is observed specific rotation?
the specific rotation of the particular sample. For example:
How can you calculate enantiomeric excess?
can be calculated from the observed specific rotation:
What are erythro enantiomers?
enantiomers with the hydrogens on the same side of the carbon chain
What are threo enantiomers?
enantiomers with the hydrogens on opposite sides
How can you make it easier to see for erythro enantiomers that similar groups are on the same side? (perspective formula)
by rotating the right side of the molecule about the horizontal C—C bond
How many stereoisomers can 1-Bromo-3-methylcyclobutane have?
only two because it does not have any asymmetric center.
Why are mesocompounds optically inactive?
Since each half of the mesocompound is the mirror image of the other, if the top half of the molecule rotates polarized light to the right, then the bottom half will rotate the light the same amount to the left. Thus, they will cancel each other and the compound will not be optically active.
How do you recognize a compound that will only have 3 stereoisomers?
has the same four atoms or groups bonded to two different asymmetric centers (les deux centres ont les même quatres)
will result in 1 mesocompound and two enantiomers
For cyclics, how do you recognize a compound that will only have 3 stereoisomers?
The two substituents attached to the two asymmetric centers are the same
If a 6 carbon ring with two asymmetric centers and two same substituents on them is mostly found in chair conformer (no plane of symmetry) is it still a mesocompound?
Yes, if a compound has a conformer with a plane of symmetry, the compound is achiral, even if the conformer with the plane of symmetry is not the most stable conformer.
Are mesocompounds always drawn in a way that we can see their plane of symmetry?
No, depending if they are drawn in staggered or eclipse formation, we sometimes need to move groups around to see it.
Can other atoms than carbon be asymmetric centers? Which ones?
yes: any atom with four different groups attached to them
most common for this class other than C are N (nitrogen) and P (phosphorous)
How can you picture amine inversion?
to think of an umbrella that turns inside out in a windstorm
What happens during amine inversion?
through a transition state in which the sp3 nitrogen becomes an sp2 nitrogen.The three groups bonded to the sp2 nitrogen lie in a plane, and the lone pair is in a p orbital perpendicular to the plane.
What is a receptor?
protein that binds a particular molecule
Do enantiomers bind the same way on receptors?
Because proteins are chiral, a receptor binds one enantiomer better than the other.
ex: the receptor binds the R enantiomer, but it does not bind the S enantiomer, just as a right hand prefers a right glove.
What makes us able to distinguish different smells?
receptors located on the exteriors of nerve cells in the nose are able to perceive and differentiate the estimated 10,000 smells to which they are exposed.
The reason (R)-(-)-carvone (found in spearmint oil) and (S)-(+)-carvone (the main constituent of caraway seed oil) have such different odors is that each enantiomer fits into a different receptor.
What is chrematography?
In this method, the mixture to be separated is dissolved in a solvent and the solution is passed through a column packed with a chiral material that adsorbs organic compounds. The two enantiomers move through the column at different rates because they have different affinities for the chiral material—just as a right hand prefers a right-hand glove to a left-hand glove—so one enantiomer emerges from the column before the other.
