Ch 4: Structure and Stability (Princeton Review) Flashcards
What is an achiral object?
example?
An object that IS superimposable upon its mirror image (after being rotated). The best example is a coffee cup.
(Khan Academy video)
What is a chiral object?
An object that is NOT superimposable upon its mirror image. (Remember that the word chiral comes from Greek kheir which means hand, and our hands are chiral objects.)
What is a constitutional isomer?
Isomers that have the same molecular formula, but different connectivity (also known as structural isomers)
If the connectivity is the same, but the molecules have a different spatial arrangement, they are stereoisomers.
p. 68, 77
What is a configurational isomer?
These are stereoisomers that cannot be interconverted by rotation about single (sigma) bonds.
If they CAN be interconverted, then they are conformational isomers.
What is geometric isomerism?
Two configurational isomers are geometric isomers if the isomerism occurs at a double bond.
If this is not the case, they are optical isomers.
What are enantiomers?
Two optical isomers are enantiomers if they are non-superimposable mirror images of one another. (comes from Greek for “opposite”)
If they non-superimposable non-mirror images of each other, they are diastereomers.
Define stereoisomers.
molecules that differ in the 3-D arrangement of atoms
What is a chirality center?
a tetrahedral carbon (sp3 hybridized) that has 4 different groups attached.
1 chiral center implies 2 different stereoisomers
2n is the formula used to calculate the number of possible stereoisomers, where n is the # of chirality centers present in a molecule
In looking at 2 conformational isomers, it is important to recognize that there are an ________ number of conformations for a molecule that has free rotation around a C-C bond. Also, any change in the rotation of one carbon, relative to its adjacent neighbor, is a change in _________ ____________.
infinite
molecular conformation
p. 70
Which is more stable? A staggered conformation or an eclipsed conformation and why?
The staggered conformation is more stable bc of electronic repulsion. This shape places the sigma bonds as far apart as possible.
p. 70
The more unable the conformation, the higher the ________ ______.
potential energy
The staggered conformation is referred to as the ____ __________ and arises when the 2 largest groups attached to adjacent carbons are 180 degrees apart. This produces the most sterically favorable, and hence the most energetically favorable (______ energy) conformation.
anti conformation
lowest
p. 71
A ______ conformation arises when the two largest groups on adjacent carbon atoms are in a staggered conformation 60 degrees apart.
gauche
When in the eclipsed conformation with the 2 methyl groups directly aligned behind one another and therefore in the most crowded position, we refer to this as the ___ __________.
syn conformation
p. 71
It is possible for the gauche conformation to be more stable than the anti conformation because of intramolecular hydrogen bonds.
p. 72
Considering cyclohexane, the most stable conformation is a very puckered molecule referred to as the ____ ____.
chair form
p. 73
When analyzing the structure of cyclohexane, it is more favorable for large groups to occupy the __________ position rather than a crowded axial position.
equatorial
p. 75
In determining absolute configuration (R, S) types, what are the 3 rules?
- Prioritize the 4 groups attached to the chirality center; the higher the atomic #, the higher the priority.
- Orient the groups so that the lowest priority group is projecting away from you.
- Determine if the sequence 1-2-3 is going clockwise (R), or counterclockwise (S).
(remember R stands for “RIGHT”, as in going around the clock the right way)
If you are determining the absolute configuration of a molecule that contains 2 isotopes, which should get higher priority?
the heavier isotope
p. 81
Where do the abbreviations ‘R’ and ‘S’ come from and what do they mean?
R comes from Latin “rectus” meaning right
S comes from Latin “sinister” meaning left
p. 82
In a Fischer projection, _______ lines are assumed to go back into the page, and _______ lines are assumed to come out of the page.
vertical
horizontal
(imagine the horizontal lines as arms, coming out to give you a hug)
p. 82
How do two enantiomers differ from one another?
Most properties such as melting point, boiling point, polarity, and solubility are the same. One difference though, is their optical activity.
In the eclipsed conformation (think Newman projection of butane), the “dihedral angle” or “torsion angle” is __ degrees.
0 degrees
A compound that rotates plane-polarized light is said to be _________ ______.
optically active
p. 87
If an optically active compound rotates light clockwise, it is said to be ____________.
dextrorotatory (d), also denoted by +
p. 87
If an optically active compound rotates light counterclockwise, it is said to be ____________.
levorotatory (l, also denoted by -)
p. 87
The magnitude of rotation of plane-polarized light for any compound is called its _______ _______.
What is the formula?
specific rotation
p. 87
A pair of enantiomers will rotate plane-polarized light with equal magnitude, but in ________ __________.
opposite directions
p. 87
The specific rotation of a 50/50 mixture of enantiomers –- a _______ mixture -– is _ degrees. Therefore, this mixture, known as a _______, is not optically active!
racemic
0 degrees
racemate
p. 87
Remember that the R and S designation has nothing to do with (+) and (-). These two notations describe separate, unrelated properties.
Diastereomers are stereoisomers that are not __________. That is, they are non-superimposable, non-mirror images.
enantiomers
p. 89
True or False?
Diastereomers can have different physical properties, including melting and boiling points.
True
In the chair conformation of cyclohexane, where will the larger groups “want” to be in order to prevent gauche interactions?
equatorial with the ring; bc of this, the hydrogens should be axial
Enantiomers like (R)-carvone and (S)-carvone may have the same melting point, boiling point, and density, but they have very different ______.
scents
A molecule is said to be saturated if it contains no…
…pi bonds and no rings.
p. 46
A saturated compound with n carbon atoms has exactly….
….2n + 2 hydrogens.
p. 46
If a molecule is optically active, it must be ______.
chiral
To experimentally to determine if a molecule is chiral, you can….
….shine plane-polarized light through the sample.
Nucleophiles are species that have ________ _____ of electrons or _______ and, frequently, a negative or partial negative charge. As the name nucleophile implies, they are “nucleus-seeking” or “nucleus-loving” molecules. Since nucleophiles are electron pair donors, they are also known as Lewis bases.
unshared pairs
pi bonds
p. 63
Nucleophilicity increases as negative charge _______.
increases
p. 63
Nucleophilicity ________ going down the periodic table within a particular group.
increases
This trend is directly related to polarizability, or how easy it is for the electrons surrounding an atom to be distorted. As you go down any group in the periodic table, atoms become larger and generally more polarizable and more nucleophilic.
p. 63
Nucleophilicity increases going _____ in the periodic table across a particular period.
left
This trend is related to the electronegativity of the nucleophilic atom. The more electronegative the atom is, the better it is able to support its negative charge. Therefore, the less electronegative an atom is, the higher its nucleophilicity.
p. 63
Nucleophilic substitution properly describes the nucleophilic attack of an -OH group onto a ________ to create an ester.
carbonyl
The general ranking for organic compound acidity is:
- ) strong acids
- ) sulfonic acids
- ) carboxylic acids
- ) phenols
- ) alcohols and water
- ) aldehydes and ketones (α hydrogens)
- ) sp hybridized C-H bonds
- ) sp2 hybridized C-H bonds
- ) sp3 hybridized C-H bonds
What is an epimer?
a subclass of diastereomers that differ in their absolute configuration at a single chiral center (only 1 stereocenter is inverted)
p. 94
Just like R and S, D and L are entirely ________ to optical activity.
unrelated
p. 94
