©THEBRIGHT EXAM STUDY SOLUTIONS 8/22/2024 12:54 PM
MCAT Organic Chemistry Exam Questions
And Correct Answers
What are the four rules for writing resonance structures? - answer✔✔1. Atoms must not be
moved. Only electrons.
2. All resonance atoms must lie in the same plane.
3. Only valid Lewis structures are allowed.
4 . The number of unpaired electrons must remain constant.
What are the two conditions for resonance? - answer✔✔1. An atom must have a P orbital or a
lone pair of electrons.
2. It must be single bonded to an atom that has a double or triple bond.
The latter compromise conjugated unsaturated systems.
Huckel's Rule - answer✔✔Planar monocylic rings with (4n+2) Pi-electrons must be aromatic
(have resonance).
What is the condition for a chiral center? - answer✔✔First, the participant in question cannot be
a halogen, a CH2 group, an Oxygen, a Halogen, or any Carbon participating in a double or triple
bond.
Then list the groups attached to a Carbon. They MUST be four different groups. If the conditions
are met, then the Carbon in question is a chiral center.
How do you determine Absolution Configuration? - answer✔✔First, prioritize the atoms in
terms of atomic weights (if two atoms are the same element, then their substituents are compared
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to determine priority. Substituents on double and triple bonds count 2 and 3 times, respectively).
Now, rotate the sigma bond of the Carbon so that the lowest priority group faces away. Next,
order the numbers in terms of increasing priority. If it increases clockwise, then the atom has "S"
absolute configuration. if it increases counterclockwise, it has "R" absolute configuration.
Does absolute configuration determine rotation of plane polarized light? - answer✔✔No
A chiral molecule's mirror image has the _______ absolute configuration - answer✔✔Same
What are the conditions of relative configuration? - answer✔✔The two atoms in question must
differ in the attachment of only one substituent, and the remaining substituents must be oriented
identically relative to the Carbon.
Observed Rotation - answer✔✔The degree to which compounds rotate plane polarized light.
How does a polarimeter work? - answer✔✔A polarimeter screens out all but one group of
photons that share the same electromagnetic field orientation.
What is a racemic mixture? - answer✔✔A mixture of equal stereoisomers that yields optically
inactive compounds (optically inactive compounds also arise from chirality).
If a compound is labeled "+" or "D", what does that mean? - answer✔✔It rotates plane polarized
light to the right. (Dexter = right in Latin)
If a compound is labeled "-" or "L", what does that mean? - answer✔✔It rotates plane polarized
light to the left. (Laevus = left in Latin)
What are the conditions of stereoisomers? - answer✔✔Molecules with the same molecular
formula and bond to bond connectivity, but are not the same compound.
Stereoisomers must be what (exclude geometric isomers from this definition)? - answer✔✔They
must have a minimum of 1 chiral center in the same relative three dimensional location.
Enantiomers are? - answer✔✔Enantiomers have the same molecular formula, the same bond to
bond connectivity, are mirror images of one another, but are different molecules.
Optical Purity is? - answer✔✔The ratio of observed rotation to pure sample rotation.
What is enantiomeric purity? - answer✔✔The ratio of pure enantiomer to racemic mixture.
Diastereomers - answer✔✔Same molecular formula, same bond to bond connectivity, not mirror
images, not equal molecules.
Why do geometric isomers arise? - answer✔✔Due to a mitigated ability to rotate about a bond,
either via ring closure or multiple bonding.
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List physical property differences between Cis and Trans isomers? - answer✔✔Cis isomers have
a net dipole moment and thus experience a higher boiling point than Trans isomers. However,
because a Cis isomer has lower possible permutations to form a crystal lattice relative to a Trans
isomer, Cis isomers do not form crystals as readily.
Higher order alkanes can be differentiated via substituent by the E-Z method. Elaborate. -
answer✔✔One were to prioritize the substituents on a Carbon. Then, if one were to note that the
higher priority substituent exists on opposite sides, it gets the E label (Entgegen: German for
Opposite). If the higher priority substituents are on the same side, it gets the Z label (Zusamen:
German for Together)
What is a meso compound? - answer✔✔Chiral centers that offset one another due to symmetry.
The net result is an optically inactive compound. In essence, meso compounds are actually a
chiral. They can be identified by finding a plane of symmetry that divides the compound into two
mirror images.
What is the formula for maximum number of optically active isomers in a single compound? -
answer✔✔2^n, where n is the number of chiral centers.
How many functional groups are methyl, primary, secondary, and tertiary alkyl groups
connected to, respectively? - answer✔✔0, 1, 2, and 3, respectively.
List some physical properties of alkanes. - answer✔✔They have low densities. The more
molecular weight an alkane has (more Carbon chains), the higher its boiling point due to
intermolecular London Dispersion forces. Branching decreases the boiling points because the
surface area of the molecule is lowered, and the molecule cannot form a more relatively polar
transient dipole. Unbranches alkanes also exhibit an increase in melting point in molecular
weight, but not as predictable as the boiling point of alkanes.
What is ring strain? - answer✔✔The strain caused by a cycloalkane deviating from the optimal
SP3 109.5 degree angle. Ring strain is usually alleviated by the movement of certain bonds away
from a flat three dimensional plane.
List the relative stabilities of cyclohexane conformers when undergoing a ring flip. -
answer✔✔Chair (low)
Half Chair (highest)
Twist (lower than Half Chair)
Boat (higher than twist but lower than Half Chair)
Twist of the other side
Half chair of the other side
Chair of the other side <--- Ring flip done
