MCAT Organic Chemistry
Questions and Answers 100%
Correct
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 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.
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
When a cyclohexane ring flips conformation, what happens to the equatorial and
axial Hydrogens? - ANSWER - They trade conformations. Axials become equatorials,
and equatorials become axials.
Where are cyclohexane substituents more energetically favored? - ANSWER - At the
equatorial position.
