MCAT Organic Chemistry UPDATED
ACTUAL Questions and CORRECT
Answers
What are the four rules for writing resonance structures? - CORRECT 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? - CORRECT 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 - CORRECT ANSWER- Planar monocylic rings with (4n+2) Pi-electrons must
be aromatic (have resonance).
What is the condition for a chiral center? - CORRECT 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? - CORRECT 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? - CORRECT
ANSWER- No
A chiral molecule's mirror image has the _______ absolute configuration - CORRECT
ANSWER- Same
What are the conditions of relative configuration? - CORRECT 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 - CORRECT ANSWER- The degree to which compounds rotate plane
polarized light.
How does a polarimeter work? - CORRECT ANSWER- A polarimeter screens out all but one
group of photons that share the same electromagnetic field orientation.
What is a racemic mixture? - CORRECT 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? - CORRECT ANSWER- It rotates
plane polarized light to the right. (Dexter = right in Latin)
If a compound is labeled "-" or "L", what does that mean? - CORRECT ANSWER- It rotates
plane polarized light to the left. (Laevus = left in Latin)
What are the conditions of stereoisomers? - CORRECT 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)? - CORRECT
ANSWER- They must have a minimum of 1 chiral center in the same relative three
dimensional location.
,Enantiomers are? - CORRECT 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? - CORRECT ANSWER- The ratio of observed rotation to pure sample
rotation.
What is enantiomeric purity? - CORRECT ANSWER- The ratio of pure enantiomer to
racemic mixture.
Diastereomers - CORRECT ANSWER- Same molecular formula, same bond to bond
connectivity, not mirror images, not equal molecules.
Why do geometric isomers arise? - CORRECT 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? - CORRECT 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. -
CORRECT 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? - CORRECT 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?
- CORRECT 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? - CORRECT ANSWER- 0, 1, 2, and 3, respectively.
List some physical properties of alkanes. - CORRECT 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? - CORRECT 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. -
CORRECT 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? - CORRECT ANSWER- They trade conformations. Axials become equatorials,
and equatorials become axials.
Where are cyclohexane substituents more energetically favored? - CORRECT ANSWER- At
the equatorial position.
What is a combustion reaction? - CORRECT ANSWER- The mixing of alkanes (and similar
oxygen containing "CH" groups) with molecular oxygen and an activation energy to produce
Carbon Dioxide, Water, and large quantities of heat.
Heat of Combustion - CORRECT ANSWER- Change in enthalpy of a combustion reaction.
High heats of combustion means that a molecule needed a large amount of energy to override
