MCAT Organic Chemistry exam with 100%
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 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.
What is a combustion reaction? - 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 - ANSWER-Change in enthalpy of a combustion reaction. High
heats of combustion means that a molecule needed a large amount of energy to
override its intrinsic energy, meaning it contained a large amount of energy. This large
amount of energy implies that it relatively unstable. Therefore heats of combustions can
be used to compare relative stabilities of compounds. Note the difference between
molar heat of combustion that determines if the group were to combust by itself, and the
"per CH2" heat of combustion, which includes factors such as ring strain and steric
hindrance.
Free radical reactions with Alkanes are possible with: - ANSWER-All the Halogens,
except I (so F, Cl, Br, but NOT I) can react in the presence of heat or light to form a free
radical, which is atom with a single electron on it.
What are the steps of halogenation? - ANSWER-1. Initiation- the halogen is cleaved by
light or heat from a diatomic molecule to a free radical.
2. Propogation- The halogen radical removes a proton from the alkane, forming an alkyl
radical. The alkyl radical can bind with a halogen radical to create an alkyl halide. This is
propogation, which can continue indefinitely or:
