MCAT Organic Chemistry Exam Questions with Correct Solutions Latest Version (Already Passed)

MCAT Organic Chemistry Exam Questions with Correct Solutions Latest Version (Already Passed) What are the four rules for writing resonance structures? - Answers 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? - Answers 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 - Answers Planar monocylic rings with (4n+2) Pi-electrons must be aromatic (have resonance). What is the condition for a chiral center? - Answers 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? - Answers 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? - Answers No A chiral molecule's mirror image has the _______ absolute configuration - Answers Same What are the conditions of relative configuration? - Answers 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 - Answers The degree to which compounds rotate plane polarized light. How does a polarimeter work? - Answers A polarimeter screens out all but one group of photons that share the same electromagnetic field orientation. What is a racemic mixture? - Answers 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? - Answers It rotates plane polarized light to the right. (Dexter = right in Latin) If a compound is labeled "-" or "L", what does that mean? - Answers It rotates plane polarized light to the left. (Laevus = left in Latin) What are the conditions of stereoisomers? - Answers 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)? - Answers They must have a minimum of 1 chiral center in the same relative three dimensional location. Enantiomers are? - Answers 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? - Answers The ratio of observed rotation to pure sample rotation. What is enantiomeric purity? - Answers The ratio of pure enantiomer to racemic mixture. Diastereomers - Answers Same molecular formula, same bond to bond connectivity, not mirror images, not equal molecules. Why do geometric isomers arise? - Answers 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? - Answers 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. - Answers 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? - Answers 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? - Answers 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? - Answers 0, 1, 2, and 3, respectively. List some physical properties of alkanes. - Answers 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? - Answers 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. - Answers 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? - Answers They trade conformations. Axials become equatorials, and equatorials become axials. Where are cyclohexane substituents more energetically favored? - Answers At the equatorial position. What is a combustion reaction? - Answers 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 - Answers 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: - Answers 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? - Answers 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: 3. Termination- Either two alkyl radicals or two halogen radicals bind together (contact with container's wall), or the remaining alkyl radicals can bond with the remaining halogen radicals. Rank the stability of alkyl radicals (same as carbocation stability) from highest to lowest - Answers Tertiary Secondary Primary Methyl What are the rankings of halogenation in terms of reactivity(high to low)? - Answers F, Cl, Br, and I. Br requires heat I is not reactive at all. What are the rankings of halogenation in terms of selectivity (how "picky" a halogen radical is when choosing a position on an alkane)? - Answers The reverse of reactivity. True or false. Combustion and halogenation reactions are both exothermic, but only halogenation is radical. - Answers False- Both combustion and halogenation are exothermic AND radical. What is special about five Carbon rings and less with respect to optical properties? - Answers The Cis isomers are meso compounds. Do substituents necessarily attach to the Hydrogen of the most stable Carbocation all the time to form the major product, even if a relatively selective halogen is being used? - Answers No, the "supply" of Hydrogen atoms must also be considered WITH selectivity. For example, Chlorine reacting with 2-Methylbutane would make one assume that the product is 2-chloro-2methyl butane, the tertiary alkyl halide. However, mass action would explain that there is a chance that Chlorine would attach to 9 Hydrogens on primary Carbons and 2 Hydrogens on secondary Carbons. Those must also be taken into account. What is an alkene? - Answers A Carbon chain with a Carbon-Carbon double bond Are alkenes more acidic than alkanes? Why? - Answers Yes. Because alkenes have double bonds that can stabilize their conjugate bases. The double bond is longer than the single bond of an alkane, and more negative in charge than the single bond of an alkane. This large surface area of negative charge makes alkenes attractive to electrophiles. What is the rule for thermodynamic stability of alkenes? - Answers The more highly substituted an alkene, the more thermodynamically stable. Why is it that more stable alkenes are more reactive when reacted with an electrophile? - Answers Because when an alkene reacts with an electrophile, an intermediate Carbocation forms. This Carbocation is stabilized when there are more alkyl groups to donate their electrons to the Carbocation. List some physical properties of alkenes. - Answers Alkenes exhibit an increase in boiling point due to stronger London Dispersion forces. Branching decreases boiling point due to a lower surface area, which creates a less polarized dipole, lowering the effective energy needed to overcome the alkene to separate it to a gaseous phase. Alkenes are slightly soluble in water due to the double bond (acidity implies slight polarity, which implies slight solubility) From the behavior of alkenes, predict the solubility of alkynes. - Answers Alkynes should be slightly more polar, and thus slightly more soluble in water than alkenes. What is the product for dehydration of an alcohol? - Answers An Alkene What is the mechanism for E1 reaction of an alcohol? - Answers The acid protonates the hydroxyl group on the alcohol, making water, which is a good leaving group. Water takes all the electrons of the water-Carbon bond and leaves, leaving behind a Carbocation (this is the slowest, rate determining step). The Carbocation may or may not rearrange to its most stable form. Finally, a water deprotonates a Carbon adjacent to the Carbocation, and a double bond between the two parties forms. Define Saytzeff's Rule - Answers The major product of elimination will be the most substituted alkene. Dehydrahalogenation produces what? - Answers An alkene. With respect to base used, what is the difference between E1 and E2 mechanisms? - Answers E1 uses a weak base, E2 uses a strong bulkier base. What is the mechanism of an E1 dehydrohalogenation reaction? - Answers First the halogen drops off, leaving behind a Carbocation. Then hydrogen is removed by a weak base. The extra electron pair left behind the hydrogen is "shared" with the Carbocation, resulting in a double bond and an alkene. What is the mechanism of an E2 dehydrohalogenation reaction? - Answers The strong base removes a proton from the Carbon next to the halogen containing Carbon, creating a carbocation. The halogen drops off as well, all in one step. The extra electron pair is shared with the Carbocation, resulting in a double bond and an alkene. What does catalytic hydrogenation due to an alkene? List the parties that take part in the process. - Answers An alkene is converted to an alkene. Hydrogen gas is supplied to form the saturated hydrocarbon. A heterogeneous catalyst (usually metal shavings of Nickel, Palladium, or Platinum) serves as a catalyst. Syn-Addition adds addition of a substituent where? This results in what type of geometric alkane? - Answers One the same side of two adjacent Carbons. This produces a cis alkane (the hydrogens are added on the SAME side). Alkane oxidation can produce glycols. What are glycols? - Answers Hydroxyl groups on adjacent Carbons. What does ozonolysis do to the alkene double bond? - Answers Cleaves it. Ozonolysis with alkynes produces what type of organic compound? - Answers Carboxylic acids. What is an electrophile? What properties must it have? - Answers An electron loving species. It must have a partial positive charge, even if it is from a dipole.