Organic Chemistry For the MCAT

Many students taking organic chemistry one day plan to write the Medical College Admission Test (MCAT). A common question students have for me is what type of thinking is required in order to do well on this exam. I plead ignorance, so to help educate myself and others I am happy to present an expert in this area, who will merely go by the name “Org Chem Prof” for our purposes. Dr. Org Chem Prof has kindly offered to write a guest post on MOC to describe the key features of organic chemistry for the MCAT . I hope this gives you a sense of what to expect.

 Organic Chemistry For The MCAT, by “OrgChemProf”

Background.: Since most (if not all) U. S. medical schools require applicants to take the Medical College Admission Test (MCAT), this exam often represents the last hurdle that an aspiring medical school applicant must overcome in order to begin to fulfill his or her ambitions. A huge amount of information is available that is designed to help students prepare to take the examination; I defer to the expertise of the many authors who offer this assistance.

My own involvement with MCAT stems from long experience as a reviewer and content provider to the agency that prepares materials for inclusion in the Organic and Bio-organic Chemistry sections of the MCAT examination. My purpose in writing this brief essay is to share some of my experience with students who plan to take the MCAT exam and who wish to gain some relevant philosophical insight and possibly also some practical insight. Since my experience is necessarily limited, I shall confine my remarks to the Organic Chemistry portion of MCAT.

Why Do Medical Schools Rely Upon MCAT? Students should understand that medical schools seek to admit applicants who can demonstrate the ability to think independently and to reason critically. As a result the MCAT examination does more than simply test an examinee’s knowledge of a particular subject.

One should consider the situation from the standpoint of the medical school’s admissions committee. They are deluged with applications that appear, at least on paper, to have been tendered by outstanding students.

In the case of any one particular applicant, how can the members of the committee be reasonably assured that their faculty’s investment of time, instructional effort and materials ultimately will be rewarded? Training future MDs is an expensive business; so the admissions committee can’t afford to make many mistakes. That’s where the MCAT comes into play.

The bottom line: High achievement on the MCAT, when taken together with solid personal recommendations and an impressive grade point average in university coursework (and other factors too numerous and too nebulous to mention), is considered to offer a reliable predictor of the applicant’s suitability to pursue a career in medicine and potential for eventual success.

Mechanics and Philosophy. A typical Organic Chemistry unit consists of a ca. 250 word narrative passage followed by 10 questions that are related to topics, experimental results, and/or conclusions that either are presented or discussed explicitly in the passage or can be inferred from information contained therein.

The narrative passage might present research results and also might include conclusions derived from the experimental results. Alternatively, the passage might simply present information about a particular reaction or set of reactions. Occasionally the passage will present arguments to favor one reaction mechanism relative to other alternative mechanisms, in which case examinees are likely to be asked to evaluate those arguments.

Questions generally are related in one way or another to the content of the passage. Occasionally the questions are straightforward and rely primarily upon an examinee’s knowledge of the subject gained from Organic Chemistry lecture and laboratory courses. In this case, the examinee will be required to apply that knowledge to new or unfamiliar situations.

Then there is another type of question that generally requires a higher level of understanding of the subject matter contained within the passage. Questions of this type may require examinees to predict the outcome of a reaction or set of reactions based upon information contained in the passage. Other questions of this type may require examinees, e. g., to evaluate experimental results, to interpret graphical or pictorial representations of results, to interpret absorption spectra (UV, IR, NMR) or perhaps to evaluate conclusions derived from experimental results. Examinees may be asked to recognize a set of conditions that can be used to differentiate among mechanistic alternatives.

Can You Give Me An Example? Sure! I have prepared a sample unit that deals with the topic of nucleophilic substitution in allylic systems. The ten questions that accompany the narrative offer varying degrees of difficulty and probe different aspects of the information and conclusions presented in the passage.

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Sample Organic Chemistry MCAT Passage and Exam Questions

Topic: Nuceophilic Substitution in Allylic Systems

The stereochemistry of nucleophilic substitution reactions that involve allylic halides and alllylic alcohols as substrates has been studied extensively.

Results of Experiment 1: Reaction of optically active cis-5-methylcyclohexen-3-ol (Compound 1) with thionyl chloride in dry diethyl ether was found to afford optically active cis-3-chloro-5-methylcyclohexene (Compound 2) as the exclusive reaction product. The absolute configurations of Compounds 1 and 2 are shown in Figure 1.

Figure 1.        Reaction of optically active cis-5-methylcyclohexen-3-ol with thionyl chloride

Results of Experiment 2: Acetolysis of optically active trans-3-chloro-5-methylcyclohexene (Compound 3) was allowed to proceed through one reaction half-life. The reaction mixture then was quenched, and the starting material was isolated. Compound 3 thereby recovered was found to have racemized.

Results of Experiment 3: The kinetics of acetolysis of optically active trans-3-chloro-5-methylcyclohexene (Compound 3) were followed through several half-lives. The rate of racemization of Compound 3 was found to be more than four times the corresponding rate of acetolysis. Solvolysis resulted in the formation of racemic cis-3-acetoxy-5-methylcyclohexene (Compound 4, Figure 2).

Figure 2.        Acetolysis of optically active trans-3-chloro-5-methylcyclohexene

Question 1. Which of the following conclusions is consistent with the results of Experiment 1 cited in the passage?
A.        Compound 2 is formed via S_N2 displacement of OS(O)Cl by Cl^– with inversion of configuration at C-3.
B.        Compound 2 is formed via S_Ni displacement of OS(O)Cl by Cl^– with retention of configuration at C-3.
C.        Compound 2 is formed via S_N2′ displacement of OS(O)Cl by Cl^– with concomitant allylic rearrangement.
D.        Compound 2 can result either via S_Ni or S_N2′ displacement of OS(O)Cl by Cl^–.

Question 2. Which of the following statements is most consistent with the results of Experiment 2 cited in the passage?

A.        The reaction proceeds via formation of a chiral carbocation that subsequently reacts with HOAc with concomitant racemization.
B.        The reaction proceeds via formation of an achiral carbocation.
C.        Homolysis of the C-Cl bond in Compound 3 results in the formation of an allylic radical.
D.        The reaction proceeds via formation of an achiral radical.

Question 3. Which of the following statements as applied to the reaction mechanism indicated below is most consistent with the results of Experiment 3 cited in the passage?

A.        A chiral intermediate is formed reversibly during acetolysis of Compound 3; the relative rates of disappearance of this intermediate: k₂ <  k_-1.
B.        A chiral intermediate is formed reversibly during acetolysis of Compound 3; the relative rates of disappearance of this intermediate: k₂ >  k_-1.
C.        An achiral intermediate is formed reversibly during acetolysis of Compound 3; the relative rates of disappearance of this intermediate: k₂ <  k_-1.
D.        An achiral intermediate is formed reversibly during acetolysis of Compound 3; the relative rates of disappearance of this intermediate: k₂ >  k_-1.

Question 4. The absolute configuration of optically active Compound 3 is shown in Figure 2. What are the Cahn-Ingold-Prelog R,S designations of carbon atoms C-3 and C-5 in this molecule?

A.        3R, 5R
B.        3R, 5S
C.        3S, 5R
D.        3S, 5S 

Question 5. Compound 1 is allowed to react with hydrogen gas over palladized charcoal, and aliquots are withdrawn at various reaction time intervals. What change(s) in appearance of the infrared spectrum of the reaction mixture are expected to occur as the reaction progresses?
A.        Absorption signals at 3060 cm^-1 and at 1660 cm^-1 become less intense as the reaction progresses.
B.         Absorption signal at 3060 cm^-1 becomes more intense, while the absorption signal at 1660 cm^-1 becomes less intense.
C.        Absorption signal at 1660 cm^-1 becomes more intense, while the absorption signal at 3060 cm^-1 becomes less intense.
D.        Absorption signals at 3060 cm^-1 and at 1660 cm^-1 become more intense as the reaction progresses.

Question 6. Which specifically-deuterated optically active substrate could be used to determine whether the reaction discussed in Experiments 2 and 3 proceed with or without concomitant allylic rearrangement?

Question 7. Which series represents the correct order of ¹H chemical shifts (1 = highest field, 3 = lowest field) of the indicated protons in the proton NMR spectrum of  cis-3-chloro-5-methylcyclohexene (Compound 2)?

     Question 8. The reaction of diethylamine with α-methylallyl chloride, which affords Compound 5, is first order in both reactants. Both α-methylallyl chloride and Compound 5 are stable under the reaction conditions employed. What is most likely to be the rate-determining step of this reaction?

A.        Nucleophilic attack by diethylamine at the α-carbon atom in α-methylallyl chloride
B.        Nucleophilic attack by diethylamine at the β-carbon atom in α-methylallyl chloride
C.        Nucleophilic attack by diethylamine at the γ-carbon atom in α-methylallyl chloride
D.        Heterolysis of the C-Cl bond in α-methylallyl chloride

Question 9.  Consider again the reaction of diethylamine with α-methylallyl chloride, which affords Compound 5. What change in hybridization occurs at the α-carbon atom in α-methylallyl chloride as a result of this reaction?

A.        sp → sp²
B.        sp² → sp
C.        sp² → sp³
D.        sp³ → sp²

Question 10.  In general, alcohols are not useful as substrates for bimolecular nucleophilic substitution reactions. Reaction of a primary alcohol, R-OH with which one of the following reagents affords a derivative that is suitable substrate for bimolecular nucleophilic substitution reaction with NaI?

Click here for the answer key (PDF)

Reality Check. In order to maximize the effectiveness of the sample unit I recommend strongly that you first work through the entire examination without looking at the answers and accompanying explanations. In this way some clarification of the philosophical points discussed above may emerge after you have subjected yourself to a practice session that resembles actual test conditions.

Wishing you the best of success in all endeavors.

“OrgChemProf”