MCAT Organic Chemistry Quiz

Once you’re familiar with what kind of Organic Chemistry questions you’ll come across on the MCAT, take this quiz to see how your prep is coming!

Question 1
Consider (E)-2-butene and (Z)-2-butene. This is a pair of what type(s) of isomers?
I. Cis–trans isomers
II. Diastereomers
III. Enantiomers
A. I only
B. II only
C. I and II only
D. I and III only

C: (E)-2-butene can also be called trans-2-butene; (Z)-2-butene can also be called cis-2-butene. As such, they are cis–transisomers. Remember that cis–trans isomers are a subtype of diastereomers in which the position of substituents differs about an immovable bond. Diastereomers are molecules that are non-mirror-image stereoisomers (molecules with the same atomic connectivity). These are not enantiomers because they are not mirror images of each other.

Question 2
An electron is known to be in the n = 4 shell and the l = 2 subshell. How many possible combinations of quantum numbers could this electron have?
A. 1
B. 2
C. 5
D. 10

D: An electron in the n = 4 shell and the l = 2 subshell can have five different values for ml: –2, –1, 0, 1, or 2. In each of these orbitals, electrons can have positive or negative spin. Thus, there are 5 × 2 = 10 possible combinations of quantum numbers for this electron.

Question 3
All of the following are true with respect to carbonyls EXCEPT:
A. the carbonyl carbon is electrophilic.
B. the carbonyl oxygen is electron-withdrawing.
C. a resonance structure of the functional group places a positive charge on the carbonyl carbon.
D. the π electrons are mobile and are pulled toward the carbonyl carbon.

D: The reactivity of the carbonyl can be attributed to the difference in electronegativity between the carbon and oxygen atoms. The more electronegative oxygen atom attracts the bonding electrons and is therefore electron-withdrawing. Thus, the carbonyl carbon is electrophilic. One resonance structure of the carbonyl pushes the π electrons onto the oxygen, resulting in a positively charged carbonyl carbon.

Question 4
Butanoic anhydride can be produced by the reaction of butanoic acid with which of the following compounds?
A. Butanoic acid
B. Ethanoic acid
C. Butanol
D. Methanal

A: Butanoic anhydride is an anhydride with two butane R groups. Anhydrides are produced by the reaction of two carboxylic acids with the loss of a water molecule. Therefore, butanoic anhydride would be produced by the reaction of two molecules of butanoic acid.

Question 5
What would be the charge of aspartic acid at pH 7?
A. Neutral
B. Negative
C. Positive
D. There is not enough information to answer the question.

B: The amino acid in question is aspartic acid, which is an acidic amino acid because it contains an extra carboxyl group. At neutral pH, both of the carboxyl groups are ionized, so there are two negative charges on the molecule. Only one of the charges is neutralized by the positive charge on the amino group, so the molecule has an overall negative charge.