Day 57 MCAT Practice Question

Passage 10: Compound X

It is theorized that a bioactive compound, Compound X, resides within a carnivorous

plant in the Amazon rainforest. Interested in its anticancer properties possibly due

to its unique chemical structure, researchers set out to isolate Compound X from

the plant it was hypothesized to be contained within. However, the compounds’

clear dual affinity for hydrophobic and hydrophilic environments posed a challenge.

Plant material was first collected and dried. Once dried, it was ground into a fine

powder, which was then subjected to the following extraction process to isolate the

compound of interest.

FIGURE

Step 1

Hexane was used as the solvent in which dried plant material was added to for 24

hours. Following extraction, the solvent was evaporated under low pressure, yielding

a crude extract.

Step 2

Minimal ethanol and equal volume of distilled water dissolved the crude extract.

This mixture was transferred to a separatory funnel and the aqueous layer was

removed. To further purify the ethanol-soluble fraction, a series of liquid-liquid

extractions was performed using tetrahydrofuran (THF) and an acidic aqueous

solution. The organic phase was collected and the aqueous phase was discarded;

this step was repeated several times to ensure maximum extraction of Compound X

into the THF layer.

Step 3

Rotary evaporation was utilized on the THF layer to remove the solvent. The semipurified extract that remained was then subjected to distillation. It was at this step

that the researchers decided to restart the process and adjust their method of

distillation because the constituents of the remaining extract had boiling points that

were too close to perform simple distillation. Compound X was then collected.

Step 4

Column chromatography was used to further purify the obtained Compound X.

Fractions were collected and analyzed using thin-layer chromatography (TLC) to

verify the presence of Compound X.

Step 5

The final purified fraction was analyzed with polarimetry. It was observed that no

angle of incident light had any effect on the intensity of light that passed through

the sample. A chiral resolving agent was used to form diastereomeric salts,

which were then separated using recrystallization which yielded the individual

enantiomers of Compound X.

Final Analysis

The resultant enantiomers were characterized using HPLC with a chiral column to

identify any enantiomeric excess. Nuclear magnetic resonance (NMR) spectroscopy

and mass spectrometry (MS) elucidated the structure of Compound X and

vindicated the hypothesized isolation process.

During TLC analysis, the researchers observed a retention factor (Rf) of 0.35 on

a silica gel TLC plate using a hexane/ethyl acetate (7:3) solvent system. What

changes should be made to increase the Rf of Compound X to around 0.5?

A) Increase the proportion of hexane

B) Increase the proportion of ethyl acetate

C) Use an alternative stationary phase with a higher polarity

D) Use a hexane/methanol (9:1) solvent system instead of hexane/ethyl acetate

(7:3)