Consider the reaction below. Which of the following observations would indicate the reaction product is pure?

Question

Consider the reaction below.  Which of the following observations would indicate the reaction product is pure?

Quizplus · Accepted Answer

11ecba2d_1288_4d72_a3bf_fb5acb266afd_MD0008_00 Several techniques can be used to evaluate the purity of a compound, including thin-layer chromatography (TLC), which separates compounds in a mixture based on polarity. In TLC, the tested sample is dissolved in an organic solvent and a small amount is spotted onto the stationary phase (commonly a polar silica [SiO₂]) coated on a TLC plate. After spotting, the TLC plate is placed in a solvent chamber, and the mobile phase (organic solvent) travels up the plate and carries the spotted compounds along at different rates, which separates the mixture components. Nonpolar compounds have less affinity for a polar stationary phase and will travel further up the plate than polar compounds. The conversion of theobromine to caffeine results in N-methylation (conversion of N-H to N-CH₃). The N-H in theobromine can hydrogen bond to the polar stationary phase on the TLC plate whereas the N-CH₃ in caffeine cannot hydrogen bond. Because theobromine can hydrogen bond, it is more polar and has a smaller R_f value than caffeine. Therefore, TLC can be used to assess product purity by comparing a standard of pure theobromine to the isolated product. If a single spot with a larger R_f than that of theobromine is visible on the product sample track on the TLC plate, then the isolated product is pure. (Choice A) Size-exclusion chromatography separates compounds by size; therefore, compounds that are similar in size (eg, theobromine, caffeine) elute together. An absorbance peak indicates only the presence of a compound, not the number of compounds contributing to the absorbance. (Choice B) Although high-performance liquid chromatography (HPLC) can be used to determine product purity, a pure reaction product would show only one peak (not two) on the HPLC chromatogram. (Choice C) There are three unique methyl (−CH₃) groups in caffeine; therefore, three unique −CH₃ signals would be seen in its ¹H NMR spectrum. Two unique −CH₃ signals would indicate the product had not formed because theobromine has only two unique −CH₃ groups. Educational objective: Thin-layer chromatography (TLC) is a technique used to evaluate a compound's purity. The components of a mixture are separated based on polarity, and a single spot on a TLC plate is indicative of a compound's purity.

Consider the Reaction Below