Qualitative and Quantitative Analyses of Synthesized Short-Chain Fatty Acid Phenyl Esters Using Fourier-Transform Infrared Spectroscopy

Abstract

Fourier-transform infrared spectroscopy (FT-IR) is a widely used technique to qualitatively determine the molecular structure of organic compounds; however, using quantitative FT-IR (qFT-IR) for the compositional analyses of mixtures is less common. To reinforce instrumental use in undergraduate laboratories, we have devised a multipart experiment that not only combines the qualitative and quantitative aspects of FT-IR but also exposes students to computational and synthetic organic chemistry. The objectives of this experiment are to synthesize a series of phenyl esters (PhEs) of various molecular weights; use qualitative FT-IR to characterize and compare the synthesized products with standards, databases, and with theoretical spectra computed using the cost-efficient B97-3c functional; and determine the weight percent (wt. %) composition of a binary mixture. We report on the methodologies used to synthesize and purify four PhEs; characterize them using FT-IR, conduct theoretical calculations and compare their FT-IR spectra with experimental ones; and determine the wt. % composition of phenyl acetate (PhAc), phenyl propionate (PhPr), phenyl butyrate (PhBu), and phenyl hexanoate (PhHex) in binary mixtures ranging from 0% to 100%. The results show a strong, linear correlation of gravimetrically calculated wt. % composition of a selected compound in a binary mixture using qFT-IR. This experiment demonstrates the applicability of qFT-IR as an educational tool for the undergraduate chemical laboratory and combines four different branches of chemistry: computational, instrumental, organic, and analytical.