EKU Faculty and Staff Scholarship


Spectroscopic Differentiation and Chromatographic Separation of Regioisomeric Indole Aldehydes: Synthetic Cannabinoids Precursors



Document Type


Publication Date



The compounds in this study are the six regioisomeric 2-, 3-, 4-, 5-, 6- and 7-formyl indoles and the corresponding six regioisomeric N-n-pentylindole aldehydes. These compounds can serve as precursor chemicals and synthetic intermediates for a number of synthetic cannabinoid drugs. These two sets (the six regioisomeric indole aldehydes as well as the six regioisomeric pentylindole aldehydes) each have identical elemental compositions and differ in the position of attachment of the aldehyde group on the indole ring.

The electron ionization mass spectra for the indole aldehydes were essentially identical. However, the vapor phase infrared spectra showed differences in the absorption frequencies for the NH and carbonyl groups based on intramolecular interactions. The associated NH absorption band occurs as low as 3467 cm−1 while the free band is as high as 3517 cm−1. The aldehyde carbonyl band for the indole aldehydes varies from 1713 cm−1 to 1686 cm−1. Substitution of the aldehyde group on the pyrrole ring for the N-n-pentylindole aldehydes yields lower carbonyl absorption bands. The EI mass spectra for the pentylindole aldehydes are identical with little information for differentiation among these six regioisomeric compounds. The six compounds were separated on a capillary column using gas chromatography and the elution order appears to be related to the steric crowding of the indole ring substituents.

Graphical abstract

Journal Title

Forensic Chemistry

Journal Volume


Article Starting Page


Article Ending Page