Comparison of three methods for the methylation of aliphatic and aromatic compounds

Abstract

RationaleMethylation protocols commonly call for acidic, hot conditions that are known to promote organic 1 H/ 2 H exchange in aromatic and aliphatic C-H bonds. Here we tested two such commonly-used methods and compared a third that avoids these acidic conditions, to quantify isotope effects with each method and to directly determine acidic-exchange rates relevant to experimental conditions. MethodsWe compared acidic and non-acidic methylation approaches catalyzed by hydrochloric acid, acetyl chloride and EDCI (1-ethyl-3-(3-dimethylaminopropyl)carbodiimide) / DMAP (4-dimethylaminopyridine) respectively. These were applied to two analytes: phthalic acid (an aromatic) and octacosanoic acid (an aliphatic). We analyzed yield by gas chromatography flame ionization (GC/FID) and hydrogen and carbon isotopic composition by isotope ratio mass spectrometry (GC/IRMS). We quantified the 1 H/ 2 H exchange rate on dimethyl phthalate under acidic conditions with proton nuclear magnetic resonance ( 1 H-NMR) measurements. ResultsThe δ 2 H and δ 13 C values and yield were equivalent among the three methods for methyl octacosanoate. The two acidic methods resulted in comparable yield and isotopic composition of dimethyl phthalate; however, the non-acidic method resulted in lower δ 2 H and δ 13 C values perhaps due to low yields. Concerns over acid-catalyzed 1 H/ 2 H exchange are unwarranted as the effect was trivial over a 12-hour reaction time.This article is protected by copyright. All rights reserved. ConclusionsWe find product isolation yield and evaporation to be the main concerns in the accurate determination of isotopic composition. 1 H/ 2 H exchange reactions are too slow to cause measurable isotope fractionation over the typical duration and reaction conditions used in methylation. Thus, we are able to recommend continued use of acidic catalysts in such methylation reactions for both aliphatic and aromatic compounds.

Reconstructing Arctic Precipitation Seasonality Using Aquatic Leaf Wax δ2H in Lakes With Contrasting Residence Times

“…The cores were split and stored at 4°C until analysis. Age models (Table S1) and analytical methods are described in the supporting information (Lee et al, 2017).…”

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“…We checked the linearity in isotopic determination across a range of peak amplitude (1 -10 V) daily, and only the measurements from compounds with peak amplitudes within the range of linearity were accepted. δ 13 C values were normalized to the Vienna Pee Dee (Lee et al, 2017) to report the δ 13 C and δD values of n-alkanoic acids.…”

Summer sea-ice variability on the Antarctic margin during the last glacial period reconstructed from snow petrel (Pagodroma nivea) stomach-oil deposits

“…FAME fractions were then taken to dryness in a stream of N2. The isotopic value of the methanol was determined through methylation of a phthalic acid with known isotopic value (Lee et al, 2017).…”

“…Reference material F8-3 was also used to determine the usable amplifier signal range, which minimized the residuals, and (ranging from 0.21 to 0.34‰, n=21). Fatty acid  13 C was calculated through mass balance corrections of the measured FAME  13 C and known  13 C of the added methyl group (Lee et al, 2017) and reported relative to Vienna PeeDee Belemnite (V-PDB).…”