📌 Atom Labeling Used in This Article

   H₃C(a)         O
         \         ‖
          N --- C(c) --- H(d)
         /
   H₃C(b)
        

a = N-methyl group trans to C=O (upfield)  |  b = N-methyl group cis to C=O (downfield)  |  c = carbonyl carbon  |  d = formyl H

💡 Identifying DMF in a reaction mixture: The formyl proton singlet at ~7.95 ppm in CDCl₃ is the most diagnostic signal. In aliphatic regions where N-CH₃ signals appear, other compounds may overlap - but the formyl H at ~8 ppm is rarely confused with other common organic signals. The two-singlet pattern of the N-methyl groups (0.1–0.15 ppm apart) further confirms DMF identity.

DMF ¹³C NMR Summary - Predicted Spectrum

Schematic ¹³C spectrum - chemical shift (ppm from TMS) →

💡 Temperature-dependent NMR behavior: At room temperature, the two N-CH₃ groups give two distinct singlets due to restricted C–N rotation (coalescence temperature ~120 °C in CDCl₃). If you heat the sample above 100 °C in a high-temperature NMR probe, the two peaks coalesce into one singlet - a classic demonstration of dynamic NMR / rotational isomerism. This is a useful teaching example and also a diagnostic tool: coalescing behavior on heating confirms the signal is from an amide bond.

📊 C=O Stretch Comparison - Why DMF's Carbonyl Is Red-Shifted

📌 Key MS identifiers for DMF: Molecular ion at m/z = 73 (base peak in EI), combined with the fragment at m/z = 44 ([M−29], loss of formyl) and m/z = 42 ([N(CH₃)₂]⁺), is diagnostic for DMF. In GC-MS (headspace or direct injection), DMF is identified by retention time on the reference column AND the m/z 73 molecular ion confirmed by the 44/42 fragmentation pattern.

💡 Important for residual solvent testing: DMF elutes relatively late in headspace GC due to its high boiling point (153 °C) compared to common Class 3 solvents. Always verify DMF retention time against an authentic reference standard on the specific column batch used. The DMA impurity (dimethylamine, bp 7 °C) elutes much earlier than DMF and must be resolved from solvent front - use a longer oven hold at initial temperature or a lower initial temperature if DMA must also be quantified.

✅ Combined confirmation criteria for DMF: GC RT consistent with DMF reference + m/z 73 (M⁺) in MS + ¹H NMR singlet at δ 7.95 ppm (CHO) + IR band at 1666 cm⁻¹ (C=O). Any three of these four criteria, especially when using standardized reference data, constitutes robust analytical identification of DMF in a sample.

Q1 · What is the ¹H NMR of DMF?

DMF shows three signals in ¹H NMR (CDCl₃, 400 MHz): (1) a singlet at approximately δ 7.96 ppm (1H) for the formyl proton HC=O - this is the most distinctive and diagnostic signal; (2) a singlet at δ 3.01 ppm (3H) for the N-methyl group cis to the C=O; and (3) a singlet at δ 2.88 ppm (3H) for the N-methyl group trans to the C=O. The two separate N-CH₃ signals arise from restricted rotation around the amide C–N bond - a hallmark of tertiary amides including DMF.

Q2 · What is the IR absorption of the DMF carbonyl?

The amide carbonyl (C=O) stretch of DMF appears at 1666 cm⁻¹ - this is the most intense and diagnostic IR band. This wavenumber is significantly lower than aldehyde (1720–1740 cm⁻¹) or ketone (1705–1725 cm⁻¹) carbonyls because nitrogen lone pair resonance donation partially reduces the C=O bond order. The second most diagnostic IR band is the amide C–N stretch (Amide II band) at approximately 1502 cm⁻¹. These two bands together are highly characteristic for DMF identification.

Q3 · Why does DMF show two N-methyl peaks in NMR?

DMF shows two separate N-methyl signals because the two methyl groups are in different chemical environments - one is positioned cis to the C=O oxygen and one is trans. This inequivalence arises from restricted rotation around the C–N amide bond, which has partial double bond character due to nitrogen lone pair resonance. At room temperature, interconversion between the two rotational forms is slow on the NMR timescale, so each methyl group gives a distinct singlet (~3.01 and ~2.88 ppm in CDCl₃). If the sample is heated above ~100 °C, the peaks broaden and eventually coalesce into a single signal as rotation becomes fast.

Q4 · What is the molecular weight of DMF?

The molecular weight of DMF (N,N-dimethylformamide, C₃H₇NO) is 73.09 g/mol. This can also be confirmed by mass spectrometry - the molecular ion (M⁺•) appears at m/z = 73, which is the base peak in the EI mass spectrum. Per the nitrogen rule, an odd molecular weight with one nitrogen atom is expected for DMF.

Q5 · What are the residual DMF NMR peaks in CDCl₃ and DMSO-d₆?

When DMF is present as a trace residual solvent in a sample dissolved in CDCl₃, it appears at: formyl H at δ 7.96 ppm, N-CH₃ at δ 3.01 ppm and δ 2.96 ppm. In DMSO-d₆: formyl H at δ 7.95 ppm, N-CH₃ at δ 2.96 ppm and δ 2.89 ppm. These values should be verified against the Gottlieb-Nudelman-Vega reference table (J. Org. Chem. 1997) for the most authoritative values. The formyl proton signal at ~7.95 ppm is the most distinctive and least likely to overlap with product signals.

🏭 Sinolook Chemical Co., Ltd.  |  sinolookchem.com  |  Export to 50+ countries  |  ISO-certified supply chain