Physical and Chemical Properties of NEP
Dielectric, Polarity, Hansen Parameters, NMR Behaviour - Complete Technical Reference
This technical reference covers the physical and chemical properties of N-Ethyl-2-Pyrrolidone (NEP, CAS 2687-91-4) in the detail that formulators, process chemists, and application engineers actually need 🔬. Unlike a typical marketing data sheet, we include temperature-dependent property curves, Kamlet–Taft and Hansen solubility parameters, ¹H and ¹³C NMR chemical shifts, partition coefficients, and reactivity notes - everything you would look up in a CRC Handbook or manufacturer technical bulletin, consolidated in one place.
If you are new to NEP, start with our complete guide to N-Ethyl-2-Pyrrolidone for the high-level introduction. This article assumes you already know what NEP is and want the working numbers.
- 🧪 Identity, Structure, and Core Constants
- 🌡️ Thermodynamic Properties
- 💧 Density and Viscosity vs Temperature
- ⚡ Electrical & Polarity Parameters
- 🎯 Hansen Solubility & Kamlet–Taft Parameters
- 🧲 ¹H and ¹³C NMR Behaviour
- 🔥 Flammability, Stability, and Reactivity
- 🧬 Solubility and Partition Data
- ❓ Frequently Asked Questions
1. 🧪 Identity, Structure, and Core Constants
NEP is a five-membered γ-lactam (2-pyrrolidinone / pyrrolidin-2-one) bearing an ethyl substituent on the ring nitrogen. The carbonyl group (C=O) at C-2 provides the strong hydrogen-bond-accepting character that underpins NEP's solvency. The N–C=O amide bond exhibits partial double-bond character and restricted rotation at room temperature, visible as coalescence phenomena in variable-temperature NMR.
| Identifier | Value |
|---|---|
| IUPAC name | 1-ethylpyrrolidin-2-one |
| CAS Registry No. | 2687-91-4 |
| EC / EINECS No. | 220-250-6 |
| PubChem CID | 21140 |
| Molecular formula | C₆H₁₁NO |
| Molecular weight | 113.16 g/mol |
| InChIKey | HPFVBGJFAYZEBE-UHFFFAOYSA-N |
| SMILES | CCN1CCCC1=O |
| Structural class | γ-Lactam (cyclic amide), 5-membered ring |
| Functional groups | Tertiary amide C=O; N-ethyl substituent |
| Solvent classification | Polar aprotic (dipolar aprotic) |
The amide carbonyl is the active site for NEP's solvent behaviour. The nitrogen lone pair is delocalised into the carbonyl, giving the C=O oxygen significantly enhanced Lewis basicity and hydrogen-bond accepting power. Meanwhile the N-ethyl group blocks any H-bond donation (aprotic character) and provides modest steric / lipophilic balance. This combination - strong acceptor, no donor, modest lipophilicity - is exactly what makes NEP (like NMP) dissolve both polar organic molecules and ionic salts.
2. 🌡️ Thermodynamic Properties
The thermodynamic profile of NEP is dominated by its high boiling point, low freezing point, and low vapour pressure - all practical advantages in liquid-phase processing.
| Property | Value | Units / Notes |
|---|---|---|
| Normal boiling point | ≈ 212 | °C at 1 atm |
| Melting / freezing point | ≈ −78 | °C; remains liquid in unheated warehouses |
| Flash point (closed cup) | ≈ 94 | °C; combustible, not flammable per UN |
| Auto-ignition temperature | ≈ 245 | °C |
| Enthalpy of vaporisation (ΔHvap) | ≈ 55 | kJ/mol at boiling point |
| Heat of combustion | ≈ 3,390 | kJ/mol |
| Specific heat capacity (Cp) | ≈ 2.0 | J/(g·K) at 25 °C |
| Thermal conductivity | ≈ 0.175 | W/(m·K) at 25 °C |
| Surface tension | ≈ 39 | mN/m at 25 °C |
Vapour pressure as a function of temperature
NEP has noticeably lower vapour pressure than NMP across the entire operating range - approximately 40–60 % of NMP's vapour pressure at any given temperature. This translates to slower evaporation in coatings, longer open times in formulations, and reduced VOC emissions during drying.
| Temperature (°C) | Vapour Pressure (hPa) | NMP Comparison (hPa) |
|---|---|---|
| 20 | ≈ 0.13 | 0.32 |
| 50 | ≈ 1.3 | 3.1 |
| 100 | ≈ 25 | 56 |
| 150 | ≈ 160 | 340 |
| 180 | ≈ 460 | 870 |
| 212 (BP) | ≈ 1013 | - (NMP BP 202 °C) |
3. 💧 Density and Viscosity vs Temperature
Density and viscosity data are essential for process design - pump sizing, heat-transfer calculations, mixing-time predictions, and coating-application rheology all depend on these numbers. NEP's density is slightly lower than NMP's (thanks to the longer alkyl chain and slightly larger molar volume), and its viscosity is slightly higher.
| Temperature (°C) | Density (g/mL) | Viscosity (cP) | Kinematic Viscosity (cSt) |
|---|---|---|---|
| 0 | ≈ 1.005 | ≈ 3.8 | ≈ 3.78 |
| 20 | ≈ 0.990 | ≈ 2.3 | ≈ 2.32 |
| 25 | ≈ 0.985 | ≈ 2.0 | ≈ 2.03 |
| 50 | ≈ 0.965 | ≈ 1.3 | ≈ 1.35 |
| 80 | ≈ 0.940 | ≈ 0.85 | ≈ 0.90 |
| 100 | ≈ 0.925 | ≈ 0.65 | ≈ 0.70 |
Practical process implications
- Coating operations: NEP viscosity at 25 °C (~2.0 cP) is slightly higher than NMP (~1.65 cP). For thin-film coating (wet-film < 50 µm), you may need to reduce line speed by 5–10 % or raise coating-head temperature modestly.
- Pump sizing: Slightly higher viscosity + marginally lower density mean roughly equivalent pumping power to NMP at the same volumetric flow.
- Heat transfer: Thermal conductivity and Cp are close to NMP; heat-exchanger sizing generally translates one-for-one.
- Handling at low temperatures: NEP's −78 °C freezing point is a major advantage over sulfolane (MP +27 °C) and a modest advantage over NMP (−24 °C) for cold-climate outdoor storage.
4. ⚡ Electrical & Polarity Parameters
The electrical parameters determine NEP's solvation of ionic compounds and its behaviour as a reaction medium for polar-transition-state reactions (SNAr, SN2, amide couplings, etc.).
| Parameter | NEP | NMP | DMF | DMSO |
|---|---|---|---|---|
| Dielectric constant ε (25 °C) | ≈ 28 | 32.2 | 36.7 | 46.7 |
| Dipole moment (D) | ≈ 4.1 | 4.09 | 3.86 | 3.96 |
| ET(30) polarity (kcal/mol) | ≈ 42 | 42.2 | 43.8 | 45.1 |
| Donor number (DN) | ≈ 27 | 27.3 | 26.6 | 29.8 |
| Acceptor number (AN) | ≈ 12 | 13.3 | 16.0 | 19.3 |
| Refractive index (n²⁰D) | ≈ 1.470 | 1.470 | 1.430 | 1.479 |
The electrical picture: NEP sits slightly below NMP on dielectric constant and ET(30) but is nearly identical on dipole moment and donor number. For practical purposes - dissolving salts, solvating transition states, supporting polar chemistry - NEP and NMP behave as "same-class" dipolar aprotic solvents.
5. 🎯 Hansen Solubility & Kamlet–Taft Parameters
Hansen solubility parameters (HSPs) and Kamlet–Taft parameters are the most useful tools for predicting polymer solubility and reaction solvent behaviour, respectively. They are the first things an experienced formulator checks when evaluating a new solvent.
Hansen Solubility Parameters (HSPs)
| Parameter | NEP | NMP | Meaning |
|---|---|---|---|
| δd (dispersion) | ≈ 18.0 | 18.0 | Van der Waals contribution - MPa⁰·⁵ |
| δp (polar) | ≈ 11.5 | 12.3 | Dipole–dipole contribution - MPa⁰·⁵ |
| δh (H-bonding) | ≈ 7.0 | 7.2 | H-bond contribution - MPa⁰·⁵ |
| δt (total) | ≈ 22.4 | 22.9 | Total cohesive energy - MPa⁰·⁵ |
HSP interpretation: NEP and NMP occupy nearly the same point in Hansen space (within ~ 1 MPa⁰·⁵ on each axis). This is precisely why NEP can be expected to dissolve the same polymers that NMP dissolves - PVDF, polyimide, aramid, polyamide-imide, polyetherimide, and most engineering thermoplastics.
Kamlet–Taft Parameters
| Parameter | NEP | NMP | Meaning |
|---|---|---|---|
| α (H-bond donor) | 0.00 | 0.00 | Aprotic - no H to donate |
| β (H-bond acceptor) | ≈ 0.76 | 0.77 | Strong H-bond acceptor (carbonyl oxygen) |
| π* (polarity / polarisability) | ≈ 0.91 | 0.92 | Very high polarity, similar to NMP & DMF |
Kamlet–Taft interpretation: the (α = 0, β ≈ 0.76, π* ≈ 0.91) profile places NEP in the same "dipolar aprotic" cluster as NMP, DMAc, and DMF. This makes NEP a strong candidate for SNAr reactions, anion-promoted chemistry, and PVDF dissolution.
6. 🧲 ¹H and ¹³C NMR Behaviour
NEP's NMR spectra show two characteristic features: (1) the restricted-rotation amide behaviour that gives some signals broadened or split at room temperature, and (2) the clear ethyl-group patterns at N–CH₂–CH₃.
¹H NMR (400 MHz, CDCl₃) - typical chemical shifts
| δ (ppm) | Multiplicity | Integration | Assignment |
|---|---|---|---|
| 1.12 | t | 3H | N–CH₂–CH₃ |
| 2.00 | m | 2H | ring C-4 H₂ |
| 2.38 | t | 2H | ring C-3 H₂ (α to C=O) |
| 3.35 | q | 2H | N–CH₂–CH₃ |
| 3.43 | t | 2H | ring C-5 H₂ (α to N) |
¹³C NMR (100 MHz, CDCl₃) - typical chemical shifts
| δ (ppm) | Assignment |
|---|---|
| 174.8 | C=O (amide carbonyl) |
| 44.3 | ring C-5 (N–CH₂) |
| 39.5 | N–CH₂ of ethyl group |
| 31.0 | ring C-3 (α to C=O) |
| 18.0 | ring C-4 |
| 12.8 | N–CH₂–CH₃ |
Like all tertiary amides, NEP shows restricted rotation about the N–C(O) bond. Below about 60 °C in CDCl₃, the two methylene signals of the ethyl group may appear slightly broadened relative to an idealised quartet. This is not an impurity signature - it is normal amide behaviour. Elevated-temperature NMR (above the coalescence temperature, ~ 80–100 °C depending on solvent) produces sharp, well-resolved peaks.
7. 🔥 Flammability, Stability, and Reactivity
Flammability
- Closed-cup flash point: ~ 94 °C (combustible, above UN 60 °C threshold for flammable liquids - not a UN-regulated dangerous good)
- Auto-ignition temperature: ~ 245 °C
- Lower flammable limit: ~ 1.3 vol % in air
- Upper flammable limit: ~ 9.5 vol % in air
Stability & storage
- Thermally stable up to ~ 200 °C in the absence of strong oxidisers or acids.
- Hygroscopic - absorbs atmospheric moisture quickly. Keep containers sealed; in analytical or electronic-grade applications, use under nitrogen headspace.
- Shelf life: 24 months in original sealed container at 5–30 °C, away from direct sunlight.
- Colour drift: APHA may increase over time when exposed to air and light. Material that develops a strong yellow-brown colour should be re-checked for peroxide content and, if necessary, distilled before reuse in sensitive applications.
Reactivity
- Hydrolysis: slow hydrolysis of the amide bond occurs with strong acids (H₂SO₄, HCl) or strong bases (NaOH, KOH) under prolonged heating. For most reactions at pH 2–12 and < 100 °C, NEP is effectively inert.
- Oxidation: reacts with strong oxidisers (peroxides, nitric acid, permanganates). Avoid contact.
- Peroxide formation: unlike some ethers, NEP does not readily form peroxides on storage.
- Materials compatibility: generally compatible with stainless steel 304/316, PTFE, PVDF, fluoroelastomers (Viton / Kalrez). Attacks many polyurethanes, nitrile rubber (NBR), natural rubber, and some polyolefins on extended contact.
8. 🧬 Solubility and Partition Data
Miscibility with common solvents
| Solvent | Miscibility with NEP |
|---|---|
| Water | Fully miscible |
| Methanol, ethanol, isopropanol | Fully miscible |
| Acetone, MEK, MIBK | Fully miscible |
| THF, dioxane, diglyme | Fully miscible |
| DMF, DMSO, DMAc, NMP | Fully miscible |
| Ethyl acetate, butyl acetate | Fully miscible |
| Dichloromethane, chloroform | Fully miscible |
| Toluene, xylene | Fully miscible |
| Hexane, heptane, pentane | Partial (amphiphilic) |
Polymer solubility
NEP dissolves essentially the same polymer set as NMP: PVDF (20 – 30 wt % solutions attainable), polyimide and polyamide-imide wire enamel resins, aramids (at elevated temperature), polyetherimide (PEI / Ultem), polysulfone (PSU), polyethersulfone (PES), and polyphenylsulfone (PPSU). It will also dissolve most polyacrylates, epoxy pre-cures, and alkyd resins - which is what makes it effective in paint-stripper formulations.
Partition & environmental parameters
| Parameter | Value |
|---|---|
| log P (octanol–water) | ≈ 0.45 (slightly lipophilic) |
| Water solubility | Fully miscible |
| Henry's law constant | Very low (strongly retained in aqueous phase) |
| Biodegradability (OECD 301) | Readily biodegradable (> 60 % in 28 d) |
| Bioaccumulation (BCF) | Very low - not bioaccumulative |
| Hydrolysis in water | Stable across pH 4–9 at ambient temperature |
If you are replacing NMP with NEP in an existing formulation, start with a 1-for-1 mass substitution. Expect coating-rheology adjustments of ±10 %, minor evaporation-profile tuning, and possibly a small recipe tweak if the product contains water (NEP's slightly different water activity can affect formulation stability). For reaction chemistry, NEP usually performs within 5 % yield of NMP - occasionally better, rarely worse.
9. ❓ Frequently Asked Questions (FAQ)
🔹 Q1. What is the boiling point of NEP?
Approximately 212 °C at 1 atmosphere. This is about 10 °C higher than NMP (202 °C) and is one of the key practical differences - NEP gives you slightly more thermal headroom for atmospheric-pressure chemistry and slower drying in coating processes.
🔹 Q2. Is NEP a polar or non-polar solvent?
NEP is a strongly polar, dipolar aprotic solvent. Dielectric constant ≈ 28, dipole moment ≈ 4.1 D, π* (Kamlet–Taft polarity) ≈ 0.91. For solvation purposes it sits in the same cluster as NMP, DMAc, and DMF - strong enough to dissolve ionic salts and polar polymers, with no protic donor character.
🔹 Q3. What is the density of NEP?
≈ 0.99 g/mL at 20 °C, slightly lower than NMP (1.03 g/mL). The density decreases linearly with temperature at a rate of approximately 0.0008 g/(mL·°C) between 0 and 100 °C.
🔹 Q4. What is the viscosity of NEP?
≈ 2.0 cP at 25 °C, slightly higher than NMP (1.65 cP). Viscosity drops to ~ 0.65 cP at 100 °C. For most industrial purposes - pumping, coating, mixing - the difference versus NMP is small and rarely requires process redesign.
🔹 Q5. What is the dielectric constant of NEP?
Approximately 28 at 25 °C, slightly below NMP (32.2) and DMF (36.7), and well above THF (7.5) or dichloromethane (9.1). This puts NEP firmly in the "high dielectric" class suitable for dissolving ionic salts and supporting polar-transition-state reactions.
🔹 Q6. Is NEP miscible with water?
Yes, fully miscible in all ratios. It is also miscible with most polar and semi-polar organic solvents - alcohols, ketones, ethers, esters, chlorinated solvents, aromatic hydrocarbons. Only short-chain aliphatic hydrocarbons (pentane, hexane, heptane) show partial miscibility.
🔹 Q7. What polymers does NEP dissolve?
Essentially the same set NMP does: PVDF, polyimide, polyamide-imide, aramid (at elevated temperature), polyetherimide, polysulfone, polyethersulfone, polyphenylsulfone, polyacrylates, and epoxy / alkyd pre-cures. It does not dissolve polyolefins, PTFE, or fully-cured epoxy thermosets.
🔹 Q8. Is NEP flammable?
NEP is classified as combustible, not flammable. Its closed-cup flash point is ~ 94 °C, above the 60 °C UN threshold for flammable liquids, so it is not a UN-regulated dangerous good for transport. Auto-ignition occurs around 245 °C. Standard sea-freight and inland-logistics rules apply.
📚 Related Articles in the NEP Series
Overview of structure, synthesis, applications, and regulation.
When to switch, when to stay with NMP (coming soon).
Toxicity, CLP classification, Annex XVII update (coming soon).
🔗 Authoritative External References
- PubChem Compound CID 21140 - 1-Ethyl-2-pyrrolidinone (full property data): pubchem.ncbi.nlm.nih.gov/compound/21140
- NIST WebBook - thermodynamic data for 1-Ethyl-2-pyrrolidone: webbook.nist.gov
- Hansen Solubility Parameters in Practice (Charles Hansen): hansen-solubility.com
- BASF Product Information - N-Ethylpyrrolidone-2: products.basf.com
- SDBS (AIST) - spectral data for organic compounds: sdbs.db.aist.go.jp
- Kamlet–Taft solvent parameter database (ACS publications): pubs.acs.org
Need Grade-Specific NEP for Your Process?
Sinolook Chemical supplies N-Ethyl-2-Pyrrolidone in industrial, electronic, and pharmaceutical grades - each with batch-specific COA, full spectroscopic identification, and regulatory documentation (China-GHS SDS, EU eSDS, US HCS 2024, Prop 65 warning for US buyers, ICH Q3C statement for pharma). We can supply against custom specifications for water content, APHA colour, or trace metal limits.
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