What Is the Melting Point of Acetanilide?

The melting point of pure acetanilide is 113–115 °C (235–239 °F; 386–388 K).

You’ve just finished recrystallizing acetanilide in the lab, loaded a capillary tube, and watched the crystals melt at 108 °C. The textbook says 114 °C. Did you fail?

Probably not. That lower reading tells you something useful: your sample isn’t pure yet. The melting point of acetanilide is a reliable purity check, and knowing exactly what it should be — and why — turns a confusing number into a clear next step in the experiment.

The Accepted Melting Point of Acetanilide

Pure acetanilide (C₈H₉NO) has a sharp, reproducible melting point of 113–115 °C. That range appears consistently across the most authoritative chemical databases.

PubChem lists 113–115 °C. The OECD HPV chemical dataset reports 113.7 °C. The United States Pharmacopeia sells an official melting point standard with a reference value around 114 °C. Even the NOAA chemical datasheet gives 237.7 °F — which converts to roughly 114.3 °C.

When multiple Tier-1 sources agree within a degree or two, that range becomes the benchmark. Any measured value that falls outside it signals that the sample contains impurities.

Why Melting Point Tells You About Purity

Impurities disrupt the orderly crystal lattice of a solid, making it easier for the molecules to break free and melt at a lower temperature. That’s why a wide or depressed melting range is the simplest indicator of an impure compound.

• Sharp vs. broad range: Pure acetanilide melts over a 1–2 °C window. A range wider than 2 °C usually means contamination.
• Lower than expected: If your sample melts at 108–110 °C, it’s not pure. Recrystallization can remove the contaminants and raise the melting point back toward 114 °C.
• Same as literature but wide: Rarely a mixture can coincidentally melt at the right temperature but over a broad range. Always note the range, not just the start.
• Slightly above literature: This is less common but can happen if a different polymorph forms or if the heating rate is too fast. A properly calibrated instrument with slow heating gives the most reliable result.

The takeaway is straightforward: the melting point of acetanilide isn’t just a number to memorize — it’s a diagnostic tool for every recrystallization lab.

How Recrystallization Affects the Melting Point

Recrystallization works by dissolving the crude product in a hot solvent, then cooling slowly so that pure crystals reform while impurities stay dissolved. The result is a solid whose melting point should match the accepted value.

Before recrystallization, crude acetanilide might melt at 105–110 °C. After a proper recrystallization (typically from water, since acetanilide is much more soluble in hot water than cold), the product should melt at 113–115 °C. That shift from a lower, broader range to a higher, narrower one confirms that the purification worked.

Cleveland Clinic‑quality sources like the UC Santa Cruz lab manual explicitly state that you assess purity by comparing the observed melting point to the literature value. You can find the exact standard range in the PubChem acetanilide melting point entry, which is the most commonly cited reference in teaching labs.

What if the melting point hasn’t changed?

If your recrystallized sample still melts low, one of three things happened: the solvent wasn’t the right one, the cooling happened too fast (trapping impurities inside the crystals), or the starting material was too contaminated to clean up in one pass. A second recrystallization often fixes it.

Other Physical Properties of Acetanilide

Knowing the melting point alone is helpful, but the full set of physical constants gives you additional ways to identify and characterize the compound. Here are the key properties from authoritative databases.

These numbers help in lab safety too — the high vapor density means acetanilide vapor will sink and accumulate in low spots, which matters for ventilation planning.

How to Measure Melting Point in the Lab

Getting a reliable melting point for acetanilide requires consistent technique. Follow these steps for a result you can trust against the literature.

1. Dry the sample thoroughly. Residual solvent or water will depress the melting point. Place the recrystallized acetanilide on a watch glass in a 60 °C oven for 15–20 minutes.
2. Pack a capillary tube. Tap the closed end to get about 2–3 mm of finely powdered sample. Too much sample or uneven packing can widen the range.
3. Set a slow heating rate. Heat the melting point apparatus at about 1–2 °C per minute near the expected range. Rapid heating causes a lag between the thermometer reading and the actual temperature of the sample.
4. Record both start and end. Note the temperature when the first droplet of liquid appears and when the last solid disappears. That range is your melting point.
5. Compare to the standard. If your range includes 113–115 °C and is no wider than 2 °C, the sample is pure. If not, consider another recrystallization.

The NOAA chemical datasheet provides an alternative Fahrenheit reading of 237.7 °F, which you can find on its melting point Fahrenheit page if your lab uses a non‑metric thermometer.

The Bottom Line

The melting point of acetanilide — 113–115 °C — is more than a number to look up. It’s the quickest check on whether your recrystallization succeeded. If your sample melts low, recrystallize again; if it hits the range with a narrow window, you’re done.

Every lab instructor expects you to compare your experimental value to the literature standard. For help interpreting a borderline result or choosing a better recrystallization solvent, your teaching assistant or lab supervisor can walk through your specific procedure and data.