Caffeine
Properties
| State | Solid (white crystalline powder with bitter taste) |
| Color | White |
| Solubility | Moderately soluble in water (22 g/L at 25°C); more soluble in hot water |
| Melting Point | 235°C (sublimes) |
| Boiling Point | Sublimes at 178°C |
About Caffeine
Caffeine is 1,3,7-trimethylxanthine, a methylated purine alkaloid whose mechanism is one of the cleanest receptor-pharmacology stories in pharmacology. The molecule is a near-perfect mimic of adenosine at the A1 and A2A receptors — the bicyclic purine scaffold is shared, and the methylation pattern at N1, N3, and N7 occupies just enough of the binding pocket to act as a competitive antagonist without triggering downstream Gi signaling. When adenosine accumulates in the basal forebrain over a waking day and binds A2A receptors there, it inhibits the wake-promoting neurons that gate sleep onset. Caffeine displaces adenosine, and the disinhibition is what we feel as alertness. The Ki at human A2A is about 2.4 µM, which is comfortably below the 5–8 µM peak plasma concentration after a 200 mg dose, so a regular cup of coffee actually saturates a meaningful fraction of A2A receptors. The molar mass is 194.19 g/mol, the melting point is 235 °C with sublimation, and the 22 g/L water solubility at 25 °C jumps to 660 g/L in boiling water — which is why coffee and tea are made hot. Friedlieb Ferdinand Runge isolated caffeine from coffee beans in 1819 after Goethe handed him a box and asked him to figure out what the active principle was. Beyond beverages, caffeine is added to OTC analgesics like Excedrin and Anacin because it potentiates the analgesic effect of aspirin and acetaminophen by roughly 40% (probably through cerebral vasoconstriction and adenosine-receptor antagonism in pain pathways), and the caffeine-from-tea-leaves DCM extraction is the most-performed liquid-liquid extraction in undergraduate organic teaching labs.
Where you'll encounter it
If you've ever extracted caffeine from black tea in a sophomore orgo lab — boil tea bags in dilute Na2CO3, partition with dichloromethane in a separatory funnel, dry over anhydrous MgSO4, rotovap, sublime the off-white residue to crystalline white needles — you've worked with caffeine as the prototype natural-product extraction. In a clinical setting, IV caffeine citrate is the standard treatment for apnea of prematurity in NICU infants; the half-life in neonates is 100 hours versus 5 hours in adults because their CYP1A2 expression is still ramping up. In a sports-nutrition lab, caffeine pharmacokinetics matter for the WADA threshold (formerly 12 µg/mL in urine, now de-listed but still monitored) and for the well-replicated 2–6% endurance-performance benefit at 3–6 mg/kg doses.
Common Uses
- Adenosine A1/A2A receptor antagonist in coffee, tea, and energy drinks
- Analgesic adjuvant in OTC headache combinations (Excedrin, Anacin)
- IV caffeine citrate for apnea of prematurity in neonatal intensive care
- Standard liquid-liquid extraction target in undergraduate organic teaching labs
- Ergogenic aid at 3–6 mg/kg for endurance-sport performance
Safety Information
Harmful if swallowed at high doses. GHS H302 (harmful if swallowed). The acute LD50 in humans is roughly 150–200 mg/kg — about 10 g for a 70 kg adult, equivalent to ~100 cups of brewed coffee, but as little as 1–2 teaspoons of pure caffeine powder. The FDA has issued specific warnings against consumer-grade pure caffeine powder after several teen deaths from miscalculated doses. Therapeutic and recreational doses (50–400 mg) cause sympathetic stimulation, mild tachycardia, increased gastric acid secretion, and diuresis; chronic intake produces tolerance via A2A receptor upregulation, and abrupt discontinuation produces a 12–48 hour withdrawal syndrome of headache, fatigue, and impaired concentration.
This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals.