Preparazione Aspirina

PREPARAZIONE DELL'ACIDO ACETILSALICILICO DA ACIDO SALICILICO

Mm = 180
p.f. = 136 °C

- MATERIALE DI CONSUMO: Acido salicilico anidro (5gr.)
Anidride acetica (7 ml)
Acido solforico conc. poche gocce

-MATERIALE: Beuta da 250cc con refrigerante ad aria

Buchner

Beuta da vuoto

Termometro
Cilindro 100cc

-APPARECCHIATURE: bagnomaria termoregolatore

-MODO DI OPERARE: Si mette della beuta l'acido e l'anidride acetica e si aggiungono 2-3 gocce di acido solforico concentrato, come catalizzatore (reazione esotermica, controllare con il termometro). Si applica alla beuta il refrigerante ad aria, si agita qualche minuto fino a portare in soluzione il tutto e si scalda a b.m. a 50.60°C per circa 15 minuti agitando spesso; infine si lascia raffreddare la miscela, si aggiungono 75 ml di acqua per idrolizzare l'eccesso di anidride acetica, si agita bene e si filtra su buchner. Il prodotto ottenuto si può purificare per cristallizzazione con benzolo, oppure con una miscela di 15 ml di etanolo e 40 ml di acqua. Il prodotto si decompone facilmente, quindi non bisogna mai prolungare il tempo di ebollizione durante la cristallizzazione.

Abstract:

The Synthesis of Aspirin

Purpose: To synthesize aspirin, a common analgesic drug. The experiment actually involves three parts: The synthesis of aspirin, the isolation and purification of aspirin, and the estimation of the purity of the final product.
Synthesis: The synthesis involves the reaction of salicylic acid and acetic anhydride in the presence of a catalyst, phosphoric acid H2SO4

Isolation and Purification: Once the aspirin is prepared it must be isolated from the reaction solution and purified. The aspirin is insoluble in cold water, and can be isolated by filtering the chilled reaction solution. Purification is necessary to remove any unreacted salicylic acid and acetic anhydride, as well as the acetic acid product and H2SO4. Acetic anhydride is caused to decompose by the addition of water once the formation of aspirin is complete:The acetic acid and H2SO4 are water soluble and can be removed by washing the aspirin with chilled water. Salicylic acid is only slightly soluble in water and is not completely removed in the washing step. Final purification is accomplished by the process of recrystallization. The impure aspirin is dissolved in warm ethanol. The solution is then cooled slowly, and the aspirin crystallizes out of solution leaving the salicylic acid and other impurities behind.

Estimation of Purity: The melting point of a compound can be used to identify it and also to estimate its purity. Generally an impure compound will exhibit a melting point which is lower than that of the pure compound. Therefore, if your aspirin melts at a temperature below the accepted melting point two possibilities exist: either your product is impure or it is not aspirin. A pure substance will melt sharply over a range of 1 or 2 degrees celcius. That is, the temperature at which melting first occurs is only 1 or 2 degree less than the temperature at which the sample is completely melted. An impure compound will melt over a wider temperature range.

Safety Considerations: This experiment uses salicylic acid, acetic anhydride and sulfuric acid. The salicylic acid and aspirin may cause irritation to your skin or eyes, but are basically not hazardous. An excess of these can be disposed of in the sink or if packaged, in the trash. If you spill some, wipe it up with a wet paper towel and throw the towel in the trash. The acetic anhydride and phosphoric acid can cause bad burns. Use them in the hood. Be sure to wear gloves and safety goggles when using these chemicals. Excess chemicals must be disposed of in the plastic tub of water. This will convert the acetic anhydride to vinegar and dilute the phosphoric acid. If you spill a lot of either of these, notify your instructor.

Bibliografia:
1)http://www.journalarchive.jst.go.jp/jnlpdf.php?cdjournal=cpb1958&cdvol=33&noissue=7&startpage=2641&lang=en&from=jnlabstract
2)http://www.chem.uky.edu/xray/people/Parkin/Parkin_Papers_pdfs/226_JPSv97n4p1361.pdf
Dalle ricerche bibliografiche effettuate si è trovato che l'acido acetil salicilico (aspirina) cristallizza nel gruppo spaziale monoclino P21/c. La cella elementare contiene quattro molecole; la struttura è fatta da dimeri centrosimmetrici nei quali due molecole sono legate da una coppia di legami idrogeno tra i loro due gruppi carbossilici.