Lab Report

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\lhead{CHEM 201 --- Organic Chemistry Lab}
\rhead{Fall 2025}
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\title{\textbf{Synthesis and Characterization of Aspirin\\(Acetylsalicylic Acid)}}
\author{
  Jessica M.\ Torres and Ryan K.\ Patel\\[4pt]
  \textit{Department of Chemistry, University of Virginia}\\
  \textit{Lab Section 004, Wednesdays 1:00--4:50 PM}\\
  \textit{Instructor: Dr.\ Amanda Liu}
}
\date{November 12, 2025}

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\begin{abstract}
Aspirin (acetylsalicylic acid) was synthesized via the acetylation of salicylic acid using acetic anhydride as the acetylating agent and phosphoric acid as a catalyst. The crude product was purified by recrystallization from hot ethanol and water. The final product was characterized by melting point determination and infrared (IR) spectroscopy. A yield of \SI{78.4}{\percent} was obtained. The melting point of the purified product was \SIrange{134}{136}{\degreeCelsius}, in close agreement with the literature value of \SIrange{135}{136}{\degreeCelsius}. IR spectroscopy confirmed the presence of the ester carbonyl stretch at \SI{1749}{\per\centi\meter} and the absence of the broad O--H stretch characteristic of salicylic acid, indicating successful acetylation.
\end{abstract}

\section{Introduction}

Aspirin (acetylsalicylic acid, \(\text{C}_9\text{H}_8\text{O}_4\)) is one of the most widely used pharmaceutical compounds in the world, employed as an analgesic, antipyretic, and anti-inflammatory agent. First synthesized by Felix Hoffmann at Bayer in 1897, aspirin functions by irreversibly inhibiting the cyclooxygenase (COX) enzymes, thereby reducing the production of prostaglandins and thromboxanes involved in pain and inflammation signaling.

The synthesis of aspirin serves as a classic esterification reaction in which the hydroxyl group of salicylic acid reacts with acetic anhydride to form an ester bond:

\begin{equation}
  \text{C}_6\text{H}_4(\text{OH})(\text{COOH}) + (\text{CH}_3\text{CO})_2\text{O} \xrightarrow{\text{H}_3\text{PO}_4} \text{C}_6\text{H}_4(\text{OOCCH}_3)(\text{COOH}) + \text{CH}_3\text{COOH}
\end{equation}

The objectives of this experiment were to: (1)~synthesize aspirin from salicylic acid and acetic anhydride, (2)~purify the product via recrystallization, and (3)~characterize the product using melting point analysis and IR spectroscopy.

\section{Materials and Methods}

\subsection{Materials}

Salicylic acid (\SI{2.00}{\gram}, \SI{14.5}{\milli\mole}), acetic anhydride (\SI{3.0}{\milli\liter}, \SI{31.8}{\milli\mole}), 85\% phosphoric acid (5 drops), ethanol (95\%), distilled water, and ice. All reagents were obtained from the University of Virginia Chemistry Department stockroom and used without further purification.

\subsection{Procedure}

Salicylic acid (\SI{2.00}{\gram}) was placed in a \SI{125}{\milli\liter} Erlenmeyer flask. Acetic anhydride (\SI{3.0}{\milli\liter}) was added, followed by 5 drops of 85\% phosphoric acid as a catalyst. The mixture was heated in a water bath at \SIrange{75}{80}{\degreeCelsius} for 15 minutes with occasional swirling.

After heating, the flask was removed from the water bath and \SI{2}{\milli\liter} of distilled water was cautiously added to decompose excess acetic anhydride. Once the reaction subsided, an additional \SI{20}{\milli\liter} of distilled water was added. The solution was cooled in an ice bath until crystallization was complete (approximately 10 minutes). The crude product was collected by vacuum filtration using a Buchner funnel, washed with two \SI{5}{\milli\liter} portions of cold distilled water, and air-dried.

Recrystallization was performed by dissolving the crude product in \SI{5}{\milli\liter} of hot ethanol, adding \SI{15}{\milli\liter} of warm distilled water, and allowing the solution to cool slowly to room temperature before placing in an ice bath. The purified crystals were collected by vacuum filtration and dried in a \SI{70}{\degreeCelsius} oven for 20 minutes.

\section{Results}

\subsection{Yield Calculation}

\begin{table}[H]
\centering
\caption{Summary of masses and yield.}
\label{tab:yield}
\begin{tabular}{@{}lS[table-format=2.2]l@{}}
\toprule
\textbf{Quantity} & \textbf{Value} & \textbf{Unit} \\
\midrule
Mass of salicylic acid & 2.00 & g \\
Moles of salicylic acid & 14.5 & mmol \\
Theoretical yield of aspirin & 2.61 & g \\
Mass of crude product & 2.28 & g \\
Mass of purified product & 2.05 & g \\
Percent yield & 78.4 & \% \\
\bottomrule
\end{tabular}
\end{table}

The theoretical yield was calculated based on the 1:1 stoichiometry of salicylic acid to aspirin (\(M_{\text{aspirin}} = \SI{180.16}{\gram\per\mole}\)):
\begin{equation}
  \text{Theoretical yield} = \SI{14.5}{\milli\mole} \times \SI{180.16}{\gram\per\mole} = \SI{2.61}{\gram}
\end{equation}

\subsection{Melting Point Analysis}

\begin{table}[H]
\centering
\caption{Melting point data.}
\label{tab:mp}
\begin{tabular}{@{}lcc@{}}
\toprule
\textbf{Sample} & \textbf{Melting Range (\si{\degreeCelsius})} & \textbf{Literature (\si{\degreeCelsius})} \\
\midrule
Crude aspirin & 122--128 & --- \\
Purified aspirin & 134--136 & 135--136 \\
Salicylic acid (starting material) & 158--160 & 158--161 \\
\bottomrule
\end{tabular}
\end{table}

The narrow melting range of the purified product (\SIrange{134}{136}{\degreeCelsius}) and its close agreement with the literature value confirms high purity. The broad melting range of the crude product indicates the presence of impurities before recrystallization.

\subsection{IR Spectroscopy}

Key absorption bands observed in the IR spectrum of purified aspirin:
\begin{itemize}
  \item Broad O--H stretch (carboxylic acid): \SIrange{2500}{3300}{\per\centi\meter}
  \item Ester C=O stretch: \SI{1749}{\per\centi\meter}
  \item Carboxylic acid C=O stretch: \SI{1689}{\per\centi\meter}
  \item C--O stretch (ester): \SI{1224}{\per\centi\meter}
\end{itemize}

The absence of a sharp, broad O--H stretch near \SI{3300}{\per\centi\meter} (characteristic of the phenolic hydroxyl in salicylic acid) and the presence of the ester carbonyl at \SI{1749}{\per\centi\meter} confirm that acetylation was successful.

\section{Discussion}

The synthesis of aspirin proceeded successfully with a percent yield of 78.4\%. The loss of material can be attributed primarily to the recrystallization step, where some product remained dissolved in the mother liquor. The use of phosphoric acid as a Br\o{}nsted acid catalyst facilitated the nucleophilic acyl substitution mechanism, in which the hydroxyl group of salicylic acid attacks the electrophilic carbonyl carbon of acetic anhydride.

The melting point analysis provides strong evidence for product purity. The purified product's melting range of \SIrange{134}{136}{\degreeCelsius} is within \SI{1}{\degreeCelsius} of the literature value, and the narrow \SI{2}{\degreeCelsius} range indicates minimal impurity. The crude product's broader range (\SIrange{122}{128}{\degreeCelsius}) is consistent with the presence of unreacted salicylic acid and acetic acid.

IR spectroscopy provided complementary structural confirmation. The appearance of a distinct ester carbonyl peak at \SI{1749}{\per\centi\meter}, separate from the carboxylic acid carbonyl at \SI{1689}{\per\centi\meter}, is diagnostic for aspirin. A potential source of error is incomplete reaction, which would result in residual salicylic acid; however, no evidence of this was observed in the IR spectrum or melting point data of the purified product.

\section{Conclusion}

Aspirin was successfully synthesized from salicylic acid and acetic anhydride in 78.4\% yield. Recrystallization effectively purified the product, as confirmed by melting point analysis (\SIrange{134}{136}{\degreeCelsius} vs.\ literature \SIrange{135}{136}{\degreeCelsius}) and IR spectroscopy. The experiment demonstrates a practical application of esterification chemistry and illustrates the importance of purification and characterization techniques in synthetic organic chemistry.

\section*{References}

\begin{enumerate}
  \item Pavia, D.~L.; Lampman, G.~M.; Kriz, G.~S.; Vyvyan, J.~R. \textit{Introduction to Spectroscopy}, 5th ed.; Cengage Learning: Stamford, CT, 2015.
  \item Williamson, K.~L.; Masters, K.~M. \textit{Macroscale and Microscale Organic Experiments}, 7th ed.; Cengage Learning: Boston, MA, 2017.
  \item CRC Handbook of Chemistry and Physics, 104th ed.; Rumble, J.~R., Ed.; CRC Press: Boca Raton, FL, 2023.
\end{enumerate}

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