Preparation of Aspirin

Objectives:1. To synthesize aspirin using salicylic acid and acetic anhydride as substrates2. To calculate the percent yield obtained based from computed theoretical and actual yields

Schematic DiagramA. Synthesis of Aspirin Sample

1.9- 2.2 g Salicylic Acid

Acetic acid vapor producedFilter, dry and weighCrystals of aspirin appearsAdd 18-20 ml water. Cool.Place in ice bathOrange-Brown solution color formedHeat at 75C for 15 minutesAdd 2 ml waterReaction CompletedAdd 5 drops of 85% H3PO4No visible reaction formed Add 5.0- 5.5 ml of Acetic Anhydride

Crystallization hastens

Aspirin weight recorded

Results/Discussion The table presented below denotes the pertinent data needed for the determination of the percentage yield of the synthesized aspirin sample.Table 1. Data for the determination of the percent yield in synthesized aspirin sampleSubjectAmount (g)

Mass of Erlenmeyer Flask91.3653

Mass of flask and salicylic acid93.3454

Mass of salicylic acid1.9801

Mass of watch glass33.2348

Mass of watch glass and aspirin34.4261

Mass of aspirin (actual yield)0.4571

Mass of aspirin (theoretical yield)2.5831

Percentage Yield (%)17.70

The determination of the percentage yield of the ASA sample by using the formula

The theoretical yield of the aspirin sample produced can be determined by using the formula

where ASA stands for acetylsalicylic acid (Aspirin). Based from Table 1, there is 1.9801 g of salicylic acid used. Applying, we get

Meanwhile, the actual yield produced can be determined by the formula

Looking back again in Table 1, it could be known that the aspirin sample is placed in a watch glass (used as a container). Subtracting the values, we get

Since the two variables are already known, hence,

Based from the result obtained, it could be easily determined that the percentage yield was comparatively lower compared to the expected yield. This huge error may have been due to the crystallization process, where not all ASA were retrieved during the crystallization has been carried out, or simply due to the loss of sample due to mishandling in storage. In addition, the synthesized aspirin crystals were colored peach-like instead to that of expected white. The rationale can be pointed out to the excess H3PO4 added that gave of a brown solution color.

ConclusionThe synthesized aspirin sample was prepared using salicylic acid and acetic anhydride. Results shown that there is a percentage yield of 17.70% ASA content in the sample, which implies that the prepared aspirin did not reach the expected amount that will be produced. It can be reasoned out to the immature crystallization or simply due to the improper handling and storage of the sample produced.

Reaction MechanismsThe synthesis of acetylsalicylic acid (aspirin) is carried out through this manner:

This reaction covers a series of steps, which can be enumerated as follows:

1) In the first reaction, a proton (H+) from the acid attacks the oxygen in acetic anhydride. This causes two electrons in the double bond to delocalize across the two oxygen atoms on the acetic anhydride.

2) SA now acts as a nucleophile, with a free pair of electrons on the oxygen of the hydroxyl group attacking the carbon of the acetic anhydride. The delocalized electrons rearrange, creating a temporary bond between the two reactants. Due to the extra proton attached to the oxygen atom, the oxygen is positively charged.

3) In the third reaction, the proton from the hydroxyl group of the phenol attacks the oxygen in the original OH portion of the acetic anhydride,forming a positively charged oxygen atom.

4) In reaction 4, due to the now positive charge on the oxygen, electrons bonding the C and O+ atoms are donated to the oxygen atom. This breaks the bond between the two and forms acetic acid, a by-product of the synthesis of ASA. At the same time, electrons between the OH and C rearrange to reform a double bond between C=OH.This causes the OH group to become positively charged due to loss of negative electrons.

5) Finally, the 5th stage illustrates how the positively charged -OH group loses a proton to reform the H3PO4 catalyst, leaving the ester, ASA, behind.