Question

Synthesis of Aspirin Post-lab Questions February 2, 2018 Discussion of the aspirin product including, 1. The calculated percent yield of your aspirin product and comments on its purity based on the literature melting point and literature IR spectrum. Label all major peaks in the IR spectrum of your aspirin product. How close does your spectrum match to the literature spectrum of aspirin? What are the differences in the two spectra, and how do you explain them? You may find the literature IR spectrum of aspirin at the following website: Spectral Database for Organic Compounds, SDBS Please attach your interpreted IR spectrum with your notebook pages. 2. Hydrochloric acid is about as strong a mineral acid as sulfuric acid, why would HCl not be a satisfactory catalyst in this reaction? Please draw a reaction scheme to support your answer.

The weight of aspirin is 1.6 g and the reagents used to perform this synthesis was 1.48 g salicylic acid, 3 mL ethanoic anhydride, 3 drops H2SO4, and acetylsalicylic acid

Answer 1

Solution:

Values provided in the question are;

Weight of salicylic acid = 1.48 g, weight of aspirin = 1.6 g,  3 drops of H₂SO₄

Volume of ethanoic anhydride = 3 mL. Density of ethanoic anhydride = 1.08 g/mL.

Hence weight of ethanoic anhydride = 3 mL x 1.08 g/mL = 3.24 g

1 a) Yield calculation:

Theoretically, 1 mole of salicylic acid reacts with 1 mole of ethanoic anhydride and produces 1 mole of aspirin. This will result in 100% yield.

Experimental yield can be calculated using the number of moles of salicylic acid input & aspirin output.    

Number of moles of salicylic acid (Exp) = input mass / molar mass

= 1.48 g / 138.121 g/mole = 0.01072 mole ........> A

Number of moles of aspirin (Exp) = output mass / molar mass

= 1.6 g / 180.158 g/mole = 0.00888 mole .........> B

%Yield of the reaction = (B / A) x 100 = (0.00888 mole / 0.01072 mole) x 100

= 0.8285 x 100 = 82.85 = 83%

Hence the yield 83%. According to Vogel's Textbook of Practical Organic Chemistry, yields above 80% are very good.

Further the theoretical melting range of aspirin is 138 - 140^oC. The range difference is only 2^oC shows the compound is pure.

b) IR spectrum:

Functional group Bond Frequency, cm-1 Frequency, cm-1 Remarks

in literature IR in Exp IR

Esters C=O stretch 1754(s) 1748(m) Comparable

Carboxylic acids   C=O stretch 1693(s) 1698(m) Comparable

Aromatics C-C stretch (in-ring) 1483 - 1420(m) 1481 - 1417(m) Comparable

Carboxylic acids/esters C-O stretch 1190(s) 1181(m)   Comparable

Aromatics C-H 'oop' 918(s) 914(s) Comparable

The IR spectrum of exp is comparable with literature. However, C=O stretching are medium. Hence the compound appears to be crude and requires further purification.

2. Concentrated HCl is 65% water by weight, while concentrated sulfuric acid is only about 1% water by weight. The ethanoic anhydride is moisture sensitive and it will get hydrolized to 2 mole of acetic acid. The reaction is moisture sensitive.

(CH₃CO)₂O + H₂O ...................> 2CH₃COOH

Hence sulfuric acid is used in the reaction instead of hydrochloric acid.