Aspirin

[dinosaur93]

Could anyone help me understanding each step of the procedure in forming aspirin, tnx heaps guys...






Procedure:

1. Weigh accurately about 1.0g of salicylic  acid and transfer all of it into a 250 mL flask.




2. Carefully add 8mL of acetic anhydride and 1mL of concentrated sulphuric acid. Swirl the flask without heating until all the solid has dissolved.

• The presence of an sulphuric acid is to speed up the reaction between acetic anhydride and salicylic acid as it acts as a catalyst, what would happen if concentrated sulphuric acid is accidentally replaced with a a diluted sulphuric acid?
• What would happen if a student accidentally placed too much or too less of the concentrated sulphuric acid, would it affect the yield?
• What if a student misread procedure 2, and heated the flasks when swirling it, what would happen to the end result? Why is the reaction not heated?

The volume of concentrated hydrochloric acid will not effect the percent yield as it does not take part in the reaction therefore, regardless of the volume added, you would still be able to obtain the same amount of percent yield???



3. All the flasks to cool for 5 minutes. Pour 100mL of ice cold water into the 250mL flask

• What is the purpose of adding ice cold water into the reaction when forming aspirin?

Could water is used so that it would help lower the solubility of the aspirin, in this way, the aspirin would not [or prevented to] react with the water added and form back into the salicylic acid.



4. At this stage, aspirin might begin to crystallise out of solution spontantaneously. If it doesn't use a glass rod to gently disrupt the surface of the liquid to encourage precipitation.

• In the process of precipitation, incomplete crystallisation often occur, why is this so?

5. Allow the precipitation to continue for a few minutes, or longer if necessary until no further crystals are seen to appear.



6. Collect the precipitate by vacuum filtration, washing the 250mL flask beaker with small amounts of cold deionise water and adding the washing to the filter funnel.



7. Place the filter paper and crystals in an oven to dry.

• What would happen if the filter paper and crystals was overheated in the oven; What happens if your filter paper was heated for a long period of time?

8. weigh the filter paper and crystals to determine the mass of the aspirin produced.

• How would you determine whether the -OH group in the salicylic acid has reacted with the acetic anhydrate after weighing the filter paper? How would you check if, base on the filter paper and crystals, aspirin has formed successfully

[dinosaur93]

anyone?

[vea]

The volume of concentrated hydrochloric acid will not effect the percent yield as it does not take part in the reaction therefore, regardless of the volume added, you would still be able to obtain the same amount of percent yield???

Do you mean sulfuric acid?  In Unit 4, you will learn that catalysts never increase yields.
Also, the amount of the catalyst you add does matter up to a certain point but once you have that certain amount of catalyst, any more of it will not increase the reaction rate.

Could water is used so that it would help lower the solubility of the aspirin, in this way, the aspirin would not [or prevented to] react with the water added and form back into the salicylic acid.

Lower temperature makes it harder for substances to dissolve in. Think about it in terms of Milo into milk, when you put it into cold milk it does not dissolve as instantly as it does when you put it in warm milk. (sorry, couldn't think of a better analogy). By lowering the solubility of aspirin, we are able to keep more of it as a solid to get a higher yield. This is generally the aim of these precipitation reactions.

[dinosaur93]

anyone else?

How about this?

reaction 1: salicylic acid + acetic anhydride ---> aspirin and ethanoic acid

reaction 2: salicylic acid + ethanoic acid ---> aspirin and water

1. reaction 2 is slower than reaction 1, therefore reaction 1 is more preferred to speed up the rate of the reaction and yield, who is this so?

2. Based on reaction 1, what metal/solution are we supposed to use to detect whether aspirin has formed and all the -OH groups in the salicylic acid has reacted...

3. Explain 5 possible explanations as to how a student has obtain a yield of less than 100% (eg. solubility, reaction formation, etc.)

[Shenz0r]

1. Notice that there is an ester linkage in the aspirin molecule. If you hydrolyse it, what happens? You'll go back and produce salicylic acid and ethanoic acid again. If you use the reaction 2 pathway, the water you produce with the aspirin will also make the reaction go backwards, thus this makes the pathway more slow and less-yielding. With reaction 1 you have no water produced, and hence the aspirin won't be hydrolysed.

2. They use what's called a Ferric Chloride Test. Salicylic acid has a OH group, while aspirin does not. In the Ferric Cholride test, drops of Ferric Chloride are added to the sample. If the solution turns purple then it indicates the presence of phenol groups with OH, meaning that the sample is impure. The less intense the colour, the more pure it is, as there will be less OH reacting to produce the purple colour. When your solution is colourless, that's when you know all the OH groups in the salicylic acid has reacted to produce aspirin.

[charmanderp]

phenol groups with OH

I'd just like to point out that it's not a phenol group unless it has an OH :p

[pi]

phenol groups with OH

I'd just like to point out that it's not a phenol group unless it has an OH :p

Yep, phenyl + OH = phenol

[AllAboutTheLGs]

Yield less than 100%:

Incomplete reaction
Loss of precipitate during filtration/transfer
Decomposition of precipitate during heating

Yield more than 100%:

Impurities such as leftover ions on precipitate due to not thoroughly washing
Precipitate not fully dry