CHEM 235 - Experiment 4 Salicylic Acid Synthesis by Saponification

What were the main techniques of this experiment?

- Reflux via thermowell

- Neutralizing a compound

- Suction filtration

- Recrsytallization

- TLC analysis

Basically what happened in this lab

- We first had to do the reaction in the RBF to form the methanol

- In the RBF with boiling chip we added NaOH and methyl salicylate and then refluxed this. Then allowed to cool

- Then we placed the solution in an ice/water bath and added H2SO4 to acidify the solution and completely precipitate the salicylic acid

- We then suction filtered this to obtain the crude salicylic acid compound

- Next, recrystallization with water was performed to recrystallize the salicylic acid

- Then TLC analysis was performed with the crude salicylic acid, pure salicylic acid, and methyl salicylate starting material

What are the reaction conditions?

- Basic conditions

During the reaction, what is the white precipitate that has formed

- Methyl salicylate! since this one is not soluble in water. It disappears as the reaction proceeds and its salt is formed.

Why are we adding acid to the reaction mixture after reflux?

- Because after reflux we have in our RBF methanol and the CB of salicylic acid

- So we add H2SO4 to protonate the CB and get salicylic acid

- We do this until it turns acidic to ensure that all CB of SA has been protonated to SA

During acidification step, what is the white precipitate forming?

- It is the salicylic acid

- The aqueous conditions are well suited for the CB of SA as it is ionic and will interact here, but as SA is formed, it is no longer soluble in the conditions and so is precipitated

Why did we have to suction filter the first time? We recrystallized anyways didn't we?

- Well in our RBF we have salicylic acid yes, but we also have methanol, water, etc. that we don't want so we suction filter these off to obtain the crude SA

What does the TLC plate tell us?

- Well the number of spots in a lane tells us how pure the product is. If we have more than one spot that means that the spotting solution was composed of more than one compound as some was able to travel further than others

- Comparing lanes to each other, we expect the lanes with the synthesized products to have different spots and Rf values than the starting material to indicate that something has happened and our synthesized product is indeed different than the original material

Does Rf values indicate compound identity?

- NO!

- they are not physical constants as they differ as to how you set up your plate

- Rather they more provide insight on purity and whether or not a new product was synthesized

Waste:

a) NaOH

b) Methyl salicylate

c) H2SO4

d) Salicylic acid

e) Petroleum ether

f) Ethyl acetate

g) Acetic acid

a) aqueous waste

b) organic waste

c) aqueous waste

d) solid waste

e) organic waste

f) organic waste

g) aqueous waste

What was the limiting reagent?

- Methyl salicylate

What phase is the development chamber made of?

- Mobile phase

What is the silica gel

- The stationary phase

- The TLC plate

What is the purpose of the 20% sulphuric acid for this experiment?

1. Increase the conductivity of water for electrolysis

2. Acts a strong dehydrating substance that forces water out of various compounds

3. Increases the polarity of the developing solvent and decrease the amount of streaking on TLC plates

4. Protonates the conjugate base of the desired product

5. Acts as the acid in the acid-catalyzed hydrolysis of esters

4

Why is streaking undesirable for TLC analysis?

1. It indicates that not enough of the compound was spotted on the plate

2. The irregular shapes will make it more difficult to measure the Rf values

3. It suggests that the compound is too soluble

4. It indicates that the wrong compound was synthesized if there was streaking

2

Which of the following is not a component of a TLC developing chamber?

1. 100 mL beaker

2. 5-8 mL of mobile phase

3. Watch glass

4. Half-moon shaped filter paper

5. Boiling chip

5.