I\'m currently taking ochem part 2. I need to perform an acid-base extraction to

Question

I'm currently taking ochem part 2. I need to perform an acid-base extraction to separate out a mixture of solid organic compounds consisting of 50% 2-chlorobenzoic acid; 40% methyl 4-aminobenzoate; 10% 1,4-dibromobenzene. I need to create a procedure to first separate the compounds. I also need to recrystallize each compound after separating the compounds; therefore, I need to know which solvent to use for each compound. I also need to prove that I separated my compounds successfully using TLC (therefore I need to know which solvent to use for TLC). I need to do this as a micro-scale procedure (therefore, I will not be given large quantities of reagants). I'm given the reagants methanol, ethanol, methylene chloride, hexane, 1M NaOH, 6M NaOH, 1M HCl, 6M HCl, Anhydrous sodium sulfate, saturated sodium chloride, acetone, sodium bicarbonate, diethyl ether. I believe we will be given about 2 grams of the solid organic compound mixture.

I have actually figured out the procedure for acid-base extractions, but I have a few technical questions and procedural questions in regards to acid-base extraction, recrystallization and conducting a TLC:

Q1: I was considering using diethyl ether instead of methylene chloride as my solvent for acid-base extraction? Is that okay? The prior person who answered my question said it was, but I just wanted to make sure.

Q2: For a microscale recrystallization, do I need to use a Craig tube or can I use other glass test tubes to do a recrystallization?

Q3: Can test tubes be placed directly on the aluminum block during recrystallization?

Q4: It was suggested by another person at chegg to run TLC with hexane-ethyl acetate mixture. He/she stated, “TLC is run in hexane-ethyl acetate mixture as the eluting solvent. 1,4-dibromobenzene being least polar moves fastest, methyl-4-aminobenzoate is more polar than 1,4-dibromobenzene and most polar of all 2-chlorobenzoic acid stays close to the base on the tlc plate”. However, ethyl acetate is not provided out of the reagants in our experimental lab at school. Out of all the other reagants provided for this experiment: methanol, ethanol, methylene chloride, hexane, 1M NaOH, 6M NaOH, 1M HCl, 6M HCl, Anhydrous sodium sulfate, saturated sodium chloride, acetone, sodium bicarbonate, diethyl ether, what can be used to run the TLC for the purified organic compounds (1) methyl 4-aminobenzoate (2) 2-chlorobenzoic acid (3) 1,4-dibromobenzene. (a) What is your suggestion for the eluting solvent? (b) I had an idea of using a mixture of 70% hexane and 30% acetone?

Q5: How do you run a TLC plate using solid organic compounds? Once I run an acid-based extraction, and then run a recrystallization, if the compound is in its pure solid form, how do I do the TLC? Earlier in other experiments, I only did TLC’s on liquid compounds for spotting.

Q6: How do I prepare the reference (the mixture of the 3 compounds) and in what ratios for conducting my TLC?

Explanation / Answer

ANSWERS:

Q1) Yes, you can absolutely use diethyl ether as a solvent for the acid-base extraction. However, this completely depend on the solubility of the compounds present in the reaction mixture (2-chlorobenzoic acid, methyl 4-aminobenzoate and 1,4-dibromobenzene). And moreover, one has to be more careful when using diethyl ether for the extraction as it is more volatile than other solvents (may develop pressure very quickly during the shaking process using separating funnel).

Q2) As you are planning for recrystallisation, it is better to use the Craig tubes as they are best suited for centrifuge with which we can easily take out the solvent using a simple pipette.

Q3) No, you are not allowed to use the test tubes for the recrystallisation process by placing them directly on the aluminum block as they may tend to break very easily at higher temperatures.

Q4) (a) I would suggest you to use a combination of methylene chloride and methanol as an eluting solvent for the column chromatography.

     (b) However, you can also use mixture of 70% hexane and 30% acetone as an eluent. But please make sure that you will get better separation (between spots) in the TLC and choose accordingly.

Q5) There is no such thing like "running a TLC plate using a solid organic compound". You have to dissolve your compound in appropriate solvent and then only you can use that liquid compound for spotting on TLC. The reason is your compound must be absorbed by the silica before running the TLC plate using mobile phase. This is only possible in liquid phase and not possible in solid state.

Q6) Preparing a reference for TLC is just intended for qualitative purpose but not for quantitative purpose. So, you can use whatever ratio you want but make sure that even the least weighed compound should appear on the TLC plate with good intensity on running using the mobile phase (eluent).

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