[aberneth]

I'm doing a pretty typical O-chem lab right now, trying to synthesize, isolate, and purify Methyl Benzoate. It's created via Fischer Esterification of methanol and benzoic acid. My set up has a round-bottom flask in a self-regulated hot water bath, with a liebig condenser above it. From what I gather, the product is supposed to be a fruity, slightly floral aromatic with a light yellow tinge.

My product, however, smells like the fake spearmint used in those sweetheart candies that say "I wuv you" and such, or like the flavoring used in spearmint lifesavers. It also has shades varying from faint blue to perfectly clear to yellow. The unexpected scent and the unexpected coloring are concerning. I verified that the alcohol I'm using is methanol. Its boiling point is about 64 degrees, as expected. The benzoic acid, though, looks like it could be old. I'm confident that it is benzoic acid, despite its age.

I guess if you haven't figured this out already, the question is "Why doesn't my product have a fruity/floral scent, and why does it smell like a life saver and have an off color?"

Thanks for reading (and hopefully responding)

[skyjumper]

Sounds like you may have made methyl salicylate. "oil of wintergreen"

[aberneth]

Is it possible that I'm oxidizing the benzoic acid to make salicylic acid?

I did a freezing point depression test and came up with a molar mass of approximately 120g/mol, which is about that of benzoic acid. (MM=122g/mol)

[skyjumper]

It may be possible, partly. It could be an impure product, where the wintergreen is overpowering the fruit...

[aberneth]

That seems ridiculous, though. Adding an extra oxygen to a benzoate group isn't a simple process, I don't think it could be simply accomplished by refluxing benzoic acid with methanol and sulphuric acid.

[skyjumper]

I don't see any other way you could have gotten a mint smell. Unless, again the acid was impure, or glassware not clean from someone who made wintergreen before, etc. etc.

[aberneth]

Well, thanks anyway.

[Jorriss]

You might want to see about getting this moved to the organic section, you'll get more input if you need more help.

[Borek]

Good idea.

[bromidewind]

It sounds like the starting benzoic acid might be your problem. If the container is exposed to air, it could react very slowly with moisture in the air to form either peroxybenzoic acid or salicylic acid (not sure which of these would predominate). If you're getting a mint odor, then it's probably forming salicylic acid which would react with the methanol to form methyl salicylate.

My advice would be to go back to that benzoic acid since you said it looked old. If it looks old, then it probably is. Try purifying some and give your reaction another go.

[aberneth]

Thoughts on purifying the benzoic acid?

[bromidewind]

Well, first try to get a melting point. If it's around 124ºC, then contamination's not your problem. If you do have a big deviation from the melting point, you've definitely got some contamination.

Once you've determined contamination, heat some DI water up to around 30ºC (benzoic acid becomes soluble in water at 25ºC), then pour some of the hot water over the benzoic acid. Keep the benzoic acid beaker on a hot plate set to around 30ºC and keep adding water until all your crystals are dissolved. Let it cool to room temperature. You should start to see some new crystals form. Once its at RT, put it an ice bath to keep it cool. Once it's pretty durn cold, vacuum filtrate it. You can wash your filter with some ice-cold water as you go along. Once you've got your crystals, dry them and run another mel temp determination. If it's in the right range, then you've got pure crystals.

I'd recommend doing a small sample, like 30 mg, first so you can know if that it is your problem. I'd hate for you to spend the time to recrystallize enough for your experiment, then find out it isn't good period. And if it turns out there is a contamination, offer to purify the whole thing for your lab instructor for bonus points

[aberneth]

Thank you!

[aberneth]

Another thought: If there is any salicylic acid in my benzoic acid, shouldn't a dissolved sample be positive for phenols (per a ferric chloride colorimetric test)?

[discodermolide]

I'm doing a pretty typical O-chem lab right now, trying to synthesize, isolate, and purify Methyl Benzoate. It's created via Fischer Esterification of methanol and benzoic acid. My set up has a round-bottom flask in a self-regulated hot water bath, with a liebig condenser above it. From what I gather, the product is supposed to be a fruity, slightly floral aromatic with a light yellow tinge.

My product, however, smells like the fake spearmint used in those sweetheart candies that say "I wuv you" and such, or like the flavoring used in spearmint lifesavers. It also has shades varying from faint blue to perfectly clear to yellow. The unexpected scent and the unexpected coloring are concerning. I verified that the alcohol I'm using is methanol. Its boiling point is about 64 degrees, as expected. The benzoic acid, though, looks like it could be old. I'm confident that it is benzoic acid, despite its age.

I guess if you haven't figured this out already, the question is "Why doesn't my product have a fruity/floral scent, and why does it smell like a life saver and have an off color?"

Thanks for reading (and hopefully responding)

Dissolve benzoic acid in methanol add an equivalent volume of heptane warm to around reflux temperature and add concentrated sulphuric acid. Continue heating until complete (follow by GC or TLC) (3-4 hours). Separate the heptane phase, wash with sodium bicarbonate (care gas evolution) until neutral. Remove heptane by distillation, distill product if required.