[Murka]

Hey guys,

I've synthesised Isopentyl Acetate (Banana Oil) using Isopentyl alcohol and Acetic Acid, the latter protonated with concentrated Sulphuric Acid.

Both the reactants and products are colorless, but the intermediate, after the reflux, is sort of yellowish-golden-brown, and after the final product distillation those yellow-golden-brown impurities remained in the distillation flask.

Our instructor asked us, what could those brown impurities be?
My best guess is that it has something to do with side reactions with Sulphur, as it kinda makes sense with the color, but I'm not sure.
I've also thought that some grease from the distillation apparatus could have entered the distillation flask but it is unlikely as I'm careful with my work and all the other students got the same results, even assuming I alone was somehow careless with my greasework it's unlikely everyone would have gotten the same result.

What do you guys think?

[Archer]

I do that same reaction with acetic anhydride and isoamyl alcohol with a catalytic amount of pyridine and get no colouration so it must the something the sulphuric acid is doing.

It is very common to get discolouration in chemical reactions even at room temperature. They start colourless and darken over time, after purification of the product you get a water white oil and a small amount of brown / black sludge.

I have never speculated as to what this might be but it certainly isn't carelessness in your case.

[Murka]

That's my purpose here exactly  to speculate as to what this might be...

If the brown stuf would've been just a discoloration of the reactants, it wouldn't have stayed in the flask after the distillation, no?
i.e. those have to be different materials entirely, perhaps made through some unknown (to us, not generally) side-reactions?

[TwistedConf]

You're right to suspect something with the sulfuric acid. It's most likely happening due to some junk or impurity in the acid (not at all uncommon in a teaching lab).

Run it again under ideal conditions with completely clean reagents and you probably won't see that color. In the end, no one really worries about it for simple esterifications because (as you saw) it's easy to get rid of.

[camptzak]

what ratio of acetic acid to isopentyl alcohol to sulfuric acid did you use? what solvent did you use? tell me about your methods as well and maybe we could figure something reasonable out.

[Archer]

Your product, isopentyl acetate, is (very slightly) enolisable thus you could, in theory, form a small quantity of isopentyl acetoacetate by reaction with protonated acetic acid (-H₂O). This could then further react in a similar way to form larger moleules so you could end up with some large and complex polyketides which have much lower pointing points and high congation and, therefore, colour.

[Archer]

Your product, isopentyl acetate, is (very slightly) enolisable thus you could, in theory, form a small quantity of isopentyl acetoacetate by reaction with protonated acetic acid (-H₂O). This could then further react in a similar way to form larger moleules so you could end up with some large and complex polyketides which have much lower pointing points and high congation and, therefore, colour.

*boiling points, not pointing :/