[rs20]

Hi folks,

I hope this is a fun question for you.

While idly browsing wikipedia, I came across ethyl butyrate. I was struck by the fact that this apparently pleasant-smelling (fruity, pineapple-like) compound can be produced by reacting two unpleasant/neutral-smelling compounds: butyric acid (smell of vomit) and ethanol, and thought that this could be core of an intriguing demonstration, perhaps for kids*. However, as many of you will be aware, it's not a simple as mixing these two compounds**; the water produced by esterification must be removed, typically using concentrated H₂SO₄/silica gel, and even then heating, agitation and reflux seems to be often be called for. This all greatly impacts the practicality and safety of this experiment outside the lab, especially the concentrated acid.

So, I was wondering if anyone could kindly suggest some simpler odour-based reactions that are safe and easy to perform outside the lab? I suspect thinking outside the box of esterification might be required, although of course esters are unfortunately a goldmine of pleasant smells. Basically, the pattern I'm looking for is:

(unpleasant/neutral aroma compound) + (unpleasant/neutral aroma compound) (pleasant aroma compound)

One thought I had was that if I start with a pre-prepared ester that has an unpleasant smell, and then slightly modify it to make it pleasant. That way the two reactants can be anhydrous or non-acidic, and hydrolisation of the esters can be avoided. But really I'm open to any and all suggestions!

* You know, the ones who are unimpressed by mixing two clear liquids to yield a violet liquid.
** Indeed, this is obvious from everyday experience -- for ethanol is frequently mixed with butyric acid in an alcoholic's stomach, yet no pineapple odour is evident...

[phth]

isopentylacetate smells strongly like bananas, and it is bee pheromone...I would not want to take responsibility for students smelling acyl chlroides or acetic anhydride. Isoamylacetate smells extremely strong, so you could possible do it without letting them smell the starting materials...

[rs20]

Thanks for your reply phth! I hadn't considered acetyl chloride -- while obviously quite a nasty compound and will require considerable attention to safety (seems to combine the aggressive affinity for water that H₂SO₄ has with some bonus flammability), I do like the idea that using the acyl chloride will simplify and speed up the reaction considerably. Especially if, as suggested, the reactants are kept out of reach of students. Do you think the reaction would progress sufficiently* by simply mixing the alcohol with the acyl chloride at room temperature, or would the addition of a base be required to produce sufficient* isoamylacetate?

* Where I define sufficient in this case to mean "sufficient for the banana odour to fill the room or become the main odour of the mixture"

EDIT: Actually, maybe scratch that. Unless I can be convinced otherwise, it seems that acetyl chloride requires a fume hood to be safely handled. Still open to other ideas though!

[wildfyr]

An acid chloride will react quickly with an alcohol to high yield yield and give off HCl. To prevent evolution of HCl and make the reaction quantitative, you can use 1 equivalent of triethylamine (safer than pyridine, its used sometimes as fruit fly anesthetic). This would be good because otherwise your final reaction mixture will be saturated with HCl. You can do it neat or in something like ether that can be gently evaporated on a hot plate, and be left with an oily mix of ester and triethylammonium chloride (no smell).

I wouldn't worry too much about using acid chlorides in a school setting. The "Nylon Rope" trick is a common (and awesome) school demonstration and it uses dihexanoyl chloride. The HCl smell is obvious, and the carboxylic acid is stinky too, but in small amounts its just that: stinky. Acetyl chloride isn't the best choice because its so low molecular weight that it is truly volatile, but butyric and propionic acid/chloride are in the sweet spot of smelling awful but not being volatile enough to cause concern. Just don't let anyone jam their nose in it, a solid waft will be more than enough. I know bromopropionic acid (and its acid chloride, because when the acid halide hits your nose it becomes carboxylic acid) smells like very very fresh dog turd.

[rs20]

The wikipedia page on butyryl chloride seems to be doing its best to dissuade the idea of using it neat*, but yes doing the following reaction in ether looks like a brilliant reaction you've arrived at, wildfyr, mixing three unpleasant things to make something ostensibly nice:

Butyryl chloride (l) (in ether) + ethanol (l) + triethylamine (l) ---> Ethyl butyrate (l) + Triethylammonium chloride (s)

Or, in terms of the odours

Burned butter/vomit + ethanol + fishy ammonia --->  Fruity orange + odourless

I think that's a pretty excellent solution, but I'd be curious to hear any other ideas? Here are some incomplete thoughts I had:
-- Synthesis of vanillin (I mean, not all the way from lignin/guaicol, because that's be too complicated. But I failed to identify any simple reactions using easily obtainable reactants).
-- Oxidation of geraniol (rose oil, essentially) to citral (corresponding aldehyde, citrus scent) or vice versa. Taking the oxidation only as far as aldehyde seems tricky.)

* "...It also reacts violently when mixed with ... alcohols. These reactions tend to result in explosions and fire..."