What do carboxylic acids smell like?
Formic (methanoic) and acetic (ethanoic) both smell very strongly of vinegar. Butyric (butanoic) acid smells apparently of rancid butter, but I’ve always found it to smell like a mix of sweat and vinegar. Yes, you do tend to get a significant amount of acid leftover when you make an ester on a simple laboratory scale. Because of the strong smell and high volatility of the acids, they tend to mask the smell of the esters. As for safety, you should be ok as long as you waft the odour towards you and sniff, rather than sticking your nose directly above the flask. Can’t promise the smell will be very pleasant though. You could try making esters of carboxylic acids which don’t have strong smells, like benzoic or salicylic acid. Methyl salicylate and methyl benzoate should both have nice smells. You could try some acetic or pentanoic acid esters, but definitely avoid butyric acid esters if you want to be able to smell the ester.
