The old name for butanoic acid is butyric acid. Butanoic acid dissolves in water, and is weak acid, because its dissociation is only partial. It smells like vomitus or rancid butter - most of the smell of rancid butter is butanoic acid. The smell lasts long even in a ventilated area, and it attaches to clothes and stays there for hours. It's quite fun how the simple esters of it smell sweet and fruity. For example, methyl butanoate is known as the smell of pineapple.

There is an isomer to butanoic acid, which was called isobutyric acid. It has also four carbons, but its proper name is 2-methyl propanoic acid. It has the same relative molecular mass of 88.12 g mol-1 and the same smell.

I modelled the structure of the acid in the school and told about its smell. One girl said: "Do you really have to know all about how different molecules smell and look like?" She found the answer to the question by means of empirical investigation: she left a bottle of butanoic acid open in a classroom with little ventilation. Thank you. Thank you very much. All the students in the class "smelled like shit" - as one guy said - for three hours.

The structure:

butanoic acid       2-methyl propanoic acid

CH3-CH2-CH2-C=O     CH3-CH-CH3
            |   and     | 
            OH          C=O

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