Tuesday, 8 January 2013

Geeky post alert

I'm allowed to be geeky every now and again.

Yesterday evening I found Garlic Snail finally (Oxychilus alliarius)

One of the ID features of this species is that it exudes a strong Garlic smell upon handling (or fingering as quoted in many places). Once Dr Boden had stopped sniggering, he found the compound responsible: n-propanethiol (below). Nothing like the compound allicin that gives garlic its smell (also below).

n-propanethiol is very volatile (it boils at about 60 degrees) so evaporates very quickly thus why the smell from the snail disappears quickly unlike garlic. Interestingly, allicin is not found in garlic as such. When you crush or cut garlic, allicin is formed enzymatically from alliin.

n-propanethiol is very similar in structure to n-butanethiol (just an extra CH2 in the chain) and n-butanethiol is one of the compounds that give Skunks their smell. Rearrange the chain of n-butanethiol slightly and you get the compound below - tertbutylthiol. This is the compound that is added to domestic gas supplies so you can smell the gas (methane is odourless).


For anyone who has had the pleasure of eating white truffles (or even truffle oil), you will know that they also smell a bit like domestic gas but this is due to another (mixture) of compounds, one of the principle compounds being 2,4-dithiapentane (below). All of these compounds are very volatile and contain sulfur (they are known as VOSCs - volatile organic sulfur compounds) and the slight differences in their structures give them different odours. In this family is also DMS, dimethylsulfide, the attractant they mix with chum to attract seabirds.

For a really good example of how little changes in chemical structure can give completely different smells, look at the two compounds below:


The top compound is eugenol, the volatile oil characteristic of cloves while the other is vanillin, the odor of vanilla. These two are very similar chemically (mix eugenol with wood pulp waste from paper making and it becomes vanillin) but have a completely different odor.

Enough geekiness - I'll try and stick to organisms for a bit.


  1. As an A-level chemist slash naturalist, I'm loving this! Revision... systematic name 3-methoxy-4-hydroxybenzaldehyde for the last one? :)


  2. Other way round - 4-hydroxy-3-methoxybenzaldehyde. Alcohols take precedence over ethers in IUPAC ordering