Re: Why is ethanol the only relatively safe alcohol for human consumption?

Relatively safe? Well, the emphasis would be on relatively!

But there is an implicit assumption in your question - that ethanol is the only alcohol that is safe to consume. This is not the case at all. Chemists (and by default biochemists) classify alcohols as any organic compound containing the "-OH" functional group. That is, CH3CH2OH is ethanol but any compound R-OH is an alcohol. The number of compounds that belong to this class is really quite large - everything from food colouring to margarines contain hydroxy groups and are "alcohols". Some are perceptually harmful. Take cholesterol, for example. Eating cholesterol is perceived in our modern society as a "bad thing" but if you don't eat it, well, your liver will make it for you! Every cell in your body has cholesterol as part of its cell membrane. It provides the fluidity. Without it, you would be as flexible as, say, broccoli.

However, I gather that your question really is asking why we can drink ethanol - whereas both methanol and proponal are relatively toxic. Indeed, methanol is extremely toxic, requiring as little as 10 mL to kill an adult.

Alcohols - small molecule alcohols - are dealt with in the liver by an enzyme called "alcohol dehydrogenase". This is an enzyme that is capable of converting alcohol into "acetaldehyde" - a two carbon aldehyde molecule - which can be used as a source of energy and in a number of other reaction pathways. Acetaldehyde can yield acetylated species which are useful as both building blocks and energy sources. (As a side note, we are an "even numbered" species - two carbon units are the preferred form for addition or removal of carbons.)

Methanol and alcohol dehydrogenase result in the formation of formaldehyde which, needless to say, is toxic. It is very reactive and capable of interacting with a host of molecules in the body, shutting down a variety of enzymatic pathways. Formaldehyde reacts, for example, with the light receptors in the eye, rendering a person blind at about half the dose that would be required to kill.

Propanol and higher alcohols can't fit very well into the pocket or active site on alcohol dehydrogenase. The body doesn't have a good route for quick disposal of these compounds. The consequence is that they hang around until they can react with other molecules causing damage along the way. Your body has to get rid of them somehow and the normal excretory path for short chain alcohols is not available. Consequently, they are toxic but less so than methanol.

As a final note, you always have a background level of ethanol in your blood as a result of biochemical reactions. It is not very much and well below legal "tolerance" levels. However, it is detectable. So, what does a zero tolerance policy on drunk driving mean since we can never have zero? Just a thought.

Hope this helps.