[mightybob]

Hello dear forum members, i have a question that i've asked a lot of chemistry professors, and it still remains unanswered.

The question is, when naming organic compounds, say, hydrocarbons, why is it that double bonds are priored over triple bonds?

When there is a choice in numbering, the double bonds are given the lowest numbers, why is this?

It would be greatly appreciated if somebody here knew the answer.

[Arctic-Nation]

Convention. At some time a bunch of chemists sat together, discussed, wined and dined, and in the end said 'screw it, we'll fix the rules this way and whoever disagrees is politely asked to remove himself from the room and never disgrace us with his presence ever again.' This, of course, was during an age when there were no female chemists yet.

Seriously, there's probably no other reason than convention. You can discuss all you want about how a triple bond can be counted as a double double bond and thus should have higher priority, but at some time a IUPAC conference (most probably) just drew up some rules based on earlier practice or tradition, streamlined them and unleashed them upon an unsuspecting world.

OK as you can see I find it hard to keep a straight face on this. Sorry 'bout that.

[mightybob]

Hehe, ok thanks a lot for your answer, it is a little better then one i got from a professors once, hehe, he literally said ``ask our lord``. The only thing i was curious about was if there was any logical explanation involved, but obviously there isn't. Again, thanks for replying.

[Schrödinger]

I beg to differ from Arctic-Nation. Double bonds are more reactive than triple bonds and so they act as the functional groups, not the triple bonds (I'm not wrong, am I?).

Hence, it is only logical that they be given more priority

[orgopete]

I agree with Arctic-Nation that it was convention. However, I don't think he expressed the rational for the nomenclature particularly well. I believe the motivation of the participants of the nomenclature conventions was to create a system of nomenclature that would result in a single name for any given compound. Prior to the use of computers, compound searching was done on the basis of molecular formula or name. To enable efficiency in searching, single consistent names would be advantageous. In practice, I encountered Chemical Abstracts naming more frequently that IUPAC.

Once a committee determined to create a set of rules, it is not difficult to rationalize the priority of groups by degrees of unsaturation or oxidation states. Doing so will integrate well with common names and relative numbers or importance of compounds. For example, the oxygen of ethanol can participate in many more reactions than the groups attached to ethane. That chemistry is recognized by making it a higher priority group. However, as Arctic-Nation said, this is just convention. The committee could have assigned the priorities in the opposite manner.

[Schrödinger]

Please forgive me for my ignorance. I always reason why a particular thing is so, sometimes without complete knowledge of the facts, and I ended up doing the same here too. 

[orgopete]

Please forgive me for my ignorance. I always reason why a particular thing is so, sometimes without complete knowledge of the facts, …

Me too!