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r/chemhelp•
reenigne

IUPAC naming of 2,5-Dichlorohexane. Why the "di"?

Hi,

From what I understand the IUPAC name for this (http://www.chemspider.com/Chemical-Structure.24059.html) molecule is 2,5-dichlorohexane.

Is there a reason for adding the prefix "di" in front of the chloro? I know it's there because there are 2 chlorine substitutions on the hexane backbone, but doesn't the fact that there are two numbers (2 and 5) render the "di" redundant? In other words, what information does the "di" add that the 2,5 does not?

Thanks!

Posted Nov 14, 2013, 4:16 PM UTC

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lilshu•

because 2,5-dichlorohexane and 2,5-chlorohexane are two different compounds. 2,5-chlorohexane suggests a strange five membered ring with an endocyclic chlorine.

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reenigneOP•

Can you draw what 2,5-chlorohexane would look like?

I'm drawing a 5-membered ring on marvin sketch with a chlorine as part of the ring and 1 chlorine substitution on different parts of the ring and marvin's IUPAC naming function does not give me anything approximating 2,5-chlorohexane. See here: http://imgur.com/FpvAk4m

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TonyDanzaPhD•

The compound 2,5-chlorohexane doesn't look like anything, because it isn't a proper name.

I know what you're asking, but there isn't really a good way to answer it. The name says there's one chloride substituent, but specifies that it's at two locations. I guess you might try bonding it to both C2 and C5:

http://i.imgur.com/HPLlYoL.png

But the particle would really be a relative of bicyclo[2.2.1]hexane, with the position of C7 taken by a chloronium cation. I can't figure out the real prefix for a substituted chloronium heteroatom during my lunch break (and I've tried!)...the best I can find is a neutral chlorine bound to the structure, which would be chlorio (see p. 55 here). If I HAD to use a name to convey it I would probably call it a

7-chloroniumbicyclo[2.2.1]hexane cation

Sorry, that doesn't really answer your question at all, but hopefully you find it interesting!

e: u/lilshu 's answer above is what I was trying to get at I guess, but I tried to make it a cyclohexane instead. I'll leave this here though in case anyone wants to see my thoughts.

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reenigneOP•

Thank you. Even though, as you say, there is no molecule with a name of 2,5-chlorohexane, I think everyone has clarified my misconception enough that I understand where my thinking was wrong.

The "di" prefix does actually convey meaning, whereas I originally thought all of its meaning was embedded in the "2,5" and so it seemed like it was pointless to include "di".

Obviously, it doesn't have to be that way. i.e., You can have a single substituent making multiple attachments to a backbone and the prefixes like "di", "tri", "tetra" would then provide clarity as to what's actually happening.

One of those nagging questions I wish I had asked when I was taking orgo.

By the way, you have the greatest username ever. I went to grad school w/ a chem grad student who looked like Sylvester Stallone, and your username is close enough that it brings back fond memories of talking w/ a guy who looked like Stallone and was incredibly bright.

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TonyDanzaPhD•

I guess the way I consider prefixes is that you can convey different levels of meaning by including or omitting them. For instance, something like this molecule can be named with different combinations of prefixes, like

a butane derivative
a halobutane
a chlorobutane
a 2-chlorobutane
(2R)-2-chlorobutane

If I'm talking about, say, a substitution reaction where the chloride makes the chiral center vulnerable to attack, I would want to specify the conformation because it's important. But it might not matter what halogen is there since any would pull electron density away from the carbon, so I might decide to generalize it to "a (2R)-2-halobutane".

That was my first thought when you asked. You could call your molecule a 2,5-chlorohexane, but it suggests that you don't care whether there's one shared chlorine or two separate ones. (You might even call it a 2,5-polychlorohexane to suggest that there could be multiple chlorides on each carbon.) By saying it's 2,5-dichlorohexane you're confirming that it's the one with one chloride on each of C2 and C5.

Again, I'm not saying much, but maybe that's useful to you!

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lilshu•

2,5-chlorohexane does not two chlorines.

http://i44.tinypic.com/bdqkj9.png

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TheBlondDutchGuyPhD, Organic Chemistry•

This one, 2,5 does not explicitly mean that there are two chlorine atoms, even though Cl usually has a co-ordination number of 1, but that doesn't preclude all theoretical possibilities.

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lilshu•

I guess so. I entered 1,4-oxobutane into chemdraw as a test for my thoughts and got out THF, which is what made me think this is the case.

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opsomath•

This is wrong. OP is correct, the IUPAC naming rules are redundant to emphasize the correct structure. 2,5-chlorohexane is not a thing.

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Fiasco13•

You're right that it is kind of redundant, but it's been the standard for so long that it's just the way its done. It's useful for acronyms though as we tend to drop the numbers, e.g. something like 1,2-dichloroethane - despite what the wiki page says it's universally referred to as DCE.

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day_kid_older_sisterchemical education/inorganic chemistry•

It's not redundant, really, because the numbers indicate which carbons the chlorines are on. Without this information, you would not be able to tell which isomer of dichlorohexane someone was referring to. In cases of multiple substitutions on a wide variety of atoms, you will read/hear the prefix easier than you will keep track of how many numbers are displayed.

And merely saying "2,5-chlorohexane" ignores the rules we established when we learned to name binary compounds.

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reenigneOP•

It's not redundant, really, because the numbers indicate which carbons the chlorines are on.

I wasn't saying that the 2,5 was superfluous. I was saying that the 2,5 seems sufficient and the "di" seems superfluous because all of the information that the "di" is carrying is also carried by the "2,5". The "2,5" carries additional information b/c it tells you the "address" for each chlorine attachment, but it also tells you that there are 2 chlorines.

But your last sentence makes it seem like the "di" is there to maintain consistency between other naming schemes devised for binary compounds, which I suppose makes sense.

Thanks.

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opsomath•

You are right (as I said in my reply to the guy with the incorrect top comment above) it is redundant, but done for consistency and emphasis and for historical reasons. Also, it lets you say "dichloropropanes" and mean all the different isomers.

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