View Full Version : Hydrocarbon nomenclature question.

02-18-2004, 11:53 PM
Chemistry class has been hard lately. We've been doing a bit of organic chemistry. Carbon's pretty important, I guess.

We've covered the many, many variations of CH there are. Methane through decane is just the foundation; we can do cyclopentane and 3-methyl-5-ethyl-nonane. I'm surprised to find myself interested in it.

Could two branches (does one call them 'branches?') of carbon come off the same side of the chain (even though it doesn't usually like that) and bond to each other? Say, something like

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(the hydrogens are implied.)

My teacher (a knowledgable guy) didn't know the answer.

02-18-2004, 11:56 PM
Yes, in fact this is the basis for the formation carbon nanotubules (http://www.foresight.org/Conferences/MNT05/Papers/Sinnott/), and other nanotechnology structures.

02-19-2004, 12:13 AM
Yes, they can and do, to the best of my knowledge. Naming them is where it gets tricky. I would call what you've got 1,2-dimethylpentane. It may be a heptane, IIRC. Any naming experts?

One of the fun things about organic chemistry is that if you can think of a crazy way of putting things together, it can usually be done. ;-)

02-19-2004, 12:13 AM
What you have there is a four carbon ring - cyclobutane - with three more carbons coming off of it.

02-19-2004, 12:48 AM

Wow, already three answers! Thanks. I guess there are at least two ways of looking at it, say smeghead and mercury. My real question
Real Question
is: what would be the rules of naming these kinds of hydrocarbons?

02-19-2004, 01:15 AM
OK, for the short answer: Take the largest straight chain structure (propane) and add prefixes naming and locating the attachments (1-cyclobutylpropane). One might also call it n-propyl cyclobutane -- I'm not sure which would be preferred. It's NOT dimethylpentane.

Now, the rebuttal. What you've got there is an unlikely molecule, but it exists. It's unlikely because carbon doesn't like those acute bond angles. What you're more likely to see, as far as networks go, is carbon atoms arranged in tetrahedra (diamonds), in sheets of honeycomb-shaped cells (graphite/fullerenes), and just higgledy-piggledy (coal).

In general, there are many common cyclical structures, which your teacher should have known (but he was probably confused by the way you phrased the question). Naming conventions are established by IUPAC (International Union if Pure and Applied Chemists), and by agreement within various national chemistry groups (most chemists do not rely on IUPAC names, but on various traditional names and trade names, including those established by the drug industry.

02-21-2004, 08:33 PM
Don't cyclical structures receive higher priority? It's propylcyclobutane, no number needed.

If I remember right, though, cyclobutane is fairly unstable (though not too badly so because it bends into a vaguely tetrahedral shape.)

02-21-2004, 09:36 PM
JFTR, the two carbons at the bottom of the 2-D drawing in the OP are, apparently, joined, which would make it propyl-cyclobutane, if I'm correct.

If it was not intended to join them, then it would be 3-methylhexane (one of the heptanes, in reality), since diagrams are intended to represent the 3-D snaky sequence of a hydrocarbon chain, and the fact that the diagram "turns" at the left is immaterial to the actual shape of the molecule, in which all four bonds are effectively equidistant. Moving everything to the right one notch, and bringing that lower left carbon up to line up with the five-carbon row, would be identical in actual 3-D reality.

The rules from IUPAC are, in my own paraphrase:
1. Identify and count the C atoms in the longest carbon chain; specify the appropriate name. (Propane, butane, pentane, hexane, etc.)
2. Identify any non carbon, non-hydrogen atoms or radicals attached to that chain, and specify atom of the main chain they're attached to. Prefix this designation to the name identified in #1, with the location on the main chain prefixed and set off with a hyphen.
3. Identify any side chains, which will not be located on the first or last atoms of the main chain (or they'd actually extend it), and designate them by the appropriate -yl ending: C- becomes methyl, C-C- becomes ethyl, etc.
4. Prefix the side chains to the base name identified in #1, with a number prefixed and set off with a hyphen according to which atom of the main chain it is attached to.
5. Identify any non-carbon, non-hydrogen atoms or radicals attached to the side chains, and prefix them as in #2 to the name designated in #3.
6. In selecting the number to identify what atom of the main or side chain something is attached to, always use the lowest possible number. I.e., if your drawing has a chlorine attached at the next-to-rightmost atom of a hexane chain, that's not 5-chlorohexane, but 2-chlorohexane, counting from the right end rather than the left.

02-23-2004, 04:57 PM
Polycarp gave a pretty clear explanation of the rules. Your molecule is propylcyclobutane, and the name "1,2-dimethylpentane" makes no sense. Cyclobutane is a stable molecule but it has a lot of strain energy. Stranger things, like cubane, are possible. Link: http://www.chem.ox.ac.uk/dhtml/default.html

Cubane has a lot of energy locked up in those horribly deformed bonds but the activation barrier to breaking the first bond in the decomposition reaction is very high. Cubane itself is not explosive, for this very reason. It's a kind of waxy solid and has a really high vapour pressure.

Ultimately, nomenclature, while useful, is not what organic chemistry is about. Computers can name molecules, but manipulating them is where the action is :)