Hey I got a quick question for problem 2.34 it says draw structures and names for all of the cycloalkanese with formula C6H12

Answer 1

Approaching this problem systematically can help you a great deal. I will give guidance as to how to answer the question, but not provide a full answer. There are several reasons to do this, not least being I can't draw structures in this text box.

Since the question asks for "cycloalkanes", you know that there is at least one ring. From the suffix you know there are no double or triple bonds, since they would have a different suffix ("-ene" and "-yne" respectively), so specifying "-ane" specifies their absence.

With one ring in 6 carbons, there are only 4 possible ring sizes. The ring could include all 6 carbons in the cycle, which would be drawn like a hexagon, and would be named "cyclohexane". "Cyclo-" because it is cyclic, "-hex-" because the ring includes 6 carbons, and "-ane" meaning alkane, or rather, no double or triple bonds. There is only one "way" to make a 6-carbon cycle, so there's only one isomer like this.

But the ring could include only 5 of the 6 carbons, with the 6th carbon a substituent on the outside of the ring. This would look like a pentagon with a one carbon substituent attached to any of the 5 carbons in the cycle (it does not matter which one. Why?). This substituent would be a "methyl-" substituent, which would be appended as a prefix to the parent name of the 5-carbon ring. I will leave you to construct the parent name (following the pattern above), but the substituent name "methyl-" specifies one carbon ("meth" -- no, it's not a drug, that's short for "methamphetamine") and "-yl" means it's attached as a substituent onto the parent molecule. As is the case for the 6-membered ring, there's only one "way" to do this, so there's only one C6H12 isomer with a 5-membered ring.

Making the cycle even smaller, the situation gets a little more complex. With a 4-carbon ring, there are now two carbon atoms left over, and not part of the ring, so there are several ways to attach them. Construction of the name of the parent molecule again follows the same pattern as for the 6- and 5-membered rings above -- but where 6- and 5-atom chains have sensible "roots" (hex- and pent- respectively), a 4-atom chain or ring is specified in organic nomenclature with the root "but-" where "but-" means 4 atoms. If you're a smoker, your "butane" lighter contains, as its fuel, a 4-carbon alkane.

But with the two non-ring carbon atoms, they could be attached together as a single 2-carbon substituent attached to the 4-membered ring, or they could be attached separately, as two separate 1-carbon substituents. We already said a 1-carbon substituent is a "methyl-", and in organic nomenclature, a 2-carbon group is specified by the root "eth-". (Ethanol, every college student's favorite beverage, is a 2-carbon alcohol.) When attaching a single 2-carbon ("ethyl-") substituent to the ring, it doesn't matter "where" it is attached, as in the absence of any other substituents, the 4 atoms in the ring are indistinguishable. And since there is only one "way" to do this, there is only one isomer which contains a 4-membered ring with a single 2-carbon chain attached.

But when two separate, individual 1-carbon methyl- substituents are attached, it DOES matter "where" they are attached. They could be attached to the same carbon in the ring, or they could be attached to adjacent carbons, or they could be attached diagonally with respect to each other (my mother would have said "kitty corner"). In addition, it not only matters which carbons the two methyl- substituents are attached to, it also matters HOW they are attached. And here, we get into some complexity which the author of your question (sounds like a textbook question) may or may not want to delve at this time.... Stereochemistry (isomers which have the same connectivity and differ only in arrangement of groups of atoms in space) is a topic which is often saved for later chapters. At maximum, depending on how many kinds of stereoisomers you are expected to distinguish, there are SIX different ways to attach two methyl- substituents to a 4-membered ring! Each of these would be a "dimethyl-" isomer, and each methyl group would need its own "locant" which is a number which specifies the relative position of each methyl on the atoms of the ring. In addition, again depending on how much you are expected to know about stereoisomers, there would need to be other labels to specify the relative arrangement of the methyl groups in space. These labels might include "cis-" or "trans-" -- in current society, these prefixes are used in the context of gender, but in the context of organic nomenclature, they have other specific, structural meanings. If you are expected to distinguish enantiomers (almost certainly not, but not impossible), you may need to use (R) and (S) to specify configuration at any chirality centers present. If you are not expected to distinguish enantiomers, the number of dimethylcyclobutane isomers drops to FIVE (I suspect you are not expected to distinguish the pair of enantiomers). But in that case, you WOULD be expected to label two different pairs of stereoisomers as either cis- or trans-. If, by some chance, you were not expected to distinguish cis from trans, then there would be only THREE isomers of dimethylcyclobutane.

Finally, the smallest ring size possible is 3-carbons (there is no such thing as a 2-carbon ring -- that's a double bond). With 3 carbons in the ring, the parent name would again be constructed as above, using "prop-" as the root to specify three carbons. (The "propane" tank you use when grilling hamburgers contains the 3-carbon alkane of that name as fuel.) Now, with three carbons left over (not used in the ring), there are MANY ways to attach them to the ring. As before, they could be attached as a single 3-carbon substituent (a "propyl-" substituent), or as one 2-carbon ethyl- and one 1-carbon methyl- substituent, or as three separate 1-carbon methyl substituents.

Taking the last case first (three separate methyl- substituents), there are two different "connectivites" (or "constitutional isomers" -- isomers which differ in how atoms are actually connected, rather than simply how groups are arranged in space). You could have two methyls on the same carbon and one on an adjacent carbon, or one methyl on each of the three ring carbons. There is only one "way" to do the first, but there are TWO different "ways" (these are stereoisomers) to attach one methyl group to each carbon in the 3-membered ring. So there are THREE possible "trimethylcyclopropane" isomers (they will each need THREE locant numbers, since there are three separate methyl substituents, and each one needs a number to specify its relative position). The "tri-" indicates there are three different methyl substituents. (The stereochemistry of the two stereoisomers is actually a complicated situation, and it is difficult to specify the relative orientation -- "cis-" and "trans-" are not sufficient, and even "R" and "S" fail to fully specify the relative positioning. That said, you can draw the two stereoisomers and visibly see that they are different in their orientation of the methyl groups.)

Taking the first possibility (a single 3-carbon, "propyl" substituent), there are two ways to attach it. These do not differ by which carbon of the ring to which the substituent is attached -- just like the mono-substituted cyclopentane and cyclobutane, in the absence of any other substituents, it does not matter which carbon the substituent is attached to, and no "locant" number is necessary. Instead, they differ by which carbon on the SUBSTITUENT the attachment occurs. If the propyl- group is attached by one end or the other (again, it does not matter which end), it is just a "propyl-" substituent. But the 3-carbon substituent could be attached to the ring by the MIDDLE carbon of the 3-carbon substituent! This is a "branched" substituent, and is specified as an "isopropyl" substituent. ("iso" is a prefix which generally means "branched" though its usage is less clearly applied for larger groups.) In any case, there are two "ways" to attach a single 3-carbon substituent to a 3-carbon cyclic molecule, so TWO isomers.

Lastly, the situation with one 2-carbon ethyl group and one 1-carbon methyl group has FIVE ways to attach the two substituents. There is only one "way" to attach both substituents to the same carbon, but there are actually four ways to attach them to different carbons! They could be "cis-" or "trans-" (look them up for examples of what they mean in this context), and each of those possibilities can be one of two different enantiomers (which could only be specified using the R and S labels). Again, while there are actually five isomers of ethylmethylcyclopropane, I suspect you would not be expected to distinguish the two pairs of enantiomers at this point, and so the likeliest number of isomers you would be expected to distinguish would be three.

So! Obviously, this was not necessarily a simple answer. Assuming you are NOT expected to distinguish enantiomers, but ARE expected to distinguish cis- or trans- stereoisomers, then there are a total of SIXTEEN isomers. There are EIGHT molecules with a 3-carbon ring (would be TEN if you are expected to distinguish enantiomers), then SIX isomers with a 4-membered ring (would be SEVEN with enantiomers), and ONE 5-carbon ring, and ONE six-carbon ring.

I have not provided complete names for these isomers. But there are patterns you can extrapolate from to create these systematic names.

Finally, if you have any questions, or want to check the structures you came up with (or find ones you were not able to come up with), or check your names, please don't hesitate to contact me!