Ok. There was a question on our test today, but it was bonus. It has been bothering me ALL DAY. No one else in the class I’ve talked to thinks they got it right, and no one recognized it from the notes or chapter homework.
Reaction: E-2-bromo-2-methyl-4-hexen-6-ol is diluted in an unreactive solvent. What is the product?
Ok, I know that haloalcohols like this one will bend back on each other to form cyclic ethers. So in this case, I had the O of the OH group attach to where the Br was to get a 5-membered ring, with O where one of the carbons would be, and two methyl groups coming from the O (I left the double bond where it was.)
I looked at my answer and saw that I had 4 things attached to my O, so I took the methyl groups off, and wondered where they went. Dr. Campbell only asked for the major product, so I ignored that, and though maybe they just broke off or something. Yeah, that’s it. Broke off.
I spent 10 damn minutes staring at that ring. And nothing.
Organic chemists! Help! (if my notation makes it impossible to work, LMK… I was trying to draw it in ChemDraw, but realized that it’s… complicated to use.)
Um, you didn’t replace the 2-carbon with the oxygen, did you? Didn’t you mean to make it link to the 2-carbon in place of the Br, making a six-membered ring?
The methyl groups in your product should not be on the oxygen atom, and they don’t fall off. Another aspect of the starting material you should note is that it is an allylic alcohol. I’ll look into it a bit later and post again if it hasn’t already been solved.
I don’t think anything unusual is happening. The hydroxyl group acts as a nucleophile; the bromine atom is the leaving group. Probably the reaction is Sn[sub]1[/sub] with bromine leaving to form a carbocation (which would be stabilized by the methyl groups on the tertiary 2-carbon). The hydroxyl group (the nucleophile) then attacks the carbocation to give an oxonium ion intermediate. The intermediate is deprotonated by bromide ion to give HBr and the cyclic ether, 2,2-dimethyl-3,6-dihydro-2H-pyran (a six-membered ring with oxygen in the 1-position, two methyl groups at the 2-position and a double bond at the 4-position). Allylic stabilization of the ether should still occur and the double bond will remain in the same place (an advantage of working with allylic alcohols, actually). Rearrangements do not seem likely because the carbocation intermediate is the most stable possible carbocation.
It seems simple enough, but I’m still waiting for someone to come along and write a big red NO!!! on my post. =)
That’s what I’m thinking as well. If the OP made a mistake on the test (and not just in how he worded his answer OP), it was making a five-membered ring instead of the six-membered ring. It looks simple enough. Is this an introductory organic class?
Also, I agree that ChemDraw sucks, but the other option (which I can’t remember off the top of my head) isn’t really any better. Nothing really to do but to learn to use it if you are a chem major. Somehow I’ve found the drawing tools for the structure search in SciFinder Scholar to be the easiest to use.
Well, I made a 5 membered ring, and the methyl groups… went away.
If by “introductory organic chem” you mean “organic chem I”, then yes, I’m in it. But it’s still enough to make me go cry after each class when he presents us with new stuff.
But (I don’t know how to name cyclic ethers) it looks like the two methyl groups ended up…
AAAH! I realized what I did wrong! I was looking at the carbon that the Br was attached to and for some reason put the O in its place… that’s why I was ending up with the 5-membered ring with no methyls… I had an O instead of the other carbon! :smack: God, if I had stared at that long enough, I would have gotten an extra 5% on that test. Garrr!
Yep. Rule number one in writing reactions that create a structure fundamentally different from what you started from is to always count your carbons. This sort of thing only gets worse the more advanced you get in organic chemistry. Check my sig for an example of how bad it gets.
asterion: ChemDraw could be much easier to use than it is (especially for drawing mechanisms); I actually think I liked ACD ChemSketch better, but ChemDraw is a bit more powerful. I liked the SciFinder drawing tools for doing searches, but IIRC it doesn’t do mechanisms. All of these tools are wonderful compared to HyperChem, where you basically have no choice but to roughly sketch out a molecule, hope it gets all the connections right, desperately hope you haven’t forgotten to indicate which rings are aromatic, then clean it up by doing molecular mechanics calculations on it. It’s also practically impossible to indicate stereochemistry without it messing something up.
chaoticdonkey: Naming cyclic ethers is a bit difficult because you base the names on other compounds that resemble the ring. IUPAC actually prefers another way, but I’ve never seen it. A five-membered ring with one oxygen atom is ‘tetrahydrofuran’; a six-membered ring with one oxygen is ‘tetrahydropyran’. (Furan and pyran have double bonds compounds, so you indicate the number of hydrogens needed to saturate them.) If there’s still a double bond in the cyclic ether, then you have to indicate which hydrogens have been added, as I did in the last post. 3,6-dihydro-2H-pyran means two hydrogens have been added at the 3- and 6-positions and implies that a double bond remains between the 4- and 5-carbons.
Depending on how you look at it, any future o-chem courses you take may be easier rather than harder, though it might depend on who is teaching them. The old way of teaching o-chem was to base everything on functional groups and name reactions, so it ends up requiring a huge amount of memorization. The newer way involves teaching mechanisms, though name reactions are usually still taught. I didn’t like my introductory courses since they were all about recognizing name reactions and writing out the appropriate products. I found courses based on understanding mechanisms and reactivity much easier since they didn’t depend so much on whether I had adequately memorized a particular name reaction. BTW, very few of the name reactions you will encounter early on have much practical value in modern chemistry. Real synthesis is done partly with a subset of the name reactions, but much of it is done with newer reactions that are much more versatile. Many of these reactions are named after someone, but you don’t learn them the same way.
I agree. Actually, the other package I was thinking of was Isis. HyperChem isn’t really a drawing program (though it’s great for easy and pretty 3d pictures within its capabilities) and it isn’t really a great molecular modeling program either. I know that the people here at school that are doing the hard-core theoretical chemistry aren’t using HyperChem but the beowulf cluster running Gaussian. We used HyperChem to do some really basic stuff in P Chem and I used to get the 3d form (roughly) of a molecule I came up with and would love to try and make if I ever have the time (but I think I’d basically need four or five years of grad school to make it a reality, if it’s at all possible. That’s still better than some of the professors in the department, though.)
What I would love would be the ease of use of the SciFinder drawing tools with the power of ChemDraw.
Yes… I do like the mechanism way. There are only a few reactions that we’ve just had to memorize, but I’ve found that I can recall what happens much faster if I know the mechanism for it.
asterion: Have you tried ACDLabs ChemSketch? In some ways it’s easier to use than ChemDraw. It has a periodic table tool for selecting elements, and frequently-used elements (and radicals) appear in a toolbar on the right of the screen. It’s also powerful, with various plugins available including ACS and IUPAC naming tools and NMR prediction. The basic version is freeware.
