The Periodic Table - 2018-01-21

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12:03 AM

3

Q: Clean ozone gas from NO2 and NO

I do produce ozone gas with an ozone generator based on corona discharge principle. These kind of ozone generators have the disadvantage that if N2 is present, a bit of NO and NO2 will be produced too. To produce oxygen I use a oxygen concentrator, with an output of about 90% O2 and about 10% N...

2 hours later…

2:06 AM

@TanYongBoon honestly the part about Clar's rule is pretty much spot on (the wording is a little awkward but it's very hard to describe in words, so I added a picture for you). I just really don't agree with the number of electrons bit and if you removed the second and third paragraphs of your answer (and the fourth as well since it's just a conclusion which you restated at the end) then I would change my -1 to a +1.

As I said in the comments I don't think there is any such concept of non-integral electrons in Huckel theory, and even if there is (or even if you try to use other methods to calculate this number), the link to the degree of aromaticity is... tenuous.

7 hours later…

9:25 AM

To journal article searches: when I am looking for chemical biology stuff to read, I want structural chemical biology NOT drug chemical biology

There are enough focus on drug researches already, I want something that is NOT HUMAN BODY RELATED!

2 hours later…

11:48 AM

@orthocresol I'll upvote your second to last answer if you remove the first, second and third paragraphs. Get rid of the fourth too just in case.

Hullo @lcnittl! Welcome to our chat!

12:03 PM

@M.A.R. I'll first downvote your second paragraph if you remove @orthocresol's third answer.

:O

@Loong You wanna dance, big Loong?

(ᓄಠ_ಠ)ᓄ

::fight, fight::

-_-

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can anyone give us a hand here?

I can see that the alkyl chains have to shift. But can't really see exactly what goes on

12:27 PM

@obackhouse tough question. I looked it up. A hint for you, the reagent used was formic acid.

Thanks - I guessed so based on the formylation of the alcohol there

nothing else?

If you protonate the hydroxyl group and lose water to form an allylic cation, then the alkene can attack it. (Regioselectivity is a really difficult question here. You need to try to always form intermediate tertiary carbocations, but that doesn't always help.)

it's just the weird shift that I can't wrap my head around; I've tried drawing some synthons but end up getting some strange carbanions

Ok, I'll try that, thanks. I have to rationalise regiochemistry as well here

Yup, I tried going down that route as well. It was a dead end, wasn't it... haha

12:31 PM

it's a question on a 4th year Total Synthesis paper :)

@DSVA, I'm not sure, but I'll just screenshot from Synlett 1997 (1) 121-122:

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ref 10 is Eur Patent 315895, didn't feel like looking it up, but mechanistically just formic acid gets us to the correct product, I think.

ahh I think I may see it

if you lose the water like you said, then do a Wagner-Meerwein shift to rearrange the alkyl chain?

Hmm, the mechanism suggested in the paper has lose water -> alkene attack carbocation -> alkene attack carbocation -> alkyl shift -> carbocation intercepted by formate. That makes sense to me, but I cheated by looking it up. I'm not sure if there's an alternative pathway, it might be possible.

oh

hm I guess I'm to tired but that bridging methylene group, where was it in the starting material?

I believe that it's the alkene carbon closest to the alcohol

12:41 PM

user image

I think.

ah yeah I see

bugs me that 1-2 isn't an alkene. Would make a nice electrocyclic

well you can make electrocyclic reactions with an allylic kation. but whatever I do I end up with two five memebered rings and I don't know how to shift a methyl group during teh sequences I'm drawing.

mech- i.stack.imgur.com/5tnhx.png

oh

yep

hmm, I think I've seen electrocyclisations in biochem before. They're cool.

12:54 PM

looks good. only reason I'm suspicious is because there's not really anything in that mechanism that has the content of the course in mind

so maybe my professor had something else in mind - but any viable answer is correct in the exam, so looks good

thanks for the help!

no prob, was an interesting question!

Oh, I found the electrocyclisation: vitamin D biosynthesis

:-D ok I got my mistakes. I automatically assumed the most stable cations which doesn't work out in this case:-D

Yeah I think the theme which the exam question was trying to poke at was Wagner-Meerwein shifts

Wagner–Meerwein rearrangement

A Wagner–Meerwein rearrangement is a class of carbocation 1,2-rearrangement reactions in which a hydrogen, alkyl or aryl group migrates from one carbon to a neighboring carbon. Several reviews have been published. The rearrangement was first discovered in bicyclic terpenes for example the conversion of isoborneol to camphene: The story of the rearrangement reveals that many scientists were puzzled with this and related reactions and its close relationship to the discovery of carbocations as intermediates. In a simple demonstration reaction of 1,4-dimethoxybenzene with either 2-methyl-2-butanol...

I'm so dreadful at spotting shifts that aren't functionalised or have no heteroatoms. All the bloody carbons look the exact same

I guess the driving force in this case is probably ring strain.

Hmm, I wonder if the final carbocation (before interception) is non-classical

are those bicyclo[2.1.1]hexanes particularly strained?

they look as though they would be

1:04 PM

If you take pinene and treat it with some kind of acid it apparently gives lots of rearrangement/isomerisation products. Based on that yeah I'd guess it's not particularly stable.

that bicyclo name is wrong but not even gonna try, you know what I mean haha

I suspect the problem is the four-membered ring. Hmm, it's a bicyclo[3.1.1]heptane, I think.

ah yeah, the pinene structure

yep it is - I don't know why I even tried bicyclic nomenclature

oh, there are people in my group working on bicyclo compounds so I have some basic understanding of how it works haha

I'm a theoretical chemist - out of my depth here

1:07 PM

And you're doing this sort of organic problems?

well I enjoy organic mechanisms because they make sense to me, and it's just a sort of puzzle

but I can't imagine anything worse than being in the lab

Fair enough. This one was pretty darn tough

yeah - this was the only one on the paper I couldn't even tackle using my notes

Shh, lab work isn't that bad... silently crying

hahaha

I'm too clumsy and nonchalant to be in a lab

so I stick to a computer instead. We have a backspace, whilst you guys have the emergency room..

but yeah, I take the advanced organic modules, and my academic supervisor is a synthetic chemist - I think he half expected me to go on to synthesis, and was probably a bit disappointed when I started applying for theoretical PhD places

1:14 PM

I just need to make 7 substances. 6 are not a problem, those will work out but number 7....fuck that. I'm at attempt number 15 right now. It's one of those "i actually don't want to exist" type of compounds. I can do the synthesis with about 0.2% yield but right now I cannot purify it.-.-

I will party if I finish those because then it's theoretical all the way:-D

@DSVA It's omelette, isn't it?

Those never turn out right

@M.A.R. I guess so:-D

@obackhouse Yeah. I'm hoping to move into slightly more theoretical stuff for my phd. But better not count the chickens before they hatch, I still need to get through the interviews.

it worked out on the first try, I guess pure luck. But it degraded in the freezer so I need another patch to do some measurements in different solvents.

@orthocresol good luck. I managed to secure one just this last week, looking forward to it

1:17 PM

US?

UK

oooh okay. :D

All the best!

thanks! luckily I got the place I wanted before all the other interviews so didn't have to treck around the country

1 hour later…

2:29 PM

Hell it's me again

*hello

2

Q: Why is the rate of reaction of polar EAS favored by inductive effects and the rate of less polar EAS reactions through hyperconjugation?

Reactions involving strong electrophiles (nitration) are said to be governed by Inductive effects thus the order of nitration is D>C>B>A 2.Chlorination is not a strong electrophile so it is said the be governed by hyper conjugation. ...

organic-chemistry reaction-mechanism

Regarding that question

Is it still unclear

Or is no one interested in answering it

I am more than willing to further edit the question.

Any suggestions are welcome

3 hours later…

5:29 PM

@orthocresol just out of interest, what did you use to find that reference before? Can you search for papers by molecular structure anywhere?

I did my entire degree using google scholar instead of those fancy academic search engines lel

@obackhouse wait what? No Scifinder or Reaxys (that's also the asnwer to that question ;-) )

Sweet. Is it even retrofitted for old papers?

but yeah haha I only started using other stuff when I started researching.. and never really had to do much reading of synthetic literature

if my sources are correct, scifinder has literature starting 1907, substances beginnig 1957 (and indexing of reaction some time later, always search for substances if you don't find the things you are looking for doing a reaction search)

5:56 PM

Hey everyone I'd be glad if someone can answer the question that I asked...

Jesus christ @DSVA how did I do my degree without Reaxys

I amaze myself sometimes

This far in my education with 0% common sense

6:38 PM

@obackhouse I used Reaxys. Yeah, there are lots of old papers in there.

My backup plan in life is to work as one of those people who enter data into Reaxys/SciFinder.

I imagine a machine learning script to identify old chemical graphs could work wonders

I hope they don't figure that out in my lifetime.

I mean it's probably possible now haha

Probably.

2 hours later…

8:13 PM

@AvatarShiny If you have real research problem then make it as real stuff not another homework

2 hours later…

9:58 PM

@andselisk

o/

10:38 PM

@Mithoron \o :)

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Jan '1821

The Periodic Table

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