The Periodic Table

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Loong

5:37 AM

What is generic "animal" referring to: a lion or an octopus? — Ivan Neretin 7 mins ago

M.A.R. ಠ_ಠ

6:00 AM

@Loong King Kong. Everyone knows it's King Kong.

1 hour later…

Bounty Hunter

7:09 AM

Q: Role of pH in azo C-coupling reaction, continued

In the preexisting question asked by @DhrubaBanerjee and answered by @Klaus-DieterWarzecha, I find several ends untied. That post discusses why azo coupling of aniline is conducted in acidic medium. However I am not convinced about a few things. My query is twofold: The $\ce{-^+NH3}$ group is...

1 hour later…

pH13 - Yet another Philipp

8:18 AM

@MelanieShebel I see :)

2 hours later…

William R. Ebenezer

10:18 AM

Anyone? chemistry.stackexchange.com/questions/112468/…

Need it for an upcoming d-day exam

Additional bounties negotiable. Terms and Conditions apply.

5 hours later…

Hot nitric acid (HNQ)

3:11 PM

Q: Determine direction of mass transfer

For part a) I don't really know what to look at. Diffusion occurs from low to high concentrations so by looking at $t_1$ I would say that the direction of the mass transfer is from phase II (organic phase) to phase I (aqueous phase) since the organic phase starts at a lower concentration th...

concentration phase diffusion

5 hours later…

orthocresol

8:12 PM

Q: Could McDonalds expand into the hair industry?

A few months ago I met a person in Europe who told me about an article he had recently read that questioned whether McDonalds could go into the hair industry. As I remember it, the premise of the article was that as arguably one of the largest consumers of transglutaminase (used in making chicke...

enzymes

Now, that's a weird question.

gannex

Hi all;
Anyone got any advice on forcing a transition state search to find the SN2 TS rather than SN1? I presume that SN1 is more energetically favourable, but I figure a TS must exist for both mechanisms

I am trying to find the SN1 vs Sn2 TS for the first step of substitution of PhCHBrPH with MeCN

but the bromide always flies away and I get a TS with no involvement of the bromide (essentially TS for the second step of SN1, after dissociation of the bromide)

I've all sorts of coordinate scans, QST2, QST3, etc, etc.

Maybe a TS search with the C-Br distance constrained?

2 hours later…

orthocresol

10:00 PM

@gannex conventional wisdom says that that is definitely an SN1 process. MeCN is a terrible nucleophile, and Ph2CH+ is a highly stabilised carbocation. I don't understand why you say the SN2 TS necessarily exists, although to be fair, I have rather little experience with comp chem

gannex

@orthocresol I would think that an SN2 process would exist, but that it would just be very high in energy, although still a valid TS

I agree that the actual reaction should be SN1, but I am trying to model all possible processes.

I guess maybe it is just very difficult to get standard TS searching methods to find a very unfavourable TS

orthocresol

But if your SN2 "TS" is optimising to an SN1 TS, that means that the SN2 "TS" is a second-order saddle point, no?

You'd have one imaginary frequency along the reaction coordinate that you're looking at, but also one imaginary frequency for just the bromide dissociation.

Well, take what I say with a pinch of salt. There are far more qualified people here to talk to. :)

gannex

well, I have tried doing TS searches for the SN2 TS with noeigentest and I have got second order saddle points, but none of them have the structure of an SN2 TS.

good point though. I think I will just stop looking for an SN2 TS. It's probably not relevant anyways. I just wanted to have a somewhat objective way of pointing to the energy differences.

orthocresol

10:16 PM

maybe see if somebody else wants to say something. Or post in the optimising room. I can resurrect it if you'd like...

gannex

nah don't worry about it, I think I'll just ditch this SN2 TS. Seems like too much trouble.

I think it could maybe be found with a rigid PES scan or something, but that seems too difficult.

orthocresol

phrases like "a TS for XX mechanism could not be located" are somewhat common, at least in the organic literature where people chuck in a token DFT study just to please the reviewers...

gannex

I don't like this attitude around token DFT

we should be using this where appropriate and not just adding it in to please reviewers

seems like it devalues experimentalists' view of theoretical chem

orthocresol

I don't like it either. It's not every paper, of course, there are some really good joint experimental + theoretical papers out there, but sadly nowadays in every few org lett papers you find one with about 1/3 a page of DFT supporting their proposed mechanism.

That's been my experience, at least

gannex

this is why I like spectroscopy. In spectroscopy, the interplay between theory + experiment is very sane

you can make a bunch of theoretical models and then "calibrate" them with spectra

orthocresol

10:36 PM

Any kind of spectroscopy in particular that you're thinking about, or just in general?

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