Room for Safdar and Yusuf Hasan

Even I don't have access to Gaussian these days, as it is installed on the system computers, not on my personal PC...let me see if there is someway I can get the installer for the thing, if some of my seniors in comp. chemistry have it

Safdar

@YusufHasan So could you help in calculating the MPC for 1,2-diphenyl-4-trifluoromethylbenxene?

@YusufHasan I don't need it per se.. It's just a controversial question..

5

Q: Most favorable position of attack of an electrophile on 4-trifluoromethyl-1,2-diphenylbenzene

Problem This problem is from a FIITJEE AITS paper Which position is most favorable for the attack of an electrophile? Answer My attempt The more favorable position for attack should be the on the para ring (considering ring with $\ce{-CF3}$ group as the reference ring), because of the lower i...

Yusuf Hasan

What is the controversy, can you explain briefly?

Safdar

6:52 PM

@YusufHasan the answer itself. According to Ben Norris the answer is 4. According to a very very basic <30 sec calculation on trifluoromethyl benzene you get that the inductive effect on para is greater but there is no explantion for why. This is compared to meta.

Yusuf Hasan

Since this is a highly conjugated system, and the effect of inductive effect will fade over distance, I guess a better option would be to check for a negative hyperconjugation that may be caused by the presence of CF3

If you could try making resonance structures of that situation, you can potentially see where the electron density will decrease due to the -H effect

Negative hyperconjugation

In organic chemistry, negative hyperconjugation is the donation of electron density from a filled π- or p-orbital to a neighboring σ*-orbital. This phenomenon, a type of resonance, can stabilize the molecule or transition state. It also causes an elongation of the σ-bond by adding electron density to its antibonding orbital.Negative hyperconjugation is most commonly observed when the σ*-orbital is located on certain C–F or C–O bonds, and does not occur to an appreciable extent with normal C–H bonds. In negative hyperconjugation, the electron density flows in the opposite direction (from π- or...

Safdar

@YusufHasan The whole thinf isn't conjugated though.. the system is not exactly planar..

Yusuf Hasan

Ok that's an issue

even then, a -H effect can potentially work in the first ring(the one with the CF3) itself

Safdar

@YusufHasan I feel that reverse hyperconjugation is a very controversial topic here at SE..

Yusuf Hasan

It would end up inducing positive charge density on the 2 ortho and 1 para position wrt -CF3

@Safdar I agree, but the question seems to be pointing in that direction

Safdar

6:58 PM

@YusufHasan So then meta becomes more electron rich which would be equivalent to what the basic calculation provided.

Yusuf Hasan

I mean, if ever a -H effect could operate, surely a Ph-CF3 is the best position for it to happen

yeah I mean

the (1,2,6) ring will surely become more electron deficient than the (3,4,5) ring

Safdar

https://chat.stackexchange.com/rooms/111149/discussion-between-safdar-and-anindya-prithvi

This discussion is causing the issue.. The thing is I can't counter argue since i don't know well enough.. It becomes gibberish in the middle though..

@YusufHasan Yeah so then the answer provided by Anindya doesn't work..

Yusuf Hasan

Yeah, I am pretty sure that we can say that one of (3,4,5) should be the substituted guy

out of these 3, it seems a little tough to narrow down further

Maybe, we can evoke -I i guess

Safdar

@YusufHasan Could you look at my answer.. Its pretty much the same as Ben Norris's but i deleted since i couldn't explain.

Yusuf Hasan

@Safdar Yeah, I mean...I feel that the only thing a little controversial in your answer are the resonance structures maybe

These resonance structures are for the (3,4,5) ring, right?

Safdar

7:05 PM

@YusufHasan yeah.

I took it to be similar to a benzaldehyde scenario.. so the partial positive charge is stabilised.

Yusuf Hasan

and the claim is that since the original ring with the CF3 as a whole is acting as -I substituent, so you are drawing resonanc structures with the electron density shifting away from the (3,4,5) ring right?

Safdar

@YusufHasan Yes.

The comments happened before the calculation.. that is when i changed it.. and then deleted.

@YusufHasan It is also being pulled in the 1,2,6 ring but that pull is more.

Yusuf Hasan

That's where I am a little skeptical in you actually drawing out the structures approach...I believe the electron withdrawing should happen mostly via -I, which might make the resonance structures a little problematic

I mean, it may lead to downvotes, is all

Safdar

@YusufHasan So then why meta?

@YusufHasan Honestly right now a proper answer to this question is more important since it is becoming so confused..

Yusuf Hasan

That's what I am a little confused about myself...If we don't make resonance structures, we will end up with the para position(5)

Safdar

7:11 PM

@YusufHasan The molecules are too big to run an MPC test so cannot do that either.. I am unable to even verify if the answer is correct.

Yusuf Hasan

I mean, once you had narrowed it down to (3,4,5)..how did you decide that the final answer should be 4?

I want to know what others are thinking about this problem themselves

Safdar

@YusufHasan the resonance.

I'll put a bounty on it?

Yusuf Hasan

Ok, and why did you resonate? Where is the -M group?

Since the rings are not coplanar, the CF3 ring can't be the -M substituent surely

it can only be -I

as far as I think

No wait before putting a bounty

it is still a new-ish question

Safdar

The fact that Ben Norris has a +6 answer means it is pretty much over though.. many people agreed.

Yusuf Hasan

"Since the lower ring is experiencing a -I effect, electrophilic attack at meta is most likely, so position 4 is the true correct answer."

This part of his answer is troubling me

Safdar

7:16 PM

@YusufHasan I asked why that is in the comments.. Has a +5..

Yusuf Hasan

How does it help in finalizing 4 from (3,4,5)

Safdar

It hasn't been answered yet.

Yusuf Hasan

That's not obvious as of now..even I have upvoted your comment

Safdar

@YusufHasan Can't we simulate it by building a molecule that has a -I effect but no meso and then checking?

Yusuf Hasan

The meta position can only be the answer

Safdar

7:18 PM

@YusufHasan why is that?

Yusuf Hasan

if some kind of partial resonance is taking place in the (3,4,5) ring

I mean if 4 is the answer

then this has to be

altho I don't yet see why

We can try simulations

But I don't really have the software for that

Safdar

@YusufHasan What would work though.. We can't have -H

Benzyl alcohol?

Yusuf Hasan

We can have -H as a probability

Safdar

@YusufHasan you can approximate pretty well with the demo version of webMO..

webmo.net/demoserver/cgi-bin/webmo/build_molecule.cgi

Yusuf Hasan

I mean, I have seen CCl3-Ph experience some definite -H

I would only expect the extent of it to go up in CF3-Ph

But the non planarity is the issue

I would have voted for 5 in this case

if i was going non computationally

Because in my sense, the (3,4,5) ring should only experience -I

Let me see you demo website

Have you worked on it before?

Safdar

7:22 PM

@YusufHasan Yeah..

I recently found it when researching for a question on SE..

@YusufHasan Downsides are there is a 30 sec computational limit and it can't handle big molecules.

Yusuf Hasan

Okay...But this question definitely doesn't work for an exam...FIITJEE has become too cool over the years it seems XD

Safdar

@YusufHasan two years ago actually.. so it was before your exam :P

this is from a 2018 paper.

Yusuf Hasan

ohh, I see...Lol, I never saw this before....well, let me see what I can do about this simulation part

I will let you know if I get Gaussian or something from anywhere

Safdar

@YusufHasan I ran an MPC simulation on benzyl alcohol so -I with no other effect.. Job 677761 on the demo server.

It says para is more electron less compared to meta..

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♦ Room for Safdar and Yusuf Hasan