How can you tell when a compound undergoes electrophilic aromatic substitution?

[Lan(r)12]

Ok, I know, because my book says so lol, that a Triflouromethly (CF3) group is "meta directing" in electrophilic aromatic substitution...but I don't know WHY

 

If anyone could help me out, that would be great...

And its supposed to be "deactivating" right? and not "activating"?

Or do I have this mixed up?

 

I would actually like to learn this for future reference, so info is greatly appreciated, even if its not directly linked to this question

 

Thanks for the help!

[UC]

This page is quite useful: http://www.cem.msu.edu/~reusch/VirtualText/benzrx1.htm

 

As for activating versus deactivating, more electrons make an aromatic ring more reactive. Are fluorines going to give or pull away electron density from a carbon? (look at electronegativities if you aren't sure) If that carbon is attached to an aromatic ring, is it going to give or borrow electron density to compensate for what the fluorines are doing to it? If you have any further questions, feel free to ask them here

[Kaeroll]

Halogens are a special case in substituent effects. My current work deals with this area, though not halogen compounds specifically.

 

CF3 would be expected to interact purely through inductive effects, as it cannot conjugate with the system. However, it actually exhibits a stronger substituent effect at the p- position than m- position, typical of substituents acting through the pi system. A few ideas about this have been investigated (inconclusively, as far as I'm aware) but it does seem that CF3 can release electrons into the pi system.

 

I'd be cautious of interpreting the behaviour of CF3 purely in terms of inductive effects. A nice treatment of this is given in Physical Chemistry by Isaacs. If you don't have access to this, let me know and I'll pop to the library on Thursday and get the numbers for you.

 

Out of curiosity, what level are you studying at? If you're at A level don't worry too much about it, but if you're at university it's quite and interesting problem.

[Lan(r)12]

Acutally, I'm just in the second semseter of Organic Chemistry...our teacher likes to ask us really "intellectually stimulating" questions, so that we might apply all of these rules we are learning...you should see the compounds we have to use Huckel's Rule on lol...

[Kaeroll]

Sounds like a decent tutor to me.

 

Do you have access to a decent library? If so Isaacs (or any other physical organic text) will be helpful with this kind of work. Key word: Hammett equation.

[Lan(r)12]

actually, no, unfortunately our library is of sub-par quality.

Might you have a link to some of his online resources?

 

We've only briefly touched on the physical aspects of chemistry.

And yes, he is an excellent professor.

[Kaeroll]

I'm not aware of an online physical text book I'm afraid, but I'll find the relevant information and get back to you at the weekend if you're interested.

 

This area is usually termed 'physical organic', as it's applying physical ideas and methods to organic chemistry - rather than simply saying "it's activating/deactivating", we can say how activating/deactivating it is, and so on. Louis Hammett was one of the first to quantify substituent effects, and as I mentioned above, CF3 is something of an anomaly in the data.

[Lan(r)12]

Yes, I am very much interested

 

I just...I'm not good at reading very advanced stuff...like the kind you would find in published articles...by no means am I restricted to layman's terms, but I'm still not far enough along to read science journals and understand them.

[Kaeroll]

Right... CF3. Bit of simple maths to follow, don't worry too much about it. It's a matter of interpretation.

 

What Hammett did was to take a standard reaction - the ionisation of benzoic acid derivatives at 25 C in water - and use it to compare electronic effects. pKa (-log of the equilibrium constant of the ionisation) is readily measured and can be used to determine the effect of substituents placed meta or para to the carboxylic acid group (those ortho to it would also have a steric impact upon the reaction - the benzene ring acts as a 'core', keeping steric factors constant). He came up with this equation:

[math]\frac{log(K_{X})}{log(K_{H})}=\sigma\rho[/math]

KX is the equilibrium constant of the benzoic acid with substituent X at m or p, and H is the standard - benzoic acid itself. Sigma is known as the substituent constant, and rho is the 'sensitivity' (which, for the reference reaction I'm discussing, is defined as 1). We're focusing on sigma.

 

In this reaction, electron withdrawal increases the equilibrium constant by stabilising the acid anion. This manifests as a positive sigma value (negative sigma, conversely, indicates electron release). The sigma value is influenced additionally by the mechanism of withdrawal/release - particularly whether it is inductive (due to electronegativity) or resonant (due to conjugation). It is also influenced by whether the substituent is at the meta or para position on the ring relative to the functional group.

 

Resonance effects are not detected from the meta position (try pushing 'curly arrows' round the benzene ring, and you'll see why), only para and ortho. Inductive effects, on the other hand, have an influence from both positions, and are stronger at meta as it's closer to the functional group.

 

We would therefore expect that a purely inductive group, such as [math]NMe^{+}_{3}[/math], to have a larger magnitude of sigma at the meta than para position (whether positive or negative). Indeed, the data in Isaacs bears this out: sigma at the meta position is 1.04, and at the para position is 0.88. All good so far.

 

CF3 is listed in Isaacs as another example of a purely inductive group. Dr Isaacs seems not to have checked his own data (but given the quality of the rest of the text I'll forgive him). He gives sigma meta and para values of 0.46 and 0.53 respectively for CF3. This contradicts the above discussion - which assumes CF3 is purely inductive. The data suggests that somehow, CF3 can also withdraw electrons through resonance.

 

When I found this during a literature search last year I had a (very superficial) look through the literature for an answer, and discussed it with my supervisor. We first confirmed that this was not simply an error on Isaacs' part - a data book (not sure which) my supervisor has contained values of sigma from several sources, confirming the reversal of the expected trend. I then searched through some journals. It seems several competing ideas to explain this have been tossed about, including fluoride hyperconjugation, and overlap of F lone pairs with the benzene pi system.

 

It doesn't seem that a consensus has been reached, despite this being recognised several decades ago, but I could be wrong - and as I said, it was a fairly quick search.

 

But, to relate all this back to your original question. Given the sigma values for CF3, it would be reasonable to expect it to deactivate the ring to E.A.S., and direct meta. The link provided by UC should be able to explain why this is, etc.

 

If you've got any questions on the above (or anyone else has corrections - I'm bashing this out quickly over lunch) I'd be happy to answer them.

 

Kaeroll

[Lan(r)12]

yes, my book assigns the numerical data of .46(for meta) and .53(for para) for CF3...and so ,yes, according to what you wrote, this is incorrect in certain terms.

 

But nobody has bothered to follow through on this discrepancy? I find that...odd, don't you?

 

But thanks for the excellent info!

[Kaeroll]

Not a problem. As I said, my work is in a related area, so most of the info was to hand anyway.

 

It'd be nice to see a definitive answer come through, I might have another look in the literature if I find myself bored.

 

Anyway- glad to be of help, good luck in your studies.