[SVXX]

Could someone explain to me what the ortho effect is and how and where it operates? I read about it here : http://answers.yahoo.com/question/index?qid=20071012080118AABWL03 and I suppose it has something to do with the bulkiness of ortho-substituted groups which forces the acidic/basic group out of the plane of benzene, but I'm not really satisfied by this explanation. I figured the best way to put my doubt to rest would be to post here.
Are the ortho effect and chelation different things? Which groups, when substituted at the ortho position, emulate this effect? Which groups (-COOH, -CHO, etc) does it operate on?

If anyone could give me links to a proper explanation of the ortho effect(and I don't mean ortho-para selectivity), it would be more than enough. I couldn't find much when I googled.

[SVXX]

I know double posting isn't right but...
Is this topic not worthy of replies?

[bromidewind]

I know double posting isn't right but...
Is this topic not worthy of replies?

Patience, grasshopper, patience... just kidding

Just to clarify, this is all my own speculation. I don't have a fancy doctorate or anything, and I haven't dealt much with mass spec, so this is just my best guess.

From <a href="http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6TH2-4SFXK71-4&_user=10&_coverDate=08%2F31%2F2008&_rdoc=1&_fmt=high&_orig=search&_sort=d&_docanchor=&view=c&_searchStrId=1375639517&_rerunOrigin=google&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=4fda4a744732ec1b23f4b46411ff5768">Ortho Effect in Electron Ionization Mass Spectrometry of N-Acylanilines Bearing a Proximal Halo Substituent</a>

[The ortho effect occurs when] a labile hydrogen is transferred from a donor functional group attached to an ortho position of an aromatic ring, via six-membered transition state. Consequently, a neutral molecule is eliminated to give an odd-electron diene product that usually bears the charge.

In short, an acidic hydrogen on a functional group at the ortho position on an aromatic ring jumps over to whatever is on the ortho position and kicks out an electron, generating a nice peak on your mass spec that differentiates it from meta/para substituted compounds. To answer your question as to what groups it operates on, any functional group with an acidic proton would do.

The ortho effect and chelation are completely different. Chelation involves the binding of a ligand to a metal. While it can involve ortho-substituted rings, this is not a chief requirement. Carboxylic acids are often found in chelating agents though.

If you can get access to them, the following articles might help you better understand the ortho effect. I was unable to find any copies via my school's partnership or free copies via the web, but perhaps you'll have better luck.

1. Schwarz, H. Some Newer Aspects of Mass Spectrometric Ortho Effects. Top. Curr. Chem. 1978, 73, 231–263.
2. Gross, J. H. Ortho Elimination (Ortho Effect). In Mass Spectrometry; Springer: Berlin, 2004; p. 304.
3. Sparkman, O. D. Mass Spec, Desk Reference; Global View Publishing: Pittsburgh, 2000; p. 22.

[SVXX]

Thanks for trying to help bromidewind, I appreciate it I'll try to find those. But this is exactly where I'm confused : the ortho effect is a sort of steric effect depending on the bulkiness of the ortho substituted group, which causes an alteration in conformation of the acidic group wrt the benzene ring(as per that link I supplied). It supposedly doesn't have anything to do with six membered transition states(while chelation does). It seems more plausible an explanation, because how else would you explain the increased acidity of o-nitrobenzoic acid when we compare with para and meta isomers?
Had it really been a sort of chelation, acidity would've reduced, isn't it? The acidic -H would have been locked in a six membered transition state, making it harder to be given away.

Check out nitro substituted phenols. Ideally the ortho isomer should be the least acidic(due to chelation), but it's the meta isomer which fills that position, while ortho is midway between para and meta. It's far more confusing than I earlier thought!

I had a feeling that it would work on all groups with an acidic -H, but it doesn't happen in case of phenols.
Now how does the formation of an odd-electron diene affect acidity or basicity? It's all a little too farfetched for me. Now I need some help from the experts

[uvcyclotron]

hmm.. the explanation by bromidewind seems irrelevant to both of us, because i haven't studied mass spectrometry yet, and i guess the same applies for you.

I was able to dig into my class notes, and this is what I came out with:
Ortho Effect:
when some unusual property is observed due to the presence of an ortho substituent, then it's known as the ortho effect.
may be due to:
1) SIR
2) SIP
3) sometimes intramolecular H-bonding as in o-nitrophenol.

SIR (Steric Inhibition of Resonance)
It arises due to the steric repulsion between a bulkier group with it's ortho substituent. This effect is usually seen in the bulkier groups like -COOH, -SO₃H, -NO₂, NMe₂
Ortho substituents present with these bulkier groups throws these groups out of the plane, because of which the resonance connection with benzene ring vanishes, that's why it is refereed as inhibition of resonance due to steric repulsion(SIR).
It's not applicable on smaller groups like -NH₂, OH, -CN, but they can impose their effect on the bulkier groups.

Eg:
comparison on acidic nature b/w a normal benzoic acid(NBA), and an ortho-substituted benzoic acid(OSBA),
the NBA experiences +M effect from the benzene ring which destablises the COO^-, while in OSBA, the COO^- has been turned out of ring's plane hence minimizing any destabilizing mesomeric effects on it from the ring.
Thus acidic nature of OSBA > NBA.


SIP (Steric Inhibition of Protonation)
A similar effect acting on aniline-type structures, where protonation of -NH₂ is inhibited because the cation formed is less stable due to the steric repulsion.
so otho substituted aniline would be less basic than aniline, as well as it's meta & para isomers.
Exception: SIP doesn't apply in case of -OMe and -OH. (why? we haven't been taught yet..  )

UV

[SVXX]

Nicely explained! Except that as usual we're not given the answer to the "why"s of most of our queries this explanation fits the bill nicely, even if it has some minor discrepancies(such as the SIP exception when theoretically there should be none).
If SIR and SIP are decided by the bulkiness of the ortho-substituents, then groups like -Ph should also cause such unusual properties to appear when attached at the ortho position?
For example, what would be the acidity order of o, p, and m phenyl-substituted benzoic acids?
Here are some pKa values of interest to the question I asked -:
2-phenylbenzoic acid : 3.46
I couldn't find pKas of the other phenylbenzoic acids.

[uvcyclotron]

Well since SIR depends on a bulky group being ortho-substituted, benzoic acid being substituted by phenyl ring would definitely inhibit the resonance connection with the ring, leading to acidic nature as:
o>m>p

in o-phenyl SIR applies
in m-phenyl, the -I or Inductive effect applies (somewhat stabilizing)
in p-phenyl, the +M or mesomeric effect applies (destabilizing)