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Ask iit jee aieee pet cbse icse state board community Community Discussion Question: Ortho Effect
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achyut (2)

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Please help me by explaining exactly what ortho effect is and how it affects basicity/acidity
    
arun-rashi (1136)

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if any group present on ortho position of benzoic acid it always increases acidic nature of acid because this group decreases outer resonance of ring towards acid which increases acidic nature for example orthonitrobenzoic acid is most acidic in any nitro derivative of benzoic acid.orthomethyl benzoic acid is maximum acidic than other isomers.similarly if any group present on ortho position of aniline it decreases basic nature.

Arun / Rashi - Authors Macromind MCQ of Chemistry from G.R Batla
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vichu_487 (43)

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to reduce the steric strain.......the group goes out of plane.......Thereby weakening resonance........therefore weakening acid wekeaning resonance........So it can easily be acidic
This whole thing is steric effect
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achyut (2)

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i thought resonance decreased BASICITY..
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papaichanda (5)

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vichu is correct...it is the steric hinderance....ortho effect like in orthonitro benzoic acid is not dependent on the kind of substituents whether e-withdrawing or donating.......in case of ortho-nitro benzoic acid......due to steric strain between nitro grp and carboxylate ion....the carboxylate ion is forced out of plane and that shift alters the charge into the ring and so it becomes easier to delocalize the charge in the ring easily.and that increases the acidity.

"Science is a provisional truth which is always subject to change"- Charles Darwin
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achyut (2)

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now this is confusing.. one says it strengthens resonance and one says it is weakened.. i am still confused
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shubham.123 (307)

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vichu is perfect !!!!!!!
steric hindrance is real cause !!!!!!
steric inhibition of resonance prevents +r to come
in operation and since this weakens acidic strength ,its absence results in increased acid strength !!!!!!!
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ashish040191 (142)

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COOH is a bulky group. and any group at ortho position causes steric hinderence. due to this the planarity of COOH is lost.
We know that for resonanace the molecules under consideration shod be planar. ther4 resonance is inhibited.
U can easily see that Ph stabilises COOH by resonance.(in the canonical structures u can see that -ve charge comes on 1 O.
think if H is released(rather H+) it brings -ve on he other O also.
and it leads to a large repulsiv force(between 2 O-.) ther4 release of H+ is now prohibited
 
BUT when, planarity is lost i.e resonance is stopped. ad now -ve not comes on any O. and elease of H is facilitated.
 
 
I think now u ll hv no confusion..................
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RajuGentleman (2)

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Kudos Ashish
 
Perfect ans!!!!!!

If u want a place in da sun u've got 2 put up wid a few blisters
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achyut (2)

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thanks ashish...
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