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Organic Chemistry

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19 Oct 2007 22:54:28 IST

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Ortho Effect

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Please help me by explaining exactly what ortho effect is and how it affects basicity/acidity

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arun-rashi

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19 Oct 2007 23:26:51 IST

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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.

viswesh Nrayanan

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20 Oct 2007 11:00:21 IST

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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

Achyut Bihani

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21 Oct 2007 13:54:10 IST

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i thought resonance decreased BASICITY..

Debjyoti Chanda

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21 Oct 2007 15:36:20 IST

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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.

Achyut Bihani

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23 Oct 2007 16:25:24 IST

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now this is confusing.. one says it strengthens resonance and one says it is weakened.. i am still confused

SPARTIAN

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23 Oct 2007 22:46:27 IST

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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 !!!!!!!

ashish ranjan

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23 Oct 2007 23:36:11 IST

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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..................

vimal

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24 Oct 2007 14:48:29 IST

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Kudos Ashish

Perfect ans!!!!!!

Achyut Bihani

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25 Oct 2007 14:36:37 IST

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thanks ashish...

Siddhartha Gupta

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6 Mar 2009 16:41:02 IST

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Ortho effect is shown only by..ANILINE and BENZOIC ACID...Ortho effect : Ortho isomer becomes more acidic as compared to other isomers in case of Benzoic acid ....and become less basic as compared to other isomers in case of Aniline....Suppose you have four compounds ..A: Aniline with OH on ortho position.B;Aniline with OH on Meta,,,,C with OH on PARA and D with simple aniline.then basic strength will be,,, C>B>D>A....because there is ortho effect in case of A so it become less basic as compared to simple aniline.Similar in Benzoic...it will be more acidic than all members..Ortho effect is not property of all aromatic compound..only the two that i have discussed above

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