Why ortho effect will make you question acidic character?

Ortho effect refers mainly to the set of steric effects and some bonding interactions along with polar effects caused by the various substituents which are in a given molecule altering its chemical properties and physical properties. The ortho effect is associated with benzene compounds.

What is ortho effect?

Ortho effect is a special type of effect which is shown by ortho-substituents. It works only with benzoic acid and aniline. It refers to some bonding interactions and the set of steric effects with polar effects inflicted by multiple substituents in a given molecule modifying its physical and chemical properties.

Nearly all the ortho-substituted benzoic acid are more acidic than benzoic acid. While nearly all the ortho-substituted aniline is less basic than aniline.

Where ortho effect works?

There are three major ortho effects in substituted benzene.

• In substituted benzoic acid
• In aniline
• In electrophilic aromatic substitution of disubstituted benzene compounds

It increases the acidity of benzoic acid while decrease the basicity of aniline.

Ortho Effect in case of benzoic acid

In case of benzoic acid, the pi-bond of carboxyl group must lie in plane of ring for resonance to occur. But group present at ortho position to benzoic acid forces the carboxyl group to bend. Due to this the carboxyl group whirl out of the surface or plane of benzene ring. This leads to the steric inhibition of resonance, i.e., SIR effect. This cause the the formation of only two equivalent resonating structures to be exist for ortho substituted benzoic acid. This enhances the acidity of ortho substituted benzoic acid.

To understand it better, look at the acidic character of carboxylic acids:

• o-methoxybenzoic acid > benzoic acid > p-methyoxybenzoic acid
• o-nitrobenzoic acid > p-nitrobenzoic acid  > benzoic acid

Ortho Effect in case of aniline

In case of aniline, group present at ortho position to amine (-NH₂) decreases the basicity of aniline.

Due to steric hindrance, the protonation of substituted aniline is difficult. After protonation, the hybridization of nitrogen oxides alters in amino groups from sp² to sp³ propelling the group to be non-planar. This influences the steric hindrance between the H atom of an amino group and the ortho-substituted group which makes the conjugate acid less stable, thus reducing the basicity of substituted aniline.

To understand it better, look at the basic character of aniline:

• Aniline >m-Nitro aniline>p-Nitro aniline > o-Nitro aniline
• P-Toluidine > m-Toluidine > Aniline > o-Toluidine

Ortho Effect in Electrophilic Aromatic Substitution

Ortho effect in electrophilic aromatic substitution of aromatic benzene compounds refers to the set of the steric effects that will determine the regioselectivity of an incoming electrophile in distributed benzene compounds. Here the meta directing group is meta to the ortho- para directing groups.

When a particular meta directing group is meta to the ortho-para directing group. The group that comes will go ortho to the meta directing group rather than going para to the group. This is basically called the ortho effect. A good explanation for this ortho effect has not been provided but possibly we can say that there can be an intramolecular contribution from the available meta directing group. For a good explanation of this, we can take examples such as that of aromatic nitration of 1-methyl-3-nitrobenzene affords 4-methyl-1,2-dinitrobenzene and 1-methyl-2,3-dinitrobenzene in yields 60.1% and 28.4% respectively.

You can observe similar results in the case of 3 methyl benzoic acid also.

Ortho Effect in Diels-Alder Reaction

In the normal electron demand Diels Alder reactions, the Z substituted dienophiles react with the 1-substituted butadienes to give 3,4-disubstituted cyclohexane. These are independent of the nature of diene substitutes. This effect is also known as the ortho effect.

Things to Remember

• In the ortho effect, the basic strength decreases because of the electron-withdrawing groups or electron releasing groups that are placed on the ortho position.
• There is a point that ortho-substituted aniline are weaker as compared to the normal aniline irrespective of the fact that their nature is electron-withdrawing or electron releasing.
• The acidic property of a compound in which the group is at the ortho position to the carboxyl group is considered to be more than that of benzoic acid.
• Due to the ortho effect, animosity is considered to be a weaker base as compared to aniline.
• The relative basic strength of aniline can be represented as the: aniline > meta nitro aniline > paranitro aniline > ortho nitro aniline.
• The relative basic strength of toluene can be mentioned as para toluidine > meta toluidine > aniline > ortho toluidine

Read More: What are easy ways of organic conversions?

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