Ortho Effect

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What is the Ortho Effect?

The Ortho effect is the process in which ortho contained benzoic acids are reasonably stronger than benzoic acid. It doesn't matter whether the substitute is electron-withdrawing or electron releasing. In simple words, a group in the ortho position constantly boosts the acid strength of an aromatic acid. In ortho meta and para substitute, ortho compound will be the strongest acid of all. A group present in the ortho position concerning the carboxyl group generates steric obstacles compelling the carboxyl group to rotate and step back from the benzene ring. After delocalization, a carboxyl group cannot participate in the ring resonance and so the acidity increases. 

Explanation of Ortho Effect 

The ortho effect is related to substituted benzene compounds. 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.

There are three major ortho effects in substituted benzene. 

  • Ortho effect in substituted benzoic acid

  • Ortho effect in aniline

  • Ortho effect in electrophilic aromatic substitution of disubstituted benzene compounds 

Ortho Effect in Substituted  Benzoic Acid

When a group is located at the ortho position to the carboxyl group is substituted benzoic said then the acidic property of that compound is more than benzoic acid. In most cases, ortho-substituted benzoic acid is stronger than para and meta isomers. 

General Explanation

When a group is located at ortho to the carboxylic acid group in substituted benzoic acid, the steric constraints compel the carboxyl group to whirl out of the surface of the benzene ring. This shows the resonance property of the carboxyl group with the phenyl ring which boosts the acidity level of the carboxyl group which was curtailed because of destabilizing cross conjugation. This destabilizing cross conjugation is held responsible for lower acidity in benzoic acid.

The presence of hydrogen bonds near the carboxyl group can also trigger acidity.

Ortho Effect in Aniline

When a group is existing at the ortho position to NH2 in aniline, the basic nature of the compound becomes moreover less than aniline. To understand this properly, look on to the order of basicity of the following substituted aniline. 

  • p-Aminophenol>Aniline>o-Aminophenol>m-Aminophenol

  • Aniline>m-Nitroaniline>p-Nitroaniline>o-Nitroaniline

  • p-Toluidine>m-Toluidine>Aniline>o-Toluidine

General Explanation

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

Solved Examples

Which is More Acidic Para or Ortho Nitrophenol?

In para nitrophenol, there is no H-bonding due to attachment with neighboring carbon atoms. But in ortho nitrophenol, H bonding occurs due to attachment with adjacent carbon atoms. That's the reason why para nitrophenol is more acidic than ortho nitrophenol.

Why is Chlorine (CI) Ortho Para Directing?

The -I effect of chlorine takes out electrons from the benzene ring. This leads to the destabilization of intermediate carbocation created during electrophilic substitution.  On the contrary, CI provides its lone pair of electrons to aromatic rings and increases the electron density at para and ortho positions. 

FAQ (Frequently Asked Questions)

Q1. Explain Why is Aniline Less Basic than Methylamine?

Ans: As we know aniline resonates with a benzene ring that gives it a lone pair of electrons whereas methylamine does not have its lone pair of electrons delocalized. A compound's basic character is in donating electrons. But in the case of aniline, it is difficult as the lone pair of electrons is not easily accessible. Methylamines do not face this kind of issue. 

In other words, if the conjugate acid is stable then the corresponding base will be more basic. In methylamine, the conjugate acid is stabilized by a positive inductive effect from sp2 hybridized carbon. Since sp3 is less electronegative than sp2. Hence, aniline is less basic than methylamine.

Q2. What is the Ortho Effect? Why are Almost All Ortho-substituted Benzoic Acid Stronger Acid than Benzoic Acid?

Ans: Ortho effects happen when a group located in the ortho position concerning the carboxyl group generates steric hindrances or strain leading to the rotation of the carboxylic group and shifting it out of the surface of the benzene ring. The carboxylic group cannot actively participate in ring resonance and the acidity increases as the delocalization of negative charge on the conjugate base of the acid is improved, this is known as the ortho effect. 

For groups like -OH or -NH₂, in the ortho position, the steric hindrances do not compel the carboxyl group to rotate and shift out of the benzene ring. The conjugate base has chelation stability due to H-bonding leading to add more stability to the conjugate. Thus, increasing the acidity of the ortho-substituted carboxylic acid.