What is Ortho Effect?
The ortho-substituted benzoic acids are considerably stronger acids than benzoic acid, no matter whether the substituent is electron-releasing or electron-withdrawing. This effect is known as the ortho effect.
In other words, a group in the ortho position always increases the acidic strength of an aromatic acid. Out of ortho, meta and para-substituted benzoic acids, the ortho-substituted compound will be the strongest acid, although there is no satisfactory explanation for this.
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Table of Contents
- Ortho Effect Explanation
- Ortho Effect in Aniline
- Relative Basic Strength of Nitroanilines
- Frequently Asked Questions on Ortho Effect
Ortho Effect Explanation
The phenomenon of decreasing basic strength with the presence of electron-releasing as well as electron-withdrawing groups in the ortho position is known as the ortho effect.
Although there is no satisfactory explanation for this observation
Halogens such as chlorine and bromine exert a greater base weakening effect when present in the meta position than when present in the para position. For example, the basic strengths of the meta and para chloro anilines are expressed as their pKb values.
It may be recalled that chlorine has an electron-withdrawing inductive effect and an electron-releasing resonance effect.
When chlorine is present in the meta position, chlorine exerts only the electron-withdrawing inductive effect (the resonance effect does not operate at the meta position). When present at para position, the electron-withdrawing inductive effect of chlorine is party neutralised by its electron releasing resonance effect. Therefore, the electron density on nitrogen is reduced to a greater extent when chlorine is present at the meta position than when present at the para position. As a result, m-chloroaniline is a weaker base than p-chloroaniline which in turn is weaker than aniline.
Ortho Effect in Aniline
Irrespective of electron-withdrawing or electron-releasing nature, the ortho-substituted aniline has weaker bases than aniline. This is called the ortho effect and is probably due to the combination of electronic and steric effects. However, the overall basicity of ortho, meta and para-substituted anilines depend upon the electron-donating/electron-withdrawing resonance and inductive effect.
Relative Basic Strength of Nitroanilines
aniline > m-nitroaniline > p-nitroaniline > o-nitroaniline
The relative basic strength of toluidine
p-toluidine > m-toluidine > aniline > o-nitroaniline
o-anisidine is a weaker base than aniline due to the ortho effect. Due to the +R effect of the -OCH3 group at the para position, p-anisidine is a much stronger base than aniline. The presence of the -OCH3 group at the meta position cannot exert its +R effect on the -NH3 group, but it can exert its -I effect due to the presence of electronegative oxygen. Hence m-anisidine is a weaker base than aniline and it is found to be even weaker than o-anisidine. Thus the basicity order may be written as
What about Aminophenols and Methoxyanilines
Aminophenol is the organic compound with the formula H2NC6H4OH. Aminophenol may refer to any of three isomeric chemical compounds: 2-Aminophenol · 3-Aminophenol · 4-Aminophenol.