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 ortho effect.
In other words, a group in the ortho position always increases the acid strength of an aromatic acid. Out of ortho, meta and para substituted benzoic acids, the ortho compound will be the strongest acid, although there is no satisfactory explanation for this.
Ortho Effect Explanation
Even electron releasing groups decrease the basic strength when present in ortho position. The phenomenon of decreasing of basic strength with electron releasing as well as electron withdrawing groups in the ortho position is known as the ortho effect.
Although there is no satisfactory explanation of this observation
Halogens such as chlorine and bromine exert greater base weakening effect when present in meta position than when present in 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 electron releasing resonance effect.
When chlorine is present in meta position, chlorine exerts only electron withdrawing inductive effect (resonance effect does not operate at meta position). When present at para position, the electron withdrawing inductive effect of chlorine is party neutralized by its electron releasing resonance effect. Therefore, the electron density on nitrogen is reduced to a greater extent when chlorine is present at meta position than when present at 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 are weaker bases than aniline. This is called ortho effect and is probable due to the combination of electronic and steric effect. However, the overall basicity of ortho, meta and para substituted anilines depends upon the electron donating/electron withdrawing resonance and inductive effect.
Ortho substituted anilines are usually weaker bases than aniline irrespective of the nature of the substituent. This is called ortho effect and is due to a combination of steric and electronic factors.
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 ortho effect. Due to +R effect of -OCH3 group at para position, p-anisidine is a much stronger base than aniline. Presence of the -OCH3 group at the meta position cannot exert its +R effect on -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
p-anisidine > aniline > o-anisidine > m-anisidine
Frequently Asked Questions on Ortho Effect
What is meant by Ortho effect?
The Ortho effect is specific to aniline and benzoic acids. It states that the ortho-substituted benzoic acids and anilines are respectively more acidic than benzoic acid and aniline. They can also greatly affect acidity through sterics, causing the group of carboxylic acids to bend out of the benzene ring plane.
Which compound shows ortho effect?
The Ortho effect is specific to aniline and benzoic acids. It states that the ortho-substituted benzoic acids and anilines are respectively more acidic than benzoic acid and aniline. As it is understood that the conjugation of carboxylic acid to aryl rings increases its acidity, bending out the plane causes a decrease in acidity.
Which is more basic aniline or nitroaniline?
P-nitroaniline has an electron withdrawal group in the para region and the lone pair of NH2 is more stabilized by resonance due to these EWGs and the density of the lone pair is less than that of aniline. Aniline is also more basic than p-nitroAniline.
Is NO2 an ortho para director?
Because NO2 is an electron withdrawal group, a look at the resonance structures shows that the positive charge at the ortho-para positions is concentrated. Thus these positions are deactivated in aromatic electrophilic substitution. NO2 is also a meta-director, as we have all learned in organic chemistry.
What is the ortho effect in aromatic electrophilic substitution?
Ortho effect in electrophilic aromatic replacement of disused benzene compounds (third ortho effect)- Refers to the set of steric effects that determines the regioselectivity of an incoming electrophile in disused benzene compounds where a meta-directing group is meta to an ortho-para-directing group.