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Ortho substituted benzoic acids are considerably stronger than benzoic acid. It doesn’t matter whether the substitute is electron-withdrawing or electron releasing. This effect is called the Ortho effect. The ortho positioned group continuously promotes the acid strength of an aromatic acid. In other words, in ortho meta and para substitute, ortho compounds are the stronger acid.
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Key Terms: Ortho effect, meta position halogens, ortho-substituted anilines, relative basic strength of Nitroanilines, benzoic acid, bromine, chlorine
Ortho Effect
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The Ortho effect is the process in which the basic strength decreases with electron-withdrawing or electron releasing groups in the ortho position. An electron releasing group in the ortho position decreases the basic strength.
When present in meta position halogens like bromine and chlorine exert a greater base weakening effect than in para position. The basic strength of meta and para chloro anilines is given by their pKb values.
Chlorine has both electrons releasing resonance effect and electron-withdrawing inductive effect. At the para position, the electron releasing resonance effect neutralizes the electron-withdrawing inductive effect. Therefore when chlorine is present at meta position, the electron density on nitrogen is decreased to a larger extent as compared to the chlorine present at the para position. Consequently, m-chloroaniline is a weaker base than p-chloroaniline which in turn is weaker than aniline.
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Ortho Effect in Aniline
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The ortho-substituted anilines are weaker bases than aniline irrespective of the electron-withdrawing or electron releasing nature. This effect is known as the ortho effect and one reason for this phenomenon is the combination of electronic and steric effects. The total basicity of ortho, meta, and para-substituted anilines is based upon the electron-withdrawing/donating inductive and resonance effect.
Usually, ortho-substituted anilines are weaker bases than aniline irrespective of the nature of the substituent. This process is known as the ortho effect and one probable reason for this could be the combination of electronic and steric factors.
Relative Basic Strength of Nitroanilines
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The relative basic strength of Nitroanilines
aniline > m-nitroaniline > p-nitroaniline > o-nitroaniline
The relative basic strength of Toludine
p-toluidine > m-toluidine > aniline > o-nitroaniline
Due to the ortho effect o-anisidine is a weaker base than aniline. P-anisidine is a stronger base than aniline because of the +R effect of the -OCH3 group at the para position. The presence of the -OCH3 group at the meta position is not able to exert its +R effect on the -NH3 group, but it can exert it -I effect due to the presence of electronegative oxygen.
When compared to aniline m-anisidine is a weaker base and also it is weaker than o-anisidine. The basicity order is given below.
p-anisidine > aniline > o-anisidine > m-anisidine
Ortho Effect in Substituted Benzoic Acid
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When a group is at the ortho position to the carboxyl group is substituted benzoic acid then the acidic property of that compound is more than the benzoic acid. Usually, ortho-substituted benzoic acid is stronger than para and meta isomers.
Explanation
When a group is at the ortho position to the carboxyl group is substituted benzoic acid, the carboxyl group is compelled by the steric constraints to get whirled out of the surface of the benzene ring. The resonance property of the carboxyl group with the phenyl ring is visible here which is responsible for increasing the acidity level of the carboxyl group which was reduced due to the destabilizing cross conjugation.
The destabilizing cross conjugation is the reason for the lower acidity in benzoic acid. The hydrogen bonds present near the carboxyl group are also responsible for the change in acidity.
Things to Remember
- The Ortho effect is the process in which the basic strength decreases with electron-withdrawing or electron releasing groups in the ortho position.
- The ortho-substituted anilines are weaker bases than aniline irrespective of the electron-withdrawing or electron releasing nature.
- The acidic property of a compound in which a group is at the ortho position to the carboxyl group is more than the benzoic acid.
- o-anisidine is a weaker base than aniline because of the ortho effect.
- The relative basic strength of Nitroanilines is of the order:
aniline > m-nitroaniline > p-nitroaniline > o-nitroaniline
- The relative basic strength of Toludine can be expressed as :
p-toluidine > m-toluidine > aniline > o-nitroaniline
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Sample Questions
Ques: Ortho nitrophenol has a lower boiling point than p-nitrophenol. Why? (Compt. Delhi 2012, 1 Mark)
Ans: Ortho-nitrophenol forms intramolecular H-bonding whereas p-nitrophenol forms intermolecular H-bonding, due to this Ortho nitrophenol has a lower boiling point than p-nitrophenol.
Ques: Ortho-nitrophenol is more acidic than ortho-methoxy phenol. Why? (Compt. Delhi 2012, 1 Mark)
Ans: NO2 group is an electron-withdrawing group while methoxy group is electron-donating in nature. Ortho-nitrophenol is more acidic than ortho-methoxy phenol because it is easier to remove H+ from O-nitrophenol whereas it is difficult to remove H+ from O-methoxy phenol.
Ques: Which compounds show the ortho effect? (1 Mark)
Ans: Aniline and benzoic acids show the ortho effect. The ortho-substituted anilines and benzoic acids are much acidic than aniline and benzoic acid. The conjugation of carboxylic acid to aryl rings triggers its acidity.
Ques: Is NO2 an ortho para director? (1 Mark)
Ans: NO2 is an electron-withdrawing group, resonance structures show that at the ortho-para positions the positive charge is concentrated. These positions are deactivated in aromatic electrophilic substitution. NO2 is a meta-director.
Ques: Explain the following behaviours :
(i) Alcohols are more soluble in water than the hydrocarbons of comparable molecular masses. (ii) Ortho-nitrophenol is more acidic than ortho-methoxyphenol. (All India 2012, 2 Marks)
Ans: (i) Alcohols can form Hydrogen bonds with water and break the already existing hydrogen bond between water molecules whereas hydrocarbons cannot form Hydrogen bonds with water. Due to this reason, Alcohols are soluble in water in comparison to other hydrocarbons of comparable molecular mass.
(ii) The electron density in the –OH bond decreases due to the strong -R and -1 effect of the –NO2 group, and hence they easily lose a proton. Moreover, O-nitrophenoxide ion is stabilized by resonance, 1 thereby making O-nitrophenol a stronger acid.
In O-methoxy phenol, due to + R effect of the – OCH3 group the electron density in the O–H bond increases thereby making the loss of proton difficult. Furthermore, the O-methoxyphenoxide ion is left after the loss of a proton and is destabilized by resonance because the two negative charges repel each other. Hence, O-methoxy phenol is a weaker acid.
Ques: Write the equations involved in the following reactions:
(i) Williamson ether synthesis
(ii) Kolbe’s reaction (Compt. Delhi 2014, 2 Marks)
Ans: (i) Williamson ether synthesis: In this reaction, the nucleophilic substitution of the halide ion from the alkyl halide by the alkoxide ion by an SN2 mechanism takes place.
(ii) Kolbe’s reaction: Phenoxide ion formed after treating phenol with sodium hydroxide is more reactive than phenol. It undergoes electrophilic substitution with carbon dioxide. Ortho hydroxybenzoic acid is formed as the main reaction product.
Ques: Explain the fact that in aryl alkyl ethers
(i) The alkoxy group activates the benzene ring towards electrophilic substitution and
(ii) It directs the incoming substituents to ortho and para positions in the benzene ring. (2 Marks)
Ans: (i) In aryl alkyl ethers, the electron density in the benzene ring increases due to the +R effect of the alkoxy group.
Thus, benzene is activated towards electrophilic substitution by the alkoxy group.
(ii) It can also be observed from the resonance structures that the electron density increases more at the ortho and para positions than at the meta position. Due to this, the incoming substituents are directed to the ortho and para positions in the benzene ring.
Ques: What is the ortho effect? (2 Marks)
Ans: Ortho contained benzoic acids are considerably stronger than benzoic acid. Here it doesn’t matter whether the substitute is electron-withdrawing or electron releasing. This is called as Ortho effect. We can say that a group in the ortho position continuously promotes the acid strength of an aromatic acid.
Ques: Which is more basic aniline or nitroaniline? (2 Marks)
Ans: Aniline is more basic in nature when compared to nitroaniline because P-nitroaniline in the para region has an electron withdrawal group. The lone pair of NH2 is more stabilized by resonance because of the EWGs. When compared to the density of aniline, the density of the lone pair is lesser.
Ques: Describe why aniline is less basic than Methylamine?(2 Marks)
Ans: Methylamine does not tend to delocalize its lone pair of electrons and the aniline resonates with a benzene ring and gets a lone pair of electrons. In the case of aniline, it is hard to donate electrons due to the lesser accessibility of the lone pair of electrons. There is no such problem with Methylamine.
The corresponding base will be basic in nature if the conjugate acid is stable. In methylamine, the stabilization of the conjugate acid is done with a positive inductive effect from sp2 hybridized carbon. Sp2 is more electronegative than sp3 and therefore, aniline is less basic when compared to methylamine.
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