An Introduction to Ortho Effect
Ortho effect basically refers to the set of steric effects and some bonding interactions along with these polar effects that are caused by various substituents present in the given molecule. This ortho effect not only alters the chemical properties but along with it physical properties of the molecule are also altered. In general, the ortho effect is associated mainly with the substituted benzene compounds.
The Ortho effect is the process in which ortho-containing 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 substitutes, ortho compounds 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 makes the conjugate acid less stable, thus reducing the basicity of substituted aniline.
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.
General Explanation
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-nitrobenzene 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 cyclohexanes. These are independent of the nature of diene substitutes. This effect is also known as the ortho effect.
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 anilines are weaker as compared to the normal anilines 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 nitroaniline> para -nitroaniline> ortho nitroaniline.
The relative basic strength of toluene can be mentioned as para toluidine>meta toluidine>aniline> ortho toluidine
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 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.
Conclusion
We have covered all the important points of the Ortho Effect that makes learning easy. We also covered solved examples.
FAQs on Ortho Effect
1. What is the ortho effect in chemistry?
The ortho effect is a unique phenomenon observed in ortho-substituted benzene derivatives, particularly in benzoic acids and anilines. It refers to the collective impact of steric and electronic effects from a substituent at the ortho (1,2) position. This effect typically makes ortho-substituted benzoic acids stronger acids than benzoic acid itself, and ortho-substituted anilines weaker bases than aniline, regardless of whether the substituent is electron-donating or electron-withdrawing.
2. How does the ortho effect increase the acidic strength of substituted benzoic acids?
The ortho effect enhances the acidity of benzoic acids primarily through a concept called Steric Inhibition of Resonance (SIR). An ortho-substituent, due to its proximity, creates steric hindrance that forces the carboxyl group (-COOH) to rotate out of the plane of the benzene ring. This rotation disrupts the resonance between the carboxyl group and the ring, but it crucially stabilizes the conjugate base (carboxylate ion, -COO⁻) formed after losing a proton. Since the resulting carboxylate anion is more stable, the acid readily donates its proton, increasing its acidic strength.
3. Why are ortho-substituted anilines generally weaker bases than aniline?
Ortho-substituted anilines are weaker bases than aniline due to two main steric factors:
- Steric Hindrance to Protonation: A bulky group at the ortho position physically blocks the lone pair of electrons on the nitrogen atom, making it difficult for a proton (H⁺) to approach and form a bond.
- Destabilisation of the Conjugate Acid: When the amino group (-NH₂) gets protonated to form the anilinium ion (-NH₃⁺), the geometry changes. This increases steric repulsion between the ortho-substituent and the hydrogens on the anilinium ion, making the conjugate acid less stable. A less stable conjugate acid corresponds to a weaker base.
4. What is the difference between the ortho effect and the standard ortho-para directing effect?
These are two distinct concepts in organic chemistry. The ortho-para directing effect is an electronic effect (caused by resonance or induction) that governs where an incoming electrophile will attack the benzene ring. In contrast, the ortho effect is primarily a steric effect that influences the chemical properties (like acidity or basicity) of an existing functional group due to a substituent at its ortho position. The former dictates regioselectivity in a reaction, while the latter alters the reactivity of the molecule itself.
5. Does the ortho effect apply to phenols as well? Explain with an example.
Yes, an effect is observed in ortho-substituted phenols, but the mechanism can differ. For instance, ortho-nitrophenol is more acidic than its para-isomer, but not due to steric hindrance. Instead, its increased acidity is because of the formation of a stable six-membered ring via intramolecular hydrogen bonding between the nitro group and the hydroxyl group. This stabilises the phenoxide ion (conjugate base) after the proton is released, making the parent phenol more acidic.
6. Are there any exceptions to the ortho effect in increasing the acidity of benzoic acids?
While the ortho effect almost always increases the acidity of substituted benzoic acids compared to their meta and para isomers, the underlying reason can vary. The primary exception is not in the outcome but in the mechanism. For ortho-substituents like -OH or -NH₂, which can form hydrogen bonds, the enhanced acidity is not just from steric hindrance but also from the strong stabilisation of the conjugate base through intramolecular hydrogen bonding. Therefore, while the acid is stronger, attributing it solely to the SIR model of the ortho effect would be incomplete.
7. Provide the correct order of basic strength for substituted anilines like toluidines and nitroanilines.
Due to a combination of electronic and ortho effects, the basic strength of substituted anilines follows specific trends. For example:
- Toluidines (with -CH₃ group): p-Toluidine > m-Toluidine > Aniline > o-Toluidine. Here, the ortho effect makes o-toluidine the weakest base in the series.
- Nitroanilines (with -NO₂ group): Aniline > m-Nitroaniline > p-Nitroaniline > o-Nitroaniline. Again, the ortho effect makes o-nitroaniline the least basic, even more so than the para-isomer which has a strong electron-withdrawing effect.
