What Is the Ortho Effect Definition Mechanism and Examples in Substituted Benzene Compounds
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 in Organic Chemistry Explained Clearly
1. What is the ortho effect in chemistry?
The ortho effect is the phenomenon in substituted benzoic acids where an ortho substituent increases the acidity of the acid, regardless of whether the substituent is electron-donating or electron-withdrawing. In simple terms, any group at the ortho position (adjacent to –COOH) makes the acid stronger than expected.
- Observed mainly in aromatic carboxylic acids like substituted benzoic acids.
- Results in lower pKa values compared to meta and para isomers.
- Attributed to steric, inductive, and intramolecular interactions.
2. Why does the ortho effect increase the acidity of benzoic acid derivatives?
The ortho effect increases acidity because steric hindrance and intramolecular interactions at the ortho position destabilize the neutral acid and stabilize the conjugate base. As a result, proton loss becomes easier.
- Steric hindrance twists the –COOH group out of plane, reducing resonance stabilization of the acid.
- Intramolecular hydrogen bonding may stabilize the conjugate base.
- Proximity effects enhance the effective electron-withdrawing influence.
3. What is an example of the ortho effect?
A classic example of the ortho effect is that o-nitrobenzoic acid is more acidic than m-nitrobenzoic acid and p-nitrobenzoic acid. Even though the nitro group (–NO2) is strongly electron-withdrawing in all positions, the ortho isomer shows unusually high acidity.
- Order of acidity: o-nitrobenzoic acid > p-nitrobenzoic acid > m-nitrobenzoic acid.
- This enhanced acidity is due to steric and proximity effects.
4. How is the ortho effect different from the inductive effect?
The ortho effect is a positional effect specific to ortho-substituted benzoic acids, while the inductive effect is a general electron-withdrawing or donating effect transmitted through sigma bonds. Key differences include:
- Ortho effect: Occurs regardless of the substituent’s nature (even electron-donating groups can increase acidity at ortho).
- Inductive effect: Depends on the electronegativity of the substituent.
- Ortho effect involves steric and proximity factors in addition to electronic effects.
5. Does the ortho effect apply only to benzoic acid?
The ortho effect primarily applies to substituted benzoic acids and related aromatic carboxylic acids. It is most clearly observed when a substituent is adjacent to the –COOH group on a benzene ring.
- Common in substituted benzoic acids.
- Less pronounced or absent in non-aromatic carboxylic acids.
- Requires close proximity between substituent and carboxyl group.
6. What causes the ortho effect in substituted benzoic acids?
The ortho effect is caused by steric hindrance, intramolecular hydrogen bonding, and proximity effects between the substituent and the carboxyl group. These combined factors alter acidity significantly.
- Steric strain reduces resonance stabilization of the acid.
- Intramolecular hydrogen bonding may stabilize the conjugate base.
- Field effects (through-space interactions) enhance electron withdrawal.
7. Why are ortho-substituted benzoic acids more acidic than meta and para isomers?
Ortho-substituted benzoic acids are more acidic because the substituent’s close proximity to the –COOH group enhances proton loss through steric and intramolecular interactions. In contrast, meta and para substituents mainly exert inductive or resonance effects.
- Ortho: Steric + inductive + field effects.
- Meta: Mostly inductive effect.
- Para: Inductive and resonance effects.
8. Can electron-donating groups show the ortho effect?
Yes, even electron-donating groups at the ortho position can increase acidity due to the ortho effect. Although such groups normally decrease acidity via resonance donation, their steric and proximity effects at ortho dominate.
- Example: o-methylbenzoic acid is more acidic than expected.
- Steric hindrance reduces resonance stabilization of –COOH.
9. What is the role of intramolecular hydrogen bonding in the ortho effect?
Intramolecular hydrogen bonding can stabilize the conjugate base of an ortho-substituted benzoic acid, thereby increasing its acidity. This stabilization makes proton loss more favorable.
- Occurs when a substituent like –OH or –NO2 is at the ortho position.
- Forms a hydrogen bond within the same molecule.
- Leads to a lower pKa value.
10. Is the ortho effect observed in electrophilic aromatic substitution reactions?
The ortho effect discussed in acidity is different from ortho–para directing effects in electrophilic aromatic substitution (EAS). While both involve the ortho position, they refer to different concepts.
- Ortho effect (acidity): Increased acidity of ortho-substituted benzoic acids.
- Ortho–para directing effect: Orientation of incoming electrophiles in EAS reactions.
