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. Explain Why is Aniline Less Basic than Methylamine?
As we know aniline resonates with a benzene ring that gives it a lone pair of electrons whereas methylamine does not have its lone pair of electrons delocalized. A compound's basic character is in donating electrons. But in the case of aniline, it is difficult as the lone pair of electrons is not easily accessible. Methylamines do not face this kind of issue. In other words, if the conjugate acid is stable then the corresponding base will be more basic. In methylamine, the conjugate acid is stabilized by a positive inductive effect from sp2 hybridized carbon. Since sp3 is less electronegative than sp2. Hence, aniline is less basic than methylamine.
2. What is the Ortho Effect? Why are Almost All Ortho-substituted Benzoic Acid Stronger Acid than Benzoic Acid?
Ortho effects happen when a group located in the ortho position concerning the carboxyl group generates steric hindrances or strain leading to the rotation of the carboxylic group and shifting it out of the surface of the benzene ring. The carboxylic group cannot actively participate in ring resonance and the acidity increases as the delocalization of negative charge on the conjugate base of the acid is improved, this is known as the ortho effect. For groups like -OH or -NH₂, in the ortho position, the steric hindrances do not compel the carboxyl group to rotate and shift out of the benzene ring. The conjugate base has chelation stability due to H-bonding leading to add more stability to the conjugate. Thus, increasing the acidity of the ortho-substituted carboxylic acid.
3. What is the role of the Ortho Effect in acidity?
The ortho substitute of benzoic acid is considered stronger than the benzoic acid, irrespective of whether the substituent we have is electron-withdrawing or electron releasing. This effect is also known as the ortho effect. This ortho effect, If we state it in simple words, is that a group attached in the ortho position irrespective of its nature always increases the acidic strength of the aromatic compound. The Ortho effect is one of the important effects you should memorize.
4. Where can I get notes for the ortho effect?
The Ortho effect is a tricky topic but it is also easy. You just need good notes that are specially designed by experts so that it can help you to understand the topic well. The Vedantu app provides you with such notes that are designed under the proper guidance of experts. At Vedantu website, we have PDF format to download. The main motive of the experts is to increase the knowledge of the students and along with it, their capability to deal with different types of problems. These notes will help you to increase your ability and score well in the examination. These notes, specially designed for your help, will make the ortho effect topic much clear to you.
5. What is the Ortho position in these compounds on which the ortho effect is based?
The Ortho effect is basically dependent on the position of the substituent on the ortho position of the compound. In organic chemistry, ortho position is referred to that position in which there are two same functional groups tied to the ring of benzene at positions 1 and 2. For indicating that in the compound the position is ortho position the abbreviation o- is used. For example, o- hydroquinone is 1,2-dihydroxybenzene. Here the abbreviation o- indicates that the compound is basically having an ortho-directing functional group in it.