[Chemistry Class Notes] on Ortho Effect Pdf for Exam

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

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.