Question

Give a brief description about ortho effect with example. (?)

Answer 1

Here's what I find.

The ortho effect

The ortho effect is the increase in the acidity of an ortho-substituted benzoic acid compared to its meta- and para-substituted isomers.

The effect occurs whether the group is electron-withdrawing or electron-donating.

For example, the `"p"K_text(a)` values of the nitrobenzoic acids are in the order

`o"-NO"_2 < p"-NO"_2 < m"-NO"_2" < benzoic"`
`color(white)(ll) 2.16 color(white)(l)< color(white)(ll) 3.41color(white)(ll) < color(white)(ll)3.47color(white)(m) < color(white)(m)4.19`

m- and p-nitrobenzoic acid are about 20 times as acidic as benzoic acid, but an o-nitro group increases the acidity 100-fold.

Similarly, the `"p"K_text(a)` values of the methylbenzoic acids are in the order

`o"-CH"_3 < "benzoic" < p"-CH"_3 < m"-CH"_3`
`color(white)(ll) 3.91 color(white)(l)< color(white)(ll) 4.19color(white)(ll) < color(white)(ll)4.27color(white)(m) < color(white)(m)4.36`

m- and p-toluic acid are about one-third less acidic than benzoic acid, but o-toluic acid is 50 % more acidic than benzoic acid.

The explanation

There is no single explanation. The ortho effect is a complex mixture of inductive, resonance, and steric effects, and different substituents.

Inductive and resonance effects are the most important factors in determining acidity. However, except for the most polar substituents, they roughly similar in the ortho and para positions.

Steric effects are strong but roughly the same in the acid and the anion. Except for very bulky substituents, they have little effect on the acidity.

Recent research indicates that steric effects are mostly van der Waals effects.

Electron repulsions cause the carboxyl group to twist out of the plane of the ring.

This decreases resonance contributions from the ring and increases the acidity.