Relative stability of ortho and para isomers

In summary: The para product is less stable because it has a resonance structure that is less stabilized than the ortho product.
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Generally, in organic reactions, the para isomer is found to be more stable (as it is symmetrical) because of which it is produced in greater amount than the ortho isomer. However, there are some exceptions also, for example, in some reactions the less sterically hindered ortho isomer is produced in more amount than the corresponding para isomer.

If all this is true, then,

1) In Kolbe's reaction (where phenol is first treated with NaOH and then with CO2), why is ortho isomer of Hydroxybenzoic acid (salicylic acid) - which is 2-Hydroxybenzoic acid produced in greater amount than 4-Hydroxybenzoic acid?

There's absolutely no steric hindrance present at the para position!

2)Also, when phenol is treated with dilute HNO3 at 293K, para isomer (p-Nitrophenol) is produced in greater amounts than o-Nitrophenol. Why?

PS - I'm a high school student, so please answer accordingly.

Thanks in advance!
 
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  • #2
Prashasti said:
PS - I'm a high school student, so please answer accordingly.
Has there been any discussion in class or textbook regarding "electron withdrawing groups" or "electronegativity" or "resonance structure/stabilization?"
 
  • #3
Prashasti said:
Generally, in organic reactions, the para isomer is found to be more stable (as it is symmetrical) because of which it is produced in greater amount than the ortho isomer. However, there are some exceptions also, for example, in some reactions the less sterically hindered ortho isomer is produced in more amount than the corresponding para isomer.

If all this is true, then,

1) In Kolbe's reaction (where phenol is first treated with NaOH and then with CO2), why is ortho isomer of Hydroxybenzoic acid (salicylic acid) - which is 2-Hydroxybenzoic acid produced in greater amount than 4-Hydroxybenzoic acid?

There's absolutely no steric hindrance present at the para position!

2)Also, when phenol is treated with dilute HNO3 at 293K, para isomer (p-Nitrophenol) is produced in greater amounts than o-Nitrophenol. Why?

PS - I'm a high school student, so please answer accordingly.

Thanks in advance!

You might try Googling about kinetic and thermodynamic control of reactivity. "Stability" usually refers to thermodynamics. You can produce less thermodynamically favorable products if a reaction is "kinetically" controlled -- generally means the energy landscape is such that you cannot get to the lowest energy product.

For example, suppose you know that para amino benzoic acid is more stable than ortho amino benzoic acid. (I don't know if this is the case, but suppose it is). Now suppose that you start with an ortho ester of amino benozic acid. If you hydrolyze the ester, you will form the ortho product. Why don't you form the more stable para amino benzoic acid?
 
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1. What is the difference between ortho and para isomers?

Ortho and para isomers are two types of isomers that have the same chemical formula but differ in the arrangement of atoms in their molecular structure. In ortho isomers, the substituent groups are attached to the adjacent carbon atoms, while in para isomers, the substituent groups are attached to the opposite carbon atoms.

2. Why do ortho and para isomers have different relative stabilities?

The relative stability of ortho and para isomers is determined by the interactions between the substituent groups. In ortho isomers, the substituent groups are close to each other, leading to strong repulsive interactions. On the other hand, in para isomers, the substituent groups are far apart, reducing the repulsive interactions and increasing the stability of the molecule.

3. How does the size of the substituent groups affect the relative stability of ortho and para isomers?

The size of the substituent groups can have a significant impact on the relative stability of ortho and para isomers. Larger substituent groups can cause steric hindrance and increase the repulsive interactions in ortho isomers, making them less stable compared to para isomers with smaller substituent groups.

4. What is the significance of the relative stability of ortho and para isomers?

The relative stability of ortho and para isomers can affect the physical and chemical properties of a compound. For example, a more stable isomer may have a higher melting point or boiling point, and may also react differently in certain chemical reactions.

5. How can the relative stability of ortho and para isomers be determined experimentally?

The relative stability of ortho and para isomers can be determined by measuring their energies using techniques such as nuclear magnetic resonance (NMR) spectroscopy or gas chromatography. These methods can provide information on the spatial arrangement of the substituent groups and their interactions, which can help determine the relative stability of the isomers.

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