Sciencemadness Discussion Board

Intramolecular Hydrogen bonding....

sankalpmittal - 12-1-2013 at 07:18

Why intramolecular hydrogen bonding decrease the boiling point and solubility in water ? Moreover , why this type of bonding increase the acidity of the compounds ?

smaerd - 12-1-2013 at 08:34

If a molecule exhibits hydrogen bonding with-in itself(intramolecular), then that site is less available to hydrogen bond with other molecules such as water molecules or other molecules of itself, but both of these interactions are much more rare because the intramolecular hydrogen bonding is a strong interaction and is close in proximity. There's less reason for it to hydrogen bond with anything other then itself.

So it would be less soluble in water then say a molecule that hydrogen bonds with water. Also because it won't be hydrogen bonding with other molecules around it(as the hydrogen bonding is interacting with-in itself) for it to boil it doesn't need to overcome the strength of the hydrogen bonds and can boil attracted to only itself.

[Edited on 12-1-2013 by smaerd]

sankalpmittal - 12-1-2013 at 08:51

Quote: Originally posted by smaerd  
If a molecule exhibits hydrogen bonding with-in itself(intramolecular), then that site is less available to hydrogen bond with other molecules such as water molecules or other molecules of itself, but both of these interactions are much more rare because the intramolecular hydrogen bonding is a strong interaction and is close in proximity. There's less reason for it to hydrogen bond with anything other then itself.

So it would be less soluble in water then say a molecule that hydrogen bonds with water. Also because it won't be hydrogen bonding with other molecules around it(as the hydrogen bonding is interacting with-in itself) for it to boil it doesn't need to overcome the strength of the hydrogen bonds and can boil attracted to only itself.

[Edited on 12-1-2013 by smaerd]


Thanks for your reply !!

So , in intramolecular hydrogen bonding , the hydrogen bond is in close proximity , so that the number of association sites decrease.

Understood..

Just one last question : You say that intramolecular hydrogen bonding is a strong interaction and is close in proximity. Then why is it weaker than intermolecular hydrogen bonding ?

smaerd - 12-1-2013 at 11:56

well the reason why these intramolecular force bound acids are more acidic is because the hydrogen involved in hydrogen bonding stabilizes the carboxylate ion. Think about the interacting hydrogen as a partial bond such as in a resonance hybrid. Forgot about that. Thing is there are many exceptions to this. It all depends on whether the inductive effects of a group are stronger or weaker then the hydrogen bonding occurring.

It may be weaker but it still exists to remove the pull from other molecules hydrogen bonding sites. At least that was my understanding of it.

sankalpmittal - 12-1-2013 at 20:52

Quote: Originally posted by smaerd  
well the reason why these intramolecular force bound acids are more acidic is because the hydrogen involved in hydrogen bonding stabilizes the carboxylate ion. Think about the interacting hydrogen as a partial bond such as in a resonance hybrid. Forgot about that. Thing is there are many exceptions to this. It all depends on whether the inductive effects of a group are stronger or weaker then the hydrogen bonding occurring.

It may be weaker but it still exists to remove the pull from other molecules hydrogen bonding sites. At least that was my understanding of it.


Thanks for your reply smaerd !!

I understood everything except ,

What make intramolecular hydrogen bond weaker than intermolecular hydrogen bond ?

sankalpmittal - 13-1-2013 at 23:17

P.S. , In my textbook its written that , the large acidity in ortho hydroxy benzenoic of benzene is due to intramolecular hydrogen bonding which stabilizes the salicyclate ion.
Thus, intramolecular hydrogen bonding is weaker than intermolecular hydrogen bonding.

How does it stabilize the ion ? Due to resonance ? If it stabilizes the ion , how can it be more acidic ? Moreover why weaker ?

Please help ! This seems too confusing...