Need help regarding hydrogen bonding

[Really Lost]

Hi

 

I learned something in organic chem I didn't quite understand. Normally I understand that interaction between a Hydrogen (say from a water molecule) and a Nitrogen is a hydrogen bonding due to the high electronegativity of the nitrogen. However when interacting with the nitrogen in acetonitrile the interaction was described as dipole dipole because possibly the nitrogen is triple bonded to a carbon atom.

 

Why is it a dipole dipole bond rather than hydrogen bonding? I realize it has to do with the triple bond. Does the fact that the Nitrogen is triple bonded lower its electronegavity? How does this work?

 

Thanks.

[hypervalent_iodine]

There is no difference between the two, since hydrogen bonding is essentially just a specific term for a particularly strong dipole type interaction.

[Really Lost]

Thanks for your response

 

The topic we were discussing was whether mixing Acetonitirile and Water was an exothermic or endothermic reaction. Such as when mixing the two we notice that the beaker is cold rather than warm.The logic for this was that is the heat of the seperate acetonitrile and water seperating "or break up" themselves is greater than when they are mixed together then it is an exothermic reaction. Such as there are dipole dipole bonds between the acetonitrile molecules that are constantly being broken when it is by itself and that these are dipole dipole interactions so there is less energy involved, water however has hydrogen bonds between its molecules so there is more heat/energy given off. When you mix the two there are only dipole dipole interaction between the water and acetonitrile molecules rather than hydrogen bonds so there is less heat/energy given off. This is supposed to explain why it is cooler after mixing and endothermic since the energy needed to break up the two is greater (because of the waters hydrogen bonds) than the energy released when mixing.

 

This is part of a lab for making a mobile phase for HPLC if this puts this into context.

 

I understand the hydrogen bonding and dipole dipole bonding are more or less the same but why/how does a triple bond change what should be a hydrogen bond to a dipole dipole bond so as to weaken the electronegativity?

Edited January 24, 2012 by Really Lost

[hypervalent_iodine]

Thanks for your response

 

The topic we were discussing was whether mixing Acetonitirile and Water was an exothermic or endothermic reaction. Such as when mixing the two we notice that the beaker is cold rather than warm.The logic for this was that is the heat of the seperate acetonitrile and water seperating "or break up" themselves is greater than when they are mixed together then it is an exothermic reaction. Such as there are dipole dipole bonds between the acetonitrile molecules that are constantly being broken when it is by itself and that these are dipole dipole interactions so there is less energy involved, water however has hydrogen bonds between its molecules so there is more heat/energy given off. When you mix the two there are only dipole dipole interaction between the water and acetonitrile molecules rather than hydrogen bonds so there is less heat/energy given off. This is supposed to explain why it is cooler after mixing and endothermic since the energy needed to break up the two is greater (because of the waters hydrogen bonds) than the energy released when mixing.

 

This is part of a lab for making a mobile phase for HPLC if this puts this into context.

 

Technically not a reaction, but it certainly is endothermic. Firstly, acetotnitrile doesn't have dipole interactions with itself because -CH bonds are not polar. Other than that, I'm a little ost with what you were saying. The reasons behind endothermic mixing are a little complex and the exact mechanism, not really known too well. Water-acetonitrile is an example of a binary mixture that deviates from ideality. The other one you encounter a lot is water-methanol, which shows exothermic mixing. It is thought to be explained mostly by entropic considerations, since endothermic processes are typically associated with an increase in entropy, as well as the clustering structures of water and how solvents interact with them.

 

I understand the hydrogen bonding and dipole dipole bonding are more or less the same but why/how does a triple bond change what should be a hydrogen bond to a dipole dipole bond.

 

It doesn't change anything. What changes whether you can call something a hydrogen bond or not is the atoms involved, which will change the properties of the interaction. Hydrogen bonding occurs between a hydrogen bound to some electronegative atom and the lone pair of electrons on either nitrogen, oxygen or fluorine. The strong electrostatic attraction between these atoms makes the distance between them (i.e. the length of the H-bond) smaller and the dissociation energies higher than a typical dipole bond.