Hydrogen Bond Not a Covalent Bond

[SaintJude]

Kaplan:

Hydrogen bonds are not considered covalent bonds because they do not involve sharing electrons.

Well then what do they share? Is this an electrostatic interaction?

Back to the basics....

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[Buttafuoco]

I guess all bonds are electrostatic in the sense that you can make a classical approximation where the protons and electrons on a given pair of atoms attract each other.

H-bonds are basically strong dipole-dipole interactions. Generally the types of bonds/interactions from strongest to weakest (and this is pretty intuitive) are:

ionic
covalent
ion-dipole
dipole-dipole
dipole-induced dipole
random dipole-induced dipole (classic london forces like you see between noble gases)

H-bonds are really strong dipole-dipole intermolecular attractions, but they're nowhere near as strong as a full on intramolecular covalent bond.

 

[MedPR]

I don't know if this is true, but it worked out for me. For the MCAT, the only covalent bonds are intramolecular bonds. Everything else is some other kind of bond that didn't have a general name (H-bond, van der waal, ionic, etc).

 

[thestrokes14]

Covalent bonds are intramolecular, meaning they exist WITHIN a molecule. They typically have high bond energies.

Hydrogen bonds are intermolecular, meaning they exist BETWEEN molecules. These interactions are electrostatic, but are due to a partial negative charge interacting with a partial positive charge, each due to dipole moments in respective molecules. As was mentioned, Hydrogen bonding is basically a strong form of a dipole-dipole interaction.

But yes, there is not sharing of electrons in Hydrogen bonding. It is purely an electrostatic force likely dictated by Coulomb's equation.

 

[chaser0]

Eek i didnt know ionic bonds were stronger than covalent bonds.

How come it just falls apart in polar solutions??

 

[syoung]

Eek i didnt know ionic bonds were stronger than covalent bonds.

How come it just falls apart in polar solutions??

they are solvated by the water molecules. each ion can be surrounded by 2-4 water molecules, thus spreading out the charge. Remember water can be more partial positive on its H side, thereby solvating the Cl- ion and water can be more partial negative on the O side and assist solvating the Na+ ion

 

[gettheleadout]

I guess all bonds are electrostatic in the sense that you can make a classical approximation where the protons and electrons on a given pair of atoms attract each other.

H-bonds are basically strong dipole-dipole interactions. Generally the types of bonds/interactions from strongest to weakest (and this is pretty intuitive) are:

ionic
covalent
ion-dipole
dipole-dipole
dipole-induced dipole
random dipole-induced dipole (classic london forces like you see between noble gases)

H-bonds are really strong dipole-dipole intermolecular attractions, but they're nowhere near as strong as a full on intramolecular covalent bond.

Eek i didnt know ionic bonds were stronger than covalent bonds.

How come it just falls apart in polar solutions??

Ionic bonds are, in general, weaker than covalent bonds, not stronger. The specific bond energy can vary based on the compound, but in general, covalent bonds are stronger.

http://people.seas.harvard.edu/~jones/es154/lectures/lecture_2/covalent_bond/covalent_bond.html

 

[Buttafuoco]

I think they're playing kind of fast and loose there. You certainly see a lot of weak ionic bonds and strong covalent bonds in nature since there are so many big, bulky metals and a bunch of first row elements forming covalent bonds, but if you could hypothetically construct two atoms where first they have the same number of protons and are equally sharing electrons, then you give one of the protons of one atom to the second so that they have a charge difference of 1, the bond strength increases.

 

[gettheleadout]

I think they're playing kind of fast and loose there. You certainly see a lot of weak ionic bonds and strong covalent bonds in nature since there are so many big, bulky metals and a bunch of first row elements forming covalent bonds, but if you could hypothetically construct two atoms where first they have the same number of protons and are equally sharing electrons, then you give one of the protons of one atom to the second so that they have a charge difference of 1, the bond strength increases.

Source?

 

[chaser0]

I think they're playing kind of fast and loose there. You certainly see a lot of weak ionic bonds and strong covalent bonds in nature since there are so many big, bulky metals and a bunch of first row elements forming covalent bonds, but if you could hypothetically construct two atoms where first they have the same number of protons and are equally sharing electrons, then you give one of the protons of one atom to the second so that they have a charge difference of 1, the bond strength increases.

yeah, I think we'd need to see a source on this claim
You could hypothetically construct any kind of bond with any kind of strength... But we are referring to MCAT relationships don't forget.

 

[Buttafuoco]

You need to compare some bond strengths. Here are some sigma bond energies of H to first row element bonds followed by their approximate bond dissociation energies in KJ/mol and C to first row elements.

H-C 411
H-N 386
H-O 459
H-F 565

C-C 350
C-N 310
C-O 360
C-F 490

Notice the trend that as the bond becomes more ionic in character, it gets stronger. Maddeningly, N defies the trend, which I believe is due to quantum effects if I am remembering correctly (it contains 3 unpaired P electrons in its neutral state and is thus is relatively "happy" / low energy without achieving an octet), which is one of those annoying things that makes gen chem a pain in the ass.


http://www.science.uwaterloo.ca/~cchieh/cact/c120/bondel.html

But I believe that if you get a standalone question about which is the strongest type of bond on the MCAT, ionic is going to be the correct choice.