Is H2O electrophilic?

[HydrogenBond]

If you look at H2O, the highly electronegative O withdraws some of the electron density within its covalent bonds with hydrogen. The hydrogen is induced slightly positive and the oxygen becomes slightly negative. The oxygen would not have withdrawn this electron density, if this did not offer it some measure of stability, such as helping to complete the octet. This would suggest the negative side of the dipole is more stable than the positive side of the dipole, such that the hydrogen will have a higher potential. The net affect is that water should be slightly electrophilic.

 

An easier molecule to see this affect is HCl. This is a strong acid and a weak base. Although it has a dipole with equal and opposite charge, the affect of the positive aspect of the dipole is far more potent. The negative side of the dipole is stabilize because of the high electronegativity of the Cl. This stability makes it a very weak base. So the charge dipole does not reflect the true reactivity of the either the H or Cl, since they differ.

 

With water the chemical situation is similar in that one also has a highly electronegative atom in O, but the positive potential is spread over two H. The net results is that water should have a net electrophilic potential.

[Mercedes Benzene]

I seem to remember reading that water can sometimes be electrophilic, but it's usually classified as nucleophillic.

[HydrogenBond]

One set of observational data, which suggests that water is electrophilic, is to compare the corrosion rate of Fe (iron) in dry air to the corrosion rate of Fe in air when water is present. The water-O2 combination leads to higher corrosion rates. One way to look at this, is the water lowers the activation energy allowing the Fe + O2 reaction to go to products easier. This means the electrons of Fe are being pulled up the activation energy hill by the water, making it easier for the O2 to scope them for corrosion. This reasoning is easier to support with water being electrophilic.

 

Further data that supports this, is the affect of pH on corrosion rates of Fe in water. The corrosion rate will increase when the pH lowers (acid) and decrease when the pH increases (base). As such, to duplicate the corrosion rate of Fe in dry air, within water, one would need to increase the pH to get the same lower rates. This suggests the extra -OH is needed to compensate for the electrophilic potential of neutral water. This data suggests neutral water will have a slight electrophilic potential.

 

One could sort of deduce this by comparing electronegativity differences. The O, by being more electronegative is able to stablize the electrons, better, which should decrease the nucleophilic potential of the negative side of the dipole. The H is left with no way to stabilize its side of the dipole, such that its positive charge induction is left with higher potential. This electrophilic potential in the hydrogen, will then interact with the Fe, trying to share its electron density, making corrosion easier. The acid affect adds more hydrogen potential. While the base affect lowers it.

[HydrogenBond]

The existing understanding of hydrogen bonding needs to be modernized. To add to the analysis, I would like to look at the pH affect. The pH affect amounts to the strong covalent bonding within water, breaking and reforming, easily, using energetics at the level of hydrogen bonding. The electrophilic nature of water is the cause for the pH affect.

 

In neutral water H2O --> OH- + H3O+ is called the pH affect. The reason this occur is the oxygen of water is able to stabilize the electron density it shares with the hydrogen to form OH-. What should be nucleophilic, i.e., negative side of the H2O dipole, is acting electrophilic allowing the O to stabilize even more electron density. The O is able to go all the way to O-2, in some circumstances. The practical limit of -OH, in water, is due to the electrophilic potential of the H+, trying to compete beyond OH-.

 

When the OH- starts to form, this causes the hydrogen to gain even more electrophilic potential. The O lowers its electrophilic potential to form -OH passing this burden onto its H. The only thing available to lower the potential are unshared electrons on other water molecules, resulting in H3O+. The H+ moves around quite easily, being passed from water to water. The high electronegativity of O weakens the covalent bonding. In water, the hydrogen of water carries the net burden of potential. It tries to lower this potential by reforming H2O and then by hydrogen bonds.

 

Even with perfect hydrogen bonding, the H still carries potential. The way this can be understood, is hydrogen bonding displays partial covalent character. This covalent bonding allows the higher electronegativity of the oxygen to asserts itself once again, passing some of the electrophilic burden back to the H. The net result is the electrophilic potential will be minimized, but the higher electronegativity of O makes it the winner in the end. The H is never able to fully lower its electrophilic potential.

 

When ice forms water expands. The hydrogen is pushed away from the electron density core of the oxygen, as the O reasserts itself. The concept of electronegativity is important because it tells us who wants it most and in the end who will end up with the lions share of the electron density. The continuous potential of the H is important to the dynamics of life.

[freeztar]

In neutral water H2O --> OH- + H3O+ is called the pH affect.

 

Shouldn't that be "2H2O --> OH- + H3O+"?

 

When ice forms water expands. The hydrogen is pushed away from the electron density core of the oxygen, as the O reasserts itself. The concept of electronegativity is important because it tells us who wants it most and in the end who will end up with the lions share of the electron density. The continuous potential of the H is important to the dynamics of life.

 

I'm enjoying this thread because these are new ideas for me (I didn't take advanced chemistry). The way you describe these electron interactions and potentials makes it easy for a layman like myself to understand it. The delta density of the electrons are like a delicate game of tug-o-war[is this a valid analogy?]. :shrug:

 

I suppose knowing more about the "electro" side of these atomic interactions would help me clarify what is meant by electrophilic (electron loving) and electronegativity (electron equilibrium/stasis?).

[froster]

The delta density of the electrons are like a delicate game of tug-o-war[is this a valid analogy?]. :confused:

 

Interesting analogy. I could see it being that way, except one side/atom might always be "winning" (polarity), though never "wins"...well at least in covalent bonds. And for the other side to achieve a significant pull, it'd need more people, analogous to a neighbouring molecule inducing a force.

 

The OP's question is also very interesting. I think nucleophiles and electrophiles are terms in the context of reactions. Thus some component of a molecule might be electrophilic, but maybe it is just that much better of a nucleophile that it is termed and regarded as such. In this case, the electron-hungry H+ is the electrophilic component, but maybe the O's affinity for electrons is just that much stronger?

 

This topic is right on the money for what I am currently studying...2nd year organic chemistry.

[HydrogenBond]

To make water's electrophilic potential easier to see, I need to better explain the basis of electronegativity. In a loose sense, electronegativity compares how strong atoms hold onto their outer most electron. Fluorine is the strongest and is given a 4.0 (A: GPA) on a relative scale. Oxygen is real close with a B+ to A- (GPA).

 

When these comparision experiments begins, all the atoms have equal numbers of protons and electrons. On the surface, since positive and negative charges are all equal, for all the atoms, one might expect that all atoms should get the same score. It does not turn out that way, because the electrons are in orbitals. The various orbital shapes and the shape layering, influence the final binding potential of the outer most electron. In the cases of oxygen, its last electron sees a lot of binding power, whereas hydrogen, sees less. This make oxygen more electronegative.

 

The question becomes, how can orbital shapes and layering influence the affective EM binding power of the electrons, when all the atoms start out with equal numbers of positive and negative charges?

 

An electron has a negative charge. While an electron is a negative charge in motion. The motion of a charge generates a magnetic field. The most electronegative atoms use orbital layering that causes the most magnetic attraction between the electrons. This attraction compensates for the negative charge repulsion, causing the electrons to remain easier. If one took two wires, with opposite electron current, these two wires would attract due to the attraction between the opposite magnetic fields. This magnetic attraction brings more electrons together in the two wires. The electron in orbitals are loosely analogous to complex currents.

 

If we took an atom and measured the magnetic field it doesn't exist at the levels expected based on the high velocity of the electrons. The attracting magnetic fields create waves that essentially cancel, so we don't really see much in the way for magnetic output. For simplification, if we do an energy balance of two opposite spin electrons in an orbital, each should have its negative charge and a magnetic field associated with their spin and motion. Both of these EM affects contain energy potential. But with the magnetic waves cancelling, what we we end up with are two negative charges in motion, but without any apparent magnetic field. The new energy balance is only charge, so the potential energy is lower.

 

A highly electronegative atom like O, can go all the way to O-2. The magnetic cancelling gives off so much energy, that the energy potential due to the extra charge repulsion is compensated for. In other words, one is increasing the potential energy due to charge repulsion by adding two more electrons. But at the same time, one is decreasing the magnetic potential energy of all the electrons due to the waves cancelling. As long as the magnetic wave cancelling, gives off more energy, than the extra charge repulsion, the arrangment is stable. We still see the two extra negative charges, but without the expected magnetic potential energy.

 

When we look at H2O, the oxygen is trying to cancel out its magnetic waves to lower the magentic potential energy within all its electrons. The completing of the octet gives off some much energy it have little problem overcoming what should some extra negative charge repulsion. For the O to lose that extra negative charge, will mess up its magnetic cancelling.

 

The hydrogen is always trying to lower potential. The O doesn't mind if it helps stabilze the negative charge, but if it goes too far and tries to mess with its magnetic cancelling, the O will form a partial covalent bond and assert its higher electronegativity to restore minimal magnetic energy. The hydrogen is given a sweet spot, but can't get too greedy. The oxygen does not mind the H helping to stabillze the negative charge but don't touch its magnetic cancelling. The net result is H can never fully lower potential, so gets greedy and oxygen asserts its electronegativity.

 

In the cellular water, the hydrogen has more things to choose from. But the other things are also fighting back, keeping the H energized. Nature sort of put the H between a rock and a hard place. Life keeps on evolving in an attempt to help the H lower its perpetual potential.

 

A simple visual analogy for the battle between O and H in water is to picture a bunch of poker tables in a room, with reach containing one O and two H. The O is always the better poker player ,and wins the chips. The H, to stay in the game, tries to pick the pocket of an O in an adjacent table. Although this O lose chips, it continues to win them back from its own H. At the end of the night, the O always walks out of the game with the most chips, and H always goes home frustrated with deficit.

[HydrogenBond]

This discussion of the electrophilic potential in water was an introduction to what I will call hydrogen potential, which is the electrophilic potential created in a hydrogen atom, when a hydrogen atom is covalently bonded to a highly electronegative atom like O, N, Cl, F, etc., The hydrogen will attempt to form a hydrogen bond to lower this potential.

 

One of the basic applications of this analysis is the base pairing of the DNA double helix. Below are the two base pairs in DNA.

 

 

The most important thing to note is that the top base pair of thymine and adenine have two hydrogen bonds, but three (3) hydrogen capable of forming a hydrogen bond. While the bottom base pair of cytosine and guanine form three hydrogen bonds but have five (5) hydrogen capable of forming a hydrogen bond. These extra hydrogen make DNA electrophilic.

 

It had always been puzzled why the DNA, with all those negative phosphate groups, uses negatively charged mono-triphosphates to make RNA and DNA. One might expect all these negative charges repelling and therefore inhibiting the reactions on the DNA. The extra electrophilic potential within the DNA appears to act as a compensation.

 

There are some other theoretical uses for the extra hydrogen, beyond the advantages of their ability to store electrophilic potential. One possiblity is connected to the sounds of the DNA that I vaguely introduced in a biology topic, where the resonance structures of the bases can shift to form an equilibirum with the potential state of the hydrogen bonding. The hydrogen can also get involved by alternating between the double H or by just using one to form a single hydrogen bond.

 

 

Another theoretical possibility, that should be investigated, is whether the extra H allows communication along the vertical axis of the DNA. In other words, the extra H is not restricted to an H-bond, per se. If it can't alternate to lower potential, it still contains electrophilic potential, so may need to look vertically for possible electron density. It can't form a real hydrogen bond, but any lowering of potential is helpful.

 

If this was the case, the implication is that the DNA double helix would be sort of like a bio-wire that can conduct H based signals axially. This, in turn, would imply a way for an entire chromosome to stay in contact. Breaks in the DNA double helix due to transcription, would cause the transmission to subdivide. When the double helix is restored, the signal is reintegrated, telling the chromosome some useful data.

 

This last part is extreme extrapolation. What we know is that the DNA is electrophilic due to extra H, how the H deals with this, is theoretical.