How does hydrogen bonding affect the boiling points of chemical compounds?

Question

My main question is why is the boiling point of methanol so much different from that of water?

I understand that both compound are able to develop hydrogen bonding, and obviously water can develop one more hydrogen bond than methanol.

But on the other hand:

Due to presence of an electron-releasing alkyl group, the oxygen atom of an alcohol molecule is less-electron withdrawing on the remaining hydrogen atom. Therefore the electron deficiency of the hydrogen atom is less than that of a water molecule resulting in the formation of a weaker hydrogen bond.

What does this actually mean?

Both hydrogen, in methanol and water, are connected to oxygen, which has the same electronegativity and takes electrons off the hydrogen making it have a partial positive charge.

Why does a "less electron withdrawing oxygen atom" lead to a weaker hydrogen bond?

I thought that a less electron withdrawing oxygen atom would mean that the H is more negatively charged, meaning it would be stronger for hydrogen bonding, since F, O, N want more electrons as they are more electronegative?

Answer 1

A hydrogen bond is a particularly strong dipole:dipole interaction with some covalent character. If we ignore the covalent character for a moment, we can say the higher the partial positive charge on the hydrogen, the stronger the hydrogen bond (keeping the O-H bond distance constant). If you relate "electron deficient" to positive partial charge, it might help you to make sense of the textbook answer.

You write:

Both hydrogen, in methanol and water, are connected to oxygen, which has the same electronegativity and takes electrons off the hydrogen making it have a partial positive charge.

Yes, that is the primary reason compounds like water, methanol and - say - hydrogen peroxide and acetic acid are hydrogen bond donors. But the chemistry beyond the OH group also plays a role. To illustrate, just compare the acidity of the proton connected to the oxygen in these four cases. They are very different. In a similar manner, the hydrogen bonding will be different (not as different, though).

Answer 2

See. First of all, your question is so much confusing that no one may get what you are saying or what are you confused of. I would simply answer directly to you main question. Consider the following points: 1. In methanol, the oxygen atom is directly attached to the methyl group. And we know that all alkyl group are electron releasing group.

2. This means that the methyl group attached to the Oxygen will increase the electron density on the oxygen atom, thereby decreasing the polarity of the O-H bond. (That is the partial positive charge on hydrogen atom will be lowered due to the presence of the methyl group)

3. Whereas in a water molecule, no methyl group is directly attached to the oxygen atom, so the polarity of the O-H bond will be greater as compared to that in methanol.(this means that the partial positive charge on hydrogen atom attached the oxygen atom will be greater as compared to that in methanol).

Hence in case of a water molecule, the hydrogen atom will be easily to removed. This is simply an answer that I wanted to give. Get knowledge from this and I hope that this will be helpful. Cheer up chemistry!!