Why are intramolecular hydrogen bonds weaker than intermolecular hydrogen bonds?
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1$\begingroup$ The premise of the question isn't true and OP didn't even mention why he would think so... $\endgroup$– MithoronCommented Feb 28, 2022 at 16:23
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$\begingroup$ @Mithoron I actually like the question because of its flaws and brevity. I could even see it in textbooks of a country presented as fact… (if you catch my drift). $\endgroup$– Martin - マーチン ♦Commented Mar 3, 2022 at 0:36
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$\begingroup$ @Martin-マーチン The books of that country don't state this as a fact, but many teachers keep making such nonsense statements. $\endgroup$– Nisarg BhavsarCommented Mar 3, 2022 at 3:52
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3 Answers
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It is mainly due to kinetic motion in molecules
The kinetic energy of molecules follows the Boltzmann distribution. Therefore, at room temperature molecules are constantly moving.
Intermolecular interaction, such as intermolecular hydrogen bonding only take place for a short time when two molecules are in each other's vicinity. Shortly after, the molecules separate and the interaction does no longer exist.
Intramolecular interactions on the other hand are much stronger since the two atoms forming the hydrogen bond remain in each others proximity due to structure of the molecule. In other words, they exhibit strain.
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Typically hydrogen bonds are between a hydrogen atom attached to an electronegative atom, $\ce{-XH}$, and another electronegative atom, $\ce{-Y}$: $$\ce{R^1\bond{-}XH\bond{...}Y-R^2}$$
It is mostly an electrostatic interaction, but there are orbital interactions, too. While the first is independent of direction, for the latter that is actually the crucial part.
In first order approximation the bonding in $\ce{-XH}$ is a σ-orbital with a corresponding σ*-orbital. These have their "densities" opposite of each other through hydrogen, hence their angle is $180°$. The other electronegative atom donates electron density into the σ*-orbital.
Therefore the best overlap is with a linear coordination around hydrogen.
However, orbital contribution is small even for strong hydrogen bonds.
Intermolecular hydrogen bonds typically achieve this arrangement more easily, while intramolecular hydrogen bonds are typically more bent.
You may find correlation with that. Correlation is not causation. There are many more factors at play, which must all be carefully considered.
If you take away any rule of thumb here, it should be: Strong hydrogen bonds are closer to a linear coordination around hydrogen.
answered Mar 3, 2022 at 0:33
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I don't think there is much of a difference between the strengths of intramolecular or intermolecular hydrogen bonds.
However the strength of interactions between molecules depends on the type of hydrogen bond.
Consider a typical example: ortho-Nitrophenol has a lower melting and boiling point than para-Nitrophenol.
The interactions between molecules are much stronger when there are intermolecular hydrogen bonds (as in para-Nitrophenol) because the bonds are formed between molecules. This results in more stability and higher melting and boiling points (which might appear to be because of stronger bonds) because the molecules prefer to be closer to each other.
If intramolecular hydrogen bonds are present (as in the case of ortho-Nitrophenol) the interactions within individual molecules are greater and so there is less attraction between molecules. This causes less stability and lower melting and boiling points because the molecules are not really attracted to each other much and not because the hydrogen bonds are weaker.
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$\begingroup$ My textbook said that. And also, can we say that due to "strain", intramolecular hydrogen bond is weaker ? $\endgroup$ Commented Mar 20, 2013 at 13:57
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11$\begingroup$ Intermolecular: More hydrogen bonds between molecules indeed increase stability, but it leads to higher melting/ boiling points, opposite to what you have stated. Intramolecular: Hydrogen bonds within a molecule lead to lesser interactions between molecules, hence the melting/ boiling points decrease. $\endgroup$– Martin - マーチン ♦Commented Jun 29, 2015 at 8:11
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$\begingroup$ @Martin-マーチン Do hydrogen bond within a molecule necessarily lead to fewer interactions between molecules? If so, could you please explain why? $\endgroup$ Commented Jun 5, 2021 at 18:08
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1$\begingroup$ @Wolgwang First one isn't great too, but hydrogen bonding, like in the second one, simply isn't possible. $\endgroup$– MithoronCommented Feb 28, 2022 at 17:38