# Why does hybridization significantly matter for organic chemistry?

I just recalled, and still see that almost every organic chemistry book starts with bonds, empahsizing hybridization. One thing is straight forward but the rest doesn't seem to connect well.

The thing that's clear is, energy level wise $\mathrm{sp < sp^2 < sp^3}$. When $\mathrm{p}$ character is higher, the energy is larger, the electrophilicity is higher and its affinity for reaction is high up as well. This is the opposite of what electrons constantly thriving to achieve : the lower energy state and be stable. However, hybridization allows molecules to have a shape minimizing the energy though. Through this bonding it also releases the energy (dissociation) by stabilizing itself - so bond formation is the tendency. Hybrids ($\mathrm{sp, sp^2, sp^3}$) form $\sigma$ bonds while pure-breeds (100% $\mathrm{s}$, 100% $\mathrm{p}$) form $\pi$ bond.

I had the impression hybridization regulates a standard bond among same species of atoms, e.g $\ce{C-H}$ bond in all alkanes are the same. But that's not the case when I read brief comments indicating that the $\ce{C-H}$ bond in $\ce{CH4}$ differ from $\ce{CH3-CH3}$.

What I am unable to reason is, why hybridization is so important for organic chemistry than to any other (general, inorganic..., physical)? Is it about bond strength (how fast one can bond and break) or is it about acidity or basity? Why should we (learners) or organic chemists care?

Please provide some application example. It's also great if one could point out how each $\ce{C-H}$ bond differ from alkane to alkane or any other functional group.

• Now that you ask, I don't really know why this is taught - it's more like problem of teaching system as differences in C-H bond strength aren't thaught well. Maybe because it's easy to tell about it. – Mithoron Jul 23 '15 at 12:46
• Are you certain that "pure breeds" which are 100% s form pi bonds? – Tan Yong Boon Jul 22 '18 at 23:17
• One reason why you may think that hybridisation is less significant in inorganic chemistry may be that inorganic chemistry is also made up of the study of much heavier elements, in which orbital hybridisation takes place at a much smaller extent due to larger energy gaps and size differences between the valence orbitals of different subshells. – Tan Yong Boon Jul 22 '18 at 23:20

Orbital hybridization is mostly useful in the rationalisation of molecular geometry. In fact it was invented by Linus Pauling for this purpose: to rationalise structures of organic molecules using the language of atomic orbitals. Yes, hybridization is also used to rationalise bonding properties, as well as acidity or basicity, but its primary use is to rationalise molecular geometries.

I had the impression hybridization regulates a standard bond among same species of atoms, e.g $\ce{C-H}$ bond in all alkanes are the same. But that's not the case when I read brief comments indicating that the $\ce{C-H}$ bond in $\ce{CH4}$ differ from $\ce{CH3-CH3}$.

Well, $\ce{C-H}$ bonds, of course, differ in $\ce{CH4}$ and $\ce{CH3-CH3}$, but not too much; they are approximately the same. Are you naive enough to hope that a model as simple as the orbital hybridization one would perfectly predict the molecular geometry of each and every organic molecule? Surely, it won't. It can just give a crude (but simple) approximation.

What I am unable to reason is, why hybridization is so important for organic chemistry than to any other (general, inorganic..., physical)?

Where did you get that? Hybridization is a concept of general chemistry, it is used in both organic and inorganic one. Its relatively big importance for organic chemistry is due to the fact that it is the only simple model which can explain (approximately) molecular geometry of organic compounds. The original valence bond theory couldn't do it, so it was extended.

## On explanation vs rationalisation

Now I would like to emphasize that hybridization provides a rationalisation of a molecular geometry (as well as some other properties), but not an explanation, so it can not be used to reliable predict the properties. What is the key difference between an explanation and a rationalisation in these context?

Well, an explanation of some phenomenon describes it in way that we can observe and test the consequences of our claims and premises. We can then tell from the observations are our claims and premises true or not, or, in other words, does our explanation work or not. The whole point of an explanation is to distinguish what is true from what is not true, in a sense, what corresponds to the actual reality from what does not do so.

In a rationalisation things work differently. We insist on some premises being true without testing them against the reality. We do observations, but we do not use them to tell are our premises true or not, rather on the basis of the desired premises and the observable evidence we start to make the observed outcome seem consistent with the premise.

In psychology rationalisation is also known as making excuses. It is a defence mechanism in which a controversial (from the society viewpoint) behaviour is justified "in a seemingly rational or logical manner to avoid the true explanation". With respect to scientific theories, it can also be thought of as a defence mechanism: when the real explanation is too complex for beginners we avoid it to defend them. Instead we first provide some simple rationalisation and later in the course of study the explanation is introduced.

The way rationalisation works for all these general chemistry models is as follows:

• We do know geometry or some other property of a particular molecule, say, from an experiment, and we are seeking for some rationalisation of why the things are the way they are.
• The rationalisation usually has to be as simple as possible, so that, for the case of molecular geometry, we usually start from VSEPR theory.
• If the simplest possible theory can not rationalise the observed property, we choose a more sophisticated model. For molecular geometry, for instance, we could then try to use the original valence bond (VB) theory.
• But even the VB theory (in its original formulation) won't work for organic compounds, so it has to be extended by two concepts: resonance and orbital hybridization. Note that this concepts are also required in inorganic chemistry as well, but not always: for some inorganic molecules the original VB theory is enough.

But however deep you go into this hierarchy of chemical models, all them are just rationalisation models: they provide simplified (and in many cases oversimplified) descriptions of the reality which is approximate (and in many cases rather crude). And besides all this process of seeking for an rationalisation is a bit like cheating:

• If some model doesn't work, let us try another model.
• If none of them works, let us introduce a new concept with some funky name in one of them, the purpose of which will be to rationalise this particular thing which couldn't be rationalised by the original model.

As a result, the predictive power of such models is quite small. They can be used to rationalise what is already known, but any predictions based on such models will be unreliable. And yeah, the concept of orbital hybridization is just one of these cheats mentioned above.

• Is it really only about molecular geometry? Doesn't hybridization play a role in each functional groups' reactivity? How does it affect the bond cleavage? Can you please give some examples. – bonCodigo Jul 23 '15 at 13:07
• @bonCodigo, to be honest, it is all about nothing. Orbital hybridization is an oversimplified model which has very little to do with the reality. It can be used here and there to explain some things, but there is very little generality in such explanations. – Wildcat Jul 23 '15 at 14:10
• @bonCodigo, if students realize that it is just a crude oversimplified model which can not explain everything, has a lot of exceptions and a very small predictive power, then it won't struck them. But if they naively hope that such simple model is the end of the story, then there are some bad news for them. – Wildcat Jul 25 '15 at 14:50
• @bonCodigo, grad students are involved in some scientific research and if in their research project they will try to blindly use such oversimplified models to obtain some data about molecular species under the study (geometries, bond strengths, etc.) they are guaranteed to have big problems. These simple general chemistry models can be used as a part of the so-called "chemical intuition" to quickly skip a lot of the details and possibilities, and concentrate on the most chemically possible things, but that is it. Then you need something less crude to do the actual job. – Wildcat Jul 25 '15 at 14:59
• @bonCodigo, both these theories are integral parts of organic chemistry, you can't just use a part of one of them (hybridization) to explain a particular property or a trend in it. Organic chemistry is not that simple. For some properties (like geometry) original VB theory + hybridization is all you usually need. For some other properties and some molecules you need also to consider resonance. For third properties we use MO picture, etc. There is a little generality, as I said, just read or at least skim over any textbook on organic chemistry (the whole book, not just the first chapters). – Wildcat Jul 25 '15 at 16:17