The three types of hybrid orbitals we have seen are sp, sp2 and sp3.
|Hybrid Type||Angle between orbitals||Arrangement||Example||C-C Bond Strength (kJ mol-1)||C-C Bond Length (Å)|
|sp3||109° 28′||Tetrahedral||Ethane (C2H6)||376||1.54|
|sp2||120°||Trigonal planar||Ethene (C2H4)||611||1.33|
As you might expect, the more electrons involved in the bonding, the shorter and stronger the bond becomes.
Also, although our attention has so far been focused on the C-C bond in the examples above, we must not forget that the C-H bonds will also be affected. They are formed from overlap of the hydrogen atom’s 1s orbital with a hybrid orbital from the carbon atom. The greater the amount of s character (sp>sp2>sp3), or the less the amount of p character, which ever you prefer, the more like an s orbital, and the less like a p orbital the resultant hybrid will be.
This means that, for example, the C-H bond length in ethyne is shorter than in ethane. For purposes of comparison, the three types of hybrids can be imagined as looking like this (remember this is not a mathematically correct model of the orbital function; merely a depiction):
We can clearly see here that the hybrids become more spherical, and hence less elongated (resulting in shorter bonds), as the s character starts to dominate.
|Molecule||C-H bond length (Å)||C-H bond strength (kJ mol-1)|
One further example of an observable trend due to the different properties of the hybrid orbitals is the acidity of the compounds above.
It can be generally noted that the acidity increases in the order sp3<sp2<sp. Of course, none of the compounds above is traditionally noted for its acidity, however, they do have a measurable pKa, and it is found that ethyne is the most acidic out of these molecules.
The reason for this is that despite the higher C-H bond energy (which we would suppose would make the donation of H+ less likely), the factor that most affects the acidity of these compounds is the stability of the conjugate base formed, and the most important factor here is how closely the negative charge is held to the (positively charged) nucleus. Clearly, the closer it is held, the more stable the conjugate base.
The type of orbital that places the electrons closest to the nucleus is the one with the most s character (s orbitals in fact penetrate the nucleus, whereas p orbitals have zero probability of being found there – the radial node). Hence ethyne is the most acidic, as the negative charge on its conjugate base is localised in an sp orbital.