lone pairs of electrons and hybridized orbitals

[glia25]

---

Members do not see this ad.

should lone pairs of electrons always be factored into hybridization? for example if we have NH3, is the N considered sp3 or sp2 hybridized

if this is not always the case, then when are they considered in hybridization and when are they not?

 

[Dr Gerrard]

The N is sp3 hybridized.

The O in H2O is sp3 hybridized.

Each lone pair is always considered in hybridization.

In CO2, each of the Os is sp2 hybridized, while the C is sp hybridized.

Don't know if this answers your questions, just thought examples might help.

 

[Chowdder]

I can't think of an instance off the top of my head where a lone pair would not be considered in part of the hybridization. Only one that come close would be when drawing the resonance of a molecule for example deprotonated phenol. You can draw the lone pair of O spread on the aromatic benzene, but the carbon on the benzene are still sp2 hybridized.

In your example NH3, N is considered sp3.

 

[ray keller]

**Bump** with some interesting examples of unhybridized lone pairs of electrons.

I think that this compound may cause some confusion. I know it has for me. It is called thiophene. c4h4n. easily looked up on wikipedia. Here, Sulfur has two pairs of lone electrons, much like oxygen in water. However, they are not both hybridized. This is evident in the fact that thiophene shows aromatic activity.

If both lone pairs were hybridized the Huckel number would be 8, thus anti-aromatic. This means that one lone pair is hybridized while the other is not. This makes the Huckel number 6, which is aromatic.

Another example is 1-Allylimidazole (IUPAC 1(prop-2-en-1-yl)1H-imidazole)
found http://www.chemblink.com/products/31410-01-2.htm.

In this compound, the lone pair of electrons is not hybridized. It instead remains unhybridized thus allowing it contribute to the aromaticity of the imidazole system.

I was thinking along the lines that when it is *convenient* (ie results in a more stable molecule, here having aromaticity) a lone pair of electron will not hybridize. Thoughts? Comments? Other examples?

 

[loveoforganic]

http://en.wikipedia.org/wiki/Furan

The oxygen analogue of thiophene is aromatic as well.

The hybridization of the lone pairs in both cases is 1) sp2 and 2) p

Even if both electron pairs were in p orbitals (structurally impossible given the connectivity, but roll with it), both furan and thiophene would still be aromatic. Remember your orbital geometry. Only one of those p orbitals could lay in the same plane as the pi bond p orbitals. The other would be perpendicular.

 

[chemtopper]

should lone pairs of electrons always be factored into hybridization? for example if we have NH3, is the N considered sp3 or sp2 hybridized

if this is not always the case, then when are they considered in hybridization and when are they not?

Lone pairs are always in hybridized orbitals .Central atom hybridized according to number of orbitals required to accommodate sigma bond pairs and lone pairs.Lone pairs can participate in delocalization and it can change the hybridization of the central atom.For example in thiophene two lone pairs are in sp3 hybrid orbitals and during delocalization only one pair is participating and the second pair is now in sp2 hybridized orbitals .In pyridine lone pair is in sp2 hybridized orbitals and can not participate in delocalization as pure p orbitals are perpendicular to the plane of the sp2 hybridized orbitals

 

[IMattI]

I was thinking along the lines that when it is *convenient* (ie results in a more stable molecule, here having aromaticity) a lone pair of electron will not hybridize. Thoughts? Comments? Other examples?

That's exactly correct.

If a molecule can become aromatic, it will.

If the sulfur atom hybridizes only one of it's lone pairs, then the remaining lone pair can complete the pi system of the molecule, and it will become aromatic, and therefore more stable.

 

[Trisphorin]

I think the only exception what is relevant to the MCAT is amide, where the nitrogen looks like its sp3 hybridized due to the lone pair and 3 bond but is actually sp2 hybridized because in its most stable resonance form, there is a double bond and two single bond making it a sp2 hybridized.