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Topic: Are halogens hybridized?  (Read 19579 times)

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Offline stewie griffin

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Are halogens hybridized?
« on: December 02, 2009, 08:45:31 AM »
Ok so here's a modification of a question that has popped up in the undergrad gen chem section. I don't mean to cross-post, but I'd like to hear an explanation from practicing organic chemists and see what they would answer.
The question is this: Say we've got a saturated carbon and on this carbon is a halogen (let's just pick chlorine for fun). What orbitals are involved in the making of this sigma bond? After doing organic for years I naturally want to view everything as hybridized and so I immediately said sp3 hybrid of carbon with sp3 hybrid of chlorine. However, look in any gen chem book, and sigma bonds with halogens are always described as coming from the half-filled p orbital if the halogen. So the gen chem book would say the sigma bond results from overlap with the C's sp3 with the Cl's 2p orbital.
Can anyone provide an explanation for why we don't consider the chlorine a hybrid?? Wouldn't the lone pairs on the the halogen want to be as far away from each other as possible (and therefore the halogens should hybridize)?

Offline azmanam

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Offline stewie griffin

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Re: Are halogens hybridized?
« Reply #2 on: December 02, 2009, 09:28:13 AM »
I haven't looked at all of your links yet azmanan, but the third one ways the Cl is indeed sp3. Hmmmm... I'm getting confused  :)

Offline Borek

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Re: Are halogens hybridized?
« Reply #3 on: December 02, 2009, 09:43:18 AM »
My bet is that there is no "pure" sp3 at work here. If you look at angles between bonds in H2O, H2S and H2Se they are obviously getting closer to the right angle, even if sp3 with two lone pairs seems the most logical. So you may have atoms that are only "partially" hybridized.

Any hybridized orbitals are linear combination of unhybridized orbitals - nobody said that they have to be mixed in equal proportions, they just have to remain normalized (that is probability of finding electrons on orbitals before and after hybridization must be identical).
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Offline azmanam

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Re: Are halogens hybridized?
« Reply #4 on: December 02, 2009, 09:43:52 AM »
Cl-Cl bond is made from overlap of two unhybridized p-orbitals

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Offline philmont702a

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Re: Are halogens hybridized?
« Reply #5 on: December 02, 2009, 10:04:39 AM »
So, I looked this up in my org. III undergrad text (Hoffman's Organic Chem).  There he states that the interaction of a fluorine and a chlorine atom is the interaction of two p orbitals.  However, he goes on to say that second row elements, such as silicon, phosphorus and sulfur can hybridize, but they don't have to (I would add chlorine to this list).  The obvious question out of this statement is "When do second row elements hybridize?"  I doubt that this is a yes or no answer, rather, I believe that a trend is likely to emerge, where a mixing of completely hybridized and not hybridized is likely to occur. 

So, perhaps Chloromethane is tetrahedral sp3, but perhaps when bound to a different atom, say something with a larger radius from a larger row wouldn't require the hybridization to meet the Pauli exclusion principle.

Offline stewie griffin

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Re: Are halogens hybridized?
« Reply #6 on: December 02, 2009, 10:23:49 AM »
So I'd like to add some more to the mix. My undergrad orgo book (Bruice) doesn't touch this exact issue of chloride-carbon bonds, but it does state that for all of the hydrogen halides the sigma bond is formed from the H's s orbital and the halogen's sp3 orbital. The argument is that although the bond angle here doesn't give us any direct info regarding the orbitals involved in bonding (since it's obviously linear), it is best to consider the halogen as sp3 since the electron repulsion would be minimized.
Furthermore, from Grossman's "The Art of Writing Reasonable Organic Reaction Mechanisms"  he states:

"The hybridization of an atom is determined as follows. Hybrid orbitals are used to make sigma bonds and to hold lone pairs not used in resonance; p orbitals are used to make pi bonds and to hold lone pairs used in resonance, and they are used as empty orbitals. To determine the hybridization of an atom, add up the number of lone pairs not used in resonance and the number of sigma bonds (i.e. atoms to which it is bound). If the sum is four, the atom is sp3-hybridized. If the sum is three, it is sp2-hybridized. If the sum is two, it is sp-hybridized"

I guess that's the rule that I've been following which explains why I want to call the Chlorine sp3.

Offline jinclean

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Re: Are halogens hybridized?
« Reply #7 on: December 02, 2009, 12:50:16 PM »
My teacher says the hybirlization is a mathmatical method to form the module about the chemical bonds not the exactly being.so i think the Cl must use its s and p obital to form the bond,but is it is sp3 hybird,only some data can tell us the truth

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