[sam12103]

1.   What are the electron-pair and molecular geometries? What orbitals on C, H, and Cl overlap to form bonds involving these elements?


Answer in the back of the book

Electron pair geometry= tetrahedral
Molecular geometry=tetrahedral

The H-C bonds are a result of the overlap of the hydrogen s orbital with sp^3 hybrid orbitals on carbon.
The Cl-C bonds are formed by the overlap of the sp^3 hybrid orbitals on carbon with the p orbitals on chlorine
(Lone pairs on the chlorine atoms have been omitted for the sake of clarity).


Can someone explain to me how the book came up with the answers for the second portion, I know how to find the Electron pair geometry and Molecular geometry, but I don’t understand in general how to find the orbitals that overlap to form bonds involving the elements.

Can you please explain this to me?

Thank you

[pear]

http://www.youtube.com/watch?v=PrNbhuB9W44

^-- good video




Cl is [Ne] 3s² 3p⁵
C is [He] 2s² 2p²

The LD-structure shows 3 Cl attached to the single C; C, then, needs to make 3 bonds.  Its natural orbitals allow for only 2 bonds, one with each 2p electron.  If we promote one e- from the 2s² orbitals to hybridize into sp³, 3 bonds may now be made.  Each of those pair-able electrons will do so as the 3p⁵ orbital overlaps the hybrid orbital.

[banjo]

Carbon will be the central atom.  The tricky part to me is knowing that only central atoms hybridize.  Expect single bonds for H and Cl almost always.  That means that the C will need four single bonds - sp³ hybridization.

To figure out which orbitals on H and Cl will overlap with carbon's sp³'s, then just look at the electron configs. as pear said.  H's is 1s¹ so hydrogen can form a bond with carbon by overlap of an s orbital on hyrogen with an sp³ orbital from C.  Each Cl has one 3p orbital with only 1 electron so each chlorine can form a covalent bond by overlapping a 3p orbital with one of the carbon's sp³'s.
 
             

[stewie griffin]

I'm surprised by the book's answer that the chlorine atom uses a p orbital for the sigma bond. I have always understood the chlorine to be hybridized to sp³ since it has a bond and three sets of lone pairs around it.
Can anyone set me straight here?

[banjo]

Only central atoms hybridize.  The atoms on the outside - Cl in this case - form bonds from overlap of atomic orbitals with hybridized orbitals on the central atom.

[stewie griffin]

Saying only central atoms will hybridize just doesn't make sense. Look at any organic structure that has more than one carbon... there is no "central atom" in these types of molecules. Each carbon will hybridize as will any nitrogens, oxygens, sulfurs, phosphorous, etc. So why not describe the halogens as hybridized as well?

[stewie griffin]

Well I guess I have to admit defeat here. I looked back in my old gen chem book and sure enough it describes the bonding in molecules such as BF₃, HgCl₂, and PF₃ as all having the halogen use it's p orbital rather than a hybrid. I guess after doing organic chemistry one gets used to thinking in terms of hybrids so much that you start describing everything as a hybrid.
What I don't get though is why not describe the halogens as hybrids? I don't see any reason why they can't be considered hybrids. My own answer to this, although it still doesn't really satisfy me, is Occam's razor... in other words why go through the trouble of hybridizing the halogen when we can get away with leaving it as is for the covalent bond.

[stewie griffin]

Like George Bush running against Al Gore, I'm going to have to withdraw my admission of defeat. After looking into it more, it seems this issue is more nuanced.
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"

Following Grossman's rule, the chloride should be considered sp³. Indeed several other websites and homework sets online agree that chlorine is sp³. However there are also textbooks that say in Cl₂ the bond is formed via the 2p orbitals.

[banjo]

Molecular orbital theory seems to be quoted quite often when it comes to covalent bonding.  Perhaps to deal with these tricky issues?