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why are sp orbital bonds stronger than sp3 orbital bonds

why are sp orbital bonds stronger than sp3 orbital bonds, and lead to greater acidity?
Reply 1
I'm not entirely sure what you mean.

But what I think you're asking is. Why is the sigma bond created by an sp hybrid orbital, stronger than the sigma bond of an sp3 hybrid orbital. And there are a couple of ways of thinking about why this is. A nice way is thinking about which orbital is lower in energy. And as a result, which is going to create the sigma MO that is lower in energy. Once you figure that out you can figure out which is going to be stronger (Hint the lower energy MO is going to result in a stronger bond).

If that doesn't help and additional way is thinking about which aspect of the hybrid orbitals contributes most strongly to a sigma bond. Obviously this is going to be the s. An sp orbital has much more s charcater than an sp3 orbital. So it can overlap much more efficiently with another orbital to form sigma bonds, because a greater amount of the orbital has capacity to form them.
(edited 10 years ago)
Original post by limetang
I'm not entirely sure what you mean.

But what I think you're asking is. Why is the sigma bond created by an sp hybrid orbital, stronger than the sigma bond of an sp3 hybrid orbital. And there are a couple of ways of thinking about why this is. A nice way is thinking about which orbital is lower in energy. And as a result, which is going to create the sigma MO that is lower in energy. Once you figure that out you can figure out which is going to be stronger (Hint the lower energy MO is going to result in a stronger bond).

If that doesn't help and additional way is thinking about which aspect of the hybrid orbitals contributes most strongly to a sigma bond. Obviously this is going to be the s. An sp orbital has much more s charcater than an sp3 orbital. So it can overlap much more efficiently with another orbital to form sigma bonds, because a greater amount of the orbital has capacity to form them.


Is it because sp orbitals have more s character so have a lower energy level than sp3 orbitals, so are closer to the nucleus therefore the bond is shorter and stronger because of the increased attraction it feels from the nucleus. Also it's more acidic because of this as the bond with the proton would be stronger. Is this right?
Reply 3
Original post by celina10
Is it because sp orbitals have more s character so have a lower energy level than sp3 orbitals, so are closer to the nucleus therefore the bond is shorter and stronger because of the increased attraction it feels from the nucleus. Also it's more acidic because of this as the bond with the proton would be stronger. Is this right?


The first parts right. The second part about acidity is not. A stronger C-H bond in and of itself would actually make this less acidic. What has to be considered is the increased stability of the conjugate base a triple bond is electron withdrawing. This effect increases the stability of the conjugate base by 'spreading' electron charge more over the molecule.
Reply 4
Original post by celina10
Is it because sp orbitals have more s character so have a lower energy level than sp3 orbitals, so are closer to the nucleus therefore the bond is shorter and stronger because of the increased attraction it feels from the nucleus. Also it's more acidic because of this as the bond with the proton would be stronger. Is this right?


Re the increased acidity: as limetang said, the stability of the conjugate base A- is key to determining the acidity of HA. On deprotonation, we're left with a carbanion with a lone pair in some sort of orbital. Let's look at an example:

ethane (pKa >50)
ethene (pKa ~44)
ethyne (pKa ~25)

As we go from sp3 to sp hybridization, the orbital containing the lone pair in the conjugate base gets increasing s-character (from 25% to 50%). This means the orbital is more core-like and lower in energy. If the electrons are held in a lower-energy orbital, the conjugate base is more stable and the deprotonation is therefore more favourable.

Remember that with pKa we're looking at the thermodynamic balance between the acid and its conjugate base, so the fact that the C-H bond is stronger in ethyne than ethane doesn't matter.
Reply 5
Original post by limetang
I'm not entirely sure what you mean.

But what I think you're asking is. Why is the sigma bond created by an sp hybrid orbital, stronger than the sigma bond of an sp3 hybrid orbital. And there are a couple of ways of thinking about why this is. A nice way is thinking about which orbital is lower in energy. And as a result, which is going to create the sigma MO that is lower in energy. Once you figure that out you can figure out which is going to be stronger (Hint the lower energy MO is going to result in a stronger bond).

If that doesn't help and additional way is thinking about which aspect of the hybrid orbitals contributes most strongly to a sigma bond. Obviously this is going to be the s. An sp orbital has much more s charcater than an sp3 orbital. So it can overlap much more efficiently with another orbital to form sigma bonds, because a greater amount of the orbital has capacity to form them.

Then why is a s-s orbital weaker than the others?
Reply 6
Original post by limetang
I'm not entirely sure what you mean.

But what I think you're asking is. Why is the sigma bond created by an sp hybrid orbital, stronger than the sigma bond of an sp3 hybrid orbital. And there are a couple of ways of thinking about why this is. A nice way is thinking about which orbital is lower in energy. And as a result, which is going to create the sigma MO that is lower in energy. Once you figure that out you can figure out which is going to be stronger (Hint the lower energy MO is going to result in a stronger bond).

If that doesn't help and additional way is thinking about which aspect of the hybrid orbitals contributes most strongly to a sigma bond. Obviously this is going to be the s. An sp orbital has much more s charcater than an sp3 orbital. So it can overlap much more efficiently with another orbital to form sigma bonds, because a greater amount of the orbital has capacity to form them.

Then why is a s-s orbital weaker than the others?

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