percentage s character in orbitals

[SuperSaiyan3]

If there is a HIGHER percentage of s orbitals:

- sp : 50% s
- sp3: 25% s

for example,

is it correct to say that: "greater the character of s in an orbital, LESS STABLE the molecule"

I am confused because from one practice passage in Kaplan, it states that greater the s character, MORE stable, but in the other passage, it says otherwise.

Arguments:
- s: lower energy orbital
therefore greater the percentage of s orbital in a bond, greater the stability because lower the energy character

- but I also have written down:
greater the presence of s orbitals, greater the amount of pi orbitals. For example, sp (triple bond) orbital has 2 pi bonds. Pi bonds are significantly weaker than sigma bonds, therefore, it is more nucleophilic and less stable.

Both are reasonable arguments. But which one is right? Why am I contradicting myself? Is it Tuesday or Thursday? What year am I in?

??

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[Hemichordate]

If there is a HIGHER percentage of s orbitals:

- sp : 50% s
- sp3: 25% s

for example,

is it correct to say that: "greater the character of s in an orbital, LESS STABLE the molecule"

I am confused because from one practice passage in Kaplan, it states that greater the s character, MORE stable, but in the other passage, it says otherwise.

Arguments:
- s: lower energy orbital
therefore greater the percentage of s orbital in a bond, greater the stability because lower the energy character

- but I also have written down:
greater the presence of s orbitals, greater the amount of pi orbitals. For example, sp (triple bond) orbital has 2 pi bonds. Pi bonds are significantly weaker than sigma bonds, therefore, it is more nucleophilic and less stable.

Both are reasonable arguments. But which one is right? Why am I contradicting myself? Is it Tuesday or Thursday? What year am I in?

??

Greater s character = more stable

 

[SuperSaiyan3]

Greater s character = more stable

so you're saying that alkyne are more stable than alkanes?

I just googled it, and it says alkynes are LESS stable than alkenes, and alkenes are LESS stable than alkanes...

that doens't make sense.

 

[wanderer]

The C-C bond is more stable if there is greater s character because the 2 nuclei can be closer together, but the molecule as a whole is less stable because in order to go from an alkane to an alkene (or to an alkyne) sigma bonds have to be given up to form the weaker pi bonds. Pi bonds are weaker than sigma bonds because there is less overlap of the contributing atomic orbitals in pi bonds.

Alkanes are more stable than alkenes which are more stable than alkynes.

 

[SuperSaiyan3]

The C-C bond is more stable if there is greater s character because the 2 nuclei can be closer together

I understand that greater the percentage of s orbital, shorter the bonds.

why would the C - C bond be more stable if the 2 nuclei are closer together? wouldn't their electrostatic repulsions make them be LESS stable since they're so close?

 

[levodopa]

Alkanes are more stable than alkenes which are more stable than alkynes.

You can't really make a comparison between these molecule types as alkenes and alkynes are limited in that there is no free C-H rotation. I mean, it's a fact that

H2C=CH2 + H2 --> H3C-CH3 is more energetically downhill, but what have you to compare it to? S character alone does not define molecular stability, especially when the molecules are of entirely different natures.

Would anyone mind linking me to the source that says alkanes, alkenes, and alkynes are able to be compared? I really can't figure it out.

 

[matth87]

more s character generally means a shorter bond length and a stronger bond. I think you have to be comparing bonds within the same molecule because different molecules are going to exhibit resonance and inductive affects that will also effect bond stability.

 

[SuperSaiyan3]

The C-C bond is more stable if there is greater s character because the 2 nuclei can be closer together, but the molecule as a whole is less stable because in order to go from an alkane to an alkene (or to an alkyne) sigma bonds have to be given up to form the weaker pi bonds. Pi bonds are weaker than sigma bonds because there is less overlap of the contributing atomic orbitals in pi bonds.

Alkanes are more stable than alkenes which are more stable than alkynes.

I actually understand this explanation completely now. Thank you!

-. .S orbitals
.o. ..More stable
.o. ..Therefore more s orbital composition, greater the stability
.o. ..HOWEVER, alkyne < alkene < alkane
§. .Because less sigma bonds and more pi bonds (pi bonds are weaker)
§. .Therefore since less sigma bonds are participating, the stability OVERALL will be less.
·. .Sp orbital of ONE sigma bond is less stable than sp3 orbial of 3 sigma bonds
·. .50% x 1 = 50% S characteristic in alkyne

25% x 3 = 75% in alkane. Therefore, more S, therefore more stable.


Hope this explained it better for everybody else...