bond length and strength

[inaccensa]

Molecular orbitals in hydrocarbons are formed
between the 1s atomic orbital of hydrogen and the sp,
sp2, or sp3 hybrid atomic orbitals of carbon. Which
choice correctly lists the energy level of the C-H
bonds, from lowest to highest?
A. C6H6, HC≡CH, CH4
B. H2C=CH2, CH4, C6H6
C. C6H6, CH4, H2C=CH2
D. HC≡CH, C6H6, CH4


Here they are asking for the strength of C-H and not C-C, but since we are just talking about hybridized molecular orbitals, won't the trend go CH4< C6H6<HC&#8801;CH

C-C is longer bond, less strength and is easy to break

Kaplan says that the more S character, the less energy, which seems counter-intuitive since EK says as bond length increases, the bond strength and energy both decrease. I know that triple bonds are stronger than single bonds and are at a higher energy level. Can someone plzz explain!!

---

Members don't see this ad.

 

[zmatrix]

The orbital pictures for hybridization should look like:

2p: _ _ _
=> 2sp3: _ _ _ _

2s: _

Notice how 2sp3 orbitals are closer to 2p energy levels because they are 75% 2p orbital and only 25% 2s orbital.

2sp2 orbitals are lower energy because they are 66% 2p orbital.

2sp orbitals are half way between 2p energy and 2s energy because they're 50/50

 

[loveoforganic]

The reason s orbitals are lower energy than p orbitals is because they allow closer proximity of the electron to the nucleus.

 

[inaccensa]

The orbital pictures for hybridization should look like:

2p: _ _ _
=> 2sp3: _ _ _ _

2s: _

Notice how 2sp3 orbitals are closer to 2p energy levels because they are 75% 2p orbital and only 25% 2s orbital.

2sp2 orbitals are lower energy because they are 66% 2p orbital.

2sp orbitals are half way between 2p energy and 2s energy because they're 50/50

So are we saying that the energy of sp3>sp2>sp ??

 

[eric51]

So are we saying that the energy of sp3>sp2>sp ??

Yes. An sp hybrid orbital will always be lower energy than an sp3 orbital because more s character = lower energy.

 

[zmatrix]

2p: _ _ _
=> 2sp3: _ _ _ _

2s: _

So are we saying that the energy of sp3>sp2>sp

Yes, the vertical axis of the above diagram is energy

 

[inaccensa]

Yes, the vertical axis of the above diagram is energy

So the energy of sp<sp2<sp3. In that case, the stability of the bond should be sp3<sp2<sp. However the strength of the bonds follows sp>sp2>sp3. Overall the stabillity follows alkanes >alkenes>alkynes


So high energy = higher strength = lower stability
More S character lower energy

But EK says as bond length increases, the bond strength and energy both decrease.

 

[inaccensa]

anyone want to verify the last post?

 

[loveoforganic]

EK's statement assumes you're talking about the same type of bond.

 

[Rabolisk]

The stability of the bond is sp > sp2 > sp3. High strength = shorter bond length = higher stability = higher An alkyne is more stable than an alkane, although the pi bond of the alkyne can react rather easily.