engineerd

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From a kaplan practice exam:
  1. 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?

    1. C6H6, HC≡CH, CH4

    2. H2C=CH2, CH4, C6H6

    3. C6H6, CH4, H2C=CH2

    4. HC≡CH, C6H6, CH4
I'm not if I'm interpreting the question correctly. So lowest energy level of the C-H bond is the weakest bond, right? So that would be the sp3 orbital. The highest energy level of the C-H is the strongest bond, which would be the sp orbital. So the answer should correspond to sp3, sp2, sp. But the answer here is D, which is sp, sp2, sp3. The explanation they offer is that since s is of lower energy than p, the greater the s characteristic, the lower the energy. Can someone explain?
 
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engineerd

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Yes, I understand that. But aren't triple bonds the strongest bonds, and they have sp orbitals? And single bonds are the weakest bonds, which have sp3 orbitals, right? Are bond strength and bond energy not the same thing?
 
Nov 13, 2012
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CH4 (methane) is sp3 hybridized, so lowest energy. The link above gives it as an example.

There is also an old post re: this question where it's mentioned that the concept of shorter bond length conveying increased strength/energy is dependent on comparing the same types of bonds, but all of the bonds in the question are nonpolar covalent (right?) so maybe someone else can clarify that...
 
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engineerd

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Ohh we're looking at C-H bonds, not C-C bonds. Nevermind, thank you!
 

Czarcasm

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Ohh we're looking at C-H bonds, not C-C bonds. Nevermind, thank you!
Regardless if you were considering C-C or C-H bonds, the same logic would apply. The strongest bonds are the MOST stable and LEAST reactive.

What you want to realize first is that s-orbital character is considerably stronger than p-orbital character. That's why sp hybridized orbitals are stronger than sp3 hybridized orbitals (due to s character).

When considering the sigma bond between a Carbon and a Hydrogen, in those three scenarios above, you have:

The C-H bond of a Benzene Ring: sp2 hybrid orbital - s orbital (50% s character, %50 p character)
The C-H bond of an Alkane: sp3 hybrid orbital - s orbital (40% s character, %60 p character) WEAKEST bond!
The C-H bond of an Alkyne: sp hybrid orbital - s orbital (66.66% s character, 33.33% p character) STRONGEST bond!

The strongest sigma bond (and therefore the most stable bond) is the bond with the most s-character.

Thus in terms of increasing stability: C-H Alkane (weakest) < C-H Benzene < C -H Alkyne (strongest)
In terms of increasing reactivity, it's just the opposite: C-H Alkyne (least reactive) < C-H Benzene < C-H Alkane (most reactive)

You can also look at things from the C-C bond perspective (focusing on the sigma bond in each case):

The C-C bond of a Benzene Ring: sp2 hybrid with an sp2 hybrid (33.33% s character)
The C-C bond of an Alkane: sp3 - sp3 (25% s character)
The C-C bond of an Alkyne: sp-sp (50% s character)

The same trend follows:

C-C bond of an Alkyne is stronger and more stable than the C-C bond of a Benzene Ring, which in turn is stronger than the C-C bond of an Alkane.
The reactivity is just the opposite, where Alkanes being the least stable of the three is the most reactive.

Keep in mind that we focused on the sigma bonds (above). The pi bond in (double and triple bonds) is composed of two p-orbitals and is considerably weaker than sigma bonds, so when we're making a comparison on bond strength, what we're really looking at is the sigma bonds.
 
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Mar 16, 2013
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The responses in this thread are pretty correct, but there still seems to be a bit of confusion around the problem, especially about the correlation between bond shortness and bond strength and bond dissociation energy and how that correlation seems to be at odds with the correct answer. Believe it or not, this confusion is the question's fault. This
Which choice correctly lists the energy level of the C-H bonds, from lowest to highest?
is a confusing use of the words "energy level." Inferring from the answer, what the question should say is
Which choice correctly lists the energy level of the C-H sigma bonding orbitals, from lowest to highest?
as, unlike orbitals, bonds cannot be represented on orbital diagrams by a single energy level. As such, there's little to prevent one from assuming that the phrase "energy level of the C-H bonds" refers to bond dissociation energy (BDE), an assumption which would not allow you to arrive at answer choice D, the correct choice.

As the BDE of a bond rises, bond length shrinks, and the bond strengthens. The same cannot be said for the energy level of the C-H bonding orbital.