Why does more alkyl groups make a bond weaker??

[pineappletree]

For example: the CH bond between
(CH3)3C-H Weaker
vs
CH3-H Stronger

I know that the electrons will be more stabilized if the H falls away from the tertiary carbon.

But I was wondering what the actual theory is~
Because the alkyl groups are electron donating, you would think that donating more electrons to the bond would make it stronger...

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[startswithb]

My first thought was that the carbon attached to the H has less electron density to donate to the C-H bond because the 3 alkyl groups are pulling some away from it.

I have only learned to consider electron donating/withdrawing groups on ring stabilization. I am not sure if that applies to all bonds. Can someone else comment on that?

 

[PiBond]

For example: the CH bond between
(CH3)3C-H Weaker
vs
CH3-H Stronger

I know that the electrons will be more stabilized if the H falls away from the tertiary carbon.

But I was wondering what the actual theory is~
Because the alkyl groups are electron donating, you would think that donating more electrons to the bond would make it stronger...

i think of it in terms of stability. if we were to cleave our C-H bond (homolytically) and generate a radical (which is electron deficient) then the alkyl groups would thus stabilize this species more so than if there weren't any alkyl groups. it comes down to which starting material would be more "okay" with losing an H. thus, the one who generates a more stable species has a weaker bond than the one who wouldn't be as stable.

 

[Don Draper]

For example: the CH bond between
(CH3)3C-H Weaker
vs
CH3-H Stronger

I know that the electrons will be more stabilized if the H falls away from the tertiary carbon.

But I was wondering what the actual theory is~
Because the alkyl groups are electron donating, you would think that donating more electrons to the bond would make it stronger...

As you were thinking, I believe this is because the stability of the carbocation. If you can donate more electrons to a carbocation, it is more stable and more likely to form - (CH3)3CH. If you can donate less electrons to a carbocation, it is less stable and less likely to form (CH4).

Bonds are weaker or stronger based upon how long or short they are. Not by having more or less electrons. If you donate a lone pair in resonance and gain some double bond character to a single bond, then yes the bond is stronger due to being shorter. But adding electrons via the inductive effect will not give double bond character to a single bond.

Think of it like this, carbon shares a bond with hydrogen because each want some electrons. Well, if I give carbon some extra electrons, is it more or less likely to care about hydrogen's electrons? Less likely to care, because someone is giving him electrons on the side.

 

[pineappletree]

Is this the only real thing I have to be concerned about??
The stability of the conjugate compound??


This would make sense as more EN molecules develop stronger bonds (they are more reactive conjugates)

 

[loveoforganic]

i think of it in terms of stability. if we were to cleave our C-H bond (homolytically) and generate a radical (which is electron deficient) then the alkyl groups would thus stabilize this species more so than if there weren't any alkyl groups. it comes down to which starting material would be more "okay" with losing an H. thus, the one who generates a more stable species has a weaker bond than the one who wouldn't be as stable.

That needed to be emphasized. Typically when we talk about bond strength, we think in terms of cleaving the bond into 2 radical species. The alkyl groups both donate electrons to the radical and contribute hyperconjugative stabilization via the C-H bonds.

 

[orgohacks]

That needed to be emphasized. Typically when we talk about bond strength, we think in terms of cleaving the bond into 2 radical species. The alkyl groups both donate electrons to the radical and contribute hyperconjugative stabilization via the C-H bonds.

Exactly.

 

[erimus]

This thread cleared up the exact confusion I was having. So following the same logic, would an electron withdrawing group cause the bond to be stronger?

 

[Teleologist]

Try Bent's rule.