BR O-CHEM- Bond Strength

[TheMightyBoosh]

So I understand that the inductive effect will determine the acidity of a proton. But does the inductive effect have anything to do with bond length/strength?

For instance, on example 1.4 in BR O-Chem they present a molecule and ask you to rank the bond-strengths of various carbon-carbon bonds. Between two sp2-sp3 C-C bonds, the bond that includes a less substituted carbon (methyl) is stronger than the bond that includes the more substituted carbon (sec propyl). Anyone know why that is? Is it because the secondary carbon is more electron deficient and therefore contributes less to the density to the bond?

Appreciate the help.

---

Members don't see this ad.

 

[orgohacks]

So I understand that the inductive effect will determine the acidity of a proton. But does the inductive effect have anything to do with bond length/strength?

For instance, on example 1.4 in BR O-Chem they present a molecule and ask you to rank the bond-strengths of various carbon-carbon bonds. Between two sp2-sp3 C-C bonds, the bond that includes a less substituted carbon (methyl) is stronger than the bond that includes the more substituted carbon (sec propyl). Anyone know why that is? Is it because the secondary carbon is more electron deficient and therefore contributes less to the density to the bond?

Appreciate the help.

I'd need to see the molecule to be sure, but when you're breaking a single C-C bond, the bond strength that is measured is the HOMOLYTIC cleavage (that is, going to two radicals). Since primary carbon radicals are less stable than secondary carbon radicals, their measured bond strengths are higher. Hope this helps - feel free to send me any further questions. James

 

[TheMightyBoosh]

Makes perfect sense. Thanks for the help

 

[Fifty 3rds]

Great explanation Orgohacks,

This is at least partly due to the inductive effect, correct?

My reasoning is: a methyl group is more electronegative then an alkyl group and would tend to have a greater inductive effect. It is a small difference, but this would account for some major and minor products forming.

I fear I have made it too simple. I know there is a difference between the thermodynamic product and the kinetic product, etc, I would just like to know if the reasoning above is sound.

 

[orgohacks]

Great explanation Orgohacks,

This is at least partly due to the inductive effect, correct?

My reasoning is: a methyl group is more electronegative then an alkyl group and would tend to have a greater inductive effect. It is a small difference, but this would account for some major and minor products forming.

I fear I have made it too simple. I know there is a difference between the thermodynamic product and the kinetic product, etc, I would just like to know if the reasoning above is sound.

The inductive effect is really important for sp vs. sp2 vs. sp3 C-H acidity (acetylene is a stronger acid than benzene which is a stronger acid than methane) as well as bond length/strength. So the C-CH3 bond length in toluene is going to be shorter than that for ethane based on electronegativity. For alkyl groups, not so much.

Sorry if this is confusing

 

[BerkReviewTeach]

The inductive effect is really important for sp vs. sp2 vs. sp3 C-H acidity (acetylene is a stronger acid than benzene which is a stronger acid than methane) as well as bond length/strength. So the C-CH3 bond length in toluene is going to be shorter than that for ethane based on electronegativity. For alkyl groups, not so much.

Sorry if this is confusing

Not confusing at all; EXCELLENT explanation.