TBR OChem Example 1.4

[EvenIf]

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I found a couple old threads on this question, but I am still confused. I thought alkyl groups are electron donating, which is why more substitution would lead to a stronger bond. However, the book says that "Bond a is stronger than bond c, despite both sharing an sp2-hybridized and an sp3-hybridized carbon, because bond c contains the more highly substituted carbon." Please help clarify!

 

[MedPR]

I found a couple old threads on this question, but I am still confused. I thought alkyl groups are electron donating, which is why more substitution would lead to a stronger bond. However, the book says that "Bond a is stronger than bond c, despite both sharing an sp2-hybridized and an sp3-hybridized carbon, because bond c contains the more highly substituted carbon." Please help clarify!

🙂 one of those threads was created by me! The more highly substituted carbon would be more stable if it were to break off from the main molecule (the same way a tert-butyl is more stable than a methyl). Higher stability = weaker bond.

 

[Meredith92]

Sorry just to clarify your answer- What do you mean by break off? Do you mean form a cation? A radical?

 

[Trayshawn]

Think of it this way. The two methyl groups on the end carbon in "bond c" add steric issues that prevent them from getting as close to the sp2-carbon as possible. The hydrogens on the end carbon in "bond a" have no such steric issue as there are only small hydrogens attached and so they can move closer to the sp2 carbon thus making a shorter (and stronger) bond.

 

[jw1985]

🙂 one of those threads was created by me! The more highly substituted carbon would be more stable if it were to break off from the main molecule (the same way a tert-butyl is more stable than a methyl). Higher stability = weaker bond.

Thank you!