TBR bond energy confusion

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On page 11 of berkeley review organic chemistry book, example 1.4, the correct answer is C "bond C is stronger than bond A, despite both sharing an sp2 and sp3 carbon, because bond C contains more highly substituted carbon"

However, on page 19,example 1.7 asks for the most stable bond, the correct answer is A,"an sp2-carbon to a primary sp3-carbon". But I think according to answer to example 1.4, "a sp2-carbon to a secondary sp3-carbon" seems more reasonable.

Any help?
Thanks

Originally Posted by andy96

On page 11 of berkeley review organic chemistry book, example 1.4, the correct answer is C "bond C is stronger than bond A, despite both sharing an sp2 and sp3 carbon, because bond C contains more highly substituted carbon"

However, on page 19,example 1.7 asks for the most stable bond, the correct answer is A,"an sp2-carbon to a primary sp3-carbon". But I think according to answer to example 1.4, "a sp2-carbon to a secondary sp3-carbon" seems more reasonable.

Any help?
Thanks

The question on page 19 tells you to refer to a table, not to past knowledge or knowledge of a specific case. Based on the table alone, an sp2-primary sp3 bond is the most stable. On page 11 the problem is not involving a vinyl group (CH2=CH-R), it is involving a tetra-substituted alkene. The reason bond c is weaker than bond a is because if you cleaved bond a you would end up with a methyl carbocation, which is much less stable than the isopropyl carbocation you would get if you cleaved bond c.

Originally Posted by MedPR

The question on page 19 tells you to refer to a table, not to past knowledge or knowledge of a specific case. Based on the table alone, an sp2-primary sp3 bond is the most stable. On page 11 the problem is not involving a vinyl group (CH2=CH-R), it is involving a tetra-substituted alkene. The reason bond c is weaker than bond a is because if you cleaved bond a you would end up with a methyl carbocation, which is much less stable than the isopropyl carbocation you would get if you cleaved bond c.

but bond c is stronger than bond a that's what they have as answer, it said

"Bond c is stronger than bond a, despite both sharinf an sp2 and an sp3 carbon, because bond c contains the more highly substituted carbon."

My question is by looking at

H3C-C=C-R the hydrogen have a very low electron pulling tendency compare to

(H3C)2HC-C=C-R in which the two C3H have the tendency to pull electrons more which makes the C-C bond weaker and easy to dissociate (theefore less energy required to break the bond =weakerbond)
But this doesn't match my predictions. Any Help?

Originally Posted by andy96

On page 11 of berkeley review organic chemistry book, example 1.4, the correct answer is C "bond C is stronger than bond A, despite both sharing an sp2 and sp3 carbon, because bond C contains more highly substituted carbon"

However, on page 19,example 1.7 asks for the most stable bond, the correct answer is A,"an sp2-carbon to a primary sp3-carbon". But I think according to answer to example 1.4, "a sp2-carbon to a secondary sp3-carbon" seems more reasonable.

Any help?
Thanks

Refer back to the question. It states the correct answer is B - "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."

My book is 2012...check your version i guess

That make sense...thanks

Originally Posted by Golaso

That make sense...thanks

np

yah I guess alkyl groups donate electrons which makes it easy to disociate (less energy)

On page 11 of berkeley review organic chemistry book, example 1.4, the correct answer is C "bond C is stronger than bond A, despite both sharing an sp2 and sp3 carbon, because bond C contains more highly substituted carbon"

However, on page 19,example 1.7 asks for the most stable bond, the correct answer is A,"an sp2-carbon to a primary sp3-carbon". But I think according to answer to example 1.4, "a sp2-carbon to a secondary sp3-carbon" seems more reasonable.

Any help?
Thanks

It should be noted that a substituted carbocation is more stable than a primary carbocation.
And considering we base a lot of our relative bond strengths on the stability of the reaction products, it would be easier to break a more substituted C-C bond.

I'm not exactly sure why this is in atomic terms, but perhaps extra alkyl groups donate electrons and make the bond easier to break?

The exception to this is a C=C double or triple bond, where hyperconjugation from extra alkyl groups stabilizes the pi bonds.