Couple of TBR Orgo Questions Have Me PERPLEXED

[neelyboy]

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Page 19, example 1.7. Why is an sp2-carbon bonded to a primary sp3-carbon more stable than, say, a secondary sp3-carbon bonded to a primary sp3-carbon?

Here's my thought process:
ok alkenes = more reactive than alkanes right?
and reactivity and stability are inversely proportional
right?
I thought the sp3-sp3 would be more stable, since alkenes are more reactive and thus less stable. so why the eff is sp2 to sp3 more stable than, eg, sp3-sp3.
The solution says the most stable bond is the strongest bond
and the most stable is the one that requires the greatest energy to break
basically i thought alkenes were more reactive...thus arent they less stable and not as strong??

ALSO
example 1.4, page 11. why does an sp3 primary carbon have a greater bond strength than an sp3 secondary carbon, when each of the carbons are bonded to an sp2 carbon??

 

[Catburr]

Why do you say alkenes are more reactive than alkanes? Is it because you can think of more ochem reactions involving alkenes, or is there some other reason?

The C=C double bond requires more energy to break than does a C-C single bond. Like it says, the most stable bond is the one with the highest bond dissociation energy. That's the best way to think of bond stability.

For example 1.4, I can't think of another way to explain it besides what the solution has. Double bonds are stronger than any single bond. Single bonds become weaker (easier to break) as the carbons in the bond become more substituted.

 

[neelyboy]

Why do you say alkenes are more reactive than alkanes? Is it because you can think of more ochem reactions involving alkenes, or is there some other reason?

The C=C double bond requires more energy to break than does a C-C single bond. Like it says, the most stable bond is the one with the highest bond dissociation energy. That's the best way to think of bond stability.

For example 1.4, I can't think of another way to explain it besides what the solution has. Double bonds are stronger than any single bond. Single bonds become weaker (easier to break) as the carbons in the bond become more substituted.

yeah i'm thinking of orgo reactions.

and for 1.4, i guess i've been relating it to carbocations, where more substituted = more stable = stronger. this is a wrong correlation i'm assuming?

 

[Catburr]

yeah i'm thinking of orgo reactions.

Try not to think of "reactivity" this way. Just because we learn more reactions involving a given molecule doesn't make it inherently reactive. Hope that makes sense. Think of reactivity as the opposite of stability, or as inversely proportional to bond dissociation energy.

and for 1.4, i guess i've been relating it to carbocations, where more substituted = more stable = stronger. this is a wrong correlation i'm assuming?

Not wrong, it's just that alkanes and carbocations are two different things, in terms of reactivity/stability. Two different steps in certain reactions, actually, since a carbocation is one type of reaction intermediate. It IS a broken molecule. More substituted carbocations are preferred (or, "more stable" or "easier to achieve") because the more substituted carbon was easier to make into a carbocation.