Gen Chem TBR Bond strength confusion

[Jaysi]

I found an older thread confusing about this exact question regarding whether TBR made a mistake here. This is from the Orgo Chem I book, question 8 from practice passage I on pg. 45. Edit: it's orgo not gen chem sorry

The question asks:
The GREATEST amount of energy is required to break which of the following carbon-carbon bonds? Answers: C. H2C=CH2 D. (H3C)2C=C(CH3)2
I narrowed it down to those 2 answers, and I picked C because earlier TBR text had said the more substituted carbons produce weaker bonds.Then surprise, the answer was D and the explanation for it says, "the double bond in tetrasubstituted alkene is stronger than the double bond in unsubstituted alkene". This seems to contradict another question in the same passage that I got right.

The GREATEST amount of energy is released by oxidative cleavage of an alkene that is:
Answers: A. unsubstituted .. bla bla bla and it goes all the way to D. trisubstituted
Correct answer was A. which made sense.

The explanation went further and said the lower the heat of hydrogenation is, the more stable the alkene molecule is. So I looked them up and C. had the lower heat of hydrogenation. Am I overlooking something simple here?

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[gettheleadout]

Greater substitution of carbons lowers the stability of C-C single sigma bonds. With alkenes you have the pi bond involved too, and due to the electron-accepting nature of the pi bond, increased substitution increases stability.

For example:

(CH3)2CH---C(CH3)3 is more stable than (CH3)3C---C(CH3)3

but

(CH3)CH===C(CH3)2 is less stable than (CH3)2C===C(CH3)2

For C-C bonds I just think about the bonds formed when substituted vs unsubstituted; a bond to another carbon will be longer than that to a hydrogen, so the electron density would be spread out more and less concentrated near the C-C bond in question.

For C=C bonds, on the other hand, because the pi bond acts as an electron acceptor, any alkyl group substituents can act as electron donors and increase the bond strength.

That make sense?

 

[DocMC]

Greater substitution of carbons lowers the stability of C-C single sigma bonds. With alkenes you have the pi bond involved too, and due to the electron-accepting nature of the pi bond, increased substitution increases stability.

Wow. I didn't catch this until now. Thanks!

 

[Jaysi]

I think there was a typo with your examples but I get what you're saying. When pi bonds are involved, substitution provides more stability. However, how does that explain the 2nd passage that was in the book that says unsubstituted is more stable for an alkene? Was that a mistake in TBR's part?

 

[gettheleadout]

I think there was a typo with your examples but I get what you're saying. When pi bonds are involved, substitution provides more stability. However, how does that explain the 2nd passage that was in the book that says unsubstituted is more stable for an alkene? Was that a mistake in TBR's part?

Whoops, sorry about that, just fixed the typos in the reactions.

The answer explanation says "the less stable reactant yields a greater amount of heat upon reaction."

No, there is no error, because for ethene (Choice C) hydrogenation yields 136 kJ/mol, and for 2,3-dimethyl-2-butene (Choice D) hydrogenation yields 110.4 kJ/mol.

Source: http://www.science.oregonstate.edu/~gablek/CH334/Chapter7/ene_stability.htm

 

[Patassa]

Less stable will always result in releasing more energy. It's unstable because it contains too much potential energy of some for. Think of everyday objects like a stretched rubber band, a garden hose that's been kinked off, a popcorn kernel getting hot. Cut the rubber band, undo the kinked hose, and keep the kernel in the oil/microwave, when they let loose, they let out alot of energy. Same idea with molecules and bonds.