Question about enthalpy of hydrogenation

[deleted647690]

"The greatest amount of energy is released by the oxidative cleavage of an alkene that is:
A. unsubstituted
B. Monosubstituted
C. Disubstituted
D. Trisubstitutted"

Answer was A. I'm not sure why this makes sense. I would think that the extra methyl groups on a trisubstituted compound would donate electron density and thus strengthen the bond.

Another question is

"The greatest amount of energy is required to break which of the following C-C bonds?
A. CH3-CH3
B. (H3C)3C-C(CH3)3
C. CH2=CH2
D. (CH3)2C=C(CH3)2


Answer was D. Now I'm extra confused. If the first question was saying that an unsubstituted compound would yield a greater amount of energy and upon cleavage, and thus be more stable right? Why is this question saying that a substituted compound would need more energy?

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[aldol16]

Okay, the answer to both of these questions stems from one fact. So let's target that.

What are the products of oxidative cleavage of unsubstituted versus, say, fully substituted olefins?

 

[intromo]

Okay, the answer to both of these questions stems from one fact. So let's target that.

What are the products of oxidative cleavage of unsubstituted versus, say, fully substituted olefins?

There are three possible products possible from an oxidative cleavage reaction: formaldehydes, aldehydes, and ketones. It's dependent on whether the doubly bonded carbons are =CH2, CHR, or CR2 with R representing alkyl groups. Unsubstituted alkenes form formaldehydes.

However, that's as far as I got using your approach to understanding the answers. I'm not sure where to go myself. I was thinking isn't it as easy as looking at it through heats of hydrogenation and seeing that an unsubstituted alkene will have the highest heat of hydrogenation and therefore the greatest amount of energy released? Then again I'm not sure if you can apply heat of hydrogenation values to this case since its, you know, not a hydrogenation reaction.

@aldol16 so the unsubstituted alkenes form formaldehyde and a fully substituted will form a ketone. Is it just a general concept to know that the formation of formaldehyde will release more energy?

 

[aldol16]

@aldol16 so the unsubstituted alkenes form formaldehyde and a fully substituted will form a ketone. Is it just a general concept to know that the formation of formaldehyde will release more energy?

I can see now the confusion and my question didn't serve to clear it up much so I'll try explaining it.

I believe with the first question, they are assuming full oxidation, as you would get with a strong oxidizing species like permanganate. In that case, unsubstituted alkenes will be oxidatively cleaved all the way up to CO2 whereas fully-substituted alkenes can only go to ketones. Being able to oxidize all the way up to CO2 will release a lot of energy whereas oxidizing only up to ketones doesn't release as much.

This is related to the answer to the second question. It asks which one of those bonds is the hardest to break. You're right in that the hardest one to break is D, or one most electron-rich alkene. But breaking the alkene bond is only one step in an oxidative cleavage. It's coupled to oxidation of the substrate - and in the case of unsubstituted alkenes, all the way up to CO2. For a process to be thermodynamically feasible, it only has to have an overall negative free energy change.

 

[intromo]

@aldol16
I see now. I was thinking of the first question in terms of ozonolysis which I see is not full oxidation. Thank you!

 

[aldol16]

I see now. I was thinking of the first question in terms of ozonolysis which I see is not full oxidation. Thank you!

Yes, this question is not entirely clear as to whether it's referring to a partial oxidation (as with O3) or complete oxidation (as with permanganate). In any case, that's why there's a seeming discrepancy between the first and second questions - the second refers to one step whereas the first refers to multiple.

 

[deleted647690]

I don't have my orgo book with me, I'm going to look at this again soon

 

[rabbott1971]

It's easier to think of the first question simply as asking which species is the least stable. That is the one that will release the most energy when it is broken. Stability increases with substitution, so the unsubstituted alkene releases the most energy.

Conversely, the second question asks which compound requires the most energy to break the bonds. Well, that would be the one which is the most substituted, D. It is the most stable, and so needs a lot of energy to disrupt its happy little state. So both these questions can be answered by resort to the simple principle of stability being increased with substitution, rather than an in-depth analysis of what are the products, etc.