TBR Organic Chemistry Practice Passages

[AppleRind]

Hi everyone,

I'm new to the forums but thought that this would probably be the best place to ask for clarification regarding MCAT-related practice material. I just completed the first passage in the Berkeley Review's Organic Chemistry book and am confused as to their reasoning for question number 8:

"The GREATEST amount of energy is required to break which of the following carbon-carbon bonds? Answer: (H3C)2C=C(CH3)2"

Their explanation is as follows:

"The lower heat of hydrogenation in the second chart implies that the reactant alkene molecule is more stable. The more stable the alkene compound, the stronger its pi-bond"

I understand that the higher stability makes the bond stronger, but I'm uncertain as to what they mean regarding the lower heat of hydrogenation and its relation to the stability of the alkene.

Any help would be greatly appreciated, thanks!

- Applerind

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[salim271]

I still have to review my organic material, but the lower heat of hydrogenation seems like an offshoot of why its stable, not the reason it is stable. The reason double bonds in general are more stable than single bonds alone is because there are two bonds (obviously) and double the overlap, both s and p orbitals of the carbons are overlapping, sharing electrons in covalent bonds, and therefore increasing the strength of the bond, leading to increased stability. Increased stability ----> harder to break ----> more energy needed to break the bond.

Correct me if I'm wrong on any point, anyone.

 

[AppleRind]

Thanks for the response. I understand that double bonds are more stable, in general, and require more energy to break. The other answers also included another alkene. The difference between the answer and the other choices is the degree of substitution of the alkene. I believe that the more substituted alkene will have the higher dissociation energy, but my question is more centered around the relation between the heat of hydrogenation vs stability. Thanks for your help!

- Applerind

 

[salim271]

Ah, well think of this way. A higher heat of hydrogenation means that more energy (in the form of heat) is released when the alkene is hydrogenated and the double bond is broken. Less stable alkenes ---> more energetic bonds ---> more heat released when the pi bond is broken. So the opposite is true as well, more stable alkenes ---> less energy in the bonds ---> less heat released, and therefore lower heat of hydrogenation.

And of course the stability of alkenes follows substitution rules, mono < di (cis < trans) < tri < full substituted.

 

[coyotelime]

Yeah in orgo lecture I just memorized that the least negative/smallest delta H = most stable, so stick to that and you should be okay. Good luck! Orgo is fun compared to physics =D

 

[salim271]

Yeah in orgo lecture I just memorized that the least negative/smallest delta H = most stable, so stick to that and you should be okay. Good luck! Orgo is fun compared to physics =D

Agreed, physics is my weakness.... I'm now currently in the process of trying to understand it like I do organic, but its not easy lol. Probably shoulda done it while I was taking the class... ah, easy professor, thy come backeth to bite me in the butteth.

 

[AppleRind]

Ah, well think of this way. A higher heat of hydrogenation means that more energy (in the form of heat) is released when the alkene is hydrogenated and the double bond is broken. Less stable alkenes ---> more energetic bonds ---> more heat released when the pi bond is broken. So the opposite is true as well, more stable alkenes ---> less energy in the bonds ---> less heat released, and therefore lower heat of hydrogenation.

And of course the stability of alkenes follows substitution rules, mono < di (cis < trans) < tri < full substituted.

Thanks! This post helped a lot!👍

 

[nice2012]

Agreed, physics is my weakness.... I'm now currently in the process of trying to understand it like I do organic, but its not easy lol. Probably shoulda done it while I was taking the class... ah, easy professor, thy come backeth to bite me in the butteth.

just work hard you'll make something in the end.

 

[WhiteCoatSyndrome]

Another way you could think of the question you asked is as follows:

When you have double/triple bonds there is a lot more energy present, holding the atoms together. As a result, according to bond enthalpies it takes more energy in order to break the double/triple bond because it takes more outside energy to overcome the energy already present in the double/triple bond, making it more stable.

I have always liked to apply this principle to questions of stability in chemistry, general or organic. The higher the energy present in the bond, the more energy required to break it

 

[gallons]

Another way you could think of the question you asked is as follows:

When you have double/triple bonds there is a lot more energy present, holding the atoms together. As a result, according to bond enthalpies it takes more energy in order to break the double/triple bond because it takes more outside energy to overcome the energy already present in the double/triple bond, making it more stable.

I have always liked to apply this principle to questions of stability in chemistry, general or organic. The higher the energy present in the bond, the more energy required to break it

In reality it is the opposite. High ENERGY bonds are weak bonds, with LOWER bond disassociation energies.