Ochem

[Glycogen]

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Do you guys know what is the heat of hydrogenation.I mean, I know that is the amount heat that is required to convert an alkene or alkyne to alkane but I can not distinguish them among answer choices!!!!

ex:

Which of the following would have the highest heat of hydrogenation?

a.2,3-dimethyl-2-butene
b.ethylene
c.trans-2-butene
d.2-methyl-2-butene
e.propene

Thanks

 

[osimsDDS]

Is the answer D???

 

[shoombol tallah]

The least substituted double bond has the highest heat of hydrogenation because its the most unstable. Ive seen this question before but its usually done with molecules that have the same amount of carbons but differ in the amount of substituents. So Im not sure what the answer would be but i would guess that its E.

 

[osimsDDS]

Yea i have also seen this question before but im thinking about it like this:

highest heat of hydrogenation refers to the heat developed when you reduce a molecule...in this case the higher the heat it refers to an exothermic reaction where you want to make the MOST stable molecule...so wouldnt a molecule with more substituents favor a higher heat of hydrogenation because it is stabilized by some tertiary carbon systems in the molecule...

 

[Glycogen]

No,it is B.
ok,here is the explanation they have here.I put some of here may be you guys can explain it better:
"the most stable alkene will release the least energy since it is already at a relatively lower energy level.the most stable alkene are those that have the greatest substitution since e- are donated to make the pi bond stronger.d had the most substitution.the highest heat of hydrogenaton would belong to ethylene"!!!!!

Well,I'm not quite sure about it,a bit confused?
Any comment?

 

[Mstoothlady2012]

hmm I am debating between b & d.....I guess b? not sure though!

 

[shoombol tallah]

That was my second choice but I went with E because i thought triple bonds (ethylene) are more stable than double bonds. Ethylene only has two hydrogens bonded to the carbons in the triple bond so it has the least amount of substituents and highest heat of hydrogenation.

 

[Glycogen]

hmm I am debating between b & d.....I guess b? not sure though!

You got it right.
So,what are your reasonings?

 

[osimsDDS]

No,it is B.
ok,here is the explanation they have here.I put some of here may be you guys can explain it better:
"the most stable alkene will release the least energy since it is already at a relatively lower energy level.the most stable alkene are those that have the greatest substitution since e- are donated to make the pi bond stronger.d had the most substitution.the highest heat of hydrogenaton would belong to ethylene"!!!!!

Well,I'm not quite sure about it,a bit confused?
Any comment?

haha ok got it, its the same explanation i had before after shoomballah's post but opposite because you want the GREATEST heat will come from the molecule with the least stable conformation...yea it makes sense now ethylene...propene is wrong because the double bond and the pi electrons have the ability to be delocalized so that would be partially stable...ehtylene has no substituents to form tertiary or even secondary carbons for stability and therefore it is correct...

 

[Maygyver]

I am going to go with B because it seems the least stable. No substituents attached.

 

[Glycogen]

OMG, heat is in fact energy and so when something is more stable= less energy and less stable=more energy....
I guess,I have to take a break !!!!!!

 

[futuredent22]

heat of hydrogenation is the heat released when it goes from alkene to alkane. A stable alkene won't release too much heat because it is already at a low (ie. stable) energy level. A very unstable alkene will be at a high energy level, so when it is hydrogenated to an alkane a lot of energy will be released.

The question really is which alkene is the least stable. Ethylene, CH2=CH2, is the least stable.

 

[Mstoothlady2012]

I think we both posted at the same time. AFter I posted I read your explanation which further reassured me that it is b!! Initially I was confused between b & d because they both have primary and secondary carbons. But b has 1 carbon less, so definitely b!!

Like you said less stable molecule = higher heat of hydrogenation and your explanation makes sense but I am not too sure about the chemistry behind it.

I know how to compare between more substituted vs. less substituted or cis vs. trans. But I don't know how the heat of hydrogenation is affected with increasing length of the alkene. That's why I am hesitating a little.

Can some1 confirm if b is the right answer or not?

 

[osimsDDS]

who are you talking about mstoothlady??

 

[Mstoothlady2012]

who are you talking about mstoothlady??

sorry forgot to quote! I was talking to glycogen...was answering his question

 

[osimsDDS]

yea this problem is sick actually, it branches into thermochemistry and ochem....most stable molecule wont have that much heat given off (exothermic bc its more stable) least stable molecule will give off more heat in order to be stable...

 

[osimsDDS]

If you guys want another example check out Destroyer Ochem #132 its exactly the same, try to apply thermochem to it and you will get it right...

 

[Mstoothlady2012]

yea this problem is sick actually, it branches into thermochemistry and ochem....most stable molecule wont have that much heat given off (exothermic bc its more stable) least stable molecule will give off more heat in order to be stable...

yea I know at the beginning I use to have trouble with this concept as well, I had same reasoning like you did. But then finally I figured it out lol. Anyways I posted something on your other thread about destroyer ochem....

 

[andrewami]

I also think it is B, it is similar to problem #84 in destroyer.