kaplan chemistries

[Jordwin]

Has anyone taken the Kaplan class and thought that both chemistry sections have some questions that aren't likely to be on the DAT? Specifically, I had two questions on Heats of hydrogenation for O chem, which I had never heard of before these quizzes.

 

[Jordwin]

Well if no one has taken Kaplan, has anyone at least seen a problem on heats of hydrogenation?

 

[sskater]

Well if no one has taken Kaplan, has anyone at least seen a problem on heats of hydrogenation?

Hey Jordwin!

I'm actually in the Kaplan class too and I've seen this exact question.

What we do know is that when we form a more stable alkene, more heat is released right? (ie. more exothermic, reaction more favorable), so a general rule we can see here is that the most stable alkene, when hydrogenated, will have the least/lower heat of hydrogenation since it has already released a whole bunch of heat getting formed.

What we're also doing is breaking the pi bond by hydrogenating, so think about it, if you're already stable, why would you want to hydrogenate the bond? THis would be extremely unfavorable thus the lower heat of hydrogenation.

So you would look for conjugation around the double bond or how substituted it is to rank the stability!

Hope this helps.

 

[Minion]

I actually understand your question. When we hydrogenate an alkene we do break the C-C pi bond but we also make 2 C-H sigma bonds and the overall rxn is pretty exothermic. So the lowest heat of hydrogenation is actually referring to which one is the least exothermic (least negative delta H).

The general rule is that the more stable alkene (lower energy) will have a lower heat of hydrogenation. Since it already starts out lower in energy it has less to release.

The things you should look for here that stabilize pi electrons are conjugation (especially if it's aromatic) and then the more substituted pi bond is more stable.

Hope this helps!

 

[Jordwin]

Hey Jordwin!

I'm actually in the Kaplan class too and I've seen this exact question.

What we do know is that when we form a more stable alkene, more heat is released right? (ie. more exothermic, reaction more favorable), so a general rule we can see here is that the most stable alkene, when hydrogenated, will have the least/lower heat of hydrogenation since it has already released a whole bunch of heat getting formed.

What we're also doing is breaking the pi bond by hydrogenating, so think about it, if you're already stable, why would you want to hydrogenate the bond? THis would be extremely unfavorable thus the lower heat of hydrogenation.

So you would look for conjugation around the double bond or how substituted it is to rank the stability!

Hope this helps.

I guess I was just thinking of it in terms of enthalpy and that a low heat of formation would be a negative, or exothermic number. Thanks for the explanation!

Which Kaplan class are you in?? Maybe we are in the same one!

I actually understand your question. When we hydrogenate an alkene we do break the C-C pi bond but we also make 2 C-H sigma bonds and the overall rxn is pretty exothermic. So the lowest heat of hydrogenation is actually referring to which one is the least exothermic (least negative delta H).

The general rule is that the more stable alkene (lower energy) will have a lower heat of hydrogenation. Since it already starts out lower in energy it has less to release.

The things you should look for here that stabilize pi electrons are conjugation (especially if it's aromatic) and then the more substituted pi bond is more stable.

Hope this helps!

So a heat of hydrogenation is, in a nutshell, how much energy is needed to hydrogenate a double bond? And if so, wouldn't the more conjugated system be harder to hydrogenate (higher heat of hydrogenation)?

I know I am wrong, obviously, but just figured I'd ask.

 

[sskater]

I'm in the one in southern cali/CSULB. Our last class is on monday!

Oh and I knew I've seen that explanation somewhere from Minion.
Directly copied and pasted from Chad himself!
http://wwwdotcoursaverdotcom/archive/index.php/t-3637.html?

 

[Jordwin]

Okay thanks! I couldn't find it at first, but I didn't type the http: in

 

[Jordwin]

I'm in the one in southern cali/CSULB. Our last class is on monday!

Oh and I knew I've seen that explanation somewhere from Minion.
Directly copied and pasted from Chad himself!
http://wwwdotcoursaverdotcom/archive/index.php/t-3637.html?

Wow, how did you catch that? Impressive. When is your test?

 

[sskater]

Wow, how did you catch that? Impressive. When is your test?

I looked up that question when I did it like you! But I forgot how it came to me, then I saw his post.
I'm taking mine on the 12th of August. Where are you based off at?

 

[Jordwin]

Ah nice. That's a good amount of extra study time! I'm in iowa.

 

[sskater]

Ah nice. That's a good amount of extra study time! I'm in iowa.

I'm actually going to be the first one taking it in my class =_=, so they're all waiting for me to take it (guinea pig). Good luck for yours! Pretty soon?

 

[Jordwin]

Ha well my class is online so idk when other people are taking theirs. Taking mine a little later tho. Haven't set the exact date yet. Thanks, you too!