Aamc11 #7

[bajoneswadup]

Which single bond present in nitroglycerin is most likely the shortest?

A: C-H
B: C-O
C: C-C
D: N-O

The passage tells us the bond energies, where C-H has the largest and N-O has the smallest.

Answer: a

I picked D because if you look at nitroglycerin's structure, it's got NO2 at the end, meaning there's resonance... There was a question on another practice test that essentially asked the same thing, and explained the double bond in the NO2 resonance structure makes it shorter...I get why the shortest bond obviously has the highest bond energy, but can somebody explain why the difference between the two questions!!?

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[pfaction]

Nah dude. That one was clear cut. The shortest bond will hold the most energy. Double bond resonance will actually make it LONGER than a single bond. All these answers are in single bond if you look at the deltaH.

Additionally I remember what question you're asking for, you're remembering it opposite: Since it resonated between single and double bond, it was LONGER than just the N-O single bond, which was shorter than the O-H bond.

 

[MedPR]

Which single bond present in nitroglycerin is most likely the shortest?

A: C-H
B: C-O
C: C-C
D: N-O

The passage tells us the bond energies, where C-H has the largest and N-O has the smallest.

Answer: a

I picked D because if you look at nitroglycerin's structure, it's got NO2 at the end, meaning there's resonance... There was a question on another practice test that essentially asked the same thing, and explained the double bond in the NO2 resonance structure makes it shorter...I get why the shortest bond obviously has the highest bond energy, but can somebody explain why the difference between the two questions!!?

I don't know what other question you are referring to, but the passage for this question (#7) gives you the bond energies. Ignore everything else you know and use the info given in the passage.

Also, I'm pretty sure nitroglycerin does not have any resonance structures.

 

[rjosh33]

Also, I'm pretty sure nitroglycerin does not have any resonance structures.

No. Nitroglycerin has multiple nitro groups that can undergo resonance.

I picked D because if you look at nitroglycerin's structure, it's got NO2 at the end, meaning there's resonance... There was a question on another practice test that essentially asked the same thing, and explained the double bond in the NO2 resonance structure makes it shorter...I get why the shortest bond obviously has the highest bond energy, but can somebody explain why the difference between the two questions!!?

I did the same thing on this question. Was kinda peeved to see it was wrong afterwards. I think the key is that the question asks, "Which single bond present is the shortest?" The N-O bonds of the nitro groups exist as hybrids of single and double bonds, and therefore don't count as single bonds. So you can rule these out and focus on the other three using the aforementioned bond energies.

 

[bajoneswadup]

Ah ok. Yeah I think I need to just pay more attention to how the question is worded and use the passage, that question really wasn't very hard

 

[DrRichand1]

if you look at NMR you will find C-H bonds are farthest to the left. Also of the choices given C-H have the smallest atomic radius therefore leading you to the correct answer. Bond energies were not needed but helpful in solidifiying the answer.

 

[perfect weather]

Is the correct answer really based on the bond energies? the explanation from AAMC said size was the determining factor: "Since hydrogen has a much smaller atomic radius than second period elements, the covalent bond between C and H is shorter than any of the other bonds listed"

I guess I'm wondering for the actual exam, if I was given bond energy values, am I allowed to infer relative bond lengths? or not necessarily?

 

[aldol16]

They're asking specifically about single bonds, which are sigma bonds. Resonance has to do with pi systems and double bonds. They're asking you which of the single bonds are the strongest. You do not consider pi bonds in that case and only consider BDEs. On the other hand, if they ask you if the carbon-oxygen interaction/bond is strongest in C-O, C=O, or carbon monoxide, then the answer is clear - in that case they'd be comparing single bonds to double bonds to triple bonds.