Aamc 11 #7

[chiddler]

Brutal exam. I hope real thing isn't more difficult, though.

Which single bond length in nitroglycerin is the shortest?
*A. C-H
B. C-O
C. C-C
D. O-N

Reminder image

I put D because of resonance'd SP2 nitrogen, as opposed to all the other SP3's. Answer says that the most stable bond, C-H, is the shortest. How am I to compare C-H and N-O?

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[SaintJude]

It was a tough exam indeed, but the scale was more lenient that the previous ones.

I myself don't understand how to disregard the fact that NO should have partial double bond character --and I chose D as well.

Kaplan's solution:

Since bond order is not a factor the only other info we have to go on is the bond energies provided in P 4. How is bond energy related to bond length? As bond strength increase, atoms are pulled closer together; therefore, there's an inverse relationship b/w bond strength and bond length.

Q taught me I really need to check passage for relevant info

 

[chiddler]

Agh. I didn't notice that. Is this always the case? Does bond energy always dictate length?

 

[SaintJude]

Yes. Bond energy is always an indicator of bond length. But how can we reconcile this with the resonance ?

 

[chiddler]

hmm...i don't know. resonance increases bond energy, but the bonds are shorter...

 

[DrRichand1]

Looking at the bond energies given will get you to the right answer. But for more intuition:

2 things could help you answer this or other future sim prob's.

1. has to do with atom size the biggest difference is between the H and C therefore the corresponding bond will be the shortest distance between the two.

2. C-H stretch on NMR lies far to the left, Even farther than C C triple bonds. meaning it takes a higher freq to oscilate the C-H due to stronger bonds. Shorter means stronger.

Hope this Helps.

 

[SaintJude]

1. has to do with atom size the biggest difference is between the H and C therefore the corresponding bond will be the shortest distance between the two.

Can you further explain this point please? The connection is larger size difference, shorter bond?

 

[chiddler]

Can you further explain this point please? The connection is larger size difference, shorter bond?

Yeah, just like HI and HCl. The HI has a longer bond length, remember? Because iodine is huge.

And that's a very good point i forgot to consider.

and for #2, yes! I confused high bond energy with high energy bond.

thanks a lot.

 

[SaintJude]

According to that logic, HI should have a shorter bond length....

 

[chiddler]

According to that logic, HI should have a shorter bond length....

how comes? HI has a small bond energy (takes little energy to break) so is a very long bond.

NO also took little energy to break so it also should have a very long bond.

 

[EnginrTheFuture]

Looking at the bond energies given will get you to the right answer. But for more intuition:

2 things could help you answer this or other future sim prob's.

1. has to do with atom size the biggest difference is between the H and C therefore the corresponding bond will be the shortest distance between the two.

2. C-H stretch on NMR lies far to the left, Even farther than C C triple bonds. meaning it takes a higher freq to oscilate the C-H due to stronger bonds. Shorter means stronger.

Hope this Helps.

The first thing you should look at is large differences in atomic radius... it is always the most important comparative factor in bond length. For this question you have to realize that hydrogen is EXTREMELY small relative to N and O. You are dropping down an entire shell. For atoms that are in the same row, the atomic radius difference isn't able to distinguish bond length on its own because they are so similar... but that's not the case here because H isn't in the same row. A C-H bond should be shorter than any C triple bonded to anything. This is why it is so far left on the IR spectra

 

[DrRichand1]

Clarifiying "larger size distance"
I ment H is the smallest and it's bonded to a relatively much larger atom, C.
If you look at C bonded to O you have two relatively large atoms bonded together, different than the Small H and 1 Large C.

1small+1Large bondlength < 1 Large + 1 Large bondlength

 

[DrRichand1]

The first thing you should look at is large differences in atomic radius... it is always the most important comparative factor in bond length. For this question you have to realize that hydrogen is EXTREMELY small relative to N and O. You are dropping down an entire shell. For atoms that are in the same row, the atomic radius difference isn't able to distinguish bond length on its own because they are so similar... but that's not the case here because H isn't in the same row. A C-H bond should be shorter than any C triple bonded to anything. This is why it is so far left on the IR spectra

Hea i ment to say IR😛

 

[SaintJude]

Alright, HI vs. HCl which has a larger size difference? HI, right? --> longer bond.

 

[DrRichand1]

right, that's why it's more acidic as well cuz the atoms are father apart and H can disociate more easily, and the congugate base I- can stabilize the neg charge left behind better making it a stonger acid cuz the CB is more stable. Extra* for free