Issues with EK's take on hybridization and bond angles in amides and esters

[loveoforganic]

In EK 1001 organic chem, they have a few questions concerning the hybridization of the nitrogen in amides and the ether-like oxygen in esters, and the bond angle about those atoms. In both cases, they say the atom is sp3 hybridized with a bond angle of ~109o. That's just not the case though, particularly in amides. At the very least, both the ether-like oxygen in esters and the nitrogen in amides show a strong portion of sp2 characterization, if not more sp2 than sp3, and the correlated 120o bond angle. What's the deal?

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[sv3]

In EK 1001 organic chem, they have a few questions concerning the hybridization of the nitrogen in amides and the ether-like oxygen in esters, and the bond angle about those atoms. In both cases, they say the atom is sp3 hybridized with a bond angle of ~109o. That's just not the case though, particularly in amides. At the very least, both the ether-like oxygen in esters and the nitrogen in amides show a strong portion of sp2 characterization, if not more sp2 than sp3, and the correlated 120o bond angle. What's the deal?

If they showed a picture of the molecule and the atom was in SP3 form, then perhaps thats what they meant. I know that they (the atoms your referring to) are SP2 as well although i do not know the ratio of the SP2:SP3 forms to comment on which should prevail. They just might have been referring to one type?

 

[loveoforganic]

There isn't a ratio of sp2 to sp3 (resonance forms don't exist independently, they form a hybrid). You can see that sp2 character is very present in amides because when you run 1H NMR on them, you get two peaks for the protons on the amide - they're held in different chemical environments - one near the carbonyl oxygen and one away from it. If you heat up an amide, you'll cross the energy barrier for rotation about that bond and the peaks will blur.

 

[sv3]

There isn't a ratio of sp2 to sp3 (resonance forms don't exist independently, they form a hybrid). You can see that sp2 character is very present in amides because when you run 1H NMR on them, you get two peaks for the protons on the amide - they're held in different chemical environments - one near the carbonyl oxygen and one away from it. If you heat up an amide, you'll cross the energy barrier for rotation about that bond and the peaks will blur.

Right...they don't exist independently but to get the characterization overall (SP2 or SP3) don't you take the contribution via a weighted average based on stability?

But anyway I had to use another source for orgo as I found Ek was decent but not very detailed. I felt it was a bare bones approach but I have been out of the sciences for a while

Hopefully you find a questions like this in your practice tests and can get the answer.

sv3

 

[loveoforganic]

Right...they don't exist independently but to get the characterization overall (SP2 or SP3) don't you take the contribution via a weighted average based on stability?

But anyway I had to use another source for orgo as I found Ek was decent but not very detailed. I felt it was a bare bones approach but I have been out of the sciences for a while

Hopefully you find a questions like this in your practice tests and can get the answer.

sv3

Oh I see what you're saying. Yes, I think you do, although I'm not sure how you calculate that stability. I'm using BR for organic review, but using EK 1001 to have some practice problems to work through, and that was one of the practice problems.

If it was an isolated thing I wouldn't worry about it, but they had 4 questions stemming from hybridization of those atoms >.<

 

[sv3]

I'll be reviewing orgo for two days sometime this week as i desperately need to refresh myself before diving into my last science, biology. I'll keep an eye out for it and post any findings in the case that someone else hasn't replied.

I know there are some mistakes in Eks books as well. I can't see any explanation for their answer which is odd b/c I thought it was important to realize those atoms are SP2 sometimes (i use sometimes loosely). I think they are just using the simplest case and assuming the double bond character sticks with the C and O only of the carbonyl.

Sorry couldn't be of more help........

 

[sv3]

Screw Ek I say. Was finallly reviewing my orgo and came across the peptide bond discussion. TPR states: "the peptide bond is planar and rigid b/c the resonsance delocalization of the nitrogen's electrons to the carbonyl carbon gives substantial double bond character to the bond between the carbon and nitrogen". Peptide bonds are amide bonds so theres the relation obviously

I really dont like what Ek is saying and again, agree with your initial take.

 

[loveoforganic]

I still like EK for g. chem, organic, and physics where I can identify them making a mistake, but for bio I have no clue if what they're telling me is crap or not.

 

[sv3]

I still like EK for g. chem, organic, and physics where I can identify them making a mistake, but for bio I have no clue if what they're telling me is crap or not.

Well your in good shape if you can all bs on a prep company. I'm not that confident in my abilities unfortunately and will doubt myself if in disagreement with a prep co's answer - odds are against me afterall.

Bio........grrrr.......starts tomorrow for me


sv

 

[loveoforganic]

It sucks, enjoy 🙂

;P