Berkeley Error

[doctor712]

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I think I found a mistake in an MCAT book regarding degrees of saturation of a molecule and want to confirm. Am I allowed to post if here if it's not an ACTUAL MCAT question?

I did not find this in a thread search I conducted...

Thanks
D712

 

[QofQuimica]

Yes, though this question is more appropriate to the MCAT forum and has probably already been answered there. Will move this thread for you when I get home tonight. 🙂

 

[doctor712]

Ok, thanks, so I'll put it here and will look for it to pop over to MCAT forum later. I wrote out the molecule, here it is...

The question is (and it's a simple one, thus, this is why I want to confirm this is a mistake (In BIO PT 1 BERKELEY BOOK, ASPIRIN PASSAGE XIII, Page 131)

How many degrees of saturation are in this molecule?

1
2
3
4

I answered the obvious (using the definition that I have been taught), and I won't post it until peeps can think through it; the answer, however, given in the back of the book is different. I confirmed with a smartie smart TA and she says, also, my answer, not Berks, is correct. So, we shall see...

I searched both MCAT and NONTRAD forum for this question, and have not seen it posted to date...

D712

 

[MT Headed]

This belongs in the MCAT Study Question Q&A forum.

But Berkeley is right, the answer is 4. What did you miss when you answered 3 (or 2)? The ring? The hidden pi bond in the COO- group? The phrase "degree of unsaturation" has a very specific meaning in ochem, and it is the same thing as "index of hydrogen deficiency" or "elements of unsaturation".

http://en.wikipedia.org/wiki/Degree_of_unsaturation

 

[doctor712]

Thanks MT,

QQuimica is going to move this into the MCAT thread later today.

I put 3. Blasted. deprotonation. My straight A/orgo TA/slash friend/slash tutor/slash starter of med school today, also put 3. So I figured we were right. Ring=1. VISIBLE Double Bonds =2. Since I was using the tried and true, rings + double bonds formula, I thought this was straight forward when I saw what I saw and didn't consider Oxygen as I thought it didn't contribute. I think I may have had a little mix up in my proper Degree definition. No wonder this "error" wasn't listed prior. 🙂

Also, so even since the Carboxylate Anion is stabilized by resonance, a TRUE double bond exists (?) rather than it simply being drawn, as a representation to where it may exist. I'm guessing yes per your reply. To be clear, let's pretend the MCAT askers rephrased this question as, "How many degrees of unsaturation exist in the Carboxylate Anion"? I answer 1. Correct.

D712

 

[QofQuimica]

The fourth one is the carbonyl, as already explained. A deprotonated acid like this is not a "true" double bond b/c of resonance; it's more like 1.5 bond order since resonance to both O's is equal. Think about it this way: it adds another unit of unsaturation since you're missing 2 H's.

I can't move threads using virtual SDN so have to wait till I'm on my computer. Patience, good MCATers!

 

[doctor712]

Great, thank you for explanation.

Virtual SDN? Hmmm, time to hit up Itunes and see if I've been missing something...

D712

 

[hopefuldoc97239]

I was about to post a comment that this was a topic not even covered in my o-chem class (and I can say that confidently, as a top-5 student all year long in classes of ~300 people)...

...but then I looked it up on wikipedia, and this is also known as "Index of Hydrogen Deficiency." That's how we learned it.

So anyone else who's confused by this "degree of unsaturation" stuff, maybe you also learned it by a different name.

It would be most unfortunate if you were asked a question on your actual MCAT that used unfamiliar terms to describe a concept that really had learned, but with a different name!!

 

[doctor712]

That's, in part, what SDN is all about. 🙂

 

[MD Odyssey]

I was about to post a comment that this was a topic not even covered in my o-chem class (and I can say that confidently, as a top-5 student all year long in classes of ~300 people)...

...but then I looked it up on wikipedia, and this is also known as "Index of Hydrogen Deficiency." That's how we learned it.

So anyone else who's confused by this "degree of unsaturation" stuff, maybe you also learned it by a different name.

It would be most unfortunate if you were asked a question on your actual MCAT that used unfamiliar terms to describe a concept that really had learned, but with a different name!!

Interestingly, I took an honors level course, and we didn't cover it either. I had seen it the summer before when I was reviewing for the course, but was somewhat surprised that we didn't cover it. Actually ended up being a useful item when we got to second semester and I seem to recall that the ACS exam might have had one or two questions on it.

 

[mikexima]

do you mean degrees of UNsaturation? if so 3, 1 for each double bond and 1 for the ring

 

[MT Headed]

do you mean degrees of UNsaturation? if so 3, 1 for each double bond and 1 for the ring

and 1 for the double bond in the COO-, for a total of 4.

 

[mikexima]

and 1 for the double bond in the COO-, for a total of 4.

i stand corrected.