Organic Chemistry Question Thread

[QofQuimica]

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All users may post questions about MCAT, DAT, OAT, or PCAT organic chemistry here. We will answer the questions as soon as we reasonably can. If you would like to know what organic topics appear on the MCAT, you should check the MCAT Student Manual (http://www.aamc.org/students/mcat/studentmanual/start.htm)

Acceptable topics:
-general, MCAT-level organic
-particular MCAT-level organic problems, whether your own or from study material
-what you need to know about organic for the MCAT
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-how best to study MCAT organic
-how best to tackle the MCAT biological sciences section

Unacceptable topics:
-actual MCAT questions or passages, or close paraphrasings thereof
-anything you know to be beyond the scope of the MCAT

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If you really know your organic, I can use your help. If you are willing to help answer questions on this thread, please let me know. Here are the current members of the Organic Chemistry Team:

-QofQuimica (thread moderator): I have my M.S. in organic chemistry and I'm currently finishing my Ph.D., also in organic chemistry. I have several years of university organic chemistry TA teaching experience. In addition, I teach organic chemistry classes through Kaplan for their MCAT, DAT, OAT, and PCAT courses. On the MCAT, I scored 15 on BS, 43 overall.

P.S. If you shorten "organic chemistry" to "orgo," not only will I not answer your questions, but during the BS section, your test form will backside attack you with a zillion strong nucleophiles (via the SN2 mechanism, of course).

-Learfan: Learfan has his Ph.D. in organic chemistry and several years worth of industrial chemistry experience. He scored 13 on the BS section of the MCAT, and 36 overall.

 

[Tommyk7]

when i grow up i wanna MARRY you dr. Q! 😍

 

[QofQuimica]

when i grow up i wanna MARRY you dr. Q! 😍

😳

You guys are welcome. Now get back to studying. 😉

 

[desertdr]

when i grow up i wanna MARRY you dr. Q! 😍

Actually Q and I were soulmates first (Phds in Ochem, marathons, and now medschool class of 2010)! 😍

Seriously, I just happened to come across this thread and also wanted to say, "You rock Q!" I loved teaching 2nd semester Orgo, but alas, a year of intellectual property dealing with electrochemistry takes a serious toll on one's O-Chem abilities.....Phd or not 😉

QQPh.D - Good luck on your defense (when's the date?)....I want to be among the first to welcome you to the "family"!

 

[QofQuimica]

Actually Q and I were soulmates first (Phds in Ochem, marathons, and now medschool class of 2010)! 😍

Seriously, I just happened to come across this thread and also wanted to say, "You rock Q!" I loved teaching 2nd semester Orgo, but alas, a year of intellectual property dealing with electrochemistry takes a serious toll on one's O-Chem abilities.....Phd or not 😉

QQPh.D - Good luck on your defense (when's the date?)....I want to be among the first to welcome you to the "family"!

Thanks, desert. I'm defending next week on Friday. As of the end of this month, I will finally be a doc. 😀

 

[Lests55]

Hi Q,

I have a question about polarity between a cyclohexane substituted with a carbonyl vs. one substituted with a hydroxyl group. (This question comes from AAMC 7). I get that polar substances will travel less far up the silica gel, but AAMC says that the -OH group is more polar. I certainly understand this, but I also saw something that said the dipole is reduced in the alcohol because of the dipole moment pointing towards the ring from the O-H bond (as well as away from the ring in the C-O bond). I take it that AAMC is correct of course, but what do you think of the other argument.

Also I guess the hydrogen of the alcohol can hydrogen bond with the oxygen electrons in silica?

 

[QofQuimica]

Hi Q,

I have a question about polarity between a cyclohexane substituted with a carbonyl vs. one substituted with a hydroxyl group. (This question comes from AAMC 7). I get that polar substances will travel less far up the silica gel, but AAMC says that the -OH group is more polar. I certainly understand this, but I also saw something that said the dipole is reduced in the alcohol because of the dipole moment pointing towards the ring from the O-H bond (as well as away from the ring in the C-O bond). I take it that AAMC is correct of course, but what do you think of the other argument.

Also I guess the hydrogen of the alcohol can hydrogen bond with the oxygen electrons in silica?

Bingo. The carbonyl only has dipole-dipole interactions. H-bonds win. 🙂

 

[Lests55]

Bingo. The carbonyl only has dipole-dipole interactions. H-bonds win. 🙂

What if we were talking strictly polarity? Would the oppositely directed dipole from the O-H bond cancel the polarity of the C-O bond (in the alcohol) so that the carbonyl would be more polar?

Sorry if I am nit-picking...

 

[QofQuimica]

What if we were talking strictly polarity? Would the oppositely directed dipole from the O-H bond cancel the polarity of the C-O bond (in the alcohol) so that the carbonyl would be more polar?

Sorry if I am nit-picking...

You aren't nit-picking, but you ARE going beyond what you need to know for the MCAT. Yes, a ketone does tend to have a larger dipole moment than an alcohol. Here is a link to a table of several solvents. But notice that carboxylic acids and esters like ethyl acetate tend to be lower even though they contain carbonyls. Again, this is because there are two dipoles pointing in different directions.

 

[kevin86]

a few questions.

1. could you explain to me polar aprotic vs protic, and their effects on the various sn and E reactions.
2. what do they mean by having pure s orbital or p orbital in a compound. Would this be something from both atoms in a bond or just one atom.
3. what happens when there is a deactivator and an activator on a ring. Would the next compound be ortho/para or meta.

also what are the exceptions to aromaticity rules. which ones can delocalize? do u memorize or can you figure out.

 

[QofQuimica]

a few questions.

1. could you explain to me polar aprotic vs protic, and their effects on the various sn and E reactions.
2. what do they mean by having pure s orbital or p orbital in a compound. Would this be something from both atoms in a bond or just one atom.
3. what happens when there is a deactivator and an activator on a ring. Would the next compound be ortho/para or meta.

also what are the exceptions to aromaticity rules. which ones can delocalize? do u memorize or can you figure out.

1) I've already written a post about polar protic versus aprotic in the organic explanations thread. Pop in there and take a look.

2) I have no idea what a pure s-orbital is in a compound. For the p, they're probably talking about an sp2 or sp-hybridized species that has one or two unhybridized p-orbitals. Pi bonds are made from overlapping p-orbitals. Carbocations also have empty p-orbitals.

3) It depends on the relative locations of the substituents. Sometimes their effects reinforce. If they oppose one another, the stronger one will win.

4) There aren't exceptions to aromaticity rules. Aromatic compounds are all delocalized. All aromatic compounds have to be cyclic, contain 4n + 2 electrons, be planar, and be conjugated. If one of these conditions isn't met, the compound isn't aromatic.

 

[rcd]

I'm having trouble with orbital hybridization.

I understand that a carbon atom normally combines its orbitals into sp3, or sp2/sp if 1/2 p orbitals are being used in pi bonds.

But what I don't get is what is the orbital between C2 and C3 in propene?

C3 is sp3 hybridized, and C2 is sp2 hybridized. So how can the single bond between them have 2 different hybridizations 😕?

I thought I had this down until I came up with that question 🙄.

 

[QofQuimica]

I'm having trouble with orbital hybridization.

I understand that a carbon atom normally combines its orbitals into sp3, or sp2/sp if 1/2 p orbitals are being used in pi bonds.

But what I don't get is what is the orbital between C2 and C3 in propene?

C3 is sp3 hybridized, and C2 is sp2 hybridized. So how can the single bond between them have 2 different hybridizations 😕?

I thought I had this down until I came up with that question 🙄.

It's not the bonds themselves that are hybridized; it's the atoms. The first and second carbons are both sp2-hybridized, while the third is sp3-hybridized. The first two atoms use their sp2-orbitals to make three sigma bonds each, while the leftover p-orbitals overlap to form the pi bond. The third atom makes four sigma bonds using its sp3-orbitals. The sigma bond between carbons 2 and 3 is formed by overlapping an sp2-orbital from carbon 2 with an sp3-orbital from carbon 3.

 

[MediMama23]

It's not the bonds themselves that are hybridized; it's the atoms. The first and second carbons are both sp2-hybridized, while the third is sp3-hybridized. The first two atoms use their sp2-orbitals to make three sigma bonds each, while the leftover p-orbitals overlap to form the pi bond. The third atom makes four sigma bonds using its sp3-orbitals. The sigma bond between carbons 2 and 3 is formed by overlapping an sp2-orbital from carbon 2 with an sp3-orbital from carbon 3.

Okay, now I'm confused. I thought C1 and C2 had sp hybridization. Don't C1 and C2 have to have 180deg angles?

 

[QofQuimica]

Okay, now I'm confused. I thought C1 and C2 had sp hybridization. Don't C1 and C2 have to have 180deg angles?

No, you're thinking of an alkyne, not an alkene. Check out this image:

The HCH bond on the right end, for example, is a 120 degree angle.

Contrast that with propyne, which has sp-hybridization and 180 degree angles:

 

[MediMama23]

No, you're thinking of an alkyne, not an alkene. Check out this image:

The HCH bond on the right end, for example, is a 120 degree angle.

Contrast that with propyne, which has sp-hybridization and 180 degree angles:

I was SO thinking of an alkyne!!!
Thanks, QofQuimica!!! 👍

 

[kevin86]

Last time I asked about the aromaticity, I was asking about this molecule

[img=http://img102.imageshack.us/img102/4504/17xl1.jpg]

So it's not aromatic because it has sp3 oxygen? or is it because after I delocalized the lone pairs on both oxygens to make them both sp2, it violates the huckel rule by having a sum of 8 pi electrons? So should I always use the rules after I delocalize whatever I can first.

 

[QofQuimica]

Last time I asked about the aromaticity, I was asking about this molecule

[img=http://img102.imageshack.us/img102/4504/17xl1.jpg]

So it's not aromatic because it has sp3 oxygen? or is it because after I delocalized the lone pairs on both oxygens to make them both sp2, it violates the huckel rule by having a sum of 8 pi electrons? So should I always use the rules after I delocalize whatever I can first.

I can't see the figure. Is it a seven-membered ring with three double bonds and an oxygen in it? If it has 8 pi electrons, it can't be aromatic.

 

[rcd]

I can't see the figure. Is it a seven-membered ring with three double bonds and an oxygen in it? If it has 8 pi electrons, it can't be aromatic.

*fixed*

My vote is for anti-aromatic.

 

[QofQuimica]

*fixed*

My vote is for anti-aromatic.

Thanks, rcd. And I agree with you. BTW, kevin, for the heteroatom to be part of the aromatic ring, it must be sp2 hybridized, not sp3. The p-orbital is the one that contains the lone pair that is part of the aromatic system.

 

[kevin86]

Yes I can see how it is anti because it has 8 pi electrons after delocalization, thanks. But I was having this discussion with another guy about whether the Oxygen can be counted as sp3 or sp2. Generally speaking, are lone pairs considered part of sp3 or not?

For the molecule above the lone pair does delocalize right. So it behaves like sp2, but would it lone pairs be like sp3 if it was a regular ether without double bonds?

 

[QofQuimica]

Yes I can see how it is anti because it has 8 pi electrons after delocalization, thanks. But I was having this discussion with another guy about whether the Oxygen can be counted as sp3 or sp2. Generally speaking, are lone pairs considered part of sp3 or not?

For the molecule above the lone pair does delocalize right. So it behaves like sp2, but would it lone pairs be like sp3 if it was a regular ether without double bonds?

If the oxygen is conjugated, the lone pair has to be in a p-orbital, so the oxygen atom would be sp2-hybridized. If the oxygen is not conjugated, it can be sp3-hybridized.

 

[WestTexasRambler]

QofQuimica,


What is the splitting pattern for the second most downfield carbon 1-chloro-2-methyl propane?

I am getting confused as to whether the three hydrogens on the end carbon and the 2-methyl are equivalent.

My answer was 6 because they seem equivalent, where is my thinking wrong?


Thank you,


'Rambler

 

[QofQuimica]

Those 6 H atoms are equivalent, but they are only split by the one hydrogen on the adjacent Carbon atom, so they will produce a (doublet, 6H) resonance

To answer your original question, it actually doesn't matter whether they are equivalent or not. The second most downfield Hydrogen is the Hydrogen attached to the #2 Carbon atom. It will get split by 8 hydrogens on adjacent carbons, so it will produce a (9-tet (I don't know the official term), 1H) resonance.

I think where you might've been getting confused is:

1. What makes the resonances more downfield (in this molecule, it is the proximity to an electron-withdrawing substituent -- the Cl atom). The two protons on the #1 carbon will resonate most downfield, followed by the proton on the #2 carbon, followed by the 6 methyl protons.

2. Splitting vs. Height (Integration.) The height of the peak is proportional to the number of equivalent protons, while the splitting (in simple n+1 splitting) is the number of protons on ADJACENT carbon atoms + 1

Hope that clears things up....

Jota

Actually, it does matter whether they're equivalent. You'd have a septet of a triplet because the splitting due to the CH2 isn't equivalent to the splitting from the two methyls. That is, you'll have the two methyls, which split it into a septet, and then each septet peak will be split into a triplet. On a real NMR, it would probably look like a single broad peak. Rambler, I'd call it a multiplet. 😉

 

[WestTexasRambler]

QofQuimica,


What is the splitting pattern for the second most downfield carbon 1-chloro-2-methyl propane?

I am getting confused as to whether the three hydrogens on the end carbon and the 2-methyl are equivalent.

My answer was 6 because they seem equivalent, where is my thinking wrong?


Thank you,


'Rambler

Thank you ma'am.


'Rambler

 

[Lests55]

Hey Q,

The reasoning you gave for Rambler's problem is the same as mine, but I think TPR and EK are smoking something. For instance given:

1-chloro propane....

they say C-2's splitting pattern wil have 6 carbons because "it has 5 neighbors" WOW!

In my book it would be a quartet of triplets=12 peaks. I know this is the way my textbook treats NMR, so is there some hand trick or are they smoking crack?

Also what is the difference in calling that splitting a "q of t" or a "t of q". Obviously it's going to be a fat nasty mess, so maybe just the multiplet term is more appropriate.

OK one more question (this was actually from the gen chem part, but fits nicely here....) There was a question that said that an ether can't hydrogen bond with water...um...obviously it can't h-bond with itself but it seems like those electrons on oxygen could h-bond with water's protons.

THanks for all your help, I rocked out a 14-12-15 on AAMC 9 yesterday! Still hoping for a 36-37 on the real deal...

 

[Lests55]

Uh oh...another question...

In the MCAT student manual they mention something about being able to pick out solvents to use in recrystallization. Is this just a matter of picking out a solvent that will dissolve the compound of interest?

 

[QofQuimica]

Hey Q,

The reasoning you gave for Rambler's problem is the same as mine, but I think TPR and EK are smoking something. For instance given:

1-chloro propane....

they say C-2's splitting pattern wil have 6 carbons because "it has 5 neighbors" WOW!

In my book it would be a quartet of triplets=12 peaks. I know this is the way my textbook treats NMR, so is there some hand trick or are they smoking crack?

Also what is the difference in calling that splitting a "q of t" or a "t of q". Obviously it's going to be a fat nasty mess, so maybe just the multiplet term is more appropriate.

OK one more question (this was actually from the gen chem part, but fits nicely here....) There was a question that said that an ether can't hydrogen bond with water...um...obviously it can't h-bond with itself but it seems like those electrons on oxygen could h-bond with water's protons.

THanks for all your help, I rocked out a 14-12-15 on AAMC 9 yesterday! Still hoping for a 36-37 on the real deal...

As far as I know, there is not any difference between a quartet of triplets or the reverse. It's just going to depend on which neighbors you consider first. I don't think that EK is smoking anything, but they are definitely simplifying things. This is something you have to be careful about on the MCAT; if you over-think things, you can actually get questions wrong.

I think they mean that an ether can't be an H-bond donor, which is true. But like you said, it can definitely accept H-bonds. Ethers aren't really soluble in water for the most part, though, because they get pretty greasy quickly. Once you get up into the range of having 4 or 5 carbons (which you do as soon as you reach diethyl ether), they won't mix well. Dimethyl ether is actually a gas, but it is fairly water-soluble since there are only two carbons.

Fantastic job on Test 9! 👍 When you're done with all of this, I expect you to come help me answer questions for the August studiers. 🙂

For recrystallization, you want to ideally pick a solvent that will dissolve your compound well when it's hot, but won't dissolve it well when it's cold. It would be tough to pick the solvent theoretically; I think they'd probably have to give you some experimental data and ask you which solvent would be the best for recrystallization of that compound. For example, if I tell you that you want to recrystallize compound A, and 1) it's very soluble in diethyl ether at both cold and hot temperatures, 2) it's sparingly soluble in methanol at both cold and hot temperatures, 3) it's soluble in acetone at cold temperatures but not soluble at hot temperatures, and 4) it's soluble in ethyl acetate at hot temperatures but not at cold ones, you'd pick the ethyl acetate. Does that make sense?

 

[Lests55]

As far as I know, there is not any difference between a quartet of triplets or the reverse. It's just going to depend on which neighbors you consider first. I don't think that EK is smoking anything, but they are definitely simplifying things. This is something you have to be careful about on the MCAT; if you over-think things, you can actually get questions wrong.

I think they mean that an ether can't be an H-bond donor, which is true. But like you said, it can definitely accept H-bonds. Ethers aren't really soluble in water for the most part, though, because they get pretty greasy quickly. Once you get up into the range of having 4 or 5 carbons (which you do as soon as you reach diethyl ether), they won't mix well. Dimethyl ether is actually a gas, but it is fairly water-soluble since there are only two carbons.

Fantastic job on Test 9! 👍 When you're done with all of this, I expect you to come help me answer questions for the August studiers. 🙂

For recrystallization, you want to ideally pick a solvent that will dissolve your compound well when it's hot, but won't dissolve it well when it's cold. It would be tough to pick the solvent theoretically; I think they'd probably have to give you some experimental data and ask you which solvent would be the best for recrystallization of that compound. For example, if I tell you that you want to recrystallize compound A, and 1) it's very soluble in diethyl ether at both cold and hot temperatures, 2) it's sparingly soluble in methanol at both cold and hot temperatures, 3) it's soluble in acetone at cold temperatures but not soluble at hot temperatures, and 4) it's soluble in ethyl acetate at hot temperatures but not at cold ones, you'd pick the ethyl acetate. Does that make sense?

Thanks for the encouragement, I sure have gotten enough out of your services, that I should definitely try to "give back" and I would be happy to join you!

The recryst. makes sense, but I am still a little lost on EK's explanation. I don't see how I could think about in in that I should expect to only see 6 peaks. If "multiplet" were there answer, ok...but they say 6 peaks. Sorry for harping on this but there were two NMR questions on AAMC 9.

 

[QofQuimica]

Thanks for the encouragement, I sure have gotten enough out of your services, that I should definitely try to "give back" and I would be happy to join you!

The recryst. makes sense, but I am still a little lost on EK's explanation. I don't see how I could think about in in that I should expect to only see 6 peaks. If "multiplet" were there answer, ok...but they say 6 peaks. Sorry for harping on this but there were two NMR questions on AAMC 9.

They are assuming that the splitting from both neighboring sets of protons is equivalent. You and I know that it really isn't, but that's what they're assuming.

 

[Lests55]

They are assuming that the splitting from both neighboring sets of protons is equivalent. You and I know that it really isn't, but that's what they're assuming.

Ooooohhhhhhhhhhhhh......... 🙂
I guess that begs the question of which assumption to make on the big day?

 

[bravotwozero]

Hi,

I'm trying to learn the mechanisms of the imine/enamine reactions, and I am required to know these reactions in reverse, which the prof decided to let us figure out on our own.

But when I try to draw the mechanisms in reverse, what I can't figure out is what reagents to use for protonation and deprotonation. So, if the imine/enamine reaction occurs under anhydrous acidic conditions, what do I use in the reverse reaction to

i) Protonate the amine to convert it into a leaving group?
i) De-protonate the water molecule, so can it can re-form a ketone?

 

[QofQuimica]

Ooooohhhhhhhhhhhhh......... 🙂
I guess that begs the question of which assumption to make on the big day?

I'd base it on the context. See, this is what I'm talking about....too much knowledge is not always a good thing. 😛

 

[QofQuimica]

Hi,

I'm trying to learn the mechanisms of the imine/enamine reactions, and I am required to know these reactions in reverse, which the prof decided to let us figure out on our own.

But when I try to draw the mechanisms in reverse, what I can't figure out is what reagents to use for protonation and deprotonation. So, if the imine/enamine reaction occurs under anhydrous acidic conditions, what do I use in the reverse reaction to

i) Protonate the amine to convert it into a leaving group?
i) De-protonate the water molecule, so can it can re-form a ketone?

Use the acid itself to protonate it. Use the conjugate base of the acid (if it's not strong) or a solvent molecule (if it's polar) to deprotonate it. You can also use the amine to deprotonate.

 

[Dave_D]

If the oxygen is conjugated, the lone pair has to be in a p-orbital, so the oxygen atom would be sp2-hybridized. If the oxygen is not conjugated, it can be sp3-hybridized.

Sorry about not responding to this one quicker but I'm a bit confused Q.(And my curiousity is peaked.) Why would that molecule be anti-aromatic?(BTW what was the name of that compound anyway. After finishing orgo I haven't kept up my naming skills.) What I mean is you seem to suggest that the oxygens could be either sp2 or sp3. So why wouldn't at least one of the oxygens stay sp3? Wouldn't that make it non-aromatic and therefore more stable?(Or is there some other effect that forces both oxygens to be sp2 and therefore anti-aromatic?) Just curious since like I've said, I'm done with orgo YIPPIE 😀

 

[QofQuimica]

Sorry about not responding to this one quicker but I'm a bit confused Q.(And my curiousity is peaked.) Why would that molecule be anti-aromatic?(BTW what was the name of that compound anyway. After finishing orgo I haven't kept up my naming skills.) What I mean is you seem to suggest that the oxygens could be either sp2 or sp3. So why wouldn't at least one of the oxygens stay sp3? Wouldn't that make it non-aromatic and therefore more stable?(Or is there some other effect that forces both oxygens to be sp2 and therefore anti-aromatic?) Just curious since like I've said, I'm done with orgo YIPPIE 😀

Ok, there are two different issues here. One is that for a compound to truly be aromatic, EVERY atom in the ring must be conjugated. That means you cannot have any sp3-hybridized atoms in the system. (There actually is a special name for conjugated rings with an sp3-hybridized atom in them, but you don't have to know that for the MCAT.) The other is that an aromatic system must obey Huckel's Rule (have 4n + 2 pi electrons). So you are damned if you do, damned if you don't here. If the oxygen is sp3-hybridized, you will not have a fully conjugated system, and it will not be aromatic. If the oxygen is sp2-hybridized, you will disobey Huckel's Rule and STILL not have an aromatic compound.

In this case, I agree with you that technically the oxygen would stay sp3-hybridized. The system would be homoaromatic (sorry, I said I wasn't going to tell you the special name, but there you have it!), which is more stable than it being antiaromatic. But again, you really don't need to know any of this for the MCAT. They seem to pretty much just want you to recognize if a system is aromatic or not aromatic. This system would be classified as "not aromatic." Like I told Lests, too much knowledge on the MCAT can be a dangerous thing.

The IUPAC name for that molecule is 1,4-dioxine. You don't need to know that for the MCAT either.

 

[md2011]

QofQ, I feel so sorry for myself that I saw this thread so late. I had tons of questions about OChem before, and couldn't find anyone help me out. If I knew you were here, I could have got great help, sigh? But I am very glad I find it before the big test day🙂

Do you have AAMC test 7? If not, I will post it out for you to see. I cannot understand question #209. My questions are: 1) what is "cross product" of a reaction? 2)Why the chemist 1's mechanism is absent of cross products, while chemist 2's mechanism has cross products? 3)The last paragraph said....What this chemist 3 is trying to do? Can you explain the process?

Thanks a million!

 

[kevin86]

a simple question, if you can help me. What is the product of reacting an primary alcohol with h2so4. The answer seems to suggest it goes through e1 or sn1, but is the would that really work with the steric hinderance of the hso4 anion?

Oh by the way your answer to that aromatic question helped a lot.

 

[Dave_D]

a simple question, if you can help me. What is the product of reacting an primary alcohol with h2so4. The answer seems to suggest it goes through e1 or sn1, but is the would that really work with the steric hinderance of the hso4 anion?

Oh by the way your answer to that aromatic question helped a lot.

Wait a sec. You're doing a reaction with a group on a primary carbon and you're thinking E1 or SN1?

 

[md2011]

QofQuimica, does resonance always refers to a structure that has double bonds?

Another question is about gas chromatograph. In a test, it shows a graph with several hills, the first one is 45, followed by 3, 1, 0.5, 0.5. The question asked for what percentage of the second most abundant product was isolated. And the answer is 6%, because 45+3+1+0.5+0.5=50. And then 3/50=60%. I understand the calculation as it's easy, but I don't understand how gas chromatoph works. Can you explain that to me?

Thank you very much.

 

[Dave_D]

Thanks Q. I know weird I'm asking since I'm done with orgo but it's nice to hold on to this stuff just in case.(Ok, just in case I ever have to re-take the MCAT but damn I hope I don't.) Also it's interesting since I checked out some of those names on yahoo, never realized that compound is related to dioxin.

Hey quick question for you Q. Why does every orgo book out there state Huckel's rule that way when they could just tell you "Divide the number of pi e- by 2 and see if the result is odd. If so you have a Huckel number." (Ok, so I found my way easier to remember.)

Ok, there are two different issues here. One is that for a compound to truly be aromatic, EVERY atom in the ring must be conjugated. That means you cannot have any sp3-hybridized atoms in the system. (There actually is a special name for conjugated rings with an sp3-hybridized atom in them, but you don't have to know that for the MCAT.) The other is that an aromatic system must obey Huckel's Rule (have 4n + 2 pi electrons). So you are damned if you do, damned if you don't here. If the oxygen is sp3-hybridized, you will not have a fully conjugated system, and it will not be aromatic. If the oxygen is sp2-hybridized, you will disobey Huckel's Rule and STILL not have an aromatic compound.

In this case, I agree with you that technically the oxygen would stay sp3-hybridized. The system would be homoaromatic (sorry, I said I wasn't going to tell you the special name, but there you have it!), which is more stable than it being antiaromatic. But again, you really don't need to know any of this for the MCAT. They seem to pretty much just want you to recognize if a system is aromatic or not aromatic. This system would be classified as "not aromatic." Like I told Lests, too much knowledge on the MCAT can be a dangerous thing.

The IUPAC name for that molecule is 1,4-dioxine. You don't need to know that for the MCAT either.

 

[QofQuimica]

QofQ, I feel so sorry for myself that I saw this thread so late. I had tons of questions about OChem before, and couldn't find anyone help me out. If I knew you were here, I could have got great help, sigh? But I am very glad I find it before the big test day🙂

Do you have AAMC test 7? If not, I will post it out for you to see. I cannot understand question #209. My questions are: 1) what is "cross product" of a reaction? 2)Why the chemist 1's mechanism is absent of cross products, while chemist 2's mechanism has cross products? 3)The last paragraph said....What this chemist 3 is trying to do? Can you explain the process?

Thanks a million!

I don't have the AAMC tests, and I think that it is probably not a good idea for us to post anything from them. Technically we should not even be posting questions from test prep company books because of copyright issues, but posting real MCAT questions is definitely a big no-no....on the other hand, even though the AAMC tests are real MCAT questions, they HAVE been released, so it's probably ok for us to discuss them. I think the best thing to do would be for you to word the questions in a general way, and not post any AAMC questions or passages word-for-word. (I am removing the actual sentences that you posted for these reasons.) I will do my best to answer what you asked without having seen the passage or question.

1) Some reactions can form more than one product depending on how the two molecules react. These multiple products are the cross products.

2) Without knowing what reaction you are even talking about, all I can tell you is that the chemist with no cross products must have done something to control the regioselectivity (which atoms form bonds in the product) of the reaction. The chemist with multiple products did not control regioselectivity; that's why he ended up with multiple products.

3) Again, without reading the passage, I would have to guess that the chemist is trying to figure out the reaction mechanism by using radioactive labels. He labels the starting material, and then studies where the label went.

 

[QofQuimica]

a simple question, if you can help me. What is the product of reacting an primary alcohol with h2so4. The answer seems to suggest it goes through e1 or sn1, but is the would that really work with the steric hinderance of the hso4 anion?

Oh by the way your answer to that aromatic question helped a lot.

Like Dave_D said, it will be tough to do an E1 on a primary alcohol because that mechanism goes through a carbocation, and primary carbocations are usually unstable. On the other hand, you may be able to force it under extreme conditions....For MCAT purposes, I would say that a primary alcohol under strongly acidic conditions won't eliminate, though. (Again, this is technically not necessarily true, but it is as far as what you guys are supposed to know.)

 

[QofQuimica]

QofQuimica, does resonance always refers to a structure that has double bonds?

Another question is about gas chromatograph. In a test, it shows a graph with several hills, the first one is 45, followed by 3, 1, 0.5, 0.5. The question asked for what percentage of the second most abundant product was isolated. And the answer is 6%, because 45+3+1+0.5+0.5=50. And then 3/50=60%. I understand the calculation as it's easy, but I don't understand how gas chromatoph works. Can you explain that to me?

Thank you very much.

Hmm, I suppose technically no, it doesn't. It could be triple bonds instead. 😀 But even then, one of the resonance forms will have double bonds. I will try to think of an example of multiple resonance structures where NONE of the contributors has any double bonds, but off-hand, I can't think of any.

Gas chromatography (GC) works a little differently than the LC you are probably more familiar with. LC separates compounds based on polarity, but GC separates them more based on boiling point instead, with some separation based on polarity as well. Basically what you are doing is injecting a compound (or mix of compounds) onto a very long column that is kept inside an oven. The temperature of the column can thus be varied. As the temperature rises, the lower-boiling compounds will vaporize first and travel down the column. The columns are typically not packed like LC columns are, but instead are open in the center to allow the gas mobile phase to move through. The mobile phase is usually helium or some other inert gas like nitrogen. The important thing to understand about GC for the MCAT is that you cannot ever use it to separate compounds that are A) nonvolatile (no biomolecules!), or B) heat-labile (nothing that decomposes at high temperatures).

 

[QofQuimica]

Hey quick question for you Q. Why does every orgo book out there state Huckel's rule that way when they could just tell you "Divide the number of pi e- by 2 and see if the result is odd. If so you have a Huckel number." (Ok, so I found my way easier to remember.)

Because that doesn't sound as elegant? I don't know. But your method will also work fine. 🙂

 

[md2011]

Hmm, I suppose technically no, it doesn't. It could be triple bonds instead. 😀 But even then, one of the resonance forms will have double bonds. I will try to think of an example of multiple resonance structures where NONE of the contributors has any double bonds, but off-hand, I can't think of any.

Gas chromatography (GC) works a little differently than the LC you are probably more familiar with. LC separates compounds based on polarity, but GC separates them more based on boiling point instead, with some separation based on polarity as well. Basically what you are doing is injecting a compound (or mix of compounds) onto a very long column that is kept inside an oven. The temperature of the column can thus be varied. As the temperature rises, the lower-boiling compounds will vaporize first and travel down the column. The columns are typically not packed like LC columns are, but instead are open in the center to allow the gas mobile phase to move through. The mobile phase is usually helium or some other inert gas like nitrogen. The important thing to understand about GC for the MCAT is that you cannot ever use it to separate compounds that are A) nonvolatile (no biomolecules!), or B) heat-labile (nothing that decomposes at high temperatures).

Appreciate the reply. What does those numbers (45, 3, 1, 0.5) stands for?

 

[md2011]

I don't have the AAMC tests, and I think that it is probably not a good idea for us to post anything from them. Technically we should not even be posting questions from test prep company books because of copyright issues, but posting real MCAT questions is definitely a big no-no....on the other hand, even though the AAMC tests are real MCAT questions, they HAVE been released, so it's probably ok for us to discuss them. I think the best thing to do would be for you to word the questions in a general way, and not post any AAMC questions or passages word-for-word. (I am removing the actual sentences that you posted for these reasons.) I will do my best to answer what you asked without having seen the passage or question.

1) Some reactions can form more than one product depending on how the two molecules react. These multiple products are the cross products.

2) Without knowing what reaction you are even talking about, all I can tell you is that the chemist with no cross products must have done something to control the regioselectivity (which atoms form bonds in the product) of the reaction. The chemist with multiple products did not control regioselectivity; that's why he ended up with multiple products.

3) Again, without reading the passage, I would have to guess that the chemist is trying to figure out the reaction mechanism by using radioactive labels. He labels the starting material, and then studies where the label went.

With you explanation of cross product and reading the passage again, I think I figured out some of it. I still don't know what the chemist 3 trying to do, and don't know how to phrase it, as it contains very unfamiliar molecules to me. Since the question never asked it, I will just skip it.

I have a question about regioselectivity you mentioned. Regioselectivity only concerned about which atoms to form bond, but don't worry about the orientation of each bonding, is it right? What about stereoselectivity? can I say stereoselectivity only concerned about how the bonds are oriented, but don't care which atom to bond (kind of opposite to regioselectivity)?

 

[QofQuimica]

Appreciate the reply. What does those numbers (45, 3, 1, 0.5) stands for?

Those are the areas under the curve.

 

[QofQuimica]

I have a question about regioselectivity you mentioned. Regioselectivity only concerned about which atoms to form bond, but don't worry about the orientation of each bonding, is it right? What about stereoselectivity? can I say stereoselectivity only concerned about how the bonds are oriented, but don't care which atom to bond (kind of opposite to regioselectivity)?

You've got the general idea. Regioselectivity is concerned with which atoms react with which other atoms to form which new bonds, while stereoselectivity is concerned with how the molecules are spatially oriented when they form those bonds. A reaction that is not regioselective will give you constitutional isomers as products, while a reaction that is not stereoselective will give you stereoisomers as products.

 

[kevin86]

constitutional isomers, is that the same as structural isomers? last post was my mistake, I meant a secondary alcohol with h2so4. So would it be either sn1 or e1. Is the purpose of the acid only catalytic and only to remove the OH by protonate it? Would the hso4 anion be a good nucleophile or base?

 

[Dave_D]

constitutional isomers, is that the same as structural isomers? last post was my mistake, I meant a secondary alcohol with h2so4. So would it be either sn1 or e1. Is the purpose of the acid only catalytic and only to remove the OH by protonate it? Would the hso4 anion be a good nucleophile or base?

I'm thinking hso4 is a crummy(IE weak) base since it's the conjugate base of a strong acid.(Of course assuming I haven't completely forgotten all of my gen chem II) So I'd expect E1 with possible re-arrangement. (Q is probably going to come by now and let me know how wrong I am 😀 )