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
-how best to approach to MCAT organic passages
-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.

 

[BrokenGlass]

What's the difference between nucleophilic addition and nucleophilic substitution?

Here is an example of nucleophilic addition: adding HCl to a double bond of an alkene. H adds to one C and Cl adds to the other
C of the double bond. The double bond is no longer present in the product. Addition is kind of the reverse of elimination.

Here is an example of nucleophilic substitution: substituting a H on the benzene with something else. All the double bonds of
the benzene are intact in the product (because benzene wants to stay aromatic). Other examples of nucleophilic substitution are
Sn1 or Sn2.

 

[rcd]

Here is an example of nucleophilic addition: adding HCl to a double bond of an alkene. H adds to one C and Cl adds to the other
C of the double bond. The double bond is no longer present in the product. Addition is kind of the reverse of elimination.

Here is an example of nucleophilic substitution: substituting a H on the benzene with something else. All the double bonds of
the benzene are intact in the product (because benzene wants to stay aromatic). Other examples of nucleophilic substitution are
Sn1 or Sn2.

Thanks

 

[rcd]

How does the heat of combustion of alkenes compare to that of alkanes and why?

Thanks

 

[BrokenGlass]

How does the heat of combustion of alkenes compare to that of alkanes and why?

Thanks

Combustion is an oxidative process which releases energy. More reduced compounds release more energy upon complete oxidation (simply because they can be oxidize more). Therefore alkane carbons should have a higher heat of combustion as compared to more oxidized (less reduced) alkene carbons.

 

[prmdbeach17]

Hi, I was wondering if anyone had a list of strong/weak deactivators/activators? It's not really for the MCAT..it's for ochem 2 class and I wasn't sure if I could ask here but I couldn't find another organic chem thread. And if anyone is allowed to answer questions, to the poster above..in nucleophilic subsitution, there is a good leaving group which leaves so the nucleophile, which attacks the (+) charge, can be added. In nucleophilic addition there is no good leaving group and the nucleophile just adds on at the (+)and there is no subsitution..if that helps

 

[prmdbeach17]

Okay wow so somehow I totally missed the ending posts that someone had already answered the question. Sorry! Lol...but if anyone has a list of strong/weak deactivators/activators that would be really helpful. Thanks!

 

[BrokenGlass]

Okay wow so somehow I totally missed the ending posts that someone had already answered the question. Sorry! Lol...but if anyone has a list of strong/weak deactivators/activators that would be really helpful. Thanks!

Electron density can be donated or withdrawn in 1 of 2 ways: via resonance or via inductive effect. Lets condiser a monosubstited benzene ring.

If the atom directly attached to the benzene ring has a lone pairs of electrons, the substituent donates electron density to benzene via resonance (e.g. a hydroxy group).

If the atom directly attached to the benzene ring is also connected to some other atom via Pi bond(s), the substituent withdraws electron denstity from benzene via resonance (e.g. a carboxyl group of benzoic acid).

If the atom attached to the benzene has a formal positive charge, it withdraws electron density from benzene with inductive effect
(e.g. a nitrogen with 4 things attached).

If the substituent attached to the benzene ring is a (relatively) highly electronegative atom, it withdraws electron density from benzene via inductive effect (e.g. Chlorine).

If the substituent is an alkyl group, it donates electron density to benzene via inductive effect (e.g. -CH3).

Notice that halongens donate electron density by resonance (because they have lone pairs) and at the same time withdraw electron density via inductive effect (because they are electronegative).

I don't think there is much more to know about electron donating and withdrawing groups...

 

[BerkReviewTeach]

It must be 4. Only sp3 hybridized atoms can be chiral. If an sp3 hybridized atom has 4 different substituents, then we can assign an R or S absolute configuration to this chiral center.

Actually, there is a an exception to sp3 hybridization/chirality, but it almost never comes up.

Obscure exception indeed. The typical exception is allene with unequal substituents at the two ends. For example:

(H)ClC=C=C(H)Cl​

This molecule is chiral (can exist as one of two enantiomers-nonsuperimposible stereoisomers), but technically there is no R or S assignment for any atom. It is a chiral molecule with no stereogenic centers.

BTW, in the octet/resonance argument, you both have strong points in describing a non-exact set of definitions. Charged carbons are a great example because of their potential to appear on the MCAT. Carbanions have a carbon carrying a charge that fits the octet rule, while carbocations do not fit the octet rule. Both types of charged sites can be stabilized by resonance (if it is possible in the structure). So, the octet rule and resonance should be treated as seprated entities.

Where the two concepts collide is in ranking the stability of the various resonance structures. Resonance structure that do not violate the octet rule are considered more stable than those that do violate the octet rule. A great example is carbon monoxide, where the most stable resonance structure has charged atoms and full octets around carbon and oxygen:

-:C=O:+​

 

[rcd]

EK says you can reduce alcohols to alkanes. Is that true? How would go about it?

 

[MSTPbound]

EK says you can reduce alcohols to alkanes. Is that true? How would go about it?

Do they say you can do it in one step? I'm not sure how to do it in one step if there is a way, but I guess you could dehydrate the alcohol with acid, and then reduce the double bond with a platinum catalyst.

 

[killinsound]

Do they say you can do it in one step? I'm not sure how to do it in one step if there is a way, but I guess you could dehydrate the alcohol with acid, and then reduce the double bond with a platinum catalyst.

you can reduce them to halo alkanes in one reaction

 

[BrokenGlass]

EK says you can reduce alcohols to alkanes. Is that true? How would go about it?

You add a strog acid catalyst which protonates the hydroxy group of the alcohol, making water (a good leaving group). Next water leaves, leaving
behind a carbocation, which may rearrange if a more stable carbocation can be formed (via an alkyl shift or a hydride shift). Next a hydrogen is removed
from the carbon adjacent to the carbocation and an alkene is formed. As a last step, we can do catalytic hydrogenation to convert an alkene to an alkane...

There could be other ways of doing this...

 

[BerkReviewTeach]

you can reduce them to halo alkanes in one reaction

Conversion of an alcohol to an alkyl halide (halo alkane) is a substitutin reaction, not a reduction. The oxidation state of the active carbon has not changed when an alcohol has been converted into an alkyl halide.

The mulitple-step conversions described by both Broken Glass and MSTPbound are the only common methods (at the level of the MCAT) that can be used.

 

[phospho]

quick question please 🙂:

Which is more stable: trans-2-Butene or 2-Methylpropene ?

I had to get their heats of combustion to know that the latter was more stable - however, my question is if you could have known that by just looking at them... i.e. you know the small rules like how you want the alkyl groups trans to each other, and how alkyl groups typically stabilize, etc... I couldn't use any rule to be able to distinguish between those two, however. I wish I could draw them for you, because you'll see what I mean.

Thanks so much in advance🙂

 

[Foghorn]

quick question please 🙂:

Which is more stable: trans-2-Butene or 2-Methylpropene ?

I had to get their heats of combustion to know that the latter was more stable - however, my question is if you could have known that by just looking at them... i.e. you know the small rules like how you want the alkyl groups trans to each other, and how alkyl groups typically stabilize, etc... I couldn't use any rule to be able to distinguish between those two, however. I wish I could draw them for you, because you'll see what I mean.

Thanks so much in advance🙂

I would think trans-2-butene is more (thermodynamically) stable due to less sterics between the methyl groups.

My book (L.G. Wade) gives 2-methylpropene releasing 28.0 kcal/mole and trans-2-butene releasing 27.6 kcal/mole for heats of hydrogenation i.e. the trans is more stable.

Generally, the more substituted an alkene is the more stable the molecule.

Stability trends for alkenes: trans > cis
more substituted > less substituted
larger substituents > smaller substituents (Ex: ethyl > methyl)

Ok, my other book (Solomon's) says that for alkenes that are structural isomers, determining which one is more stable must be done empirically by heats of combustion because the products (water and CO2) are the same. Using heats of hydrogenation isn't useful because the molecules being hydrogenated are structurally different and give different products. Basically, you can't tell visually given the starting materials.

 

[phospho]

I would think trans-2-butene is more (thermodynamically) stable due to less sterics between the methyl groups.

My book (L.G. Wade) gives 2-methylpropene releasing 28.0 kcal/mole and trans-2-butene releasing 27.6 kcal/mole for heats of hydrogenation i.e. the trans is more stable.

Generally, the more substituted an alkene is the more stable the molecule.

Stability trends for alkenes: trans > cis
more substituted > less substituted
larger substituents > smaller substituents (Ex: ethyl > methyl)

LOL!

now i'm really confused though, because if your book says that, then you are right, the trans one would be more stable. However, my book (Carey) says that the heat of combustion for trans-2-Butene is 2707 kj/mol, and that for 2-methylpropene it is 2700 kj/mol. That would mean that the trans one would be the LESS stable one according to my book.😱

 

[phospho]

I would think trans-2-butene is more (thermodynamically) stable due to less sterics between the methyl groups.

My book (L.G. Wade) gives 2-methylpropene releasing 28.0 kcal/mole and trans-2-butene releasing 27.6 kcal/mole for heats of hydrogenation i.e. the trans is more stable.

Generally, the more substituted an alkene is the more stable the molecule.

Stability trends for alkenes: trans > cis
more substituted > less substituted
larger substituents > smaller substituents (Ex: ethyl > methyl)

Ok, my other book (Solomon's) says that for alkenes that are structural isomers, determining which one is more stable must be done empirically by heats of combustion because the products (water and CO2) are the same. Using heats of hydrogenation isn't useful because the molecules being hydrogenated are structurally different and give different products. Basically, you can't tell visually given the starting materials.

you just edited your post, right?
thank you!! i feel so much better 🙂

 

[philios]

help! Ive been reading Examkrackers Organic Chem book and have a question about bond energy & stability. The answer to lecture question 3 (p 141) says that the most stable bond is the one with the highest energy...however on page 11 in the lecture it states that "since the electrons in a sigma bond are close as possible to the two sources of positive charge (nuclei) a sigma bond has the lowest energy and is the most stable form of covalent bond...
😕😕😕

 

[philios]

haha...found the answer to my own question:
"high bond energy indicates a bond with electronrs at very low energy, and is a stable bond" (p17)

 

[BrokenGlass]

haha...found the answer to my own question:
"high bond energy indicates a bond with electronrs at very low energy, and is a stable bond" (p17)

Your question has to do with the difference between bond energy and bond dissociation energy. High bond dissociation energy means it takes a lot of energy to break a bond (because the bond is very stable). Bond energy, on the other hand, is referring to the stability of a bond with respect to electronic configuration, steric constraints, ring strain, etc., so sigma bonds are in lower energy configuration than Pi bonds...

 

[scotties123]

im not understanding the concept of an azeotrope. EK says its when there is an exact ratio of 2 liquids in a solution (during distillation), and then gives an example of 5% ethanol and 95% water solution being an azeotrope. So confused.....how is that an azeotrope? What IS an azeotrope? THANKS!

 

[spicedmanna]

im not understanding the concept of an azeotrope. EK says its when there is an exact ratio of 2 liquids in a solution (during distillation), and then gives an example of 5% ethanol and 95% water solution being an azeotrope. So confused.....how is that an azeotrope? What IS an azeotrope? THANKS!

Well, my understanding of an azeotropic mixture is that when the liquid mixture is distilled, the composition of the distillate is the same as the composition of the original (1). An azeotropic mixture that boils at a lower temperature than any of the original components that comprise the mixture is called a postive azeotrope and an azeotropic mixture that boils at a higher temperature than any of the original components is called a negative azeotrope (2).

In a liquid mixture, molecules of the two components will have a certain affinity toward one another and to themselves (2). In an ideal mixture, there is no significant differential between affinities (2). However, in an azeotropic mixture, the affinities are significantly unequal; for example, in a binary azeotropic mixture there may be a significantly greater affinity of the two components for each other than they have for themselves. (2).

 

[SB100]

im not understanding the concept of an azeotrope. EK says its when there is an exact ratio of 2 liquids in a solution (during distillation), and then gives an example of 5% ethanol and 95% water solution being an azeotrope. So confused.....how is that an azeotrope? What IS an azeotrope? THANKS!

I think you mean 95% ethanol/5% water.

 

[spicedmanna]

I think you mean 95% ethanol/5% water.

Haha, yeah, you are right; thanks for pointing that out. I didn't catch that when I was reviewing his or her post.

 

[AngedAmour326]

Can someone explain halogenation and SN1 and SN2??? I'm soo confusedddd...

 

[shadow27]

When is it important to consider syn vs anti addition reactions, for example, Hydroboration vs. Oxymercuration/Demercuration?

To be sure, syn=substituents add to same side of alkene
anti=substituents add to opposite sides of alkene

I thought one situation might be when doing additions to something in a ring structure where there is limited rotation around sigma bonds...but that might be a stretch.


I guess my real question is if this is important to know and how it may be tested on the MCAT.

Thanks!

 

[spicedmanna]

When is it important to consider syn vs anti addition reactions, for example, Hydroboration vs. Oxymercuration/Demercuration?

To be sure, syn=substituents add to same side of alkene
anti=substituents add to opposite sides of alkene

I thought one situation might be when doing additions to something in a ring structure where there is limited rotation around sigma bonds...but that might be a stretch.


I guess my real question is if this is important to know and how it may be tested on the MCAT.

Thanks!

I'll take a stab at this question. I would say know the basics, but don't stress over it too much. You probably don't need to focus on every scenario, or variation. I think as long as you understand the basic concept, know how to apply it, and know how it might come about as a consequence of a given mechanism, then you should be okay. I don't know how this particular concept might be tested, but I think that the MCAT writers seem to like to ask you to draw on simple concepts and use them in an unfamiliar context.

For example, you may get a passage that describes a very long and complex looking mechanism sequence, one that you've never seen before (and hope to never see again). You might quickly see that while the whole sequence is complex, if you look at it more closely the sequence shares similarities to several smaller and more familiar mechanisms. You may have a series of questions that ask you to make predictions based on the given mechanism, such as what the consequences of a given step in the mechanism might be, an explanation of a given step, what would happen if a modification were made in that step, the resultant product if different starting material/reaction were used, an interweaving of spectroscopy-related concepts and questions, etc. You get the point. For a passage of this sort, you basically need to draw on your outside knowledge and apply it in the context of the unfamiliar mechanism.

There are different types of passages. There may also be experiment-type passages that use the experiment, or study, as the context, different types of information-based passages that describe organic syntheses, a sequence of mechanisms, or complex concepts, or passages that include contrasting theories, etc., whatever. The context shouldn't matter.

Anyway, as you take more practice tests, particularly of the AAMC variety, you'll begin to get a feel for how concepts are tested and questions are asked on the MCAT.

 

[scotties123]

I came across a question on an AAMC exam that asked which had a lower boiling point, N2 or O2. I chose the answer choice: "N2 because it has a triple bond, where O2 only has a double bond." The correct answer choice was "O2 because it has a higher molecular weight." I Know that higher molecular weight increases boiling point, but the molecular weights only differ by 4. I thought little of a difference made the molecular weight difference negligable. I guess what I'm asking though is how do double and triple bonds effect boiling point? or dont they...

 

[BrokenGlass]

I came across a question on an AAMC exam that asked which had a lower boiling point, N2 or O2. I chose the answer choice: "N2 because it has a triple bond, where O2 only has a double bond." The correct answer choice was "O2 because it has a higher molecular weight." I Know that higher molecular weight increases boiling point, but the molecular weights only differ by 4. I thought little of a difference made the molecular weight difference negligable. I guess what I'm asking though is how do double and triple bonds effect boiling point? or dont they...

Boiling point is affected by molecular weight and INTERmolecular forces. Covalent bonds don't affect boiling point because they only provide INTRAmolecular forces. Boiling is a physical process, not a chemical reaction, so the only bonds that are affected are bonds BETWEEN molecules, NOT bonds WITHIN a molecule.

 

[jochi1543]

Can someone explain halogenation and SN1 and SN2??? I'm soo confusedddd...

Sn1 - 2-step, a carbocation intermediate is involved, so tertiary, allylic, and benzylic substrates do best. Best in protic, polar solvents. Nucleophile doesn't matter, though a strong nucleophile (think OR-) may cause a greater share of E1 product in the resulting mixture. A racemic mixture or otherwise optically inactive product results.

Sn2 - 1-step, nucleophile attacks the substrate from behind at the same time as the leaving group dissociates. Polar, aprotic solvents are best, as are less hindered substrates (primary, secondary), because they have a central atom that is easier for the attacking nucleophile to attack.

 

[streetlight]

Hydroboration of alkynes. What's the mechanism here? I can't really understand how Kaplan explains it.

Thanks!

 

[jochi1543]

Hydroboration of alkynes. What's the mechanism here? I can't really understand how Kaplan explains it.

Thanks!

You don't need to know anything about alkynes on the MCAT. However, for gen ed purposes:


BH3 will add to the same atom (the less substituted one) TWICE. Then there will be two OH groups replacing the two BH3 substituents. Then, if I remember correctly, one OH group leaves and a double bond to the remaining OH forms. In the end, an aldehyde is produced (with a terminal alkyne).

I'm ALMOST sure about the aldehyde thing (just speaking from memory now), but I know the 2 OH groups on the same atom is definitely what happens at first.

 

[voirlesetoiles]

In EK organic chem pg 17, it says "thus, high bond energy indicates a bond with electrons at very low energy and is a stable bond. recall from inorganic chemistry that this is because the high energy bond is really a high negative energy bond."

I'm confused with the last sentence in the quote... Could someone please clarify the last line? What is meant by a high negative energy bond?

 

[BrokenGlass]

In EK organic chem pg 17, it says "thus, high bond energy indicates a bond with electrons at very low energy and is a stable bond. recall from inorganic chemistry that this is because the high energy bond is really a high negative energy bond."

I'm confused with the last sentence in the quote... Could someone please clarify the last line? What is meant by a high negative energy bond?

This is from EK forum:

A stable bond is one in which the electrons are happy, and the geometry of the bond is optimal. Thus it would take alot of
energy to break this bond and ruin the stability. A highly unstable bond is one that takes little energy to break because the electrons are very reactive and the geometry of the bond may not be optimal. Generally, the magnitude of the bond energy
corresponds to the vertical drop that appears in a potential energy well at the lowest point, or most stable position, in the
curve an energy diagram. Thus this drop is sometimes considered a negative movement in the energy diagram.

 

[phospho]

Good Morning,

Would someone be so kind enough to tell me what this compound is called, and also explain to me why in the world it is aromatic? From what I can see, it only has 4 pi electrons, i.e. it doesn't follow Huckel's rule.

Many thanks in advance.

 

[phospho]

Good Morning,

Would someone be so kind enough to tell me what this compound is called, and also explain to me why in the world it is aromatic? From what I can see, it only has 4 pi electrons, i.e. it doesn't follow Huckel's rule.

Many thanks in advance.

hmm, just found out that it's actually pyrrole... but I still can't see why it would be aromatic...🙄

 

[phospho]

hmm, just found out that it's actually pyrrole... but I still can't see why it would be aromatic...🙄

I'm talking to myself again:

I'm stupid... i totally forgot about heterocyclic aromatic compounds, which is also why the nitrogen in this case would be considered to be sp2 hybridized in this compound.

Please disregard my original question - I'm not going to delete the post in case someone has/had a similar question (unless someone wants me to delete it).

 

[Foghorn]

hmm, just found out that it's actually pyrrole... but I still can't see why it would be aromatic...🙄

The molecule's pyridine not pyrrole.

 

[phospho]

The molecule's pyridine not pyrrole.

oops, actually i just realized I scanned the wrong one...lol...i was definitely talking about pyrrole though... thanks for the heads up! 🙂

 

[roycer]

I had a question: what is the hybridization of the nitrogen in pyrrole?

http://upload.wikimedia.org/wikipedia/commons/thumb/1/15/Pyrrole.png/300px-Pyrrole.png

Is it sp3 or sp2 since it is a conjugated system?

 

[Comdessert]

Hi there,

A question in EK's 1001 OC begins as, "a neat solution of 1-phenylethanol..."

What is a "neat solution"?

Thanks,

- CD

 

[scotties123]

Is it true that Sn1 and E1 reactions are UNImolecular and Sn2 and E2 reactions are BImolecular? Also, what does uni and bi refer to? thanks.

 

[spicedmanna]

Is it true that Sn1 and E1 reactions are UNImolecular and Sn2 and E2 reactions are BImolecular? Also, what does uni and bi refer to? thanks.

It refers to the rate law of the determining step of a given reaction. The rate for unimolecular reactions is determined by one species of the reaction, while the rate for a bimolecular reaction is determined by two species.

e.g.,

Unimolecular reaction: Rate = k[X]
Bimolecular reaction: Rate = k[X][Y]

 

[scotties123]

so its completely UNRELATED to sn1/sn2, etc, etc

 

[spicedmanna]

so its completely UNRELATED to sn1/sn2, etc, etc

No, it's definitely a related, but independent concept. It is used to describe the kinetic behavior of these reactions. Essentially, how it applies is this:

The rate of SN1 & E1 reactions depend only on the concentration of the reactant and not the nucleophile, while the rate of the SN2 & E2 reactions depend on both. This has implications.

 

[scotties123]

So as long as it ISNT zeroeth order for a reactant/substrate, then the basic rule is Sn1/E1 UNI, and Sn2/E2 BI? thanks by the way, i really appreciate your help with this. And sorry for having posted this in the wrong spot.

No, it's definitely a related, but independent concept. It is used to describe the kinetic behavior of these reactions. Essentially, how it applies is this:

The rate of SN1 & E1 reactions depend only on the concentration of the reactant and not the nucleophile, while the rate of the SN2 & E2 reactions depend on both. This has implications.

 

[spicedmanna]

Hi there,

A question in EK's 1001 OC begins as, "a neat solution of 1-phenylethanol..."

What is a "neat solution"?

Thanks,

- CD

My guess is that it is referring to an undiluted solution (1).

 

[spicedmanna]

So as long as it ISNT zeroeth order for a reactant/substrate, then the basic rule is Sn1/E1 UNI, and Sn2/E2 BI? thanks by the way, i really appreciate your help with this. And sorry for having posted this in the wrong spot.

No problem. I'm glad I could be of assistance. It is accurate, I think, to say kinetic studies support that SN1 & E1 reactions are unimolecular with respect to rate and that SN2 & E2 reactions are bimolecular. I think you'd be safe in assuming that this is a basic rule of thumb for these types of reactions.

 

[BrokenGlass]

Is it true that Sn1 and E1 reactions are UNImolecular and Sn2 and E2 reactions are BImolecular? Also, what does uni and bi refer to? thanks.

My 2 cents: BI-molecular means 2 molecules are colliding in the rate-determing step (it's an associative mechanism). UNI-molecular means there is only 1 molecule in the rds (it's a dissociative mechanism).

 

[Comdessert]

My guess is that it is referring to an undiluted solution (1).

Hi spicedmanna,

Thanks for the response and the correction 😀

- CD