TPR o-chem passage a reasonable reflection of MCAT?

[circulus vitios]

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I'm using TPR organic chemistry review and I have a feeling that some of the passages are ridiculous. Here's an example. Is this reflective of o-chem difficulty on the MCAT?

Answers: C, D, B, C, A, A

 

[MedPR]

I'm using TPR organic chemistry review and I have a feeling that some of the passages are ridiculous. Here's an example. Is this reflective of o-chem difficulty on the MCAT?

Answers: A, B, A, C, C, D

This type of attitude is why people don't prepare well enough (and don't do as well as they could have) for the MCAT. Everything in prep-books is fair game on the MCAT. It's too hard for you so you think it won't show up on the MCAT? You need to drop that train of thought.

 

[slz1900]

This type of attitude is why people don't prepare well enough (and don't do as well as they could have) for the MCAT. Everything in prep-books is fair game on the MCAT. It's too hard for you so you think it won't show up on the MCAT? You need to drop that train of thought.

Uh, I've generally gotten 80+% in BR organic and got 0 of these right. Could you explain them?

 

[Kimiko Totani]

I'm using TPR organic chemistry review and I have a feeling that some of the passages are ridiculous. Here's an example. Is this reflective of o-chem difficulty on the MCAT?

Answers: A, B, A, C, C, D

Yeah, hun, the MCATs are hard. BUT--you may get lucky, and not have much org on the test. The MCAT sometimes is nice. Make sure you study hard and know the material. And when you go, get a good rest and a nice meal. It's a fun experience if you let it be.

 

[Kimiko Totani]

This type of attitude is why people don't prepare well enough (and don't do as well as they could have) for the MCAT. Everything in prep-books is fair game on the MCAT. It's too hard for you so you think it won't show up on the MCAT? You need to drop that train of thought.

But...it is also true that some books make things harder so that you actually do better on the test than you did with them. Good business, but that must make some people afraid of the test when they shouldn't be.

But, yeah, you should always study your best. You don't know what they'll ask that day when you're taking the MCAT.

 

[circulus vitios]

This type of attitude is why people don't prepare well enough (and don't do as well as they could have) for the MCAT. Everything in prep-books is fair game on the MCAT. It's too hard for you so you think it won't show up on the MCAT? You need to drop that train of thought.

I have a fixed amount of study time. There's no point in wasting hours studying arcane bull**** if it's never going to show up on a subsection of a subsection of the MCAT.

 

[circulus vitios]

Uh, I've generally gotten 80+% in BR organic and got 0 of these right. Could you explain them?

Sorry, I pasted the wrong answer key. It's fixed now.

 

[slz1900]

Sorry, I pasted the wrong answer key. It's fixed now.

OK I still did really badly but at least I feel a little better now.

 

[Postictal Raiden]

I'm using TPR organic chemistry review and I have a feeling that some of the passages are ridiculous. Here's an example. Is this reflective of o-chem difficulty on the MCAT?

Answers: C, D, B, C, A, A

With all fairness, this passage is not that bad compared what I had on my MCAT last July. All of the questions could be answered with basic knowledge of substitution and elimination rules. Besides, the passage was very short.

Be prepared to see worse than this. It was a big mistake not giving ochem the attention it deserves. Take it from a retaker.

P.S. I suck at ochem.

 

[Temperature101]

I'm using TPR organic chemistry review and I have a feeling that some of the passages are ridiculous. Here's an example. Is this reflective of o-chem difficulty on the MCAT?

Answers: C, D, B, C, A, A

Be prepared my friend... Be prepared!

 

[Temperature101]

I'm using TPR organic chemistry review and I have a feeling that some of the passages are ridiculous. Here's an example. Is this reflective of o-chem difficulty on the MCAT?

Answers: C, D, B, C, A, A

Just be prepared to see stuff like that on the test...I have seen an orgo passage worst than that.

 

[Trayshawn]

Nice. Just did it and I got all right but one. What don't you get?
Its totally fair game. Although it certainly helps that I'm an orga TA and have taken inorganic (relied quite a bit on my knowledge of transition metals). But you could still totally figure these out whith a bit of reasoning and I think this would be a solid MCAT passage.

 

[circulus vitios]

Nice. Just did it and I got all right but one. What don't you get?
Its totally fair game. Although it certainly helps that I'm an orga TA and have taken inorganic (relied quite a bit on my knowledge of transition metals). But you could still totally figure these out whith a bit of reasoning and I think this would be a solid MCAT passage.

The underlying concepts aren't difficult, it's just that the passage is so weird that I was thrown off and frustrated. Question 1 is simple but poorly worded; I had no idea what the hell "the final" alkene meant. Question 5 is confusing because the possibility of the solvent complexing with AlCl didn't occur to me.

 

[regeneration]

For #3 why would amines be more stable than ethers? I would think the O has more electronegativity to give, and would thus be a better lewis base?

For #5, can someone explain 😕. When I looked at those answer choices I saw 3 nonpolar & 1 polar solvent and thought to just jump on the polar.

 

[circulus vitios]

For #3 why would amines be more stable than ethers? I would think the O has more electronegativity to give, and would thus be a better lewis base?

For #5, can someone explain 😕

#3. I don't know what you mean by "more electronegativity to give." A is invalid because O is more electronegative then N, so any argument about N being too electronegative is invalid. C is invalid because amines make good ligands (I did not know this.) D is invalid because a tetracoordinated amine will have a +1 formal charge.

#5. Diisopropyl ether complexes with the aluminum in AlCl. The reaction is the same as scheme 1 or scheme 2. You want a solvent that doesn't contain an oxygen because it would compete with diisopropyl ether and attempt to complex with AlCl. CH2Cl is the only solvent that doesn't contain an oxygen. Simple but really not obvious at all.

 

[regeneration]

#3. I don't know what you mean by "more electronegativity to give." A is invalid because O is more electronegative then N, so any argument about N being too electronegative is invalid. C is invalid because amines make good ligands (I did not know this.) D is invalid because a tetracoordinated amine will have a +1 formal charge.

#5. Diisopropyl ether complexes with the aluminum in AlCl. The reaction is the same as scheme 1 or scheme 2. You want a solvent that doesn't contain an oxygen because it would compete with diisopropyl ether and attempt to complex with AlCl. CH2Cl is the only solvent that doesn't contain an oxygen. Simple but really not obvious at all.

For #3 -- Doesn't the nucleophility & electronegativity of an atom correlate with its lewis bascity? I guess I'm asking, why are amines better lewis bases than ethers?

 

[heartsavior]

I am a Physics and Organic Chemistry instructor for TPR, what medPR stated above is exactly what we tell students:

This type of attitude is why people don't prepare well enough (and don't do as well as they could have) for the MCAT. Everything in prep-books is fair game on the MCAT. It's too hard for you so you think it won't show up on the MCAT? You need to drop that train of thought.

 

[Trayshawn]

For #3 -- Doesn't the nucleophility & electronegativity of an atom correlate with its lewis bascity? I guess I'm asking, why are amines better lewis bases than ethers?

Amines are known to be quite strong bases. Much moreso than ethers. Think about what happens after the amine and ether are protonated (aka after they do what bases do). Which one is more stable? This is usually the best way to compare basicity.

If you assume that both the amine and ether were nuetral before binding to the metal (tertiary amine and secondary ether) than they both end up with positive charges. The less electronegative (or more electropositive) amine is better able to handle this formal charge than ethers and is thus a better base since its conjugate acid is more stable.

Even if you don't make the nuetrality assumption, and rather say that both the ether and amine are secondary, than the amine has a full negative charge whereas the ether is nuetral. Clearly, the amine will be a stronger base. (the conjugate acid stability explanation can be used here too).

In short, amines are stronger bases than ethers and are thus stronger ligands in T.S. complexes

 

[Trayshawn]

For #5, can someone explain 😕. When I looked at those answer choices I saw 3 nonpolar & 1 polar solvent and thought to just jump on the polar.

Not quite. Both ethanol and CH2Cl2 are polar. So two polar and two nonpolar. Eliminate the nonpolar choices. SN2 and SN1 both require polar solvents. Polar protic solvents (polar + H-bonds) favor SN1. Ethanol is polar protic. Passage says its SN2 so you eliminate ethanol and get your answer. You can answer it without even realizing that an ether as a solvent could lead to side reactions.

 

[slz1900]

Not quite. Both ethanol and CH2Cl2 are polar. So two polar and two nonpolar. Eliminate the nonpolar choices. SN2 and SN1 both require polar solvents. Polar protic solvents (polar + H-bonds) favor SN1. Ethanol is polar protic. Passage says its SN2 so you eliminate ethanol and get your answer. You can answer it without even realizing that an ether as a solvent could lead to side reactions.

But there is also an E1 reaction that occurs which prefers polar protic so how do we decide which part of the reaction to choose a solvent for 😕

 

[Trayshawn]

The SN2 is first, so thats the primary concern. Once it occurs, the cation will form no matter what.
Unimolecular reactions (the ones ending in "1") are all thats possible after the cation is formed. So E1 will occur next even though the solvent might favor an SN2 if it had the option (and it doesn't).

 

[Bru]

Not quite. Both ethanol and CH2Cl2 are polar. So two polar and two nonpolar. Eliminate the nonpolar choices. SN2 and SN1 both require polar solvents. Polar protic solvents (polar + H-bonds) favor SN1. Ethanol is polar protic. Passage says its SN2 so you eliminate ethanol and get your answer. You can answer it without even realizing that an ether as a solvent could lead to side reactions.

THF isn't non-polar. It is a polar aprotic solvent so in a way you got luck with your reasoning I think.

I think the more accurate reason would be that you wouldn't want an ether as your solvent considering the reaction.

 

[Trayshawn]

ehh...THF isn't very polar at all and is known to dissolve quite a few nonpolar compounds, but I see what you're getting at. I guess its best to consider the side reactions of other ethers.

 

[Laureatebarrel]

Wow this is mind-blowingly hard! Can somebody explain to me #1 and #2?

 

[Trayshawn]

its not man. it an unfamiliar topic, yes, but you know more than enough to answer it all.

in 1, a carbocation forms after the initial SN2. Theres no carbocation in the product, but rather a double bond. Whats the only real reaction (on the MCAT anyways) that makes a double bond from a carbocation? an E1.

in 2, the passage says the product chirality comes from the chirality of the organostannane. And the organostannane gets its chirality from the steric hindrance of the hydrogens facing each other.
Well in the molecule in question 2, there is no steric hindrance since the rings are pointing away. Thus the compound isn't chiral and won't lead to one particular enantiomer over the other. If it makes both enantiomers of the product equally, then you'd get a racemic mixture.

 

[circulus vitios]

its not man. it an unfamiliar topic, yes, but you know more than enough to answer it all.

in 1, a carbocation forms after the initial SN2. Theres no carbocation in the product, but rather a double bond. Whats the only real reaction (on the MCAT anyways) that makes a double bond from a carbocation? an E1.

in 2, the passage says the product chirality comes from the chirality of the organostannane. And the organostannane gets its chirality from the steric hindrance of the hydrogens facing each other.
Well in the molecule in question 2, there is no steric hindrance since the rings are pointing away. Thus the compound isn't chiral and won't lead to one particular enantiomer over the other. If it makes both enantiomers of the product equally, then you'd get a racemic mixture.

The passage states the sterically hindered reagent in scheme #2 gives a stereoselective product. Question #2 supposes a non sterically hindered reagent. The product is still chiral but since there is no steric hindrance in the reagent, there is no stereoselectivity -- the product is still chiral and you get a racemic mixture. You can't get a racemic mixture of non-chiral products; that wouldn't make sense.

 

[Trayshawn]

The passage states the sterically hindered reagent in scheme #2 gives a stereoselective product. Question #2 supposes a non sterically hindered reagent. The product is still chiral but since there is no steric hindrance in the reagent, there is no stereoselectivity -- the product is still chiral and you get a racemic mixture. You can't get a racemic mixture of non-chiral products; that wouldn't make sense.

Huh? I was referring to the orgostannane reagent in the question NOT the product. The reagent was chiral in the passage but is achiral in the question.
Of course you can't have a racemic mixture of an achiral compound...
An achiral precursor however, will always lead to a racemic mixture if its product is chiral. Thats the case here.

 

[Laureatebarrel]

Thanks! Now time to brush up on organic chem

 

[regeneration]

Amines are known to be quite strong bases. Much moreso than ethers. Think about what happens after the amine and ether are protonated (aka after they do what bases do). Which one is more stable? This is usually the best way to compare basicity.

If you assume that both the amine and ether were nuetral before binding to the metal (tertiary amine and secondary ether) than they both end up with positive charges. The less electronegative (or more electropositive) amine is better able to handle this formal charge than ethers and is thus a better base since its conjugate acid is more stable.

Even if you don't make the nuetrality assumption, and rather say that both the ether and amine are secondary, than the amine has a full negative charge whereas the ether is nuetral. Clearly, the amine will be a stronger base. (the conjugate acid stability explanation can be used here too).

In short, amines are stronger bases than ethers and are thus stronger ligands in T.S. complexes

Thanks for the explanation. I got confused because I was thinking that an alkoxy (O-) would be more basic than an amine, which it is, instead of thinking of an ether.