MP 38 : Breaking Down Orgo MCAT Discrete Questions

Session 38

This week Bryan from Next Step Test Prep is going to help us break down organic chemistry discrete questions so you can crush your MCAT!

The MCAT has been re-balanced with this new format exam where they really toned down organic chemistry. It used to comprise 40% of one of the sections of the exam but now it has been reduced to a lowly 16% of one of four sections. But even though the mighty has fallen, we can't ignore this entirely.

[01:55] Question:

Which of the following alkyl chlorides is most likely to undergo SN1 reaction:

  • (A) Chloromethane
  • (B) Chloroethane
  • (C) 2-Chloropropane
  • (D) 2-Chloro-2-Methylpropane

Bryan's Insights:

Very few of these will show up in the MCAT but by far, the most common is our classic SN1 and SN2 reactions, nucleophilic substitution both unimolecular and bimolecular.

Remember that a tertiary carbon or a secondary carbon is going to be favored by SN1 whereas a methyl carbon or a primary carbon goes to SN2. In this case, for SN1, we're looking for a tertiary or a secondary carbon that would get attacked by the nucleophile.

The right answer here is (D) 2-Chloro-2-Methylpropane because the chloro is on the second carbon and there's also a methyl group on that second carbon so that's what we call a tert butyl group in the kind of common nomenclature. It's a classic substrate for SN1.

[03:35] A Quick Breakdown of SN1 and SN2 Reactions

The key to understanding this is starting with the mechanism itself where SN2 is the classic, backside attack. The nucleophile comes in, attacks the carbon, and kicks off the living group all in a single step. Just google SN1 versus SN2 and you will find a million charts out there. Rather than just memorizing, go down the list of all the different factors and figure out why this would favor a backside attack. Why would a methyl or a primary carbon favor a backside attack? And that's because there is less stearic hindrance. Or why would a polar aprotic solvent prefer SN2? So the nucleophile attacks the carbon instead of attacking the solvent. And so on and so forth.

Go down all the various factors and ask yourself how does this help the SN2 backside attack. As opposed to the SN1 mechanism, which is a two-step process, where first the living group just leaves. It's totally on its own and forms the carbocation intermediate. That's the big phrase you want to remember with SN1.

Again for SN2, remember the backside attack and for SN1, remember carbocation intermediate. And when you review these charts, don't just bluntly memorize them but try to understand how does this solvent or whatever favor forming that carbocation intermediate. If you can plug everything back into the mechanism, you will be much more likely be able to apply it correctly under pressure on test day as opposed to having memorized generic stuff in the abstract.

[06:30] Question 31:

If 3,000 molecules of triglycerides are hydrolyzed into their component molecules, what would the resulting mixture contain?

  • (A) 3,000 fatty acid molecules and 3,000 glycerol molecules
  • (B) 9,000 fatty acid molecules and 3,000 glycerol molecules
  • (C) 3,000 phospholipid molecules and 3,000 glycerol molecules
  • (D) 9,000 phospholipid molecules and 3,000 glycerol molecules

A triglyceride is your lipid, your classic energy storage that the body uses. Remember the nomenclature triglyceride, if you drew this, is E-shaped where there's these big long tails making up the three prongs of the E. So that's three fatty acid molecules and all of them are just plugged together in these little ester linkages to a glycerol backbone.

So if you take 3,000 fats and chop them up, you're going to get 3,000 glycerol backbones and 9,000 fatty acids. Simple math.

[08:15] Question #44:

For the reaction below, which solvent will best promote an SN2 mechanism of reaction. (Then it shows us a figure of an alcohol being attacked by a hydrochloric acid and the chloride is attaching on and H2O is the byproduct.)

  • (A) H2O
  • (B) Methanol
  • (C) Acetone
  • (D) Toluene

Bryan's Insights:

You should be able to walk into the test knowing what kind of solvent favors SN2 and you really don't have to look at the reaction.

On the MCAT, anytime two answer choices say the same thing, you can throw them both out. In this case, methanol has an OH group attached to a CH3. That alcohol, OH on the methanol and for H2O, the OH is in water, those are chemically very similar as far as their solvent behavior goes. Both of them have hydrogen bonding so you can throw out both choices (A) and (B) together because if it were H2O then methanol would be the same answer as they both behave the same way.

Now, we just have to decide between acetone and toluene. Keep in mind that in order for the reaction to go, to dissolve the hydrochloric acids, dissolve the propanol pictured here, you actually have to have a polar solvent. So the right answer here is acetone which is your classic polar, aprotic solvent, that SN2 is favored by.

For toluene, you wouldn't be even be able to dissolve the reagents because toluene is just a purely organic solvent.

[10:10] Polar and Nonpolar Molecules

And by polar, remember that a molecule can be either polar or nonpolar depending on the bonds found in the molecules. If you had a really electronegative atom bonded into a much less electronegative atom, like oxygen and hydrogen, that creates a polar bond. But that's just a polar bond. However, for a molecule to be a polar molecule, you have to add up all the polar bonds it has and they add as vectors. This has a little bit of physics as you have to know how to add vectors in physics. And if all of the polar bonds add up to an overall polar molecule then that's what you've got.

Whereas for example in carbon tetrachloride CCl4, (where chlorine is a lot more electronegative than carbon so a single bond from carbon to chlorine would be a polar bond) you have essentially a four-way tug-of-war with no winner. Therefore, a molecule ends up being non-polar because no one gets to win that tug-of-war on the electrons.

[11:45] About Next Step Test Prep

Next Step Test Prep is known for their one-on-one tutoring and if this is something you're looking into, go check them out. Additionally, they have an online course to study for the MCAT which includes tons of materials where you have access to ten of their full-length exams, all the AAMC material, five days of live office hours with their top instructors, and 100+ hours of videos covering everything you need to know for the MCAT at a much cheaper price than other companies out there. Use the promo code MCATPOD to save some money.

Links:

Khan Academy (sn1 versus sn2)

Next Step Test Prep (Use the promo code MCATPOD to save some money)

Transcript

Introduction

Dr. Ryan Gray: The MCAT Podcast, session number 38.

A collaboration between the Medical School Headquarters and Next Step Test Prep, The MCAT Podcast is here to make sure you have the information you need to succeed on your MCAT test day. We all know that the MCAT is one of the biggest hurdles as a premed, and this podcast will give you the motivation and information that you need to know to help you get the score you deserve so you can one day call yourself a physician.

Welcome to The MCAT Podcast, my name is Dr. Ryan Gray, and as always I am joined by the wonderful Bryan from Next Step Test Prep. Alright Bryan, we're back for some more MCAT question fun, this time organic chemistry.

Bryan Schnedeker: Yeah.

Dr. Ryan Gray: Anything exciting here?

Bryan Schnedeker: Oh gosh, no not in the entire field of organic chemistry. You know Ryan, you and I have talked a bunch on this podcast and on www.MedSchoolHQ.net's podcast about how the MCAT kind of got rebalanced with this new format exam, and how they really, really toned down the organic chemistry. You know what used to be back in the dinosaur days of when the MCAT was on paper, it used to be almost 40% of one of the sections of the exam has now been reduced to a lowly 16% of one of four sections. So the mighty have really fallen when it comes to organic chemistry. But having said that, we can't ignore it entirely. So we've been doing- gosh like a half a year, almost a year of these podcasts now, and I feel like organic chemistry deserves a little bit of love, a little bit of attention.

Dr. Ryan Gray: Just a little bit.

Bryan Schnedeker: Yeah.

Orgo Discrete Question #1

Dr. Ryan Gray: Alright so I'll read this first one. ‘Which of the following-‘ maybe you should read this first one because I don't want to screw up these words.

Bryan Schnedeker: Fair enough, okay. So ‘Which of the following alkyl chlorides is most likely to undergo an SN1 reaction? Chloromethane? Chloroethane? Two chloropropane? Two chloro, two methyl propane?' So of the various named reactions in MCAT land, you know in organic chem we learned like a billion of these things. Very, very few of them will show up on the MCAT, and by far- by far the most common is our classic SN1 and SN2 reactions. So nucleophilic substitution both unimolecular and bimolecular. So we want to remember that for SN1, that's favored by a tertiary carbon or a secondary carbon, is going to be favored by SN1. Whereas a methyl carbon or a primary carbon, that's going to probably go SN2. So in this case again, among the answer choices for SN1 we're looking for a tertiary or a secondary carbon that would get attacked by the nucleophile. And so again, our answer choices were chloromethane, chloroethane, two chloropropane, or two chloro, two methyl propane. And that last one is the right answer here because the chloro is on the second carbon, and there's also a methyl group on that second carbon. So that would actually be what we call a tert-butyl group in the kind of common nomenclature, and that's a classic sub-strain for SN1.

Dr. Ryan Gray: And if you could, a quick breakdown of the SN1, SN2 reactions?

Bryan Schnedeker: Sure so you can find a bazillion charts out there that give you like a rundown of what favors SN1 versus SN2, but the key to really not just memorizing it but understanding it is starting with the mechanism itself, where SN2 is the classic what they call back side attack. The nucleophile comes in, attacks the carbon, and kicks off the leaving group all in a single step. And when you memorize- Khan Academy has got it, Next Step has it, you can literally just Google ‘SN1 versus SN2' and find a million charts out there. Rather than just memorize it, go down the list of all the different factors and say, ‘Why would this favor a back side attack?' So why would a methyl carbon or a primary carbon favor a back side attack? Well there's less steric hindrance. Why would for example a polar aprotic solvent prefer SN2? Oh so the nucleophile attacks the carbon instead of attacking the solvent, and so on and so forth. You go down all the various factors and ask yourself, ‘How does this help the SN2 back side attack as opposed to the SN1 mechanism which is a two step process where first the leaving group just leaves, it just totally on its own decides, ‘See ya,' and takes off, and forms the carbocation intermediate, and that's the big phrase we always remember with SN1. So SN2 we remember back side attack, SN1 we remember carbocation intermediate. And so again, when you review these charts, don't just bluntly memorize them, try to understand how does this solvent, or whatever, or whatever, how does this favor forming that carbocation intermediate? And if you can plug everything back into the mechanism you'll be much more likely to be able to apply it correctly under the pressure of test day, as opposed to having just kind of memorized some generic stuff in the abstract.

Dr. Ryan Gray: So listening to you explain all of that, I just can't help but think that it's really organic chemistry's fault for why we are violent as human beings. All these attacks, and everything else.

Bryan Schnedeker: They really do- well I guess it was a field when it was first being invented 200 years ago that only let men in, so everything had to be all about attacking, and leaving, and I don't know, like some sort of little war game.

Dr. Ryan Gray: Yeah.

Bryan Schnedeker: It really could be so peaceful, it's all bonding. Bond-forming, it could be cooperating and sharing electrons.

Dr. Ryan Gray: They came in and held hands, they didn't attack.

Bryan Schnedeker: Right, exactly. The nucleophile wants to share its love of positive charge.

Dr. Ryan Gray: Alright we're going to re-write organic chemistry books.

Bryan Schnedeker: There you go.

Orgo Discrete Question #2

Dr. Ryan Gray: Alright 31 here. ‘If 3,000 molecules of triglycerides are hydrolyzed into their component molecules, what would the resulting mixture contain?' 3,000 fatty acid molecules and 3,000 glycerol molecules? 9,000 fatty acid molecules and 3,000 glycerol molecules? 3,000 phospholipid molecules and 3,000 glycerol molecules? Or D, 9,000 phospholipid molecules and 3,000 glycerol molecules? I don't know.

Bryan Schnedeker: Yeah we've got to know what a triglyceride is, that's just a fat, your lipid, your classic energy storage that the body uses. And we want to remember that that nomenclature there, triglyceride or triacylglyceride it's sometimes called tells you that there's three- and they draw it as like kind of an E shape where there's these big long tails making up the three prongs of the E. So that's three fatty acid molecules, and all of them are just plugged together in these little ester linkages to a glycerol backbone. So if you take 3,000 fats and chop them up, you're going to get 3,000 glycerol backbones, and just like choice B says, 9,000 fatty acids.

Dr. Ryan Gray: Makes sense, simple math.

Bryan Schnedeker: There you go, yeah.

Dr. Ryan Gray: Just got to think about it. It's interesting, again on this side of it now when we test for triglycerides in blood and stuff, thinking about this is a little more fun.

Bryan Schnedeker: Yeah absolutely. I mean it does ultimately- it gets you clinical eventually, right? Like you've got to push and pull, but this stuff is found in the body.

Dr. Ryan Gray: Yeah, alright go ahead and read question 44 here.

Orgo Discrete Question #3

Bryan Schnedeker: So we're back over in SN1 and SN2 land. So 44 says, ‘For the reaction below, which solvent will best promote an SN2 mechanism of reaction?' And then it shows us an alcohol being attacked by hydrochloric acid, and then chloride is attaching on, and H2O is the byproduct. So the actual reaction picture, you can see it of course if you want to go and get the handout from the show notes, but really you should just walk into the test knowing what kind of solvent favors SN2, and you don't even really have to look at the reaction. So the solvent choices we have listed here are H2O, methanol, acetone, and toluene. So first things first, on the MCAT anytime two answer choices say the same thing, you can throw them both out. So methanol, it has an OH group, it's attached to a CH3 because it's methanol. But that alcohol, OH on the methanol, and the H2O, the OH is in water, those are going to be chemically very, very similar as far as their solvent behavior goes. Both of them have hydrogen bonding, so you can throw out both choice A and B together. It's not H2O and it's not methanol because if it were H2O then methanol would be the same answer, like they would both behave the same way. So really we just have to decide between acetone and toluene, and we want to keep in mind that in order for the reaction to go, to dissolve the hydrochloric acid, to dissolve the propanol pictured here, you actually have to have a polar solvent. So the right answer here acetone is your classic polar aprotic solvent that SN2 is favored by. Toluene, the issue is you wouldn't even be able to dissolve the reagents because toluene is just a purely organic solvent.

Dr. Ryan Gray: And by ‘polar,' what do you mean?

Bryan Schnedeker: Right so we of course want to remember that when- a molecule on the MCAT can be either polar or non-polar, and that depends on the bonds found in the molecule. So if you have a really electronegative atom bonded to a much less electronegative atom, like say oxygen and hydrogen, that creates a polar bond. The polar bond itself though has to- that's just a polar bond. For a molecule to be a polar molecule, you have to add up all the polar bonds that it has and they add as vectors. So that goes to a little bit of physics, we have to know how to add vectors in physics. And if all of the polar bonds add up to an overall polar molecule, then that's what you've got. Whereas if the polar bonds cancel each other out, like for example carbon tetrachloride, CL4, chlorine is a lot more electronegative than carbon so a single bond from carbon to chlorine would be a polar bond. But in CCL4 you have essentially a four-way tug-of-war with no winner, right? The overall molecule ends up being non-polar because no one gets to win that tug-of-war on the electrons.

Final Thoughts

Dr. Ryan Gray: Alright there you have it, how to crush orgo discretes on the MCAT with some passages. If you head over to www.MedicalSchoolHQ.net and look for The MCAT Podcast session number 38, you can download the show notes- the handout for this episode and follow along with us.

If you don't know about Next Step Test Prep, if you just listen to this podcast and you're thinking to yourself, ‘Who the heck is Next Step Test Prep?' Let me tell you. They are known for their one-on-one tutoring. So if that's something you're looking into, a one-on-one tutor, go check them out. But they also have an awesome online course for you to study for the MCAT. This course has tons, tons, tons of material. You get access to all ten of their full length exams, you get access to all the AAMC material, you get access to five days of office hours- live office hours with their top instructors, and you get 100 plus hours of videos covering everything that you need to know for the MCAT for a price that's much, much cheaper than any of the other companies out there. Go check them out at www.NextStepMCAT.com, use the promo code MCATPOD, that's all capital letters MCATPOD to save some money.

Don't forget to check us out next week and every week here at The MCAT Podcast. Have a great week.

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