MP 33 : Breaking Down an Organic Chemistry MCAT Passage

Session 33

This week, Bryan and Ryan break down a passage from Next Step covering Organic Chemistry, Follow along and learn how Bryan teaches you to do this.

Organic chemistry is now Bryan's favorite after they changed the MCAT last year because it used to be a slog having to memorize a million different reaction mechanisms as well as “proper noun” names for the reaction.

Apparently, the new MCAT has a strong focus on real, general principles, lab techniques, and experimental procedures, which are good since these tend to stick with you so much better than some random stuff from a book. Students are 10x more likely to remember it if they do things themselves and you give them something to carry on an activity than having to randomly read something out of a textbook.

[03:18] Passage #1:

The characteristic fragrance, Chanel No. 5, one of the world's most well-known perfumes, is due almost wholly to 2-Methylundecanal, a compound found naturally in kumquats. The compound exists in two enantiomeric forms (Figure 1 and then they show us a picture of 2-Methylundecanal.

A chemistry class carried out an experiment to separate 2-Methylundecanal from 2-Methylundecanoic acid (“-al” means it's an aldehyde as opposed to “-oic” acid which means it a carboxylic acid).

They try to separate these two by carrying out a distillation of a liquid consisting solely of these two components. Due to the high boiling points of these compounds, the class was instructed to carry out a vacuum distillation. Students began by placing… (distillation steps given). We should certainly walk into the MCAT being comfortable with distillation procedures.

[04:22] Question #11:

Another possible method of separating 2-Methyundecanal and 2-Methylundecanoic acid could be based on:

(A) Their differences in the rotation of plane polarized light

(B) Mass spectrometry analysis

(C) Extraction based on their differing solubilities

(D) The very different scent profiles of each molecule

[05:05] Bryan's Insights:

When you think about it for a moment, it actually ends up being straightforward. But if your rush and let yourself fall into a panicky trap and pick up the first thing that seems reasonable, you're going to get it wrong. In fact, less than 40% of students get this right so this would be classified as the “most often gotten incorrect questions.”

More than 45% of students pick answer choice A. It's possible that this aldehyde and this carboxylic acid have different rotations of plane polarized light. R versus S, D versus L, and so on. But you've got to go back. It's important to answer exactly the question they asked you. In this case, the question was “a method of separating” these two molecules.

All you have to know among these answer choices is which one of these is a separation technique.

Answer choice [A] is an analysis technique. It's what you have in the beaker, not how to separate things out.

Answer choice [B] is again an analysis technique. It's what you have here and not a separation technique.

Answer choice [C] is the only separation technique listed here. Hence, this is the right answer.

Answer choice [D] is just a random filler answer choice considering the first sentence talks about a perfume.

So if you read the question exactly, it ends up being much more straightforward than it initially seems. Students think the MCAT is there to trick them but Bryan thinks it can be tricky there's never a trick question

[7:30] Question #12:

Boiling chips and vacuum distillation, respectively, are used in distillations to…

Bryan's Insights:

Here is one question you shouldn't even need the answer choice if you've prepared well for the MCAT, if you reviewed the basics of lab techniques, if you were a good organic chemistry student yourself, and if you remember these basic lab techniques.

We all remember the viral YouTube videos way back in 2009 maybe where they would pop Mentos candies into Diet Coke and a sudden fountain of bubbles could come out. This is due to the irregular surface of a Mentos that looks a lot like a boiling chip that provides all these little nooks and crannies where bubbles can form (whether in a soda or in a boiling fluid).

The point of the boiling chip is to provide “nucleation sites,” where little nucleus of gas can form and boil off. It prevents the fluid from becoming overheated.

Incidentally, a Hot Pocket in a microwave oven get insanely hot because the water in there doesn't have any nucleation sites to start boiling off or steaming off. So it actually heats the water past 100 degree Celsius which is very common in a microwave.

Now, it narrows us down to choices [A] and [D]. We just have to know the point of vacuum distillation and there is a clue from the passage which says, “due to high boiling points of these compounds, the class was instructed to carry out a vacuum distillation.”

What the vacuum apparatus does is suck off and remove the atmospheric pressure which helps lower the boiling points of the substances. This way, you don't need a Bunsen burner that gets to a million degrees. You can just use a normal desktop Bunsen burner. So the answer is choice [A]. Bunsen burners get hot but nothing more than a stove-top. The question is how would you boil this thing that needs really insanely high boiling point. Then remove the atmosphere from it.

[10:00] Question #13:

The liquid remaining in the round bottom flask at the end of the procedure was most likely:

[A] A mixture consisting of equal amounts of the two components

[B] 2-Methylundecanal

[C] Water condensed from air in the lab

[D] 2-Methylundecanoic Acid

Bryan's Insights:

This question requires a bit of outside knowledge about boiling points and to know that a carboxylic acid, because it has hydrogen bonding and that OH on the end of the carboxylic acid, is going to mean that one 2-Methylundecanoic acid molecule will stick to another molecule much more strongly.

Hydrogen bonding makes the molecule stick to each other so it's hard to boil them. When you start off a distillation with a mixture of these two substances, the aldehyde is going to be relatively easy to boil off. As you warm it up, the aldehyde boils away and goes into the little collection bowl by the end of the whole process and then the carboxylic acid gets left behind. Hence, the right answer is choice [D], the 2-Methylundecanoic acid is left behind in the round bottom flask at the end.

Links and Other Resources:

MedEd Media Network

The Premed Years Session 225

ryan@medicalschoolhq.net

Next Step Test Prep (Use the code MCATPOD to save some money from their offerings.)

Transcript

Introduction

Dr. Ryan Gray: The MCAT Podcast, session number 33.

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 on your premed path, and we’re here to give you the motivation and information that you need to know to help get you the score you deserve so you can one day call yourself a medical student.

Welcome to The MCAT Podcast, I am your host Dr. Ryan Gray, and I am excited to have you here. We're going to invite Bryan on in one second to talk all about the MCAT. I know you're so excited. If you have been here before then you know the drill. If this is your first time here, we're going to dig into some questions, you can download the handouts of these questions at our show notes page, www.TheMCATPodcast.com/33 for this episode.

I want to invite you to go listen to The Premed Years, session number 225, where we talk about common questions that premeds have about osteopathic medicine. I was excited to bring on the Junior National Director of the Pre-SOMA, which is the Student Osteopathic Medical Association, the premed club, the premed version of SOMA, and I brought her on and we talked all about osteopathic medicine. You can find that episode at www.MedicalSchoolHQ.net/225. Let's go ahead and jump into The MCAT Podcast right now.

Alright Bryan, we're back for another scintillating episode of The MCAT Podcast, this week with another passage, and organic chemistry is probably now your favorite.

Bryan Schnedeker: Absolutely. Well let me say this, after they changed the MCAT last year, it became my favorite. It used to be this horrible slog of having to memorize a million different reaction mechanisms, and all of these proper noun names for the reaction because apparently every organic chemist in the world had to name his reaction after himself instead of just what it was. But fortunately on the new MCAT, and the passage we're going to look at, there's been this real strong focus of shifting over towards real general principles, and even more than that, on lab techniques and experimental procedures which I like. I love anything where they're testing us on stuff we actually did in the lab since that has a tendency to stick with us so much better than some random stuff from a book.

Dr. Ryan Gray: Alright. I hate random stuff from a book so that's good, let's good.

Organic Chemistry Passage & Question #1

Bryan Schnedeker: Yeah well that's what all the literature shows, right? If the student does something themselves, you give them something to hold in their hands and actually carry out an activity, they remember so much more- ten times more likely to remember it than if it's just something they read randomly out of a textbook. Which is why I love all this lab-based stuff that we see now on the MCAT. And let's look at the passage here. It's a real short one so I'm going to go ahead and read it, but as always, print it out if you can. So we read, ‘The characteristic fragrance of Chanel Number Five, one of the world's most well-known perfumes is due almost wholly to 2-Methylundecanal, a compound found naturally in kumquats. The compound exists in [Inaudible 00:03:32] forms,' figure one and then they show us a picture of 2-Methylundecanal. ‘A chemistry class carried out an experiment to separate 2-Methylundecanal from 2-Methylundecanoic Acid.' Okay so 2-Methylundecanal ending in ‘AL' meaning it's an aldehyde, as opposed to 2-Methylundecanoic Acid, ‘-oic' acid meaning it's a carboxylic acid. ‘So they try to separate these two by carrying out a distillation of a liquid consisting solely of these two components. Due to the high boiling points of these compounds, the class was instructed to carry out a vacuum distillation. Students began by placing blah, blah, blah,' and then it gives us a series of steps for a distillation, and we should certainly walk into the MCAT being comfortable with distillation procedures. So let's take a look at number eleven here. ‘Another possible method of separating two 2-Methylundecanal and 2-Methylundecanoic Acid could be based on A) their differences in the rotation of plane polarized light. B) mass spectrometry analysis. C) extraction based on their differing solubilities. Or D) the very different scent profiles of each molecules. Okay another method of separating the aldehyde and the carboxylic acid. Again based on differences in rotation of light, mass spec, extraction, or based on their very different scent profiles. So this is one of my favorite questions that's on the entire Next Step full length one, because it's one that when you think about it for just a moment ends up being really straightforward. But if you rush, if you let yourself fall into that panicky trap of just picking the first thing that seems reasonable, you're going to get it wrong. And in fact less than 40% of students get this right, which would classify this as one of the most often gotten incorrect questions. So more than 45% of students pick that first answer choice A, the differences in the rotation of plane polarized light. Now it's entirely possible that this aldehyde and this carboxylic acid do have different rotations of plane polarized light; R versus S, D versus L, and so on. But you've got to go back, and Ryan we talked about this a whole bunch a few weeks back, about how important it was to answer exactly the question they asked you. And in this case the question was a method of separating these two molecules. And so really all you have to know among these answer choices is which one of these is a separation technique? Answer choice A, rotation of plane polarized light, that's an analysis technique. What do I have in the beaker, not how do I separate things out? Answer choice B, mass spec. Again that's an analysis technique, what do I have here? Not a separation technique. Answer choice C, extraction is the only separation technique even listed here, so that's the right answer. And then answer choice D, the scent profiles is just kind of a random fluffy filler answer choice because the first sentence talks about a perfume. So again if you read the question exactly, it ends up being much more straightforward than it initially seems.

Dr. Ryan Gray: Yeah I think it always comes back to students thinking the MCAT is there to trick them.

Bryan Schnedeker: Yeah absolutely. It can be tricky, but there's never a trick question. You know? It's tricky in that you have to know a lot of stuff, you have to read it really precisely, but there's never a trick question. Like, ‘ha-ha, Smith was the name of the fish,' or something. Or, ‘ha-ha the house is at the North Pole.' They're never out to like trick you.

Dr. Ryan Gray: Yeah.

Organic Chemistry Question #2

Bryan Schnedeker: Yeah alright let's take a look at number twelve. ‘Boiling chips and vacuum distillation respectively are used in distillations to.’ Now here's one where you shouldn't even need the answer choices, right? If you've well-prepared for the MCAT, if you reviewed the basics of lab techniques, if you were a good organic chemistry student yourself and you remember these basic lab techniques, you know what these are for. So a boiling chip, right? We all remember the Diet Coke and Mentos- or maybe we don't, maybe I'm really horribly dating myself. But those viral YouTube videos from the dark ages of YouTube back in 2009 maybe.

Dr. Ryan Gray: That's what I would guess, yeah.

Bryan Schnedeker: Yeah where we remember that they would pop these candies into Diet Coke and a sudden fountain of bubbles would come out, and that's because the irregular surface of a Mentos is actually a lot like a boiling chip. It provides all these little nooks and crannies where bubbles can form, whether they're bubbles of soda or bubbles in a boiling fluid. And so the point of a boiling chip is to provide what we call nucleation sites; sites where little nucleus of gas can form and then boil off, and it prevents the fluid from becoming overheated. Incidentally that's why something like a Hot Pocket in a microwave gets so insanely hot, because the water in there doesn't have any nucleation sites to start boiling off or steaming off, so it actually heats the water past 100 degrees Celsius. Very common in a microwave. So provides nucleation sites, we know that, and that actually narrows us down to choices A and D. And then we just have to know what the point of vacuum distillation is, and there was a clue here. The passage itself told us, ‘Due to the high boiling points of these compounds, the class was instructed to carry out a vacuum distillation.' So what the vacuum apparatus does is suck off the atmospheric pressure, remove the atmospheric pressure, and that helps lower the boiling points of the substances. That way you don't need a Bunsen burner that gets to like a million degrees, you can just use a normal desktop Bunsen burner. And so the answer there is choice A.

Dr. Ryan Gray: So vacuum distillation helps prevent buildings from burning down.

Bryan Schnedeker: Absolutely, yeah those little Bunsen burners- I mean they get hot, but they're nothing more than a normal stovetop really. And so the question is how would you boil this thing that needs this really insanely high boiling point? Well remove the atmosphere from it. Good safety for everybody.

Organic Chemistry Question #3

Dr. Ryan Gray: Alright last one here.

Bryan Schnedeker: Yeah last one, question thirteen. ‘The liquid remaining in the round bottom flask at the end of the procedure was most likely: A mixture consisting of equal amounts of the two components. 2-Methylundecanal. Water condensed from air in the lab. Or 2-Methylundecanoic Acid. So this one requires us to do a little bit of outside knowledge about boiling points, and to know that a carboxylic acid, because it has hydrogen bonding and that OH on the end of the carboxylic acid, is going to mean that 2-Methylundecanoic Acid molecule will stick to another molecule much more strongly. Hydrogen bonding makes the molecules stick to each other meaning it's hard to boil them. So when you start off a distillation with a mixture of these two substances, the aldehyde is going to be relatively easy to boil off. So as you warm it up, the aldehyde boils away and goes into the little collection bowl by the end of the whole process, and the carboxylic acid then gets left behind. And so that's the right answer there, choice D, the 2-Methylundecanoic Acid is left behind in the round bottom flask at the end.

Final Thoughts

Dr. Ryan Gray: Alright there you go, some more questions for you. Hopefully we did a good job breaking those down, hopefully I did a good job following up with Bryan with some questions. Again it's been so long since I've studied for the MCAT, I'm basically a super, super premed when it comes to the MCAT, and learning this information. So hopefully the follow-up questions and everything else that I'm asking are relevant and helpful for you.

If you have any suggestions on how we can make this podcast better, please do me a favor, email me. Ryan@medicalschoolhq.net.

I hope you have a great week. Don't forget to check out everything that Next Step Test Prep has to offer you on your journey to crushing the MCAT. They have their new MCAT course where they have five days a week live office hours where you can meet with their top experts in MCAT prep. It's included in the price of their MCAT course, it's their new online course which rivals anything from the other test prep companies. It has more information, more availability, and it is cheaper, and you can save some money using the special promo code only given here in this podcast, MCATPOD. All capital letters, MCATPOD. Again go check them out, www.NextStepMCAT.com.

Have a great week, we'll see you next week here at The MCAT Podcast.

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