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.)

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