so fun fact i was not always pre-med um and even though it wasn't required i decided to take organic chemistry because my dad and my brother said that it was fun i dropped that class within like two days of being in it but then i did switch back to pre-med had to take organic chemistry again and actually ended up really liking it and i was an organic chemistry tutor at the end of it because i am extremely predictable so if you don't enjoy organic chemistry believe me i completely understand but there are certain passages on the mcat that are heavily organic chemistry centered and we got to get through them one of those passages is the one we're going to be going through today which is passage 8 in the cp section of the double amc sample test so let's get right into it so as you can see we've got like a pretty solid reaction down here so it's going to be pretty heavily organic chemistry we're going to start out by flow charting terpenes constitute a class of non-saponifiable liquids whose carbon skeletons are composed of isoprene two methyl one three butadiene units according to the isoprene rule adjacent isoprene units and terpenes are linked preferentially between carbon atoms located at opposite ends of the isoprene structural subunit head to tail figure one head to head and tail to tail connections when they occur are exceptions to this rule then we see in the figure caption these are the three types of linkages that it can occur between isoprene units in turbines but it says up in the passage that this one is the preferred one so you should know kind of what a terpene is um a little bit not not crazy just be able to kind of pick it out in a group of different lipids and you should kind of understand you know some iupac rules but everything else in this first paragraph you don't really have to know anything about so we are learning something new in this paragraph and they're going to ask questions either on the basic sciences we're supposed to come in knowing or the things that the passage teaches us so don't be scared off yet in human metabolism squalene a carbon 30 terpene which is in this uh figure is utilized as a precursor for the synthesis of many lipids so this is going to be a precursor this kind of looks to me like a non-connected form of a steroid you know that the distinct pattern of um the steroid with the four um rings so i don't know if it's gonna be a precursor to that or what pretty much the only thing i knew about terpenes was that it was a type of squalene so i was able to pick out squalenes if you gave me like a wax ester versus a squalene versus a phospholipid i was able to pick those out but that's pretty much all i knew mevalinate compound 2 in reaction 3 which is right here is a key intermediate in squalene biosynthesis the biosynthetic pathway by which optim optically active mevalinate so we should know optically active means chiral is synthesized in vivo from acetate has been elucidated by using specifically 14 carbon methyl groups labeled acetyl coa reactions one through three so i would not even really look at these yet carbon 14 labeled mavalenic acid compound two produced from the carbon-14 labeled acetyl coa in reactions one through three is converted in vivo into isopentinyl pyrophosphate which is compound three again not sure why that's important i'm sure we'll get a question on it but i just don't know so i'm not going to spend any time on the figures so there was really hardly any actually like no flow charting on that you could definitely um highlight some basic sciences here some things about lipids um what squalene looks like we highlighted optically active but there's just not a lot of relationships that are not flushed out here in the figures so i'm not going to stress about it i'm going to go straight into the questions 38 says what is the expected carbon 14 labeling pattern found in isopentenyl phosphate produced this way labeled carbon atoms should be found at so i'm pretty sure if i'm not mistaken this is going to be the hardest question that we come across today in my opinion so isopentanyl pyrophosphate is this molecule right here and if we'll look back at the reaction we can see that they have put a little asterisk by the carbon-14s so what this question wants you to do is to say okay well i noticed that they are labeled up here but they are not labeled down here in the target molecule so what we're going to have to do is follow the reaction and see where those labeled carbons would be in every step of the way so that by the time we get to the end product we know where the carbons are and we can see which one of these numbers is going to have the carbon 14. so i'd be hard-pressed to do this in a minute but that's the name of the game this was a short passage so perhaps you read the passage in two minutes and then you might have another minute to try to figure this question out so it looks like they are labeled up here and continuously labeled through here this is the first molecule that we get to that is not labeled with the carbon-14s so what we have to do here is to kind of just have a good basic knowledge of how nucleophiles and electrophiles react with one another and a good knowledge of carbonyl chemistry so we should know that carbonyls are partially positive at the carbonyl carbon and partially negative i have the oxygen and so typically the way that this is going to go is that there's going to be a nucleophile that's going to attack the carbonyl carbon and push these electrons up onto the carbon and sometimes the carbon will be protonated into an alcohol does that look to be the case here so it looks like yes that is the case because we see that this part is maintained right here and we see an alcohol group that was not in either one of the molecules beforehand so what i'm thinking is that this molecule is this molecule and this part of it is this part of it i just need to find my nucleophile and then follow my carbon 14 labeled carbons so i also see right here over this reaction arrow that co a has to leave which explains why we do not see this co a anywhere in the molecule except for the one that was maintained here on the right so as far as labeling my carbons go if this truly is maintained over here then this carbon that's labeled should be this carbon so we have one that's labeled where does this carbon go well if it's true that a nucleophile attacks this carbonyl pushes some electrons up and that becomes this alcohol right here it would make sense that this carbon is kind of flipped up top right here and that this is actually our new bond so this carbon will also be labeled as far as what the nucleophile in here is the more i look at it the more it kind of looks like this methyl group is the nucleophile i'm not positive because typically you don't see carbons being nucleophiles in this way but i know it's not the carbonyl carbon that is labeled and there's no other carbon so this has to be our labeled carbon so to recap this carbon is right here this labeled carbon is right here and this labeled carbon is right here so now we have to keep following that around that was the hard part they just copied and pasted this over here so we can do the same our label carbons are right there then the reaction here kind of removes this coa and this carbonyl group and replaces it with a carbon and an alcohol group but overall i think our carbons will mainly stay in the same place i don't think that they are involved in the chemistry of this reaction so i'm so going from here to here i'm going to make a quick assumption that again our carbons don't really change that much you see that this alcohols turn into phosphate groups but again i don't think our carbon-14 labeled carbons are involved in that chemistry now here's the hard part it looks like a decarboxylation reaction has occurred a decarboxylation reaction is one you should be pretty familiar with it's one of the more highly tested it's like decarboxylation aldol condensations those are kind of like the most commonly tested mechanisms and then of course being comfortable with carbonyl chemistry so in that case what a decarboxylation reaction does is basically remove co2 so this part of the molecule is going to leave and you can see that right here we also have an inorganic phosphate leave which is probably this one right here so again uh know what a decarboxylation is and kind of how the molecules are moving or the atoms are moving but if you are comfortable with that then you should be able to see that it's kind of just copied and pasted again kind of in this triangle pattern these are the labeled carbons so that's going to be carbon 2 carbon 4 and carbon 5. so again what kind of reactions do we have here what kind of reactions do you need to be comfortable with this is some kind of nucleophilic carbon addition it's mainly be comfortable with carbonyl chemistry and how a nucleophile is going to attack the electrophile to connect itself to that carbon how this alcohol is created and then like where the new bond is from here to here is a redox reaction you can see because some common players for redox are in there and we also removed a coa but again our carbons in question were not really involved in the chemistry this is a decarboxylation reaction and i would be comfortable with the mechanism of decarboxylation so did i make a couple assumptions with question 38 yes i did but they were educated assumptions and they were the the best predictions i had for where those carbons were going um based on just general like logic that's a hard question don't feel bad if you're not getting it but never give up on a question never be like uh forget it i just hope i don't get that one on my test always take the time to go through it and with review you will just get better and the next time you see a question like this you'll be a master 39 says mevalinate exists in equilibrium with compound x in aqueous solution what is the structure of compound x so i went and grabbed the structure of mavalonate and it looks like what we're going to have to do is kind of fold this mevalonade up into a ring structure that it could be in equilibrium with remember molecules are constantly moving constantly connecting and reconnecting if you think about like glucose in a fissure versus a hemiacetal it's constantly kind of flipping back and forth between those forms so how would mevalinate make a ring if it was going to that's what this question is asking i would be pretty comfortable with how ring closings occur but again this just goes back to carbonyl chemistry a lot of times and nucleophile and electrophiles and how they relate to each other and what some common nucleophiles and electrophiles are when you see carbonyls you should always think that's going to be the electrophile will it ever be the nucleophile i don't know but most commonly it's going to be the electrophile so if i do make that assumption that this partially positive is going to be the electrophile what's going to be the nucleophile remember with ring closings one thing that's important five and six membered rings are going to be the most stable conformations so is there any nucleophile that is five or six atoms down from this carbonyl let's see one two three four five six could that oxygen be a nucleophile that would be a solid bet and it's less of the oxygen and more of the lone pair on the oxygen so if i could draw my electron pushing then i would draw from that lone pair electron on the oxygen to that carbonyl carbon and again it would have to push those electrons up onto the oxygen so that it didn't break the octet rule and have five bonds to carbon another thing that i would highly recommend you do is to label your carbons and your atoms so it's going to get a little messy but i'm going to try to keep it pretty neat i'm going to refrain from labeling these up here or this one or this one just because it's not going to um play into the ring structure only these are going to play into the ring structure it's going to kind of look like that so i know my molecule is going to be a six-membered ring because there are six main players and i know that one of those atoms is going to be the oxygen here so now i'm again going to label these so 6 was my oxygen it's right there and i can just kind of go around in either way that i want to because these are all going to be carbons now what i'm going to do is draw the substituents off of each carbon in the correct place so i know that one is going to have two oxygens with negative charges on both of them remember this will not be a carbonyl coming off because we push those electrons up two does not have anything coming off of it three has a methyl and an alcohol four and five do not have anything coming off of it and then six does not have anything coming off of it now is this my completed molecule no this is quite unstable having these two negative charges right there beside each other so there's going to be a lot of charge repulsion here so what is this molecule likely going to do to get rid of this it's probably going to protonate this so instead of having these charges here it's going to have just alcohols now is this one of my options over here no but i see it's very similar to a and b but the only difference is that i can either have a carbonyl or one alcohol over here i have two alcohols so what is the likely scenario the likely scenario is that this is going to push some electrons down to make a double bond and this is essentially going to grab this proton over here so then we kind of have this scenario it's getting ugly i'm sorry but in this case we have a fantastic leaving group which is water so we can just erase that whole thing and then we have answer choice a which is the right answer i know some of y'all are like how am i supposed to do that in one minute and i'm going to be honest i'd be hard-pressed to do it in one minute as well but the only thing that's going to help is to be really common and like muscle memory quick with nucleophiles and electrophiles being able to draw arrows push electrons that kind of thing and knowing some of the common rules like that five and six membered rings are going to be way more stable than a four four-member ring and so you can immediately mark out something like d you can also see like answer choice c is ugly there's a lot of steric hindrance there and so it's probably not going to be as likely as something like a or b if that is a possible configuration also knowing some common things like good leaving groups and where protons are going to go and that kind of thing so that you can get that last little step where we had two alcohols and we had to kind of decide well what would the molecule likely do organic chemistry is all about predictions and all about having basic rules that are going to predict what certain molecules will do it's not really hard or fast if you'll remember in orgo class when you had to come up with like the four or five or six different options that a molecule could do and you would see that all those molecules were in solution just in different percent yields so what we're trying to do here is predict the most common species that's going to be in solution moving on to 40 says mevalinate is expected to display prominent absorption in which regions of its infrared spectrum so this one's asking about ir and what are the functional groups in mevalinate so ir is all about knowing the ranges of the most common functional groups that are going to be tested alcohols carbonyls amines sp3 hybridized carbons in this question aromatic rings so this one is going to be alcohol groups this one's going to be carbonyl carbons and this one's going to be conjugated like aromatic carbon to carbon bonds so do we see an alcohol in this molecule yes we do right there and right there so one's going to be in there is three in there or not because we know that we do have a carbonyl carbon right there so are there any conjugated carbon-carbon bonds here the answer is no so we are only going to see ir absorbances in the alcohol and carbonyl group out of the three that we've been given here i named like five ir absorbance ranges earlier those are by far going to be the most common but i would recommend knowing a few more just in case because they are all up for grabs for the double amc 41 says what factor explains how a single stereoisomer is formed in reaction two so this is reaction two and it looks like they're talking about um this like chiral center right here basically if we have these non-chiral molecules how did we come up with this single stereoisomer how come we didn't have a racemic mixture is it that one of the reactants is chiral or that both reactants are chiral a or b no neither one of those reactants were chiral we would see dashes and wedges if they were now it's between the solvent medium being chiral or the enzyme being chiral the solvent medium is not really supposed to play a part in the reaction and so that would not really explain why the product is chiral from two achiral reactants but an enzyme being chiral enzymes like right up there in the reaction literally like physically forming the reactants into the product so if the enzyme was specifically laid out in a certain stereochemistry and the reactants kind of come into that enzyme pocket and are created into the product or kind of pressed into the product that would explain why we only see one kind of product or one chirality of product so d is a better answer than c 42 says which substance is not a product of squalene metabolism glancing down i see glucose testosterone cholesterol and cortisone and i see something sticking out and you should too that these are all steroids or steroid like molecules molecules created with steroids as a precursor glucose is very different and you notice that this is what squalene looks like and i talked about how it kind of looked like it was a precursor maybe for steroids so this is really a question of do you know what all these things look like you should know that these are all examples of steroids and that glucose is totally different has a bunch of alcohols and things like that and so it's going to be which one of these is different basically from the rest all right the gods of organic chemistry did not smile down on us today that was a tough passage tough questions and you really need to sharpen your organic chemistry skills and your knowledge of common reactions and how nucleophiles and electrophiles react with each other if you're going to get questions like that right so if you like this video or found it helpful then click like and subscribe and leave a comment below letting us know what you want to see next i'll see you guys in the next one