Official DAT Destroyer Q&A Thread

[densaugeo]

Hi guys, since I'm currently going through the DAT Destroyer and I'm sure many of you guys are as well I felt this thread would be helpful. I usually have various questions while going through Destroyer regarding why a certain answer choice is correct or other times I am in need of a more detailed explanation.

Instead of making multiple threads each time we have questions, I thought we could just post them here and anyone can chime in with an explanation or further clarification.

Please don't post entire questions as that would be a violation of copyright. You can ask specific questions regarding a particular problem in the Destroyers.

Hopefully others find this a useful thread. Thanks!

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[DAT Destroyer]

Sorry but I was trying to redo the problem and got confused... if the mole fraction of the gas is 2x10^-18 rwouldn't that mean it's equal to (moles of gas/moles of H2O + moles of gas)? And if that is correct, and we assume water to be 1 mole, shouldn't we factor that 1 mole of water into the mole fraction equation to get moles of gas to be different than 2x10^-18?

The moles of gas are independent from the moles of water. The gas represents the moles of SOLUTE.

Hope this help

Dr. Romano

 

[StudentDoc1234]

The moles of gas are independent from the moles of water. The gas represents the moles of SOLUTE.

Hope this help

Dr. Romano

But the problem didn't give us moles of gas, so what I was trying to do was use the mole fraction and 1 mole of water to get moles of gas. I guess I'm having trouble understanding if the mole fraction is moles of gas/total moles why does moles of gas equaling the mole fraction that was given.

By the way thank you so much for your time and help!

 

[DAT Destroyer]

But the problem didn't give us moles of gas, so what I was trying to do was use the mole fraction and 1 mole of water to get moles of gas. I guess I'm having trouble understanding if the mole fraction is moles of gas/total moles why does moles of gas equaling the mole fraction that was given.

By the way thank you so much for your time and help!

Mole fraction of gas is implied to mean moles of gas.

 

[StudentDoc1234]

That clears everything up for me, thank you!

 

[DAT Destroyer]

That clears everything up for me, thank you!

Glad I could help! Always glad to help students that are working hard, I admire that!

Good luck in your studies and on the DAT

 

[FeralisExtremum]

2015 Destroyer Gen Chem #172:

The question asks for the weakest base in water. Why is the answer not A, the strong acid? I understand that ClO4-, being the conjugate of a strong acid, is going to be a really really weak base. But shouldn't HClO4 be an even worse base? Or do we not count the strong acids to be "basic" to any extent?

 

[DAT Destroyer]

2015 Destroyer Gen Chem #172:

The question asks for the weakest base in water. Why is the answer not A, the strong acid? I understand that ClO4-, being the conjugate of a strong acid, is going to be a really really weak base. But shouldn't HClO4 be an even worse base? Or do we not count the strong acids to be "basic" to any extent?

HCLO4 is a strong acid.......do not count it as a base EVER !!! You will never likely see HCLO4 acting as a base in your entire life !!!!!!

Hope this helps.

Dr. Romano

 

[FeralisExtremum]

HCLO4 is a strong acid.......do not count it as a base EVER !!! You will never likely see HCLO4 acting as a base in your entire life !!!!!!

Hope this helps.

Dr. Romano

Thanks Dr. Romano! A quick followup question: what if a weak acid had been presented instead of HClO4, like CH3COOH? Would the answer still be ClO4- or could a weak acid potentially count?

 

[DAT Destroyer]

Thanks Dr. Romano! A quick followup question: what if a weak acid had been presented instead of HClO4, like CH3COOH? Would the answer still be ClO4- or could a weak acid potentially count?

Yes.....CH3COOH is acetic acid......don't classify it as a base ! The main point of this question.......has been missed,,,,,,,I didn't mean to write a " trick " problem......The concept is this: Strong Acids give weak conjugate bases.......and weak acids give Strong conjugate bases ! If I said to you that Methane has a pKa of about 50.....what can you conclude about CH3-......the methide anion ?

 

[FeralisExtremum]

It would be very powerful as a base, right?

 

[DAT Destroyer]

It would be very powerful as a base, right?

LOL.....INDEED SO ,,,,,,doesn't get more POWERFUL !!!!!!

 

[Lovelybond]

Dr. Romano I just want to say thank you so much for your time to answer all the questions for us!!
Also I have a question from 2012 Math destroyer #39.
The solution explains that "after squaring each of the values, taking their sum and dividing by 5 (N)" instead of 4 (N-1)
Is there reason that we take N instead of N-1?

 

[DAT Destroyer]

Dr. Romano I just want to say thank you so much for your time to answer all the questions for us!!
Also I have a question from 2012 Math destroyer #39.
The solution explains that "after squaring each of the values, taking their sum and dividing by 5 (N)" instead of 4 (N-1)
Is there reason that we take N instead of N-1?

The reason we divide by N and not N-1 because we assume it's a population not a sample.

 

[ultimateballer07]

1.) Why is an alcohol not able to react with a Grignard reagent? Is the first step always to make sure we have an alkyl halide before treating it with Grignard? I was just wondering if you could give me an explanation of the process.

2.) Why is it that, when adding an acyl chloride to aniline, the acyl chloride attaches to the nitrogen atom instead of doing EAS and attaching to the benzene ring?

 

[DAT Destroyer]

1.) Why is an alcohol not able to react with a Grignard reagent? Is the first step always to make sure we have an alkyl halide before treating it with Grignard? I was just wondering if you could give me an explanation of the process.

2.) Why is it that, when adding an acyl chloride to aniline, the acyl chloride attaches to the nitrogen atom instead of doing EAS and attaching to the benzene ring?

1. An alcohol can indeed react with the Grignard in an acid -base reaction. If an alcohol is present on a carbonyl compound,,,,,,the Grignard would react with the alcohol......remove the acidic proton......then re-protonate to give on one the impression that no reaction occured. This is bad, since we wanted the Grignard to add to the carbonyl group ! In real life organic synthesis, this is easily solved by using what is know as a protecting group !

2. Great question ! Careful here,,,,, The key is not using the AlCl3 catalyst. If no catalyst, then the acyl halide reacts with the NH2 group to give the amide.....a wonderful way to deactivate the highly reactive NH2 group. If you employed the acyl halide with AlCl3.....it would NOT be a wise move .........the electrons from the Nitrogen would add to the Aluminum in a Lewis Acid base reaction......and create a charged molecule that would be highly deactivated.

I hope this helps......a fantastic question !

Dr. Romano

 

[ultimateballer07]

I understand. Thanks! So, in a Grignard reaction, would the alcohol typically need to first be converted to an alkyl halide by FeBr3 or SOCl2 before adding the new alkyl/acyl group?

For Gen. Chem in the 2013 Destroyer, could you please explain how to go about #44; I'm having a little difficulty when it comes to balancing full redox reactions.

 

[super112]

Gchem 188 (2013) In the explanations it says that anions of strong acids can't undergo hydrolysis... so in NH4Br, only NH4 ion undergoes hydrolysis. So my question is, why can't NO3 undergo hydrolysis? Is there any anion that CAN undergo hydrolysis? I'm thinking not b/c I thought the OH- of water needs to find a cation to react with. Thanks.

 

[FeralisExtremum]

In the explanations it says that anions of strong acids can't undergo hydrolysis...So my question is, why can't NO3 undergo hydrolysis?

Because it's the anion of a strong acid, HNO3.

 

[ultimateballer07]

Because it's the anion of a strong base, HNO3.

HNO3 is nitric acid

 

[FeralisExtremum]

HNO3 is nitric acid

Meant to write strong acid, that's my bad. Good catch!

 

[super112]

Because it's the anion of a strong base, HNO3.

I think you meant strong acid, b/c that's also part of the explanation in the book. But I was more wondering the reason behind why strong acid ions can't do hydrolysis.

 

[DrNoCavity]

OChem Odyssey: Ch.8 Q41: Shouldn't OH be a stronger nucleophile than SH since they are both in an aprotic solvent?

 

[FeralisExtremum]

I think you meant strong acid, b/c that's also part of the explanation in the book. But I was more wondering the reason behind why strong acid ions can't do hydrolysis.

Think about it like this: when you put a strong acid in water, it completely dissociates. So let's say theoretically, NO3 did cause hydrolysis, and picked up an H to become HNO3, what would happen? It would just immediately dissociate again, because you'd have simply reformed the strong acid. The net result in activity would be nothing.

More accurately, the conjugate bases of the strong acids are incredibly weak, to the point that they're negligible. They don't really want to deprotonate anything and won't interact with water. This is why they dissociate completely in water; their anions are very stable and unreactive on their own.

 

[DrNoCavity]

Thank you for the explanation. I was just confused because in Chad's 1.4 ochem video he states that nucleophiles in aprotic solvents increase going up a group in the periodic table.

 

[FeralisExtremum]

Thank you for the explanation. I was just confused because in Chad's 1.4 ochem video he states that nucleophiles in aprotic solvents increase going up a group in the periodic table.

That was actually the explanation for the above question...for the one you're referring to, I have the same question. I was taught that negatively charged nucleophiles in aprotic solvent get stronger going up and to the left in the periodic table, so I would have thought -OH > - SH in nuclophile strength.

 

[PocketRocket]

(Destroyer 2015) OChem # 282

Given that there are 2 alpha-Hs on the reactant, why is it that the one on the "right" of the molecule gets removed and not the one on the "left". I tried drawing out the latter situation and I get answer choice C. Does it have anything to do with induction?

Thanks in advance!

 

[StudentDoc1234]

(Destroyer 2015) OChem # 282

Given that there are 2 alpha-Hs on the reactant, why is it that the one on the "right" of the molecule gets removed and not the one on the "left". I tried drawing out the latter situation and I get answer choice C. Does it have anything to do with induction?

Thanks in advance!

I'm assuming the C2H5 will have a steric hinderance on the base removing that alpha hydrogen on the "left" carbon, so it will be easier to remove the one on the "right".

 

[DAT Destroyer]

(Destroyer 2015) OChem # 282

Given that there are 2 alpha-Hs on the reactant, why is it that the one on the "right" of the molecule gets removed and not the one on the "left". I tried drawing out the latter situation and I get answer choice C. Does it have anything to do with induction?

Thanks in advance!

Great question ! It is a bit easier to abstract the H on the " right ".....it is less sterically hindered...... it is a bit further from the ethyl group......thus is easier to abstract.

Hope this helps !

 

[PocketRocket]

Great question ! It is a bit easier to abstract the H on the " right ".....it is less sterically hindered...... it is a bit further from the ethyl group......thus is easier to abstract.

Hope this helps !

Didn't even consider that, thank you!

 

[Lovelybond]

DAT destroyer 2015 Organic chemistry #196. I understand that adding H3O+ quenches the reaction by neutralizing the base and pronates the claisen product. But when you look at the final product, it has an ester group. Is it an ester group one of the carboxylic acid derivatives? Wouldn't it be enough H3O+ to turn ester to carboxylic acid? Thank you!

 

[DAT Destroyer]

DAT destroyer 2015 Organic chemistry #196. I understand that adding H3O+ quenches the reaction by neutralizing the base and pronates the claisen product. But when you look at the final product, it has an ester group. Is it an ester group one of the carboxylic acid derivatives? Wouldn't it be enough H3O+ to turn ester to carboxylic acid? Thank you!

An ester is a carboxy derivative indeed. An ester can be hydrolyzed into an alcohol and acid upon treatment with acid or base. . HOWEVER,,,,,,in the Claisen reaction......you are doing something very different.....you are condensing 2 molecules together to make a larger one ! First, we use the base to deprotonate the alpha hydrogen, then use that enolate to act as a nucleophile for the attack on the second molecule. This sets up the leaving group, ethoxide. Only when this is all done.....does acid come in for a " quench ". Thus.....hopefully you see two very different processes have occurred. Hydrolysis breaks a molecule apart.....CONDENSATION builds a molecue with the loss of a small group such as water or usually an alcohol. This reaction, along with the Haloform, Aldol, Dieckmann, Wittig, Grignard, Diels Alder, to name a few, are CRITICAL to know for the DAT exam.

Hope this helps

Dr. Romano

 

[hopefuldent52]

^^^ literally the question I was about to ask here . Thanks for asking it @Lovelybond....

And thanks Dr. Romano for the detailed explaination

 

[tooth_hurty]

2015 Destroyer, Gen Chem #322

Is there a formula that can be used to find the number of constitutional isomers? I've never heard of one but wanted to make sure in order to save time on questions like these.

I know degrees of unsaturation formula could be used to narrow down the answers a bit but I'm not sure if there's a formula to find the number of constitutional isomers. Thanks in advance

 

[DAT Destroyer]

2015 Destroyer, Gen Chem #322

Is there a formula that can be used to find the number of constitutional isomers? I've never heard of one but wanted to make sure in order to save time on questions like these.

I know degrees of unsaturation formula could be used to narrow down the answers a bit but I'm not sure if there's a formula to find the number of constitutional isomers. Thanks in advance

No.....there are no formulas!!! Computer programs are now designed to calculate billions of isomeric structures. If a problem like this lands on the DAT.....I assure you only a few isomers exist. Calculating the degree of unsaturation certainly helps.......branching a straight chain also allows isomer construction. Remembering things such as a saturated molecule with C, H, and O.......would give alcohol and ether isomers ~~~~Practice a few .....not as bad as it looks ! Problem 322 is as tough as it would get for the DAT exam . Keep hammering away.......

Dr. Romano

 

[super112]

2013 OC question 193. If CH3O- and polar protic, wouldn't this mean E2 reaction? Is it just in this particular problem we bend the rules? B/c when I saw this I initially looked in the answers for "no reaction".

 

[super112]

Also 2013 GC question 208. If I calculate PH = 9, am I supposed to do something fancy to see if indicator with pka 4.4 is in protonated or deprotonated state? I got the right answer by knowing ph =9 is basic, so I chose the deprotonated colors.

Side question: Could I technically use any of the indicator dyes, but it is best if I use one that has pka 1/2 of ph?
Thanks

 

[cc609]

Hello!

This is a super outdated question (from Destroyer 2010) # 108 from Gen Chem Section...

I understand the N vs M for normality because Ca(OH)2 has two OHs and I understand you set up with NV=N2V2

So if 20mL of Ca(OH)2 wa added to 48 mL of HBr, I thought the volumes would be 20 mL for V1 and 68ml (20mL Ca(OH)2 + the 48mL of HBr) for V2. Hwever, in the answers it says 20 mL of V1 and 48 for mL. I know that other questions emphasized that since they are added together, you use the final volume of both solutions added together not just the volume of one. When would I combine the two volumes, and when would I only use just one of the volumes?

 

[tbogdan89]

Hi Dr. Romano,

In the 2015 DAT Destroyer Organic Chem #34 I had two question about the set of reagents/their implications to the DAT I was hoping you might be able to help with.

In my orgo class we were taught that friedel crafts alkylation will always result in poly-alkylation so we were discouraged from using that as a first step. I also remember losing a few points on an exam because I added a "deactivator" to the ring before the activator. In this case the right answer was adding a moderate deactivator then a weak deactivator I think? I guess this question reminded me of those two topics.

So my questions are, in the DAT is it okay to assume friedel alkylation will add only 1 R group? Also, does the order of adding R groups to a Benzene in terms of Activator/deactivator matter?

Thank you for your help!

 

[DAT Destroyer]

Hi Dr. Romano,

In the 2015 DAT Destroyer Organic Chem #34 I had two question about the set of reagents/their implications to the DAT I was hoping you might be able to help with.

In my orgo class we were taught that friedel crafts alkylation will always result in poly-alkylation so we were discouraged from using that as a first step. I also remember losing a few points on an exam because I added a "deactivator" to the ring before the activator. In this case the right answer was adding a moderate deactivator then a weak deactivator I think? I guess this question reminded me of those two topics.

So my questions are, in the DAT is it okay to assume friedel alkylation will add only 1 R group? Also, does the order of adding R groups to a Benzene in terms of Activator/deactivator matter?

Thank you for your help!

Great question. In the real world, polyalkylation is indeed a problem. We easily get around this using an excess of benzene. For the DAT.......don't assume any polyalkylation, but do a single monoalkylation . Ideally.....adding an activator is usually best.....however......a deactivator sometimes is the only way to go.....and the yields are decent. As an example....In the Maitland Jones text book....written by my good friend Professor Jones, Professor Emeritus at Princeton University and now at NYU, he used Nitrobenzene and nitrated it. The yield was 93% for the 1, 3-dinitro product !!!.......so that should allay your trepidation about placing a deactivating group on the ring first.

Hope this helps !

Dr. Romano

 

[tbogdan89]

Wow I wish I had known that at the time haha. Thank you, very helpful.

 

[Lnguyen7]

I have a question about Q#33 of organic chemistry version 2010. The reaction is under acidic condition, why are all alpha hydrogen replaced with chlorine? I thought this would only happen under basic solution.

 

[super112]

Gchem: Between pentane and 2,2 dimethyl propane, I initially thought pentane had higher melting point b/c of no branching. But for MP (but not BP), is it a special case for when the structure is sp3 b/c of "perfect" symmetry?

Thanks

 

[FeralisExtremum]

Gchem: Between pentane and 2,2 dimethyl propane, I initially thought pentane had higher melting point b/c of no branching. But for MP (but not BP), is it a special case for when the structure is sp3 b/c of "perfect" symmetry?

Thanks

EDIT: messed this one up, disregard message

 

[DAT Destroyer]

Gchem: Between pentane and 2,2 dimethyl propane, I initially thought pentane had higher melting point b/c of no branching. But for MP (but not BP), is it a special case for when the structure is sp3 b/c of "perfect" symmetry?

Thanks

The 2, 2-dimethylpropane isomer has a MUCH MUCH HIGHER ...melting point .....how does 110 degrees Celsius sound ? When a molecule achieves symmetry,,,,,especially in sp3 structures......melting points can be unusually high. Highly symmetrical molecules pack more tightly and efficiently in a crystal lattice.

Hope this helps.

Dr. Romano

 

[PocketRocket]

DAT destroyer 2015 GC # 248

Why is b wrong? I understand that since CaSO3 is a solid it is not included in the solubility equation but by adding more CaSO3 into solution you will get more of it dissolving, wouldn't you? leading to more SO2 gas?

Thanks you!

 

[FeralisExtremum]

2015 Destroyer Organic Chemistry #15: how come we don't assume that the Claiden Condensation would repeat a second time and add again at the ester of the product?

 

[PocketRocket]

2015 Destroyer Organic Chemistry #15: how come we don't assume that the Claiden Condensation would repeat a second time and add again at the ester of the product?

In the mechanism of this reaction we go through an enolate formation which than undergoes nucleophilic acyl substitution and so on. After getting our product the second reagent is H3O which will prevent further enolate formation by adding a proton to it right away. I believed chad talked about this in one of his videos.

 

[FeralisExtremum]

In the mechanism of this reaction we go through an enolate formation which than undergoes nucleophilic acyl substitution and so on. After getting our product the second reagent is H3O which will prevent further enolate formation by adding a proton to it right away. I believed chad talked about this in one of his videos.

The one that's beta to both carbonyls gets protonated, yea. But in the answer choice product, there's still an ester group present. Why doesn't that carbon get attacked again by another deprotonated reactant molecule?

Also:
2015 Destroyer Biology #1. I could swear A is also a valid answer because that image is a TEM, not an SEM?

 

[DAT Destroyer]

DAT destroyer 2015 GC # 248

Why is b wrong? I understand that since CaSO3 is a solid it is not included in the solubility equation but by adding more CaSO3 into solution you will get more of it dissolving, wouldn't you? leading to more SO2 gas?

Thanks you!

CaSO3 is a solid.....and will not dissolve to any extent. Even if it did dissolve...no SO2 would be produced. If a solid is not in the Keq expression...the addition or removal will have no
EFFECT !!!!!! This is an important concept to know.

Dr. Romano

 

[DAT Destroyer]

2015 Destroyer Organic Chemistry #15: how come we don't assume that the Claiden Condensation would repeat a second time and add again at the ester of the product?

No. a Claisen reaction is between two individual esters !!!! LOL.....you are a sly fox,,,,,, Another possible Claisen would not occur...the Claisen requires 2 alpha protons to react....our product has only 1. The details are not important for the DAT,,,,,,just know how to execute a Claisen. If however, you wanted to explore the mechanism, David Klein text presents it nicely...as does Maitland Jones. Bottom line...... If an ester has at least 2 alpha hydrogens and the base matches the leaving group...do a CLAISEN !!!!!

Hope this helps...

Dr. Romano