Official DAT Destroyer Q&A Thread

[densaugeo]

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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!

 

[hockeyboyrw]

It turns out for reasons not completely understood a Sulfur , Phosphorus or CYANIDE CN-, nucleophile works great in BOTH polar and aprotic solvents.

If you follow all this.....the DAT will be a piece of cake. Just remember,,,,,when using CN-...this nucleophile LOVES to do SN2, regardless of the solvent. I have been studying solvents for over 30 years,,,,,,and many unexpected outcomes do result.

So essentially the trends in nucleophilicity in polar protic vs aprotic solvents, (increase in nucleophilicity going down a group and to the left in period in polar protic; while increase in nucleophilicity going up a group in aprotic and increase going left in a period in aprotic), will be followed on the DAT, except when using CN-, Sulfur, or Phosphorus. These 3 are the exceptions and will be more nucleophilic in aprotic solvents still even if the other nucleophile(s) we compare it to is/are smaller.

 

[510586]

The first compound is 4-methyl-3-vinylcyclohexanone. The second compound you posted is called 3-allyl-2-methylcyclohexanone. Please review the functional groups.

Hope this helps.

Dr. Romano

I meant the problem that was showing us how to name the functional groups, not the 2 naming problems. http://forums.studentdoctor.net/attachments/14375021646541204583240-jpg.194231/ On h, would we name that as a vinyl or allyl? I thought it was vinyl

 

[sugarmagnolia91]

Hi Dr. Romano,

#172 in gen chem asks for the weakest base...it comes down to NH3 vs ClO4-...when comparing the conjugate acids of these two, I thought HClO4 seemed more stable/less reactive/weaker acid (giving the stronger base) as compared to NH4+ because HClO4 has resonance while NH4+ places a positive charge on an electronegative atom and has no resonance. Is HClO4 more acidic because of the oxygens? Wouldn't resonance be more important as a stabilizing factor?

Thanks!!

 

[510586]

Hi Dr. Romano,

#172 in gen chem asks for the weakest base...it comes down to NH3 vs ClO4-...when comparing the conjugate acids of these two, I thought HClO4 seemed more stable/less reactive/weaker acid (giving the stronger base) as compared to NH4+ because HClO4 has resonance while NH4+ places a positive charge on an electronegative atom and has no resonance. Is HClO4 more acidic because of the oxygens? Wouldn't resonance be more important as a stabilizing factor?

Thanks!!

Hclo4 is one of the strong acids we must memorize. Nh4+ is not one of those strong acids. Cb of it is a weak base.

 

[sugarmagnolia91]

Omg of course. I'm an idiot. Thank you!!

 

[510586]

Omg of course. I'm an idiot. Thank you!!

Np

 

[Predental1995]

In the biology section of the destroyer question number 15, one of the answer choices is Gastric Valve. Is there an actual valve called the gastric valve or is just the pyloric valve?

 

[DAT Destroyer]

In the biology section of the destroyer question number 15, one of the answer choices is Gastric Valve. Is there an actual valve called the gastric valve or is just the pyloric valve?

It is the same thing. Some people call it Gastric valve, but the scientific name is a pyloric sphincter or valve.

 

[Predental1995]

It is the same thing. Some people call it Gastric valve, but the scientific name is a pyloric sphincter or valve.

Thank you!

 

[daniho]

Could you please explain to me buffer systems/weak acids/bases/salts? I thought buffer systems were simply the weak acid and its conjugate base or a weak base and its conjugate acid... but as I work through some of the problems I can't see to understand it. Also, how you know which salts would have its solubility most affected by pH or have a pH of _____.
For example: Question 41 in Gen Chem (Destroyer 2012) and Question 81, 110, 135.
Thank you so much in advance!!

 

[DAT Destroyer]

Could you please explain to me buffer systems/weak acids/bases/salts? I thought buffer systems were simply the weak acid and its conjugate base or a weak base and its conjugate acid... but as I work through some of the problems I can't see to understand it. Also, how you know which salts would have its solubility most affected by pH or have a pH of _____.
For example: Question 41 in Gen Chem (Destroyer 2012) and Question 81, 110, 135.
Thank you so much in advance!!

The Buffer is a SOLUTION that is made to RESIST pH change. If you mix strong acid and strong base, they will neutralize each other and will not be able to resist the newly coming H+ or the OH- ions. That is why you are mixing weak base and it's salt or weak acid and it's salt to make a buffer.
The pH of the buffer will depend on the ratio of Base to acid, so if you have more base than acid , ph of the buffer will be higher than pka of the acid, and vice versa.

Now, the salts can be acidic, basic or neutral. In order to figure it out, just react a salt with water;

1) If you form strong acid and strong base, salt is neutral.

2) If you form Strong acid and weak base, salt is acidic.

3) If you form Weak acid and Strong base, salt is basic.

For example NaCl is a neutral salt, because when you react it with water, you form NaOH (Strong base) and HCl (Strong acid).

For the Ksp problem that you were talking about, you just need to know that Acids will react with 5 things:

OH-, CH3COO-, S--, F--, CO3--

So, if you have a salt that can produce any of the 5 anions written above, then it can be more soluble in the lower pH.

Hope this helps.

 

[poutinepower]

Hi Dr. Romano,

On #41 in ochem it states that the proton on the alcohol is the most acidic. Are conjugated protons not more acidic than the ones on alcohols? I thought the answer would be C because the alkene is conjugated with the benzene making it very stable.

Also, are alcohols more acidic than double ketones, like 2,4-pentanedione?

Sorry i also had a question on #3 in ochem. Why does the OH give up protons most easily? The most acidic groups would give up protons most easily but isnt CH3 more acidic than OH, or does para play a role here?

Thank you.

 

[DAT Destroyer]

Hi Dr. Romano,

On #41 in ochem it states that the proton on the alcohol is the most acidic. Are conjugated protons not more acidic than the ones on alcohols? I thought the answer would be C because the alkene is conjugated with the benzene making it very stable.

Also, are alcohols more acidic than double ketones, like 2,4-pentanedione?

Sorry i also had a question on #3 in ochem. Why does the OH give up protons most easily? The most acidic groups would give up protons most easily but isnt CH3 more acidic than OH, or does para play a role here?





Thank you.

Destroyer is correct here. You are thinking of an alcohol, which has a pka of about 16. Question 41 is asking abut Phenol, which has a pka of about 10. Double alpha hydrogens can be more acidic than an alcohol, for example malonic ester has a pka of about 13.

However, Phenols, are more acidic than a regular alcohol, because once you generate a phenoxide ion, it is resonance stabilized, unlike a regular alcohol.

#3
The hydrogen coming out of Oxygen will be more acidic than a Hydrogen coming out of the carbon, simply because Oxygen is more electronegative and is therefore more likely to stabilize the anion.

Hope this helps.

 

[poutinepower]

For #161 on GC, i understand that only changes in moles will affect pressure, temp, and volume based on PV=nRT, but if using the P(MW)=DRT equation wouldnt the differing molecular weights of Ar and He affect P and T?

 

[DAT Destroyer]

For #161 on GC, i understand that only changes in moles will affect pressure, temp, and volume based on PV=nRT, but if using the P(MW)=DRT equation wouldnt the differing molecular weights of Ar and He affect P and T?

No. Here, the ideal gas law is assumed. The formula PMW =DRT is derived from PV=nRT. That means, if the gas changes, so as it's Density and MW. Any changes is Density will be compensated for by changes in MW and vice versa. Therefore Pressure should not change.

Hope this helps.

 

[FloorMatt]

Q. 320 bio.... Savanna is apparently a grassland with low rainfall. But I remember somewhere else in the book it said grasslands get 25-30 inches a year... Is this considered low?

 

[DAT Destroyer]

Q. 320 bio.... Savanna is apparently a grassland with low rainfall. But I remember somewhere else in the book it said grasslands get 25-30 inches a year... Is this considered low?

You are confusing Savanna with a Temperate grassland biome ( it is in South Africa, Argentina, Uruguay, etc).
It has precipitation about 25-30 inches per year. Savanna (Kenya, South Africa, South America) is an equatorial and sub-equatorial region. Precipitation is seasonal and dry season can last up to 9 months.

Hope this helps.

 

[DrNoCavity]

Can someone please explain mass to charge ratio and Nitrogen rule in destroyer ochem #17? I don't remember covering it in my review material.

 

[DAT Destroyer]

Can someone please explain mass to charge ratio and Nitrogen rule in destroyer ochem #17? I don't remember covering it in my review material.

Mass to charge ratio is a mass of a compound that is missing an electron. Since electrons do not contribute to the MW of the molecule, the mass to charge ratio will be the mass of the radical that can be fragmented by the machine.

The nitrogen "rule" just means that if a compound has an even number of nitrogens, it will have an even MW, if it has odd number of Nitrogens, it has odd MW.

Hope this helps.

 

[poutinepower]

Hi Dr. Romano,

For OC #67, wouldn't HCl work because the reactant is a primary alcohol which will undergo Sn2 and not Sn1 and therefore no rearrangement, leading to a primary halide?

 

[DAT Destroyer]

Hi Dr. Romano,

For OC #67, wouldn't HCl work because the reactant is a primary alcohol which will undergo Sn2 and not Sn1 and therefore no rearrangement, leading to a primary halide?

Nice try,,,,,,but no. Normally a primary alcohol does an Sn1 process BUT not here . Why ? Once protonated, a SIMULTANEOUS shift,,,,,recall to NEVER NEVER form a primary carbocation........will result in a highly stable tertiary carbocation, hence the skeleton has rearranged and another isomeric product results. If we want to avoid any shifting......SOCl2 in Pyridine is the way to go. I have done this reaction for many years,,,,,and it always gives a solid yield. If you are still baffled, I will be happy to draw it out for you

Hope this helps.

Dr. Romano

 

[poutinepower]

Did you mean to say a primary alcohol normally does an Sn2 process but in this case it is undergoing Sn1? Also, if the original alcohol was just a propanol would there still be a shift forming a secondary carbocation, or does the shift only happen if it can form a tertiary carbocation?

 

[510586]

On Math destroyer test 10, is there a theorem that tells you why the triangles are similar? How can you tell?

 

[alexis2010]

2 triangles are similar if they have 2 angles that are congruent ( same). Similar means same shape but different sizes.

 

[alexis2010]

In similar triangles the ratio of corresponding sides is the same.

 

[510586]

In similar triangles the ratio of corresponding sides is the same.

I was asking how we know they were the same in this case

 

[alexis2010]

I don't have the math destroyer on me. Can you post a picture?

 

[510586]

Here

 

[alexis2010]

Angles C and D are right angles. Angle BAC is the same as angle EBD ( alternate interior interior angles). Now side AC corresponds to side BD. Side ED corresponds to side BC and side AB corresponds to side BE.

 

[alexis2010]

BD = 5. ED/BC = BD/AC
ED/6 = 5/8. ED = 15/4.
A = (1/2)(5)(15/4) = 75/8 = 9 3/8

 

[510586]

Angles C and D are right angles. Angle BAC is the same as angle EBD ( alternate interior interior angles). Now side AC corresponds to side BD. Side ED corresponds to side BC and side AB corresponds to side BE.

Ohh ok I've never seen alternate interior angles used in this manner. THank you!

 

[Jay20155555]

I am having trouble trying to figure out why number 249 in GEN Chem in the destroyer is what it says it is, here is the question. The answer is C, I thought it was E. The answer just talks about pka values. It said to pair the strongest base with the strongest acid. Is there a way to answer this question without memorizing Pka values? I chose E becuase HF is a weak acid, and CH3CH2COO- would be considered a weak base. I didnt choose B because Br is considered electron withdrawing so I thought it would reduce some of that negative charge.

 

[Jay20155555]

errr scratch that, didn't notice that ACID Ka chart at the top LOL

 

[KendreasyDDS]

Hello! For question 208 gen chem I'm having a hard time figuring out how the PKa of 4.4 will have a pH of 9. Can you please show me how to figure this out? Thanks so much!

 

[510586]

Hello! For question 208 gen chem I'm having a hard time figuring out how the PKa of 4.4 will have a pH of 9. Can you please show me how to figure this out? Thanks so much!

It's not. If ph is 9 then the indicator with pka 4.4 will be deprotinate

 

[DAT Destroyer]

Hello! For question 208 gen chem I'm having a hard time figuring out how the PKa of 4.4 will have a pH of 9. Can you please show me how to figure this out? Thanks so much!

The hydroxide ion concentration is 1*10^-5. If you take a negative log out of it the pOH will be 5. You can figure out the pH from it by subtracting 5 from the 14. The ph will equal to 9. Since ph of 9 is higher than pka of 4.4 , the indicator will be deprotonated.
Hope this helps.

 

[ivyred12]

Hello!

I am going through the math destroyer test 3 and was wondering if I could ask about some conceptual questions. For problem 11, my strategy differed from that in the answers and I am wondering if I am thinking about it correctly. First, I figured out what the concentration of the 30% pomegranate juice must be in the mixture through setting up the equation 30*4=x*12 and got 10. Then, I found out what the concentration of the 15% pomegranate juice must be through 15*8=x*12 and again got 10. I added the two together to get the total concentration, which was the correct answer. However, I'm not sure if you can split it up like that.

For question 15, I approached it in a similar way that the solution to practice test 1, question 27 was approached. That is, I subtracted out the both percentage from 14 as well as 25 to get the "pure" percentages for both blue eyes and left handed. I then summed up both+blue+left to get a grand total of 27, which is clearly incorrect as it isn't even an answer choice. So, I'm confused! (14-8=6%, 25-8=13%, 13%+8%+6%=27%).

Thank you for your time~

 

[RenzyBoy]

Hi,

For 2015 Ochem #62, can someone please explain why NH3 comes in with a positive charge, I was thinking because it has a lone pair that it's going to attack it with will make it a negative charge...

Thanks in advance!

I kinda saw this question, and idk if anyone answered it, but its because NH3 is a neutral molecule, and in the SN2 product, it has a positive charge. So, in the transition state, going from neutral to positive, the NH3 would have a partial positive charge as it goes in to attack the carbon. The charges on these transition states depend on what charge the molecule was, and what its going to be at the end, not on lone pairs.

 

[DAT Destroyer]

Hello!

I am going through the math destroyer test 3 and was wondering if I could ask about some conceptual questions. For problem 11, my strategy differed from that in the answers and I am wondering if I am thinking about it correctly. First, I figured out what the concentration of the 30% pomegranate juice must be in the mixture through setting up the equation 30*4=x*12 and got 10. Then, I found out what the concentration of the 15% pomegranate juice must be through 15*8=x*12 and again got 10. I added the two together to get the total concentration, which was the correct answer. However, I'm not sure if you can split it up like that.

For question 15, I approached it in a similar way that the solution to practice test 1, question 27 was approached. That is, I subtracted out the both percentage from 14 as well as 25 to get the "pure" percentages for both blue eyes and left handed. I then summed up both+blue+left to get a grand total of 27, which is clearly incorrect as it isn't even an answer choice. So, I'm confused! (14-8=6%, 25-8=13%, 13%+8%+6%=27%).

Thank you for your time~

For Question #11. your approach will not work all the time. for example if you are mixing water with another solution that has alcohol, or if you are mixing pure alcohol with a solution that is part alcohol. 4 liters that is 30% Pomegranate juice gives 4x(.3) = 1.2 L of juice. 8 Liters that is 15% Juice gives 8x(.15) = 1.2 L of juice. The total amount of juice is then 2.4 Liters and the total mixture is 12 L. the concentration is then 2.4/12 = 20%.

For question 15 check your calculations 🙂25 - 8 = 17 not 13 🙂

Hope this helps..

 

[Jay20155555]

DAT Destroyer OCHEM question number 177, also attached. I thought the answer was B, shouldn't it say oxidizing agent not reducing agent?

 

[Jay20155555]

Also one more question, for 178, why does it say E is not a correct newman representation

 

[FloorMatt]

DAT Destroyer OCHEM question number 177, also attached. I thought the answer was B, shouldn't it say oxidizing agent not reducing agent?

Reducing agents help to reduce substituents by adding on H's. In this example they add to the nitrile. It does this by getting oxidized, or losing its own H's.

Also, newman conformations are the images that look like 2 circles on top of eachother just like ABC. D is a chair conformation.

 

[DAT Destroyer]

DAT Destroyer OCHEM question number 177, also attached. I thought the answer was B, shouldn't it say oxidizing agent not reducing agent?

For question 177, remember, when you are oxidizing you are adding O's or removing H's. So, when you are converting a nitrile into amine, you are adding H's, therefore you need reducing agent, like LiAlH4.
For question 178, choice E is a Sawhorse structure, not a Newman, that is why it is wrong.

Hope this helps.🙂

 

[Jay20155555]

For question 177, remember, when you are oxidizing you are adding O's or removing H's. So, when you are converting a nitrile into amine, you are adding H's, therefore you need reducing agent, like LiAlH4.
For question 178, choice E is a Sawhorse structure, not a Newman, that is why it is wrong.

Hope this helps.🙂

Oh haha when I woke up this morning I was wondering why I asked this question, guess I was having a brain fart last night!

Also for 178 in in 2015 destroyer letter E is a newman projection, it just had 2 methyls and a ethyl group on the lines, choice D in my book is a sawhorse

 

[CornHusker]

I had a question on #185 GC 2014 Destroyer.

Which molecule would be the most polar in the gas phase?

a) I2 b) NaBr c) LiF d) NaF e) CsF

So my question is, can a ionic compound like CsF be called a molecule? In a qvault problem they say it can't and after googling this question, I found mixed results. Many claim ionic compound arnt molecules but crystal lattices composed of cubic units.

Because if it's not a molecule, then the answer would be "a" and not "e".

Would appreciate some clarification on this.

Edit: also, arnt molecules 2+ of the same atom and compounds 2+ of any atom combination?

 

[byahnoob]

Does anyone have a divide and conquer doc for ORGO and GEN CHEM?

 

[510586]

I had a question on #185 GC 2014 Destroyer.

Which molecule would be the most polar in the gas phase?

a) I2 b) NaBr c) LiF d) NaF e) CsF

So my question is, can a ionic compound like CsF be called a molecule? In a qvault problem they say it can't and after googling this question, I found mixed results. Many claim ionic compound arnt molecules but crystal lattices composed of cubic units.

Because if it's not a molecule, then the answer would be "a" and not "e".

Would appreciate some clarification on this.

Edit: also, arnt molecules 2+ of the same atom and compounds 2+ of any atom combination?

Pretty sure water molecules are a thing. Don't think molecules have to be from different compounds. Cant answer the main question with certainty however

 

[FloorMatt]

I had a question on #185 GC 2014 Destroyer.

Which molecule would be the most polar in the gas phase?

a) I2 b) NaBr c) LiF d) NaF e) CsF

So my question is, can a ionic compound like CsF be called a molecule? In a qvault problem they say it can't and after googling this question, I found mixed results. Many claim ionic compound arnt molecules but crystal lattices composed of cubic units.

Because if it's not a molecule, then the answer would be "a" and not "e".

Would appreciate some clarification on this.

Edit: also, arnt molecules 2+ of the same atom and compounds 2+ of any atom combination?

Seems to me like its an error because Ionic compounds are NOT molecules. Molecules are covalent. I would've went with choice A as well

 

[DAT Destroyer]

Pretty sure water molecules are a thing. Don't think molecules have to be from different compounds. Cant answer the main question with certainty however

Ionic species do not exist as molecules but they exist in crystal lattices. We do not use the term molecule when referring to an ionic compound. However, if in the gas phase the term molecule is appropriate since the crystal lattice is broken and discrete molecules would exist.


Hope this helps


Dr. Jim Romano

 

[510586]

Ionic species do not exist as molecules but they exist in crystal lattices. We do not use the term molecule when referring to an ionic compound. However, if in the gas phase the term molecule is appropriate since the crystal lattice is broken and discrete molecules would exist.


Hope this helps


Dr. Jim Romano

That makes a lot of sense. Thanks Dr romano!