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CrO3

Discussion in 'MCAT Discussions' started by PlayaHata, Apr 24, 2002.

  1. PlayaHata

    PlayaHata Member
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    Ok I know we discussed it several times... but my Organic Teach said its Aldehyde (yay)... then he said you must take the aldehyde and react it again... so more confusion because now it depends on the amt of Cr03 in the mix.... I think we should write to MCAT and tell them this question is messed up (I know they dont give a [email protected])... maybe there was something in the passage that gave away the answer.... Anyway I know let it rest...
     
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  3. RT

    RT Rt
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    RogerRabbit, they stated "aqueous CrO3" ie. excess CrO3 I assumed.
     
  4. PlayaHata

    PlayaHata Member
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    I dont think it was aqueous.
     
  5. jofrbr76

    jofrbr76 Senior Member
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    Although CrO3 is considered a weak oxidizer, many organic profs agreed that it would oxidize a primary alcohol to a ald and again to Carb. Acid. Most of the time if you want just aldehyde ox. You also add PCC, which limits amount of O2 available in reaction mix.
     
  6. Tweetie_bird

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    I believe the solvent was CH2Cl2 and not aqueuous. does that change the answer?
     
  7. PlayaHata

    PlayaHata Member
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    I have the FINAL ANSWER!!! (as long as someone knows the solvent Cr03 was in) IN ALL SOLVENTS EXCEPT FOR WATER THE OXIDATION WOULD END AT THE ALDEHYDE. IF THE SOLUTION IS AQUEOUS THEN THE OXIDATION WOULD CONTINUE TO THE ACID. SO WHAT WAS THE SOLVENT?
     
  8. RT

    RT Rt
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    Yay, spent the last ~4 mins on this ques, changed from aldehyde to COOH. But that doesn't change my state of bliss 'cause I thought I repeatedly indicated that it was "aqueous"? Have I not been clear? Heheh k/d...
     
  9. Blitzkrieg

    Blitzkrieg 1K Member
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    roger, wrong!

    it's COOH! I have consulted my orgo book and teachers on this.
     
  10. Blitzkrieg

    Blitzkrieg 1K Member
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    <a href="http://www.rdc.ab.ca/rdc/organic_chemistry/chemistry/251_seminars/alcohol_reactions.pdf" target="_blank">http://www.rdc.ab.ca/rdc/organic_chemistry/chemistry/251_seminars/alcohol_reactions.pdf</a>

    pwned!
     
  11. Tweetie_bird

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    ahemm...Blitz...I believe the solvent was CH2Cl2. :p
    I think it's kinda funny how we all are going insane over this and AAMC is prolly thinkin "hehehe suckazz" with the 8 million they made in one day. <img border="0" title="" alt="[Eek!]" src="eek.gif" /> <img border="0" title="" alt="[Eek!]" src="eek.gif" />
    keep it real!!
    Tweetie
     
  12. Blitzkrieg

    Blitzkrieg 1K Member
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    ahem, tweet, aldehyde is gotten with ch2cl2 ONLY with PCC....dude, CrO3= mighty oxidizer..read that link again..
     
  13. Alli Cat

    Alli Cat Flygirl
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    Hey guys,

    You didn't need outside info to answer this one... the answer was in the passage. It said that CrO3 oxidized "aldehydes and alcohols" in the flow chart, which means that both must be taken to COOH. There's no reason an alcohol would stop at aldehyde if aldehydes themselves are oxidized. Maybe the passage was wrong, because it sounds like 1o alcohols don't always go to COOH. BUT, in the passage, it said they do.

    Does anyone else remember the flow chart of reactivities from the passage, to corroborate this?
     
  14. Rapid Decomposition

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    Yes, I do - that is exactly why I chose carboxylic acid. How can you identify an aldehyde with Cr03 if the aldehyde is not oxidized to a carboxylic acid?
     
  15. Blitzkrieg

    Blitzkrieg 1K Member
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    according to the chart, in AgNO3, alkane would have formed a ppt, cos it's not soluble. However, this isnt' the case. Alkyl halide forms the ppt, even though it's soluble in AgNO3..I put alkane cos of the chart alone,and am kicking myself. :( boo
     
  16. esoteric

    esoteric Senior Member
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    H2Cr207 and Cr02 both will oxidize a primary alcohol all the way to a carboxylic acid. The aldehyde is formed as an intermediate but is further oxidized into the COOH.

    If you want to create an aldehyde from a primary alcohol you have to use PCC or PDC (I think its PDC...).

    By the way.... HF is far more reactive than HBr, HCl, and HI. If you spill concentrated HCl on you hand you can wash it off but if you spill concentrated HF on your hand it will only stop when it reaches the bone.

    Flourine is so electronegative that it doesn't care if there is a H+ there or not so it is very very very acidic.
     
  17. Alli Cat

    Alli Cat Flygirl
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    USC Premed-- I agree with you.

    Blitz-- The only way an alkane will form a precipitate in solution is if it reacts, since alkane solids don't exist at STP. The precipitate formed was not because of a failure of an alkane to dissolve in AgNO3, but because of the alkyl halide reacting with AgNO3. If you think about it, the alkane layer and the aqueous AgNO3 layer wouldn't mix, they'd just form two phases.

    esoteric-- HI is much more acidic than HF. Check your Ochem text. This is because I- is much more stable than F-. Both anions form in solution pretty readily, but much more I- is formed (pH of 1M HI &gt; pH of 1M HF) HOWEVER, since the F- is less stable, it is more reactive, which explains the violent reactivity you described.
     
  18. Street Philosopher

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    Ok the only thing I'm wondering about now is...
    Do I ever have to look at another frickin ochem reaction for as long as I live? I hope they don't cover this shizzat during medical school.
     
  19. Alli Cat

    Alli Cat Flygirl
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    I know what you mean. The thing I'm pissed about is that after studying for the MCAT, I finally get a lot of stuff that I didn't get in undergrad Ochem. Oh well :rolleyes: Med school shouldn't have too much Ochem, thank God.
     
  20. Blitzkrieg

    Blitzkrieg 1K Member
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    </font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">Originally posted by Alli Cat:
    <strong>USC Premed-- I agree with you.

    Blitz-- The only way an alkane will form a precipitate in solution is if it reacts, since alkane solids don't exist at STP. The precipitate formed was not because of a failure of an alkane to dissolve in AgNO3, but because of the alkyl halide reacting with AgNO3. If you think about it, the alkane layer and the aqueous AgNO3 layer wouldn't mix, they'd just form two phases.

    esoteric-- HI is much more acidic than HF. Check your Ochem text. This is because I- is much more stable than F-. Both anions form in solution pretty readily, but much more I- is formed (pH of 1M HI &gt; pH of 1M HF) HOWEVER, since the F- is less stable, it is more reactive, which explains the violent reactivity you described.</strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">ya, I know. That's why I'm so stupid. it was an easy question <img border="0" title="" alt="[Frown]" src="frown.gif" />
     
  21. Mish550

    Mish550 Member
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    So are we 100% sure it was HF and not HI that was the most reactive? I'm still not sure about this one!!
     
  22. Alli Cat

    Alli Cat Flygirl
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    If you're on good terms with your Ochem professor, you could ask him/her <img border="0" title="" alt="[Wink]" src="wink.gif" /> The way I understand it, I- is a much more stable anion than F-, so it's less likely to react. However, I'm pretty sure the Q wasn't as straightforward as "Which halo acid is most reactive?" I put down HI, and while I don't remember the exact Q, I remember going through this reasoning and getting HI as the answer.

    Good luck Mish!
     
  23. Rossp917

    Rossp917 New Member

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    In the argument over HF or HI being the most acidic, I'll throw in my $0.02.

    The strongest acid is the one with the most stable conjugate base, correct?!?

    That would make the Iodine ion is much MUCH more stable than the Flouride ion, thereby making HI more acidic than HF...I could be wrong on this one...but that's just my opinion.
     
  24. PlayaHata

    PlayaHata Member
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    esoteric
    HF is acidic but not as acidic as HI... HI is by far most acidic... acidity increases as you go down the table of halogens.. Im positive. HI is more acidic than HF. HF has a strong conjugate base (F-) therefore its acid is weak. Bases want to take in a proton so therefore very little H+ is available (weakly acidic). I hope that cleared it, but I'm positive I got it beat into me for a year in ORGO.
    AGAIN... the only way Cr03 can give a carboxylic acid is by having water available.... think about it how else would you get the OH of the acid... I asked my ORGO prof. Dry Cr03 or any other solvent can't give you the OH.
     
  25. Papa Smurf

    Papa Smurf Thug 4 Life
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    Acid Ka
    HF 7.2 X 10^-4
    HCl 1 X 10^6
    HBr 1 X 10^9
    HI 3 X 10^9

    So, it all depends on how you interpret this question. If you equate most reactive with the largest Ka or dissociation constant, then you would choose HI as the answer. I- is a more stable base than F-, thus making it much easier for HI to donate a proton. If you look at reactivity from an Ochem standpoint (addition of HX to an alkene), than HF is clearly the right answer. Personally, I don't even remember what the question asked anymore, but I put HF. It was in the Bio section right? I guess I was in orgo mode, so I wasn't thinking about it from a Gen Chem standpoint.
     
  26. esoteric

    esoteric Senior Member
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    Ok I'm back. I checked out some websites and I figured out where I was in error. HF is more reactive.....but less acidic. The bond dissoc. energy for H-F is huge compared to H-I.
    I have no idea what the mcat problem said but I'm sure you guys knew what you were doing. Good Job! It's too bad the problem didn't include what type of reaction we were dealing with...

    See ya.

    -esoteric
     

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