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Ochem Question

Discussion in 'Pre-Pharmacy' started by PistonFan531, Dec 3, 2008.

  1. PistonFan531

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    When you react a tertiary alkyl halide with acetone,(and nothing else) will there be elimination or substitution?
     
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  3. 161927

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    no reaction
     
  4. PistonFan531

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  5. 161927

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    Acetone is not a nucleophile, so substitution will not take place.

    Acetone is not a strong enough base to abstract a hydrogen bonded to a carbon.
     
  6. PistonFan531

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    So then will elimination take place?
     
  7. Nottingham

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    No...because it's NOT a strong enough base to abstract a hydrogen, there will be no elimination and no reaction would occur.
     
  8. Sparda29

    Sparda29 En Taro Adun
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    I was under the impression that an SN2 reaction will take place.

    For the OP, is there also Sodium Iodide in the solution?

    Because I know that there is a reaction in lab tests for alkyl halide unknowns, they use Sodium Iodide/Acetone and combine it with the alkyl halide. The tertiary halide should take the longest to react, 1 > 2 > 3.
     
  9. Organic Pharmer

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    Acetone is just a solvent for the reaction to take place. It favors Sn2 reaction because it is a polar aprotic solvent.
     
  10. OChemTA

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    Actually, I believe you're both wrong and that Nottingham is correct.

    SN2 reactions occur at a very slow rate with tertiary alkyl halides. If a substitution were to take place, SN1 mechanism would dominate because the tertiary carbocation formed is relatively stable. Tertiary alkyl halides is also less inclined to go through SN2 also because the reaction requires a backside attack... rather difficult when you have three alkyl groups there.

    Since the OP mentioned that there are there is "nothing else" other than a tertiary alkyl halide with acetone, there should be no appreciable amount of elimination reaction because as other have said, nothing in the solution is basic enough to abstract a hydrogen from the tertiary carbocation to produce 2-methyl propene. Since there is also no nucleophile to react with the tertiary carbocation (well, except for the halide) there is no SN1 subsitution as well. Therefore, there should be no reaction.
     
    #9 OChemTA, Dec 4, 2008
    Last edited: Dec 4, 2008
  11. Artful Dodger

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    agree. It's a No Reaction problem.

    acetone is polar aprotic, which works well with alkyl halides undergoing SN2 reactions (usually primary and secondary).

    and tertiary alkyl halides relies on just solvent (for SN1 & E1), which happens to be exclusively polar protic - a weak base ie water, ROH, carboxylic acid. However for E2 to occur with tertiary alkyl halides, you just need a strong base (bulky or non-bulky - it makes a difference which one you use too).

    Edit: Polar Protic sovlates nucleophiles. Polar Aprotic solvates cations (never nucleophile). And...if you look at the reaction rate equations of both SN2 and SN1, you'll see that SN2 relies on nucleophile concentration and SN1 does not. That's why solvents are put in place...to do these sort of things...and when you use them in the wrong place like tertiary alkyl halide & acetone, you get NO REACTION!

    Also, OchemTA - I learned that tertiary NEVER undergoes any SN2. My professor mentioned it on countless number of occassions. So, in my guesstimate, it's not even worth mentioning that "some" SN2 will occur with tert alkyl halide...im sure it happens in an experiment, but theoretically none should take place because a tert halide is too hindered.

    If you don't get it now, you'll get used to this stuff in Organic 2...It's pretty important. Just try understanding it while you can though.
     
    #10 Artful Dodger, Dec 4, 2008
    Last edited: Dec 4, 2008
  12. a tertiary alykl halide will not underage an sn2 reaction, it is sterically hindered. tertiary would favor an sn1 reaction.
     
  13. rycetrix

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    Tertiary alkyl halides DO NOT do SN2 reactions. Too sterically hindered. Also, acetone is NOT a nucleophile. It is an electrophile, therefore, it would NOT react with a tertiary alkyl halide to do an elimination reaction. For E1/E2 or SN1/SN2, you need a nucleophile to attack. When you learn about carbonyl chemistry in ochem, you will see why acetone is an electrophile. In this case, acetone is just the solvent so there would be NO REACTION.
     
  14. OChemTA

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    Actually, acetone CAN be a nucleophile. The LP of the oxygen is nucleophillic and will react with the electrophillic hydrogen in acids. This is how carbonyls in hemi-ketal and ketal reactions are activated.
     
  15. OChemTA

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    No, you're wrong. A tertiary alkyl halide will RARELY undergoes SN2 and SN1 reaction will dominate (I mean this in the sense that SN1 will dominate every case of tertiary alkyl halide [and analogous reactions] but that a very small percent of the reactant will react under SN2). If you go into more advanced organic chemistry, you'll learn it's not so clear cut. Sorry, but your prof use terms like "never" in lower organic chemistry classes to emphasize certain points but it is not realistic. ie. your prof may have said that all mechanisms are only E1/E2 and SN1/SN2, but there is also E1CB mechanism. So again, it is NOT "theoretically none" and in fact theory says that SN2 reaction do occur, only that kinetics dictate it only occurs at a very very slow rate (aka rarely occurs). By rarely, I also do not imply that it's so rare that it doesn't happen at all in lab, it does. Just remember that when you're trying to figure out what the product of the reactant is, you're always talking about the most PROBABLE reaction which will give the greatest yield. This (and as emphasis) is why profs may choose to use words like "never".
     
    #14 OChemTA, Dec 4, 2008
    Last edited: Dec 4, 2008
  16. Artful Dodger

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    ^ah i gotchu. didn't mean to pose an argument. im just trying to point out what i thought sounded wrong...but...it look like you've probably taken an advanced org chem. class or something

    myself? haha im currently enrolled in org 2...
     
  17. rycetrix

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    That is considered an acid-base reaction...when you are doing an abstraction of an acidic hydrogen to protonate the O on the carbonyl group. But even doing so, that makes the protonated carbonyl even more electrophilic. And tertiary alkyl halides do not have any acidic hydrogens anyway. We don't need to be that technical to confuse this guy... Just trying to answer the question. :rolleyes:

    "The chloride anion, as well as the acetone, are poor nucleophiles and very weak bases." Quoted from : http://www.chemgapedia.de/vsengine/vlu/vsc/en/ch/12/oc/vlu_organik/substitution/sn_e_konkurrenz/sn1_e1_konkurrenz.vlu/Page/vsc/en/ch/12/oc/substitution/sn_e_konkurrenz/a3_2_sn1_e1_nucleophil/sn1_e1_nucleophil.vscml.html
     
  18. OChemTA

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    Yes, it is an acid base reaction, but keep in mind that all nucleophiles are lewis bases and all electrophiles are lewis acids. You seem to underestimate the importance of "simple" bronsted lowry acid base reaction. As a simple example, just take a look at the formation of enolates, they're based on the "simple" bronsted lowry acidity of the alpha hydrogen. Again, it's a simple reaction, but one required to understand the rest of the reaction that results from the enol or enolate. So the reaction (to get the proton) is one which the acetone is a nucleophile. Even though acetone is a weak nucleophile, it CAN and do act as a nucleophile. So I still stand by my earlier statement that acetone can be a nucleophile.

    Yes, I also do know that after being protonated that the carbonyl is a better electrophile... it's why I say it is "activated" for reactions like hemi-ketal and ketal synthesis. But that's not the point, the point is the earliar BL acid-base reaction. I also wasn't trying to "confuse" this guy. If you looked at my first response, it was quite simple and already stated that no reaction would occur. Lastly, to say that acetone cannot be a nucleophile may confuse him when he learns how carbonyls are activated.

    PS: anyone celebrating the end of classes?

    Edit: I also did not say that the tertiary alkyl halide is acidic and will protonate the carbonyl. I'm simply saying that yes, acetone can be a nucleophile.
     
    #17 OChemTA, Dec 5, 2008
    Last edited: Dec 5, 2008

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