Polar Protic vs Aprotic Solvents with SN1 SN2

Discussion in 'DAT Discussions' started by crhoody12, Jun 25, 2007.

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  1. crhoody12

    crhoody12 5+ Year Member

    Jan 27, 2007
    Quick question/clarification about protic v aprotic solvents

    I know from what I have read that SN1 likes polar protic while SN2 likes polar aprotic solvents. I am confused when in Kaplan books under the SN1 reactions it says a good polar solvent for this reaction is water/acetone. My question is, isn't acetone a polar aprotic solvent and therefore good for SN2 and not SN1? I'm just confused between the two and which is good for which reaction.
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  3. Shunwei

    Shunwei 7+ Year Member

    May 26, 2007
    I think you just have to understand that for protic solvents, they "solvate" or basically cover up the ions in solution; therefore, for a SN2 reaction, this would obviously not be good as the nucleophile needs to project its electron pairs for action. On the other hand, for SN1, solvation really helps the leaving group to dissociate and facilitate the rate-limiting step of the reaction.
  4. andyjl

    andyjl 5+ Year Member

    Apr 16, 2007
    but the water is polar protic
  5. D D S 2 B

    D D S 2 B 2+ Year Member

    Jun 15, 2007
    Are you sure that it says Acetone as polar protic, because when they talk about polar protic they usually refer to water or alcohol, or substances that have hydrogen bonding. So for those you would do Sn1. And for polar aprotic they can use dmso or acetone which dont have hydrogen bonding and these are considered polar aprotic and shouldbe used for Sn2. Double Check...
  6. JamieMac

    JamieMac Elite Member 7+ Year Member

    Jul 7, 2005
    Royal Oak
    I believe for something to be polar protic it must be able to H bond...i.e. it must have an F, O, or N with an attached H, which would enable it to hydrogen bond. Ketones are not capable of Hydrogen bonding, therefore would not be polar protic. They are, however, polar aprotic (due to the dipole forces from the negatively charged O creating a partially positive charge on the corresponding Carbon.
  7. old-school

    old-school UNT '07 alum 5+ Year Member

    May 21, 2005
    McKinney, TX
    I was just studying this as well. The main point is that Protic solvents involve H-bonding which will push the sn1 because the formation of the carbocation is the rate-determining step, and they help stabilize the formation of the carbocation.
  8. old-school

    old-school UNT '07 alum 5+ Year Member

    May 21, 2005
    McKinney, TX
    Oh, and yes, acetone is aprotic.
  9. infernobutterfl

    infernobutterfl Pyro user 7+ Year Member

    May 26, 2005
    Congrats. I think you found another error in the kaplan book. :)

    Acetone is polar aprotic (does not contribute a H+ to the solution, but is highly polar due to the large electron density at the lone pair and double bond of the oxygen). It should be useful for SN2 instead of SN1.
  10. Saber tooth

    Saber tooth Hunting For The Prey:DATs 2+ Year Member

    Jun 5, 2007
    I agree with both, Polar protic needs H bonds (i.e. H20, CH3OH) which help stabilize the carboncation once it is formed. Acetone ( a type of ketone) is polar aprotice and is excellent for Sn2 rxn.
  11. Techer

    Techer 5+ Year Member

    Jul 29, 2009
    I am looking at the Kaplan page, and say:"polar solvents (water, acetone, etc.)" . nothing about acetone being protic. Here they are talking about stabilization of carbocations in polar solvents.
  12. wantVCUdental


    Aug 6, 2009
    you dug up a thread from 2007? what's your question?
    Acetone is polar aprotic :)

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