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Difference between nucleophile and base

Discussion in 'MCAT Discussions' started by TripleDegree, Apr 14, 2004.

  1. TripleDegree

    TripleDegree Joker Doctor
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    On one of the other threads, a poster had mentioned that while steric hindrance reduces nucleophilicity (if that's a word), it has no impact on basicity.

    Nucleophile is attracted to a positive charge, a base donates its electron pair so therefore it would also like a positive charge.

    Consider OH- - it is a weak base, but it also a nucelophile. In elimination reactions as a base it would attack the H, but in a substitution reaction it would directly attack the polar C atom.

    Can anyone shed more light on this?
     
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  3. liverotcod

    liverotcod Lieutenant Crunch
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    I'll give it a shot. Basicity refers to the desire of a molecule (or atom) to abstract a proton, as you said. -OH is a strong base, as is -OtBut, because the steric hindrance of the tert butyl has no effect on ability to grab protons: they're pretty small. On the other hand, Cl- is a terrible base because it's the conjugate base of a strong acide. Cl- is not at all interested in grabbing protons.

    Nucleophilicy referes to the desire of the molecule to seek out positive charges. Now the sterics come into effect. Certainly -OH is a good nucleophile, because it is able to easily sidle up to a positive carbon, for example. Also, Cl- is a pretty good nucleophile because it's quite electronegative. But -OtBut is NOT a good nucleophile, because the tert butyl group gets in the way when it tries to approach an electrophile.
     
  4. captbadass

    captbadass Senior Member
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    there are a couple important trends with nucleophiles. one is that the more negative charge they have, the greater their nucleophilicity. the other trend is one that often confuses people--nucleophilicity increases as electronegativity decreases. for example, F- would be less nucleophilic than a Cl- because it is more EN.

    Capt
     
  5. babinski bob

    babinski bob Senior Member
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    Let's make it simple folks...


    When going down a COLUMN, polarizability determines nucleophilicity. This is why I > Br > Cl > F. I- is the most polarizable and is the worlds best nucleophile.
    When going across a ROW, basicity determines nucleophilicity. So NH2- > RO- > HO- > NH3 > H2O. Remember, being highly electronegative is NOT a good thing when it comes to nucleophilicity. Electronegativity is the inherent measure of an atoms' ability to hold e- close to the nucleus. Therefore, a highly electronegative atom (like F) holds its outer e- close to the nucleus, therefore it's less willing to share it's e- and serve as a nucleophile. HOpe this helps.
     
  6. liverotcod

    liverotcod Lieutenant Crunch
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    Great explanation, babinski bob. Helped to clear it up for me too!
     
  7. kiki the alto

    kiki the alto Daughter of the 80's
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    Thanks! I'll have to use that the next time it comes up in a tutoring session...I always wondered how exactly to explain nucleophiles without comparing them to bases.
    PS - liverotcod...that Yeats quote is from one of my favorite poems...you rock. "Things fall apart;the center cannot hold"
     
  8. liverotcod

    liverotcod Lieutenant Crunch
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    Thanks, Kiki.
     
  9. blz

    blz Senior Member
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    Actually, for the halogens, Nucleophilicity depends on the solvent (or phase). F- is a stronger nucleophile in a polar aprotic solvent because it is the strongest base and therefore I- is the weakest nucleophile because it is the weakest base. However, in a polar protic solvent, ion-paring interactions reduce the nucleophilicity of F- making it the weakest nucleophile and I- the strongest. Since Ion-paring does not occur in the gas phase, since there is essentially no solvent-solute ion-paring, F- would be the strongest nucleophile and I- would be the weakest nucleophile. The MCAT probably would not test this concept because there are too many exceptions.
     
  10. DaveinDallas

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    I may be displaying some real ignorance here so help me out folks....

    I always thought electronegativity was a measure of the atoms ability to attract electrons, not a measure of how close it held them to the nucleus. In my OChem book, that's the general explanation/definition they used and always used electron density maps for a visual when explaining EN. I'm sure the two concepts are interrelated somewhere down the line. Would someone care to correct my understanding? I'm not being cheeky, just want to be sure I'm not missing an important tie in.....
     
  11. babinski bob

    babinski bob Senior Member
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    Dave in Dallas... You're correct, but in fact we're both correct because we're saying the exact same thing. :) An atoms' ability to attract electrons = an atoms' ability to hold electrons close to the nucleus. If the electron density map is more dense around a given atom (for an electronegative atom), then that atom is holding the e- close to the nucleus! Remember, the nucleus is positively charged... You understand it perfectly fine. We're just saying the exact same thing using different wording.. :)


    Blz.... This is not really correct. The classic Sn2 reaction is NaI in DMF (a polar aprotic solvent) w/ a primary substrate. It works extremely well, 100% yield. Trust me. I've done this reaction many times in the lab. However, if you try the exact same reaction w/ NaF in DMF, it just doesn't work. In fact, regioselective fluorination of alkanes is not a simple task. Most methods rely on the use of an electrophilic fluorine source rather than a nucleophilic source because fluorine is such a lousy nucleophile no matter what solvent it's in.

    For everyone else, just remember... the MCAT keeps it simple. The trend is always I > Br > Cl > F. When going down a COLUMN, polarizability wins. When going across a ROW, basicity wins. Keep things simple, rely on first principles, and you'll rock the ochem portion and save yourself more time for the much more difficult bio portion.
     
  12. TripleDegree

    TripleDegree Joker Doctor
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    BB

    I'll take your word for it since you seem to know what you're talking about. However, one question, when going across a row, how did you come to the conclusion that NH2- > RO- > HO- > NH3 > H2O? You seem to be looking at only N and O here. And how did you get that relationship?

    Thanks for everyone's answers
     
  13. babinski bob

    babinski bob Senior Member
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    This relationship between basicity and nucleophilicity can be found in any major ochem text. But just think intuitively for yourself.... The def of a nucleophile is a species that seeks out a positive center. If you were a nucleophile, would you more easily seek out a positive center if you were neutral or negatively charged? Of course you'd want to be negatively charged!!! And the negatively charges species is ALWAYS more basic than it's conjugate acid which is neutral!!!

    Therefore OH- > H2O in terms of nucleophilicity and OH- is more basic than H2O. In terms of why NH2- is more nucleophilic than OH-... oxygen is more electronegative than nitrogen and is therefore LESS willing to share its electrons and serve as a nucleophile. Therefore, N is a better nuc than O and NH2- > OH-. It also happens that NH2- is more basic than OH-!! This trend in nucleophilicity ALWAYS follows basicity when you compare nucleophiles across a ROW!!! I repeat, it only works when comparing nucs across a ROW!!!

    So for example.... RNH- > NH2- > RO- > OH- > Cl- > RNH2 > NH3 > ROH > H2O in terms of nucleophilicity AND basicity!!! *****This of course works beautifully provided R is SMALL and NOT sterically hindered (Me, Et, etc). Notice how I didn't bother to include secondary amines in there because now you can have more hindrance and may or may not impair nucleophilicity. Hope this helps. Just keep it simple...

    Nucleophilicity parallels basicity across a ROW!!!
    Nucleophilicity parallels polarizability down a COLUMN!!!
     
  14. DaveinDallas

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    Hi all,

    Wow, I'm a little intimidated by the way people here are ripping off probs and stats off the top of their head. Looks like I've got a lot of material to go over....Anyway, I asked my O Chem prof. this question today and here's what he said...

    Any negatively charged atom can act as a base. What differentiates a nucleophile from a base is that it displaces something else i.e. nucleophilic substitution reactions. Nucleophiles and bases are basically (no pun intended) the same thing, with the displacement ability of nucleophiles being the key differentiator.

    I hope I relayed his thoughts correctly and any factual errors are wholly mine, not his, obviously.

    Hope this helps. Kinda sounds like he's following the KISS principle......

    Cheers...
     
  15. TripleDegree

    TripleDegree Joker Doctor
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    Thanks!!!!

    Here's another stupid question then ..... why is I- more polarizable than F-?


    DaveInD - I like the way your professor thinks :)
     
  16. jhk43

    jhk43 Senior Member
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    cuz bob's momma said so :D
     
  17. babinski bob

    babinski bob Senior Member
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    ONe of the nice things about ochem is that IMO, most everything is intuitive unlike bio which is pretty much just memorizing... Think about I vs. F. I is much largerthan F. The outer e- of I are much further away from the nucleus than those of F. So what happens if you bring I near a positive charge vs. F? I will more readily distort it's e-density than F will. How did I come to this conclusion? Well you remember from physics that the electric force Fe = kq1q2/r^2... I think that's the right formula. So since the distance from the positively charged nucleus and outer valence e- is much greater in I v. F, the attraction betwen the two is much less.

    Now think of I as a nucleophile approaching a substrate. REmember, the def of nucleophile is a species that seeks out a positive center. The substrate C to which is leaving group is attached is the positive center. As I approaches this positive center, it's e- cloud is more easily distorted than that of F. Additionally, since I's outer e-'s are further away from the nucleus than those of F, it will more readily share it's e- to form a bond. Remember, I is less electronegative than F.

    So this is the whole concept of polarizability... The ability of an atom to distort it's e- cloud, in essence become "more negative" as it approaches the positive center (the substrate), and nucleophilically displace the leaving group. It's a challenging concept, but it you break it down piece by piece, it's not so bad..
     

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