Nucleophilicity ~ Help!

Discussion in 'DAT Discussions' started by busupshot83, Feb 28, 2007.

  1. busupshot83

    busupshot83 S.D.N. Vet
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    Kaplan refers to one method for determining the nucleophilicity of a molecule, and Barron's Organic Review says another:

    Kaplan
    1) if attacking atom is the same, then nucleophilicity depends on basicity.
    2) if attacking atom differs, and reaction occurs in protic solvent, then nucleophilicity depends on molecule size.
    3) if attacking atom differs, and reaction occurs in aprotic solvent, then nucleophilicity depends on basicity.

    Barron's Organic Chemistry Review Series
    1) when molecules have the same central atom, a charge increases nucleophilicity.
    2) larger molecules have greater nucleophilicity.
    3) strong bases are more nucleophilic than weak bases.
    4) steric effects decrease nucleophilicity.

    I'm confused on what I should know for determining the nucleophilicity of a molecule. Is Kaplan or Barron's explanation better?
     
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  3. RING12

    RING12 Member
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    do not study form barron's/ that book it;s just good for pat.for nuclophile you should look some thing like OH every thing that has negative charge is a nuclophile.

    go to this page i think it would help you.

    http://www.chem.ucalgary.ca/courses/351/Carey5th/Ch08/ch8-0.html
     
  4. busupshot83

    busupshot83 S.D.N. Vet
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    I don't know if this makes a difference, but it's not the Barron's DAT book... it is Barron's Organic Chemistry Review Series.

    The site that you gave me is good.
     
  5. busupshot83

    busupshot83 S.D.N. Vet
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    Ok, I've compiled what I learned based on Kaplan, Barron's Orgo, and that web site.

    Nucleophilicity Rules:
    1) when molecules have the same central atom, negative charges increase nucleophilicity:
    CH3O- > CH3OH (C is central atom)
    Me3P < Me2P- (P is central atom)

    2) in atoms with the same attacking atom, nucleophilicity increases with basicity (basicity increases as you move to the left on the periodic table)

    3) in atoms with different attacking atoms, and occurring in protic solvents, nucleophilicity increases with atomic size (size increases as you move to the left and bottom of the periodic table):
    I- > Br- > Cl-

    4) in atoms with different attacking atoms, and occurring in aprotic solvents, nucleophility increases with basicity (as in #2).

    5) Nucleophilicity increases with less steric effects (steric effects decrease with more accessibility to the attacking atom)

    Is this pretty accurate? Let me know.
     
  6. BodybldgDoc

    BodybldgDoc Guest

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    #5 is not really a rule. Its just common sense that a nucleophile that can attack a molecule which is less sterically hindered will have greater nucleophilicity. The remaining 4 rules sound accurate. It would also be a good idea to understand why nucleophilic strength differs between protic and aprotic solvents, just to be able to make it easier to memorize the rules.
     
  7. busupshot83

    busupshot83 S.D.N. Vet
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    Thanks buddy, that sums that up.
     
  8. CyborgNinjaX

    CyborgNinjaX Junior Member
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    Crap man.... this stuff is hurting my head... is there any easy way around this? I hate the Alkane chapter for this reason alone... I think this is probably one of my more weaker points.
     
  9. busupshot83

    busupshot83 S.D.N. Vet
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    I was looking at Organic Chemistry - Janice, Gorzynski, Smith, and here's another explanation:

    when solvent is not given:
    1) when molecules have the same central atom, negative charges increase
    nucleophilicity:
    2) nucleophilicity increases as you go left on the periodic table, or in other words, as basicity increases:
    3) nucleophilicity increases as you go down on the periodic table, or in other words, as size increases:
    4) Nucleophilicity increases with less steric effects, or in other words, with more accessibility to the nucleophiles or central atom. Look for the least-substituted nucleophile/central atom.

    when solvent is given:
    5) in protic solvents, larger atoms are better nucleophiles (see #2 above):
    6) in aprotic solvents, more basic atoms are better nucleophiles (see #3 above):

    ---

    Now test yourself and determine which molecule is more nucleophilic (highlight spaces below for the solution):
    CH3O- vs. CH3OH
    CH3O- > CH3OH (rule #1)

    -OH vs. H2O
    -OH > H2O (rule #1)

    OH vs. F-
    -OH > F- (rule #2)

    I- vs. Br- vs. Cl- vs. F-
    I- > Br- > Cl- > F- (rule #3)

    in protic solvent: HSe- vs. HS- vs. HO-
    HSe- > HS- > HO- (rule #5)

    in aprotic solvent: I- vs. Br- vs. Cl- vs. F-
    I- < Br- < Cl- < F- (rule #6)
     

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