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Discussion in 'MCAT Study Question Q&A' started by DrMattOglesby, May 25, 2008.

  1. DrMattOglesby

    DrMattOglesby Grand Master Moderator Emeritus 7+ Year Member

    Aug 15, 2007
    what is the difference between a molecule being amphoteric and it being a zwitterion ....and it being a dipolar ion?

    also, SN1 reactions are dependent on one molecule so we say they are unimolecular, right?
    then that makes SN2 bimolecular?
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  3. RySerr21

    RySerr21 i aint kinda hot Im sauna 7+ Year Member

    Dec 22, 2007
    zwitterion refers to a molecule that has both a positive and a negative charge but is still neutral over all (an amino acid has a positive charge on the nitrogen and a negative charge on the oxygen).

    amphoteric refers to a molecule that can act as either a base or an acid, depending on what it is reacting with. (water is an example)

    H2O + NH3 ---> OH- + NH4+ (here water acts as an acid)

    H2O + HCl ---> H30 + Cl- (here water acts as a base)

    your distinction bewteen SN1 and SN2 sounds correct.
  4. tncekm

    tncekm MS-1 2+ Year Member

    Jul 18, 2006
    Just to further clarify, lets give an example:

    (CH3)3CBr (in MeOH) ----> [(CH3)3C+ + MeOH <--> (CH3)3CO(H)CH3+] ---> (CH3)3COCH3

    In short, t-butyl-bromide --> methyl t-butyl ether

    The rate of this reactant is ENTIRELY dependent on the stability of the carbocation; this is why a tertiary carbocation in a polar protic solvent (like water) will favor SN1. If the tertiary carbocation is stable, the water can readily attack the positively charged carbon. If not, it wont.

    Just to clarify further:

    a tertiary carbocation is more stable than a secondary or primary (or methyl) b/c there is additional electron density to stabilize the positive charge.

    The polar protic solvent (water in this case) can also help stabilize the positive charge similar to how it would hydrate ions.

    In an SN2 reaction a base comes in and steals a proton and the electrons quickly form a double bond with the partially positive carbon, expelling the halogen.

    I.e. CH3CH2CH(Br)CH3 + -OCH3 --> CH3CH2C=CHCH3

    If the solvent is polar and protic it will stabiliize the attacking base by solvation slowing or haulting the reaction. So, we don't want that here.

    If we have higher concentrations of the base and the alkyl halide then the reaction will go faster b/c there will be more collisions!

    In summation:

    SN1 --> rate=k[Alkyl Halide]
    SN2 --> rate=k[Alkyl Halide][Base]

    The respective k values will be effected by things like temperature, solvent, etc.
  5. tncekm

    tncekm MS-1 2+ Year Member

    Jul 18, 2006
    Exactly, and to expand upon this in an MCAT relevant fashion an example of an amphoteric molecule that forms zwitterions in biological systems, we have amino acids.

    Amino acids have an amino reside and a carboxylic acid residue:


    The amine reside and the carboxylic acid residue have different pKa values, so they'll be protonated and deprotonated at different pH's.

    Lets say that the pKa of the amine residue is 10 and the pKa of the carboxylic acid residue is 3.

    At pH > 10 the amine will not be mostly protonated, but the carboxylic acid will lose a proton. So, the overall charge of the molecule is negative.
    H2NCHRCOO- ******** (see note below!!!!)

    At 3<pH<10 the carboxylic acid residue will still lose protons, and the amine group will start to pick them up here. So, the carboxylic acid reside is a carboxylate and is negatively charged, and the amine residue is an ammonium and is positively charged, thus it is a zwitterion because it has two residues that are both charged but its overall charge is neutral ******** (see note below!!!!).

    At pH<3 the carboxylic acid will no longer lose protons to the solution, so will be neutral, but the amine will be protonated and exist as an ammonium, so the overall charge of the molecule is now positive.
    +H3NCHRCOOH ******** (see note below!!!!)

    Oh yeah, and know that the pI (where it will exist as a zwitterion) is the average of the pKa of the acid and the pKa of the base residues.

    ******** Note: this moreso means that there are lots of zwitterions in this range between 3 and 10 pH, but there will be a predominant form that is not a zwitterion b/c there will still be proton losing and gaining going on. In reality, below the pI (6.5 in this case) the amino acid will be protonated and positively charged more often, and is therefore considered to be positive below the pI. Likewise, above the pI the amino acid will be deprotonated and negatively charged more often, and is therefore considered to be negatively charged.

    Also note that at "exactly" pKa amine and pKa carboxylic acid those are half equivalent points, and for the amine the concentration of protonated amine is equal to the concentration of deprotonated amine. For the carbox pKa at that point the carboxylic acid concentration is equal to its conjugate base, the carboxylate anion. So, although I gave "protonated / deprotonated" etc examples, in reality its more like a "gradient" of how the amino acids are charged, and how many are zwitterions, etc, but at the levels I indicated you can determine with relatively easy about how many zwitterions will exist, or what form of the AA predominates.

  6. DrMattOglesby

    DrMattOglesby Grand Master Moderator Emeritus 7+ Year Member

    Aug 15, 2007
    beautifully written...AND it made sense! ha
    you should be a contributor to the MCAT review guides:thumbup:

    wait a tic...the SN2 mechanism you described is actually an elimination!
    we are not supposed to get alkenes as a result of SN rxns!
    so it kinda looks more like an E2...if anything.

    maybe i should be an editor... =|
    okay i gotta go take my MCAT now....wish me luck!!!!!!! lord knows im gonna need it for the PS and VR

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