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How can you determine acidity from long hydrocarbon chains?

Discussion in 'MCAT Study Question Q&A' started by ejay286, 09.05.08.

  1. ejay286

    ejay286 Member

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    I often run into questions where it asks which of a certain group of hydrocarbons is the most acidic/basic, whats the quickest way to figure this out?
  2. CrimsonMirage

    CrimsonMirage

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    There was a previous thread on this:
    http://forums.studentdoctor.net/showthread.php?t=535952

    I just know the gist of it for MCAT q's...carboxylic acids are most acidic, alkanes are the lousiest acids...amines can act as bases so they are pretty lousy acids as well. alcohols, phenols, and most other carbonyl compounds lie somewhere in between.
  3. CrimsonMirage

    CrimsonMirage

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    Oh I just noticed you specifically said 'long hydrocarbons'...for those I just compare how many acidic groups are present...like diacid will be more acidic than an acid, having an electronegative group near an acid is more acidic than having one farther away, etc..
  4. ej37

    ej37

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    There are a couple of ways to do this.

    First of all, lets start with the definition of acidity. For the MCAT, lets just call it the ability of a molecule to give up a proton.

    For the MCAT, there are two factors that determine acidity, and they both stem from this general idea of how easily the molecule will let go of an H-atom.

    The first factor is size considerations. For example, compare
    CH3CH2OH vs. CH3CH2SH. The one with sulfur is a stronger acid. Why? S is larger than O, thus the H-O/S bond is longer (read: weaker), and the H falls off easier. This can also be reasoned through electronegativity. You should know that O is the second most electronegative element, (Memorize the 4 most electronegative elements... F, O, Cl, N, in that order. Aside: You should also know that because of this, they are the only ones capable of forming hydrogen bonds)

    Anyway, the second thing you need to consider are the resonance/inductive effects that each molecule has, and if losing a proton would provide resonance stability. Remember, resonance=very stable, and nature favors these reactions.

    So... keeping this in mind, lets use this logic to solve a couple problems. Remember, the overall goal is to provide the MOST STABLE CONJUGATE BASE.

    1) CH2CH2CH2OH vs. (CH2)2CHOH

    In a simple hydrocarbon chain, primary alcohol is more acidic. Always. The alcohol functional groups are the same, and the only other functional groups are Alkane chains.

    The conjugate base is, in general: R3O-. Keep that negative charge on the oxygen in mind. Alkyl groups are electron donating, you should remember this from aromatic substitution. So, if it is electron donating, some of the electron density from the R chain spills onto the O atom, in essence, making the charge greater. An electron donating group DOES NOT stabilize the base.

    2) CH2CHFOH vs. CHFCH2OH

    CH2CHFOH is more acidic. In this case, we have a Fluorine, which is very EWG. Again, picture the conj. base: CH2CHFO-. The Fluorine WILL help stabilize that charge, it is hungry for electrons. Hence, the closer it is to the anion the more effective it will be, and of course, the more EWG's you have the more acidic the compound is.

    3) Phenol (OH bonded to benzene ring) vs. p-cresol (OH-bonded to a toluene ring) vs. picric acid (look this one up... its also called
    2,4,6-nitrophenol)

    Ok, this one is a little more difficult, although the same reasoning applies. Aromatic rings are EWG, their pie system has can accommodate extra electron density from the O-atom. So by default, we know all of these three compounds are going to be pretty acidic, much more so than their cyclohexane counterparts. Lets rank them.

    As we determined earlier, a straight Alkyl group, no matter what it is, will always be an EDG. So in the case of p-cresol, the methyl group decreases the stability of the conjugate base. The aromatic pie system (electron withdrawing), takes on some its electron density, and hence, has less "room" to accept electrons from the Oxygen anion in the conjugate base. Therefore, we can say with certainly that Phenol is more acidic than cresol.

    Picric acid, on the other hand, has 3 EWG Nitro groups on it, and those groups actually remove electron density from the EWG pie system! Given that they are both EWG groups, you might be saying, well how do you know the electrons go into the Nitro groups and not the pie system? Well, for the purposes of the MCAT, just assume that an aromatic ring is the weakest possible EWG... that is, any other EWG will trump it: when an EWG substitute is attached to an aromatic ring electrons go into the EWG (this can get tricky when you have multiple aromatic rings inter-connected... but that is well beyond the scope of the MCAT). So anyway, now that there is more free space in the pie system of picric acid, the EWG properties of the aromatic ring are ENHANCED relative to phenol, and will take on a good amount of the negative charge on the oxygen anion, making a VERY stable conjugate base.

    FYI: Picric acid is one of the most explosive compounds known to man, it is a contact explosive and extremely unstable at room temperature, all because of the EWG Nitro groups.

    Hope this helps, sorry if it is too long and confusing. I definitely could have made it more concise... but hopefully it gets the point across to the OP and others.


  5. MBHockey

    MBHockey

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    ej37, great post!

    Simply put, acidic hydrocarbons are more likely to give up their Hydrogens because the Hydrogens are more weakly bound to the rest of the molecule than hydrocarbons that are less acidic.

    The more electronegative an atom that hydrogen is bonded to means that the other atom has a much stronger "grip" on the electron pair that constitutes their bond which makes it easier for this hydrogen to "fall off" the chain -- making the hydrocarbon more acidic. It's like if two people are playing tug of war with a rope...the stronger person (more electronegative atom) is pulling harder on the rope (electron pair) and it makes it harder for the weaker person (H atom) to hold on...so he just lets go and is a free proton.

    For example, look at these three organic acids:

    1) CH3CH2CH2OH
    2) CH3CF2CH2COH
    3) CH3CH2CH2CH3

    Organic acid #2 is going to be the most acidic.

    Even though the acidic proton in 1 and 2 is bonded to Oxygen in both cases, organic acid #2 has two Fluorines in there taking the place of two Hydrogens. The Fluorines + the Oxygen (Fluorine's effects are inductive effects -- effects felt through other bonds) are pulling on the electron pair that is connecting the H and the O much more than the Hydrogens + the Oxygen is pulling on that same electron pair in organic acid #1. This makes acid #2 the most acidic, followed by #1.

    #3 is by far the least acidic because there are no O's or F's yanking electron pairs away from any H atoms as in #1 and #2 which means less hydrogens will be falling off the chain. Remember, acidity is just a measure of hydrogen ion concentration.

    As ej37 said, size of the atoms also matters. Size matters more when comparing atoms from different periods, while electronegativity is more important for comparing atoms in the same period. The bigger the atoms, the weaker the bond, the easier it is for Hydrogens to fall off, the stronger the acid.
    Last edited: 09.20.08

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