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Author Topic: Basicity Of Hydrocarbons  (Read 3825 times)

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mylkoa

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Basicity Of Hydrocarbons
« on: December 11, 2007, 08:50:20 AM »

Hello,

I'm a second term O-Chem student and this is my first post to this forum. So to begin, I would like to say thanks to the community for being here!

My question/comment has to do with the pK values of Hydrocarbons.

-According to my O-chem book, ethane has a pKa of 50,
-According to wikipedia, Potassium Hydroxide has a pKa of 16.29 (converted from -2.29 pKb)

This seems amazing to me that ethane is much stronger a base than KOH, which I was taught in my first year of General Chemistry to be revered as one of the few "strong bases".

Can anyone offer any insight here? I'm just trying to get a better understanding. Is ethane used as a base in procedures much? If I'm not mistaken ethane is a gas at room temp and average pressure, so I'm guessing that's why my class didn't use it in our wet chemistry labs. But why would we be taught about "strong" bases like the hydroxides (KOH, Ba(OH)2, CsOH, NaOH...)

What's going on here?! :)

Andrew Mylko
Eugene, OR
Lane Community College
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ARGOS++

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Re: Basicity Of Hydrocarbons
« Reply #1 on: December 11, 2007, 12:21:58 PM »

Dear Mylkoa;

The usual simple Acid/Base term may not longer be of the same accurateness as you think.

A more accurate “Picture” is to think of the “power”/strength of a Base/Extractor it has to have to remove an H+ from the molecule. As you easily can imagine, in case of such high values the “reaction” will not be happen in water.
To do such extractions Bases like Sodium-Alcoholate, Sodium-Hydride, Sodium-Amide, and similar are required.

To give you an Idea I have added a Diagram (in German) about the Acidity of CH2 Groups.
(How we had to learn it.)

Hints:
  Acidität          =    Acidity;           Gruppen    =   Groups;
  loslössen  mit  =    extract with;    katalytische Menge = catalytic amount.
  The type of “CH2” in charge is always underlined.

I hope it gives you at least an Idea.

Good Luck!
                   ARGOS++

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     If you DON’T catch “The Picture”, you are lost, - for ever!      (A++)

There is ONLY one correct Formula for the “Hydrogen”:  —
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RBF

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Re: Basicity Of Hydrocarbons
« Reply #2 on: December 11, 2007, 05:45:44 PM »

A quote from the original post:

"This seems amazing to me that ethane is much stronger a base than KOH..."

It's not ethane that is the stronger base, it's the anion of ethane (CH3CH2-) that is a stronger base than OH-.
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mylkoa

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Re: Basicity Of Hydrocarbons
« Reply #3 on: December 11, 2007, 07:17:23 PM »

It's not ethane that is the stronger base, it's the anion of ethane (CH3CH2-) that is a stronger base than OH-.

LOL

Thank you!


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AWK

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Re: Basicity Of Hydrocarbons
« Reply #4 on: December 11, 2007, 07:20:32 PM »

Quote
[-According to wikipedia, Potassium Hydroxide has a pKa of 16.29 (converted from -2.29 pKb)/quote]
wrong conversion
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mylkoa

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Re: Basicity Of Hydrocarbons
« Reply #5 on: December 12, 2007, 10:28:45 AM »

wrong conversion

How would you convert it? Can you please show the numbers?

Thanks
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Borek

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Re: Basicity Of Hydrocarbons
« Reply #6 on: December 12, 2007, 12:03:20 PM »

Potassium Hydroxide has a pKa of 16.29 (converted from -2.29 pKb)

I wonder where did this number (-2.29) came from. Unfortunatley reference is not clear to me. My sources put pKb at 0.5 (see http://www.chembuddy.com/?left=BATE&right=dissociation_constants and links provided there).
« Last Edit: December 12, 2007, 12:36:08 PM by Borek »
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mylkoa

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Re: Basicity Of Hydrocarbons
« Reply #7 on: December 13, 2007, 08:00:26 PM »

I got mine from wikipedia, but I didn't check it any further back then that. It may be incorret, I would want to check it in a CRC or something like that.

Still, it was said that I converted wrong. I googled and found the conversion formula to be:

pKa + pKb = 14

It also works like that @: http://www.webqc.org/unitconverters.php
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