Relative acidity

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Kaleb Tesfaye 1I
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Relative acidity

Postby Kaleb Tesfaye 1I » Thu Jun 07, 2018 9:41 am

When Dr. Lavelle was explaining the characteristics of a strong acid, he mentioned how "oxoacids more readily lose H+ if resulting anion is stabilized by electron withdrawing atoms delocalize and stabilize the negative charge"? What does he mean by this?

Chem_Mod
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Re: Relative acidity

Postby Chem_Mod » Thu Jun 07, 2018 7:18 pm

If you have an acid that is going to lose a proton (H+) and leave an O- behind the resulting conjugate base will have a large localized negative charge on the oxygen. This makes the conjugate base less stable, more reactive, and therefore more likely to act as a base and gain a proton back. The less stable the conjugate base the stronger the conjugate base will be, meaning that your original acid is going to be weaker. However, if you have some way to distribute that negative charge on your oxygen you will stabilize the conjugate base making it a weaker base, thus your acid is stronger. There are several ways that the negative charge can be stabilized. One is through resonance, this is most commonly seen in carboxylic acids, which have a -COOH group that has both the double bonded oxygen and the -OH on the C. It will look like this:
Image
When the carboxylic acid loses that proton on the OH it will leave the negatively charged O, but this negative charge can be in resonance with the double bonded O that is also on the carbon. This will result in the negative charge being distributed like this:
Image
This makes the charge more balanced, the conjugate base more stable, and the original acid a stronger acid that another acid that just has -COH.
The other method of stabilizing the negative charge on the oxygen is to have an electron withdrawing group, something with a high electronegativity or that would create a dipole towards itself, attached to the same atom as the oxygen. The example used in class was Cl3COH, where the 3 chlorines are all attached to the central carbon which also has the oxygen attached. When the proton is lost the O attached to the C will become O-, however, the Cl atoms are electronwithdrawing, so they will pull the electron density towards themselves and away from the O. This creates a more even electron distribution and an overall more stable conjugate base, which means that the original acid is going to be rather strong. H3COH would be a weaker acid because it does not have the electron withdrawing ability that the Cl3COH does.

Briana Lopez 4K
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Re: Relative acidity

Postby Briana Lopez 4K » Fri Jun 08, 2018 1:22 am

Also, the bonds for a strong acid are very easy to break when they are dissociated in water. This means that the H+ ions will lose an electron and it will go to the other element, the weak conjugate base.

EX: HCL -> H+ + Cl-
Strong Acid -> H+ + Weak Conjug Base
->this WCB is stable and unreactive because its octet is full

Ex shown w/ water:
HCL + H2O -> H3O+ + Cl-
Acid + Base-> Conj Acid + Conj Base

The hydrogen loses an electron and the chlorine gains one because that is how they are most stable because they have their outermost orbitals filled. Now, the Cl-, the anion, has a negative charge but is more stable.


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