[irunwithmonkeys]

Why do certain molecules have different pK_a values?  Can anyone explain this to me?  Malonic acid and succinic acid both have 2 different values each.  Does it have something to do with molecular structure and weight?  My book doesn't go into much detail about this.

Malonic Acid                             Succinic Acid

HOOC-CH₂-COOH       COOH-CH₂-CH₂-COOH

pK_a: 2.85 & 5.70        pK_a:  4.20 & 5.60

[Yggdrasil]

For both of these molecules, it is fairly easy to remove one hydrogen atom.  However, to remove the second hydrogen atom is fairly difficult because it places two negative charges fairly close to one another.  The principle is similar to that of ionization energy.  Ionization energies increase with the number of electrons removed because it's hard to keep pilling negative charge upon an object.

[Borek]

Why do certain molecules have different pK_a values?

Your question is a little bit ambiguous to me. Do you ask why each pKa value for ONE molecule is different (Ygg already answered that) or doyou ask why pKa values for DIFFERENT molecules are different?

[irunwithmonkeys]

Whoops.  I didn't realize how open that question was.  I guess what I meant to ask is why does the 1st pK_a value of malonic acid differ so much from succinic acid and so forth.  Is it because of their molecular weight and structure that causes different pK_a values between malonic and succinic acid?

[Yggdrasil]

Here it's because of the inductive effect.  The oxygens of the second carboxyl group in malonic acid act as electron withdrawing groups and can help to stabilize the negative charge when the first carboxylic acid of malonic acid is deprotonated.  The same effect happens in succininc acid, but the second carboxyl group is farther away so the inductive effect is lessened.

[AWK]

Why do certain molecules have different pK_a values?  Can anyone explain this to me?  Malonic acid and succinic acid both have 2 different values each.  Does it have something to do with molecular structure and weight?  My book doesn't go into much detail about this.

Malonic Acid                             Succinic Acid

HOOC-CH₂-COOH       COOH-CH₂-CH₂-COOH

pK_a: 2.85 & 5.70        pK_a:  4.20 & 5.60

Compare inductive electronic effects in both molecules

[Abbie]

So, then why is the 2nd pka of malonic acid higher then the 2nd pka of succinic acid?

[Yggdrasil]

When the first proton of the dicarboxylic acids are removed, a carboxylic acid group acts as an electron withdrawing group to stabilize the anion generated by deprotonation.  When the second proton of these dicarboxylic acids is removed, a carboxylate (not a carboxylic acid) acts as the electron withdrawing group.  Because the carboxylate already has a negative charge, it is much less effective at stabilizing the negative charge.

So, while the inductive effect still happens its effect is much less.  Furthermore, you have the additional effect of electrostatics discussed in Reply #1 of this thread.  Here, the effect of the negative charge makes it harder to remove the second proton from malonic acid because the two carboxylates are very close to each other.  This effect ends up outweighing the inductive effect, making it easier to fully deprotonate succinic acid because the negative charges lie farther apart.