Login

Join for Free!
118872 members


Which Molecule is a Better Acid?

Discussion of all aspects of biological molecules, biochemical processes and laboratory procedures in the field.

Moderator: BioTeam

Which Molecule is a Better Acid?

Postby zerobladex » Sun Oct 11, 2009 5:29 pm

Between:
....H....H......O
....|....|.....//
H.-.C.-.C.-.C
....|....|.....\
....H...H.......OH

and
....H...O......O
....|...||....//
H.-.C.-.C.-.C
....|....|.....\
....H....H.....OH

Ignore the periods.
From what I think, number 2 would be the worse acid since the double bonded [O] on the 2nd carbon would probably attract and bond to H+ molecules in water, raising the pH. I'm not exactly sure however, so can someone clarify?
zerobladex
Garter
Garter
 
Posts: 7
Joined: Tue Oct 07, 2008 9:58 pm

Postby david23 » Sun Oct 11, 2009 10:08 pm

and here i thought the second one was the better acid because of the extra resonance structure it can gain with the 3 oxygens.
david23
Coral
Coral
 
Posts: 430
Joined: Thu May 03, 2007 8:15 am

Re: Which Molecule is a Better Acid?

Postby JackBean » Mon Oct 12, 2009 1:34 am

What about to writte it in some normal way like CH3-CH2-COOH? :twisted:

Than you could find out, that nothing like CH3-CHO-COOH doesn't exist ;)

Anyway, the attraction of carbonyl of hydrogen ions won't be so high to beat the COOH dissociation. On the other hand, there could be a 5-atom ring after H-bond formation between the carboxyl hydrogen and carbonyl oxygen.
On yet another hand, carbonyl will definitely attract electrons, so the O-H bond will be weakened and thus the acid should be stronger.
You choose :)

(but I think, the second one will be stronger)
http://www.biolib.cz/en/main/

Cis or trans? That's what matters.
User avatar
JackBean
Inland Taipan
Inland Taipan
 
Posts: 5690
Joined: Mon Sep 14, 2009 7:12 pm


Postby MrMistery » Mon Oct 12, 2009 3:36 am

i would say the first one, as the second one doesn't exist (you have 5 bonds to carbon). If you had for example an alcohol instead of that ketone (to make the thing possible) it would be the second one: inductive effect - OH pulls electrons away from the carbon, the carbon on the carboxyl becomes more electropositive. But i think that there wouldn't be much difference anyway - I mean, the conjugate base is stabilized by resonance in both, the inductive effect would have only a minor contribution
"As a biologist, I firmly believe that when you're dead, you're dead. Except for what you live behind in history. That's the only afterlife" - J. Craig Venter
User avatar
MrMistery
Inland Taipan
Inland Taipan
 
Posts: 6832
Joined: Thu Mar 03, 2005 10:18 pm
Location: Romania(small and unimportant country)

Postby JackBean » Mon Oct 12, 2009 3:46 am

It would have definitively some effect, especially, if it was carbonyl (CH3-CO-COOH;)
The effect of halogens on alpha carbonyl is like change in pKa of 1 ;)
http://www.biolib.cz/en/main/

Cis or trans? That's what matters.
User avatar
JackBean
Inland Taipan
Inland Taipan
 
Posts: 5690
Joined: Mon Sep 14, 2009 7:12 pm


Return to Molecular Biology

Who is online

Users browsing this forum: No registered users and 1 guest