The Vespiary

The Hive => Serious Chemistry => Topic started by: malvaxman on April 01, 2001, 01:43:00 PM

Title: How do I predict..?
Post by: malvaxman on April 01, 2001, 01:43:00 PM

How is it possible to predict looking on the stuctural formula of a two molocules to preict which one is more accidic/basic?

Title: Re: How do I predict..?
Post by: smiley_boy on April 01, 2001, 06:53:00 PM

How is it possible to predict looking on the stuctural formula of a two molocules to preict which one is more accidic/basic? It depends on the compounds, the definition of acids and bases you're using, and under what conditions. Yes that's vague, but without more information to go on, that's about as accurately as that question can be answered. There are numerous definitions of acids and bases: Arrhenius, Brønsted-Lowry, and Lewis acid/base theories are just a few that ar out there. All three are pertinant to orgnaic chemistry, but there are plenty of compounds that are defined as acidic or basic by one theory, but not another.

For example, using the Brønsted-Lowry definition of acids and bases, an acid is a species that donates a proton (that is, it donates a H⁺), and a base is one that accepts it. The acidity of a compound is determined by its pKa, or the dissociation constant - the lower the constant, the more acidic it is.

So, how do you tell how acidic an organic compound is? Look at its structure, and look for functional groups that will cause a proton to dissociate. Once the proton is dissociated, the compound will be ionic, so also look for functional groups that will stablize the deprotonated form.

I'm guessing you want more information than this, but unless you're willing to be more specific about the compounds you're interested in, this could take a while...

Title: Re: How do I predict..?
Post by: b159510 on April 05, 2001, 07:38:00 AM

Not so serious chemistry. This subject is covered in the
first semester of college o-chem. Some things that come into
play are: Hydrogen bonding, Inductive effects, Hybridizaton,
Resonance and the stability of the conjugate base.

Title: Re: How do I predict..?
Post by: Chromic on April 07, 2001, 05:44:00 PM

More important than the proton theory of acid/bases, is the Lewis (transfer of electrons) acid/base theory.

Acids are electron acceptors. (eg. H+ is an acid, as it is able to accept negatively charged electrons)
Bases are electron donors. (eg. OH- is a base, because it is able to give away negatively charged electrons)

Almost all organic reactions can be thought of as acid/base reactions in this context. Most reaction mechanisms are illustrated by the transfer of electrons (and their stabilization in the intermediates in the reaction).

Title: Re: How do I predict..?
Post by: hypo on April 07, 2001, 10:36:00 PM

AFAIK lewis acids/bases are electron _pair_ acceptors/donors. Simple electron transfer = redox.