Organic Chemistry, "Function", Part 14 - Properties of Organic Acids

Greetings,

This post focuses on Organic Acids (also known as Carboxylic Acids).  Carboxylic acids are named as such because they are derived from the Carboxyl Functional Group (-C=O(-OH)).  The name, "Carboxyl" is a combination of the names, "Carbonyl" and "Hydroxyl".  Organic Acids exist in a pure state, but this post will focus on properties of aqueous Organic Acid solutions.

Aroma

Organic Acids are generally quite foul-smelling, with Acetic Acid being among the "nicer"-smelling ones.  This knowledge comes from the Author's own personal experience; testing for the presence of organic acids in landfill samples.  Come to think of it, none of it smelled very nice!

Acidity

The primary property of interest is the acid nature of Organic Acids.  Organic Acids are weak acids: Does the reader recall what a weak acid is? Check here for the answer!  A "collection" of Weak acid molecules in water only partially ionize, meaning only a small fraction produce ions.  The learned Chemistry Student will remember what those produced ions are, namely a hydrogen ion and an anion; which is the conjugate base of the weak acid.

Explanation of the Weak Acid Nature of Organic Acids

Relative Bond Strength Explanation

Hydrochloric Acid, HCl, consists of a weaker H to Cl covalent bond than the corresponding H to O bond of, say, acetic acid.  The exceptionally high strength of the H to O covalent bond is the reason for the exceptional stability (ie unreactivity) of water.  Much less energy is needed to break the H-Cl bond than that needed to break the H-O bond of acetic acid.  It is much more statistically likely that HCl will break apart than that H-O bond of acetic acid will break.  Therefore, a much greater fraction of HCl molecules will ionize compared to acetic acid molecules.

Anion Stability Explanation

The chloride ion in water is much more stable (exists with a much lower potential energy) than the acetate ion of acetic acid.  The more stable anion is more likely to exist than the less stable ion, therefore we find a much greater fraction of free chloride ions compared to acetate ions.  More chloride ions than acetate ions means that HCl ionizes to a much greater extent (virtually all of it) compared to acetic acid (only about 0.001% of it).  The following image shows the effect.

Polarizibility Explanation

We say a molecule is polarizible when a separation of charge (and we're really talking about degrees of partial separation) is more likely for electrons to be separated across the structure.  Now, this separation (again only partial) occurs for a fleeting moment only.  The idea is that a greater separation of charge occurs, providing the ability for the molecules to more readily ionize.  Because there are so very many molecules involved, more polarized acid molecules (actually their anion part) will cause many more of them to give up their protons to water molecules.  The result is a stronger acid, with all molecules ionized to Hydrogen Ions and Anions.  The opposite effect occurs with a weak acid like acetic acid, for related reasons; the oxygen atom is not very polarizable.  This results in a shorter separation of charge, which translates to "less ionic character"; consequently, acetic acid is not readily ionizible.  And, once again, a lesser ionizible acid is a weaker one.

That's all for this post.  As always, Thank you for reading!

A Publication of http://ExcellenceInLearning.biz