Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 328

Brief Organic Chemistry

Summer 2004

Lecture Notes: 26 January

© R. Paselk 2004
 
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Functional Groups

I want to very briefly look at a few important functional groups covered in your text. In each case these are groups of atoms that are commonly found in organic molecules and which have charectereistic properties. We will look at each of them later since each is given an entire chapter of discussion in the course.

 

Acids and Bases

What are acids and bases? There are three major definitions. We will look at two (the third, the Arrhenius definition, is not needed for our study).

Although I will signify protons in water as H+, you should realize that naked protons do not exist in water - they are always hydrated. At a minimum we see the hydronium ion, H3O+. But hydronium ion is in fact also generally though to t be hydrated, so you will sometimes see hydrogen ion represented as H5O2+, H7O3+, etc.

        H2O Æ  H+  + OH- 
         acid       conj. base
                 
     H+ +  OH-  Æ  H2O  
        base    conj. acid    
                 
 H3O+   ¨  H+  +  H2O  Æ  OH- + H+ 

 

conj. acid 

     

 acid

base

  conj. base    

Strong vs. Weak Acids & Bases: These terms have nothing to do with concentration, rather they refer to the degree of dissociation of an acid or base:

Let's look at the equation for the dissociation of a weak organic acid, CH3CO2H (acetic acid, HOAc):

CH3CO2H ´ H+ + CH3CO2-

or, HOAC ´ H+ + OAc-

The equilibrium expression for this reactions is: Ka = [H+][OAc-] / [HOAc]

and Ka = 1.8 x 10-5

The values of Ka for most of the acids we will be interested are very small, and it is often convenient to express them in negative logrythmic form, analogous to pH, as pKa values:

pKa = -logKa = -log (1.8 x 10-5) = 4.76.

pKa is a general measure of acidity, or the tendency of a substance to give up hydrogen ions, and has a tremendously broad range in this sense as seen in the table in your text and below:

 

Acidity vs. Molecular Structure

So why do some substances tend to dissociate readily, loosing a hydrogen ion, whereas others hold onto their hydrogens very tightly? A number of factors contribute, but we can look at it from the overall perspective of the stability of the resulting conjugate base. Factors then include:

but the carboxylate ion is stabilised by resonance:

spreading the negative charge over a greater volume, stablizing the anion and thus making the acid stronger.


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Last modified 3 June 2004