CHEM 30B  Dr. R. Rinehart

Ch. 15  CARBOXYLIC ACIDS AND  ESTERS

 Welcome to the sweet-and-sour land of carboxylic acids, their salts, and their esters: three extremely important classes of compounds.  They play critical roles in industry, food, and biochemistry.  From the infamous “polyester” to flavors and odors to the fats in our diet [and our bodies] to the very core of our metabolism [the Krebs cycle uses 7 different acids]. this time they really are “all around us”.

I.   Carboxylic acids      
            A. Structure: contain the carboxyl [hydroxycarbonyl] group  -CO₂H  
             B. NOMENCLATURE:  
                        1.      Common names:  see tables below  
                        2. IUPAC names: The carboxyl group is our highest-priority group.
                                    a. Locate longest chain containing the  -CO₂H 
                                    b. The carboxyl carbon is automatically #1 in the chain
                                    c. Drop the -e of the stem name and add -oic  acid 
                                                thus HCO₂H, formic acid, is methanoic acid 
                                                and  HO₂CCO₂H, oxalic acid, is ethanedioic acid                     

             C. Physical properties: See Tables 5.2, p136  & 5.3, p. 137  and  Fig. 5.4, p137

                        <C₆  are water-soluble; all dissolve in 5% NaHCO₃ ; sharp, penetrating odors.

                 D. CHEMICAL PROPERTIES e
                        0.  Preparation: via oxidation of an aldehyde [or primary alcohol]  
                        1. ACIDITY [well, DUH!]  Yes, folks, they donate protons; much weaker than strong
                                  mineral acids  
                                                 RCO₂H + H₂O à H₃O⁺ + RCO₂^- 
                                                    pK_a for an isolated carboxyl group is ~4.7 .
                                        The pK_a is strongly influenced by the presence of neighboring groups.
                         2.      [as a natural consequence of 1] React with BASES to form organic  SALTS

RCO₂H + NaOH à  RCO₂^- Na⁺  +  H₂O
                     a. Nomenclature:  e.g., sodium acetate

                                    b. properties:  electrolytes, water-soluble 
                                    c. Long-chain (“fatty”) acids form soaps. 

                                    To see an interactive Chime depiction of a lipid [unfortunately, not a soap]
                                     micelle [assuming you have MDL Chime installed -- the Netscape version will
                                     work better here], go to: 
                                     http://info.bio.cmu.edu/courses/03231/micelles/micelle.htm

                         3. React with ALCOHOLS [an intermolecular dehydration or “condensation”]  to
                                     form ESTERS  [see diagram below]; Triglycerides are triple esters of glycerol with
                                     three molecules of long-chain ["fatty"] acids.

                see http://www.molecularmodels.ca/molecule/Esters.htm   
for Dave Woodcock's Chime structures of esters

                                    a. Nomenclature: alkyl alkanoate e.g., ethyl acetate;  
                                    b. Properties: usually smell nice; water-insoluble; good solvents for other organics 
                                    c. Reactions: hydrolysis with acid or with base [ “saponification”]

                      4. [just in case life wasn’t complex enough]

                                        ALCOHOLS + HYDROXY INORGANIC ACIDS [boric, nitric, sulfuric, phosphoric]  yield INORGANIC ESTERS;  Especially important for PHOSPHORIC ACID H₃PO₄

                                  5. Activated Derivatives of carboxylic acids:

`                                   a. Anhydrides            

                                    b. Acyl chlorides        

                                    c. THIOESTERS: very important in living systems      

         Coenzyme A [usually abbreviated as "CoA"], shown above, is a crucial component in the metabolism of carboxylic acids. The  thiol [sulfhydryl] group, where the thioester link is formed, is the "business end" of the molecule seen here at its right side. The remainder of the molecule serves to specifically bind to the enzymes . Acetyl-CoA is the main entry point into the Krebs cycle for carbon from carbohydrates and fats. Succinyl-CoA is an intermediate of the Krebs cycle. Fatty acyl-CoA is the form in which fatty acids are actually broken down to give acetyl-CoA.  Malonyl-CoA is used in the biosynthesis of fatty acids.

Tables of carboxylic acids are below:

You can see Dave Woodcock’s Chime structures of carboxylic acids at:
http://www.molecularmodels.ca/molecule/Acids_(organic).htm
 
PowerPoint-type presentation of oxygen-containing functional groups [aldehydes, ketones, carboxylic acids, esters, etc.]
by Warren Gallagher at U Wisconsin Eau Claire are viewable [with Acrobat Reader] at:
http://www.chem.uwec.edu/Chem150_S06/Pages/lecture-materials.html  > Lecture 5 
and bookmark the site for future reference