Identifying Carboxylic Acids and Derivatives Through Classification Tests
Bianca Therese Rivera, Camille Aliana Rivera, Zarah Mae Roxas, Ma. Rosario Teresa Saylo, Jean Darlene Semilla and Adrian Yu Department of Pharmacy, Faculty of Pharmacy
University of Santo Tomas, Espana Street, Manila 1008
Date Submitted: September 13, 2012
Carboxylic acid derivatives namely, acyl halides, acid anhydrides, esters and amides were classified through different reactivity tests. The samples used were acetyl chloride for acyl halide group, acetic anhydride for acid anhydride group, ethyl acetate for ester group, acetamide and benzamide for amide group. These derivatives were identified through the mechanism of hydrolysis, alcoholysis and aminolysis reactions. Positive results were obtained and the presence of a carboxylic acid derivative in each compound was confirmed. _____________________________________________________________________________________
Carboxylic acids and their derivatives are among the most abundant of organic compounds, both in the laboratory and in living organisms. They are a type of carbonyl group that contains a carbon double bonded to an oxygen.
Figure 1. General Structure of Carboxylic Acid
The melting points and boiling points of carboxylic acids are higher than those of hydrocarbons and oxygen-containing organic compounds of comparable size and shape and indicate strong intermolecular attractive forces. They are much weaker than mineral acids, but are nevertheless much stronger than alcohols.
The most common derivatives of carboxylic acids are acyl halides, acid anhydrides, esters, and amides. These groups of compounds also contain a carbonyl group, but now there is an electronegative atom (oxygen, nitrogen, or a halogen) attached to the carbonyl carbon. They contain an acyl group attached to a nucleophilic molecule that replaced the -OH group of a carboxylic acid. The -OH group acts as the leaving group in neuclophilic acyl substitution.
All carboxylic acid derivatives react with the same mechanism: the nucleophile attacks the carbonyl carbon, forming an unstable tetrahedral intermediate, which reforms a carboxylic acid derivative by eliminating the weakest base. The weaker the base, the better the leaving group; because a weak base does not share its electron as well as a strong base does, a weaker base forms a weaker bond- one that is easier to break. Therefore, the most reactive in carboxylic acid derivatives is acyl halides followed by esters and carboxylic acids, and the least are the amides. The types of nucleophilic acyl substitution that a carboxylic acid derivative undergoes are hydrolysis, aminolysis and alcoholysis.
RCOY + :Nu- RCONu + :Y-
Figure 2. Nucleophilic Acyl Substitution Reaction
Carboxylic acids and their derivatives are involved in many industrial processes and most biological pathways. Esters are pleasant-smelling liquids which are responsible for the fragrant aromas of fruits and flowers. They are also present in mineral fats and other biologically important molecules but the lack of strong intermolecular forces makes them more volatile at room temperature. Amides are less reactive than Esters and their stability makes them ideal linkages in peptides and proteins. Acyl chlorides and anhydrides are commonly used in the chemical and pharmaceutical industries.
The structural formulas of the sample compounds used in the experiment are as follows:
Figure 3. Structural Formula of Acetyl Chloride
Figure 4. Structural Formula of Acetic Anhydride
Figure 5. Structural Formula of Ethyl Acetate
Figure 6. Structural Formula of Acetamide...