Carboxylic Acids

Carboxylic acids contain a carboxyl group and are characterized by the general formula R-COOH, where R is some monovalent group, such as hydrogen or a carbon chain. The alcohol group on this molecule is a source of an acidic hydrogen proton that can be donated in reaction, thus making carboxylic acids Brønsted-Lowry acids. The IUPAC prefix for naming carboxylic acid compounds is “-oic acid”. The reactions of carboxylic acids and their derivatives are characterized by nucleophilic addition-elimination at their carbonyl carbons. This results in a substitution at the electrophilic carbonyl carbon (C=O). The nucleophilic attack usually occurs via the nucleophilic oxygen in COOH. A key intermediate step in the mechanism of these reactions involves the formation of a tetrahedral intermediate that reverts back to a carbonyl group after the leaving group is eliminated. When carboxylic acids are reacted with LiAlH4, the carboxylic acid undergoes a reduction reaction, converting it to an alcohol. When a carboxylic acid (RCOOH) undergoes decarboxylation, it loses CO2 to form R-H and CO2. However, this reaction tends to be extremely slow for most carboxylic acids. Specific groups must be present in the molecule for this reaction to be useful to organic chemists. Carboxylic acids that contain a carbonyl group once removed from the carboxylic acid group are known as β-keto acids which decarboxylate readily when heated to a temperature between 100°C and 150°C. There are two reasons for this: One reason is that when the acid itself decarboxylates, this occurs through a six-membered cyclic transitional state. The other reason is that when the carboxylate anion decarboxylates, it forms a resonance stabilized anion. When carboxylic acids are reacted with Ba(OH)2 and the reaction is heated, the carboxylic acid is reduced to create a ketone. When a carboxylic acid is reacted with an alcohol and acid, an ester is formed. This reaction must take place under acidic conditions in order to proceed successfully. When a carboxylic acid reacts with a diatomic halogen molecule, halogenation at the alpha carbon occurs. The alpha carbon is usually located in the 2 position for carboxylic acids. When a carboxylic is reacted with an electrophile, substitution occurs at the alpha carbon. The carboxylic acid is converted to an acyl halide which can tautomerize into its enol form through a manipulation of the alpha carbon. Next, a halogen or other electrophile gets attacked by this alpha carbon and the molecule reverts back to a carboxylic acid. Overall, the alpha hydroge on the alpha carbon gets substituted for the halogen or other electrophile.

Carboxylic Acid Summary. Source: https://www.khanacademy.org/test-prep/mcat/chemical-processes/carboxylic-acids/a/carboxylic-acid-reactions-overview

 

References

1) Bruce Alberts, A. J. (2007). Molecular Biology of the Cell.

2) Van Holde KE, M. C. (1996). Biochemistry. Menlo Park, California: Benjamin/Cummings Pub. Co., Inc.

3) NV, B. (2002). Medical Biochemistry. San Diego: Harcourt/Academic Press.

4) Suman Khowala, D. V. (2008, June 4). Biomolecules: (introduction, structure & function) . Retrieved from http://nsdl.niscair.res.in/jspui/bitstream/123456789/802/1/Carbohydrates.pdf

5) H. M. Asif, M. A. (2011, January 21). Carbohydrates. Retrieved from International Research Journal of Biochemistry and Bioinformatics: http://www.interesjournals.org/full-articles/carbohydrates.pdf?view=inline

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