Acid Derivatives

The suffix used in IUPAC naming of acid chlorides is “-oyl chloride”, for anhydrides it is “-oic anhydride, for amides it is “-amideand for esters, it is “-oate”. For these classes of compounds, since they contain a polar carbonyl group (C=O), there are dipole-dipole interactions within the molecules. Also, there will never be hydrogen bonding with acid chlorides, anhydrides or esters because they normally do not contain a hydroxyl group. However, if a hydroxyl group is added as a substituent somewhere on these molecules, hydrogen bonding will occur. As for amides, since the nitrogen is bonded to an acidic alpha hydrogen, hydrogen bonding will occur. Because of this hydrogen bonding in amide groups, amides will have higher boiling points than other classes of acid derivatives. Also, because amides contain a carbonyl group, amides will have higher boiling points than their alkane counterparts. Acid derivatives tend to have infrared absorption in a few places based on what molecule is being looked at. Acid chlorides show up between 1700 and 1800 cm1. Anhydrides show up as 2 bands between 1700 and 1800 cm-1. Amides show up in two places. The N-H bond presents around 3300 cm-1 and the C=O shows up around 1700 cm-1. Esters show up around 1700 cm-1 for the C=O carbonyl bond and around 1200 cm-1 for the C-O ether bond. The relative reactivity of acid derivatives is as follows:

Acid chloride > Anhydride > Esters > Amides

The relative reactivity of acid derivatives is based on the leaving group. Therefore, since halides are great leaving groups, acid chloride is the most reactive acid derivative. Anhydrides are the next most reactive, followed by esters and finally amides.  Amides are the most stable acid derivative because the leaving group of amides, the amine serves as a poor leaving group. The C-N bond also helps stabilize the molecule. The stability of nitrogen molecules and amines is what makes them successful in nature. For example, peptide bonds create proteins and proteins are very durable molecules.

When bulky substituent groups are added to the carbon adjacent to the carbonyl group, the reactivity of the acid derivative is hindered. The bulky group helps protect the carbon from a nucleophilic attack. Substituents that can redistribute and stabilize negative charge serve as good leaving groups. In the case of acid anhydride, for example, the carboxylate anion is able to redistribute its negative charge evenly along its hybrid structure, making it a good leaving group, and therefore making acid anhydride the second most reactive class of acid derivatives. In nucleophilic substitution, a nucleophile attacks the molecule at the carbonyl group (C=O), substituting out the original substituent for itself. This reaction decreases in speed in the following order: Acid chlorides > Anhydrides > Esters > Amides

In a Hoffman rearrangement, the carbonyl group in an amide is removed, leaving the R group. This can be any carbon chain attached to the carbonyl group that is directly attached to the amine.   In transesterification, when an ester is reacted with another alcohol, this alcohol will essentially “kick out” the existing ester group and insert itself into the molecule as a new ester group, thereby creating a new ester. Saponificaton or the hydrolysis of fats and glycerides is essentially the process of hydrolizing an ester using a base. The hydrolysis of amines is essentially the same process as the hydrolysis of esters, except an amine is the leaving group. The leaving group is not the typical NR2 as is instead a neutral amine. The opposite reaction is actually the formation of amides.

 

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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