Condensation reactions Fischer esterification

Analytical Approach and Interconversion Under mildly acidic conditions, GHB undergoes an internal condensation via Fischer esterification to produce a stable lactone. This involves two steps cyclization, followed by the elimination of water. The reverse reaction, the hydrolysis of GBL to GHB, can be driven in the presence of a strong base, which is the method used clandestinely to make GHB (Figure 7.29). The reverse reaction can occur in solution when GHB is spiked into a drink, many of which are slightly addic. However, once equilibrium between the add and the lactone form has been reached, no significant losses of GHB appear in typical exhibits over the time frame of days or weeks. ... 

This method is called the Fischer esterification. It s a condensation reaction where the loss of a water molecule accompanies the joining of the alcohol portion to the acid portion. The acid gives up the OH and the alcohol gives up the H to make the water molecule. All steps in the mechanism are reversible (that is, it establishes an equilibrium), so removing the ester as soon as it forms is helpful. Removal of the ester is normally easy since esters typically have lower boiling points than alcohols and carboxylic acids. Figure 12-20 illustrates the mechanism for the acid-catalyzed formation of an ester by the reaction of an alcohol with a Ccirboxylic acid. 

Steps (1) and (2) are the aldol condensation between the thioester (acetyl CoA) and the keto group of oxaloacetic acid. Step (3) is a reverse Fischer esterification reaction the ester is hydrolyzed to form the carboxylic acid and the thiol, CoA-SH. 

As seen in previous chapters, the term condensation is used to characterize any reaction in which two molecules undergo addition accompanied by the loss of a small molecule such as water, carbon dioxide, or nitrogen gas. Throughout this book, we have seen many condensation reactions. One such example is the Fischer esterification process, as seen in Section 21.10. 

As shown, PET is prepared by successive Fischer esterification reactions. Since the polymer is generated via condensation reactions, it is called a condensation polymer. Because PET has repeating ester moieties, the polymer is also classified as a polyester. like PET, nylon 6,6 is also prepared via condensation reactions and is also a condensation polymer. Nylon 6,6 is a polyamide, which is prepared from adipic acid and 1,6-hexanediamine. 

In this case, the repeating functional group is an ester moiety, so this condensation polymer is a polyester. Recall that ester moieties can be prepared from the reaction between a carboxylic acid and an alcohol via a Fischer esterification process. 

The forward scheme for this strategy is shown below. One equivalent of ethanol is oxidized with PCC to give acetaldehyde, which is then heated with aqueous sodium hydroxide to give an a,P-unsaturated aldehyde (via an aldol condensation reaction). Another equivalent of ethanol is oxidized with chromic acid to give a carboxyhc acid, which is then treated with ethanol under acidic conditions to give an ester (via Fischer esterification). The ester is then converted into a P-ketoester (via a Claisen condensation). The P-ketoester is then deprotonated with ethoxide to give a doubly stabilized enolate which then attacks the a,P-unsaturated aldehyde to give a Michael reaction ... 

Benzilic acid rearrangement Benzoin reaction (condensation) Blanc chloromethylation reaction Bouveault-Blanc reduction Bucherer hydantoin synthesis Bucherer reaction Cannizzaro reaction Claisen aldoi condensation Claisen condensation Claisen-Schmidt reaction. Clemmensen reduction Darzens glycidic ester condensation Diazoamino-aminoazo rearrangement Dieckmann reaction Diels-Alder reaction Doebner reaction Erlenmeyer azlactone synthesis Fischer indole synthesis Fischer-Speior esterification Friedel-Crafts reaction... 

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