how is possible that we can get esters of carbon acid anhydrides but of esters we can't make anhidrous acids?

Carbon acid anhydrides are compounds that are formed by the removal of a molecule of water from two molecules of carboxylic acids. This occurs through a condensation reaction, where the -OH group of one carboxylic acid combines with the -H of the adjacent carboxylic acid, resulting in the formation of a water molecule. The remaining oxygen atom then bridges the two carboxylic acid molecules, forming the anhydride.

Esters, on the other hand, are compounds that are formed by the reaction between a carboxylic acid and an alcohol, resulting in the removal of a molecule of water. This reaction is called an esterification reaction. The -OH group of the carboxylic acid reacts with the -H of the alcohol, resulting in the formation of water. The remaining -O-R group (where R is an alkyl or aryl group) is then attached to the carboxyl carbon, forming the ester.

The reason anhydrous acids cannot be obtained directly from esters is because esters are less reactive than carboxylic acids. In an ester, the alkyl or aryl group attached to the carbonyl carbon provides electron density that reduces the acidity of the carboxylic acid group. This makes it difficult to remove the -OH group from the ester and convert it back into the acid. The removal of water from an ester would require stronger and harsher conditions compared to the formation of anhydrides from carboxylic acids.

Instead, if one wishes to obtain an anhydrous version of a carboxylic acid, alternative methods can be used. One common method is the reaction of the carboxylic acid with a dehydrating agent, such as thionyl chloride (SOCl2) or phosphorus pentoxide (P2O5). These dehydrating agents can effectively remove the -OH group from the carboxylic acid, resulting in the formation of an anhydrous acid.

In summary, while it is possible to obtain carbon acid anhydrides by removing water from two molecules of carboxylic acids, anhydrous acids cannot be obtained directly from esters. This is because esters are less reactive and require stronger conditions or alternative methods to remove water and convert them back into the corresponding acids.