It is extremely useful in organic synthesis to utilize methods by which less reactive carboxylic acids (and their derivatives) may be converted to more reactive species, like acid chlorides. Such reactions are not always intuitive as they proceed by transforming low-energy species to more energetic states. Reagents such as thionyl chloride and phosphorus tribromide , which you have likely seen previously in examination of common alcohol reactions, readily convert carboxylic acids to acid chlorides. The reaction is energetically favorable due to the formation of sulfur dioxide, a very stable gas.

The Grignard reagent also acts as a base, and will remove any protons that have a low-enough pKa. This means that the alpha hydrogen will be removed, and the resulting carboxylate ion is stabilized through resonance and is unlikely to react with the protonated Grignard reagent.

This reaction is an acid-catalyzed esterification of a carboxylic acid. In the reaction the carbonyl is attacked by the alcohol oxygen, and the carbonyl is reformed by kicking off what was the hydroxyl group in the original carboxylic acid. What is left is an ester (of the form RCOOR', with R' containing the same number of carbons as was in the original alcohol).

A carboxylic acid can be synthesized directly into an acid anhydride by introducing the carboxylic acid to an acid chloride.

This question is asking us to determine what the product will be when a carboxylic acid is reacted with thionyl chloride, . In this reaction, the carboxylic acid is converted into an acyl chloride, with additional production of and as side products. Generally, this reaction is very useful for converting carboxylic acids into a more reactive form, so that further reactions can take place.