The chart below shows the reagents necessary for the possible interconversions of carboxylic acids and their derivatives. The compounds are arranged from most reactive at the top (acid chloride) to least reactive at the bottom (carboxylate anion).

By following the paths on this chart, any derivative can be converted into any other one in no more than two steps. For example, an ester can be converted to an amide by reaction with an excess of an amine. It could be converted to a carboxylic acid by treatment with aqueous acid. So could the amide.

Fewer pathways are available moving upwards because this is "against the flow" - moving from less reactive to more. To convert an acid to an amide requires to steps: reaction with R'OH and TsOH to make an ester, and then reaction of the ester as described above.