List the following four carboxylic acid derivatives, ester, anhydride, amide, and acid halide, in order of increasing reactivity in nucleophilic acyl substitution reactions. Start with the least reactive derivative.

Nucleophilic Acyl Substitution

A reaction mechanism in which a nucleophile displaces a leaving group attached to an acyl compound, forming a new covalent bond.

Carboxylic Acid Derivatives

A group of organic compounds, including esters, amides, and acid halides, derived from carboxylic acids by replacement of the hydroxyl group with other functional groups.