${\mathrm{C}}_2{\mathrm{H}}_5{\mathrm{NH}}_2\stackrel{\mathrm{Z}}{\to }{\mathrm{C}}_2{\mathrm{H}}_5-\mathrm{NH}-\mathrm{CO}-{\mathrm{CH}}_3;$ ‘Z’ in this reaction cannot be

${\mathrm{C}}_2{\mathrm{H}}_5{\mathrm{NH}}_2$ is a base and ${\mathrm{CH}}_3\mathrm{COOH}$ is an acid. At room temperature they form a salt but acylation cannot occur.