The UV absorptions and colors of 2,4-dinitrophenylhydrazone derivatives of aldehydes and ketones depend sensitively on the structure of the carbonyl compound. Suppose that you are asked to identify the contents of three bottles whose labels have fallen off. The labels indicate that one bottle contained butanal, one contained trans-2-butenal, and one contained trans-3-phenyl-2- propenal. The 2,4-dinitrophenylhydrazones prepared from the contents of the bottles have the following characteristics. 

Bottle 1: m.p. 187 – 188°C; λ_max 5 377 nm; orange color 

Bottle 2: m.p. 121 – 122°C; λ_max 5 358 nm; yellow color 

Bottle 3: m.p. 252 – 253°C, λ_max 5 394 nm; red color

Match up the hydrazones with the aldehydes (without fi rst looking up the melting points of these derivatives), and explain your choices.