Three IR spectra are shown, corresponding to three of the following compounds. For each spectrum, determine the structure and explain how the peaks in the spectrum correspond to the structure you have chosen.

An IR-spectrum peak at 3300 cm^-1 indicates hydrogen attached to the terminal alkyne carbon. Terminal alkyne exhibits IR stretching frequency at 2100 cm^-1. The methyl of the tertiary butyl group exhibits an IR stretching frequency of around 3000 cm^-1. Thus, the structure below is evident from the given IR spectrum from the given list of structures.

Structure resulting from the spectrum (a)

An IR spectrum peak at 1720 cm^-1 indicates the presence of a carbonyl group, and a stretching frequency around 2820 cm^-1 indicates that the carbonyl group must be of aldehyde as hydrogen attached to carbonyl carbon has IR stretching frequency around 2820 cm^-1. There is the presence of overtone also at 3440 cm^-1, which occurs in the case of the carbonyl group. IR stretching frequency at 1600 cm^-1 indicates the presence of a benzene ring. Thus, from the given list of structures, the structure which is evident from the given IR spectrum is as follows.

Structure resulting from the spectrum (b)

An IR stretching frequency at 1760 cm^-1 indicates the presence of an ester functional group. IR stretching frequency around 1600 cm^-1 is indicative of a benzene ring. Ester’s oxygen must be attached to the benzene ring, as evident from the IR stretching frequency of 1200 cm^-1. Thus, from the given list of structures, the evident structure is as follows.

Structure resulting from spectrum (c)