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Identifying an ester, C4H8O2

Structure from IR, ¹³C NMR and mass spectra

Compound **X** has molecular formula

\ce{C4H8O2}

. IR: strong peak at 1740 cm

^{-1}

, strong peak at 1240 cm

^{-1}

, no broad peak between 2500 and 3550 cm

^{-1}

. ¹³C NMR: four peaks at 171, 60, 21 and 14 ppm. Mass spectrum: peaks at m/z 88, 45, 43 (base peak) and 29.

Facts you may need

IR absorption ranges
¹³C environments and symmetry
molecular ion and fragments

(a)

Show that **X** has one double bond (or ring), and use the IR spectrum to show it is an ester, not a carboxylic acid.

(b)

Draw all four ester isomers of

\ce{C4H8O2}

. Use the ¹³C NMR data to rule out one of them.

(c)

Use the mass spectrum to identify **X**. Give the formula of the ion responsible for m/z 45 and 43, and explain why the other two remaining isomers are unlikely.

(d)

Assign each ¹³C peak of **X** to a carbon.