ScholarshipSpectroscopyFree

From soot-free combustion to a structure

Empirical formula from combustion data, molecular formula from MS, structure from IR/¹³C/MS

Compound **P** contains only C, H and O. Complete combustion of 0.250 g of **P** produced 0.500 g of

\ce{CO2}

and 0.2045 g of

\ce{H2O}

. Mass spectrum: molecular ion m/z 88; other peaks at 73, 45 and 43 (base). IR: very broad absorption 2500–3300 cm

^{-1}

, strong peak at 1710 cm

^{-1}

. ¹³C NMR: three peaks at 184, 34 and 19 ppm. M (g mol

^{-1}

): C 12.0, H 1.0, O 16.0.

Facts you may need

mass of C from CO₂ and H from H₂O
empirical vs molecular formula
IR, ¹³C NMR, MS interpretation

(a)

Determine the empirical formula of **P**.

(b)

Use the mass spectrum to find the molecular formula.

(c)

Justify the identity of **P** using the IR and ¹³C data: draw the other carboxylic acid that fits the IR data, explain how the ¹³C spectrum rules it out, and assign each ¹³C peak of **P**.

(d)

Give the formulae of the ions responsible for m/z 73, 45 and 43.

(e)

Isomer **Q**, also

\ce{C4H8O2}

, has no broad IR peak, a strong peak at 1740 cm

^{-1}

, and three ¹³C peaks. Identify **Q** and describe one chemical test that distinguishes **P** from **Q**.