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SolutionsHow to Determine the Structure of an Organic Compound

1. From composition to formula

A compound contains 40.0 % C, 6.7 % H and 53.3 % O. Its mass spectrum shows a molecular mass of 180. Calculate its empirical and molecular formulas.

C: 40.0/12.0 = 3.33; H: 6.7/1.01 = 6.63; O: 53.3/16.0 = 3.33. The ratio is 1 : 2 : 1 and the empirical formula is CH2O (mass 30).

180/30 = 6, so the molecular formula is C6H12O6 (for example, glucose).

2. Index of hydrogen deficiency

Calculate the index of hydrogen deficiency (IHD) of: a) C6H6; b) C6H12; c) C4H9N; d) C2H3Cl.

  1. (12 + 2 − 6)/2 = 4: one ring and three double bonds (benzene).
  2. (12 + 2 − 12)/2 = 1: one double bond or one ring (hexenes or cyclohexane).
  3. (8 + 2 + 1 − 9)/2 = 1.
  4. (4 + 2 − 4)/2 = 1: one double bond (vinyl chloride).

Halogens are counted as hydrogens; oxygen and sulfur do not change the value.

3. Which technique?

Which technique would you use to find out: a) whether a compound has a carbonyl group; b) its molecular mass; c) whether it is 1-propanol or 2-propanol; d) the exact arrangement of its atoms in space, if it forms crystals?

  1. IR spectroscopy (strong C=O band near 1700 cm−1).
  2. Mass spectrometry.
  3. NMR: the number, intensity and shape of the signals distinguish the two isomers.
  4. X-ray diffraction.

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