Combined Techniques (OCR AS Chemistry A)

10.00 g of an organic compound, A, was analysed and found to contain 6.21 g of carbon, 1.03 g of hydrogen and 2.76 g of oxygen. The relative molecular mass of the compound was 58.0

Determine the molecular formula of compound A.

Answer:

Therefore, the empirical formula of compound A is C₃H₆O

The empirical mass is (12.0 x 3) + (1.0 x 6) + 16.0 = 58.0

Therefore, the molecular formula of compound A is also C₃H₆O

The infrared spectrum of compound A is shown below:

Identify the peaks on the infrared spectrum

Use your molecular formula and the infrared data to suggest two possible structures for compound A

Answer

• The peaks at around 2850 cm^-1 correspond to C-H bonds

  • This matches with the idea that compound A is a hydrocarbon derivative

• The peak at around 1750 cm^-1 corresponds to a carbonyl / C=O group

  • Couple with a molecular formula of C₅H₁₀O, this suggests that compound A could be an aldehyde or a ketone

• As you are told that compound A has a straight chain structure, you can deduce that the possible structures are:

  1. Propanal

  2. Propanone

The mass spectrum of compound A is shown below:

Use the data from the mass spectrum to deduce the structure for compound A from your suggestions

Answer

• There are characteristic m/z peaks for a carbon chain:

  • m/z = 29 could represent C₂H₅⁺ or CHO⁺ 

• Therefore, compound A is propanal

  • Also, propanone would give a clear signal at m/z = 15 as it could fragment on either side of the carbonyl group

You can be expected to work with any empirical and molecular formula combined with infrared and mass spectral data to identify unknown compounds