Combined Techniques (OCR A Level Chemistry)

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Deducing Organic Structures

  • Organic chemists often use a number of techniques in combination to determine the structure of a compound

Worked example

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:4-7-3--worked-example-1-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

Worked example

The infrared spectrum of compound A is shown below:

4-7-3-propanal-infrared-spectrum

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

Worked example

The mass spectrum of compound A is shown below:4-7-3-propanal-mass-spectrum

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

Examiner Tip

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

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Richard

Author: Richard

Expertise: Chemistry

Richard has taught Chemistry for over 15 years as well as working as a science tutor, examiner, content creator and author. He wasn’t the greatest at exams and only discovered how to revise in his final year at university. That knowledge made him want to help students learn how to revise, challenge them to think about what they actually know and hopefully succeed; so here he is, happily, at SME.