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:
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:
Identify the peaks on the infrared spectrum
Use your molecular formula and the infrared data to suggest two possible structures for compound A
Answer
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- 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:
- Propanal
- Propanone
- The peaks at around 2850 cm-1 correspond to C-H bonds
Worked example
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
- There are characteristic m/z peaks for a carbon chain:
Exam Tip
You can be expected to work with any empirical and molecular formula combined with infrared and mass spectral data to identify unknown compounds
