Sequence of techniques for structure determination

To determine the structure of an unknown organic compound, chemists employ a series of analytical techniques in a specific order:

1. Elemental analysis - This method determines the percentage composition of elements within a compound, enabling the calculation of the empirical formula. This formula shows the simplest ratio of elements in a compound.
2. Mass spectra - The peak representing the molecular ion in a mass spectrum provides the molecular mass. This information, when combined with the empirical formula, helps deduce the molecular formula of the compound. Additionally, patterns of fragmentation can offer clues about the structure of certain molecular fragments.
3. Infrared spectra - The peaks in an infrared spectrum correspond to various bond vibrations, which indicate the presence of specific bonds and functional groups within the molecule.
4. NMR spectra - Nuclear magnetic resonance (NMR) spectroscopy reveals the chemical environment of hydrogen (H) and carbon (C) atoms in the compound. The splitting patterns in the spectrum can also provide information about the number of neighbouring hydrogen atoms.

Each technique builds on the information from the previous one, cumulatively offering a detailed view of the compound's structure.

Step-by-step analysis of unknown compounds

The following are two illustrative examples that demonstrate how the combined use of analytical techniques can lead to the determination of the structures of unknown compounds:

Worked example 1 - Determining the structure of an organic molecule

Using the following information provided, determine the structure of the unknown organic compound.

• Elemental analysis: C: 60.00%, H: 13.33%, O: 26.67%
• Mass spectrum:

• IR spectrum:

• ¹³C NMR spectrum:

• ¹H NMR spectrum:

Step 1: Elemental analysis and empirical formula calculation

Assuming 100 g of the compound for simplicity, convert the mass percentages to moles:

• Carbon: $ \frac{60.00}{12.0}=5.00 \text{ mol} $
• Hydrogen: $ \frac{13.33}{1.0}=13.33 \text{ mol} $
• Oxygen: $ \frac{26.67}{16.0}=1.67 \text{ mol} $

The smallest number of moles is 1.67. Dividing all by 1.67 gives:

• C: 3, H: 8, O: 1

Empirical formula is C₃H₈O.

Step 2: Molecular formula calculation

The molecular ion peak at m/z = 60 corresponds to the molecular weight of the compound, confirming the empirical formula C₃H₈O as the actual molecular formula.

Step 3: IR spectrum analysis

The broad absorption at 3340 cm^-1 indicates the presence of an -OH group, characteristic of alcohols.

Step 4: NMR spectrum analysis

¹³C NMR:

• The peak at δ 64 ppm suggests a carbon bonded to oxygen (C-O).
• The peak at δ 25 ppm suggests a carbon bonded to carbon (C-C).

¹H NMR:

• The septet at δ 4.00 ppm with a relative peak area of 1 is characteristic of a single proton in a unique environment, likely attached to a carbon that is bonded to an OH group.
• The doublet at δ 1.20 ppm with a relative peak area of 6 is characteristic of six protons, suggesting the presence of two methyl groups.

Step 5: Structure determination

Based on all the evidence, the structure can be determined as propan-2-ol.

Worked example 2 - Determining the structure of an organic molecule

Using the following information provided, determine the structure of the unknown organic compound.

• Elemental analysis: C: 54.55%, H: 9.09%, O: 36.36%
• Mass spectrum:

• IR spectrum:

• ¹³C NMR spectrum:

• ¹H NMR spectrum:

Step 1: Elemental analysis and empirical formula calculation

Assuming 100 g of the compound for simplicity, convert the mass percentages to moles:

• Carbon: $ \frac{54.55}{12.0}=4.55 \text{ mol} $
• Hydrogen: $ \frac{9.09}{1.0}=9.09 \text{ mol} $
• Oxygen: $ \frac{36.36}{16.0}=2.27 \text{ mol} $

The smallest number of moles is 2.27. Dividing all by 2.27 gives:

• C: 2, H: 4, O: 1

Empirical formula is C₂H₄O.

Step 2: Molecular formula calculation

The molecular weight from the molecular ion peak (m/z = 88) matches twice the weight of the empirical formula (2 × 44 = 88). Thus, the molecular formula is C₄H₈O₂.

Step 3: IR spectrum analysis

The strong absorption at 1,740 cm^-1 suggests a carbonyl (C=O) group.

Step 4: NMR spectrum analysis

¹³C NMR:

• The peak at δ 175 ppm suggests a carbonyl (C=O) carbon.
• The peak at δ 52 ppm suggests a carbon bonded to oxygen (C-O).
• The peak at δ 9 ppm suggests a carbon bonded to carbon (C-C).

¹H NMR:

• The singlet at δ 3.70 ppm with a relative peak area of 3 is characteristic of a methyl group (CH₃) adjacent to an oxygen atom.
• The quartet at δ 2.32 ppm with a relative peak area of 2 is characteristic of a CH₂ group adjacent to a carbonyl group on one side and a methyl group on the other side.
• The triplet at δ 1.15 ppm with a relative peak area of 3 is characteristic of a methyl group (CH₃) adjacent to a CH₂ group.

Step 5: Structure determination

Based on all the evidence, the structure can be determined as methyl propanoate.