Combining Analytical Data

In real chemistry — and in Edexcel A-Level exams — you are rarely given data from a single technique. The power of analytical chemistry comes from combining data from mass spectrometry, infrared spectroscopy, ¹H NMR, and ¹³C NMR to build up a complete structural picture. This lesson teaches you a systematic approach for tackling these combined problems.

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The Systematic Approach

When given data from multiple techniques, work through them in a logical order:

Step 1: Molecular Formula and Degree of Unsaturation (from Mass Spectrometry)

Start with the mass spectrum. The molecular ion peak (M⁺) gives the relative molecular mass. If high-resolution mass spectrometry data or elemental analysis data is provided, determine the molecular formula.

From the molecular formula CₙHₘOₓNy, calculate the degree of unsaturation (also called the index of hydrogen deficiency, IHD):

Degree of unsaturation = (2n + 2 − m + y) / 2

Where:

• n = number of carbons
• m = number of hydrogens
• y = number of nitrogens
• Oxygen and halogens do not affect the calculation (but halogens count as hydrogens for this formula)