3.3.6 Organic Analysis

Know the characteristic tests/reactions for the functional groups in the course, but learn them as evidence: reagent, condition where needed, observation and what that observation tells you about the structure.

For example, bromine tests for C=C, acidified dichromate distinguishes oxidisable alcohols/aldehydes in the right context, and Tollens’ or Fehling’s can distinguish aldehydes from ketones.

When an unknown is tested, translate each observation into a structural constraint before choosing a molecule. A positive test can support a functional group; a negative test can eliminate structures that should have reacted.

Use several observations together. One colour change rarely identifies a whole molecule uniquely, but a consistent set of functional-group evidence can narrow the possibilities sharply.

If you have a precise molecular mass from mass spectrometry, you can use it to distinguish candidate molecular formulae that share the same nominal whole-number mass. Their exact masses differ slightly because the constituent atoms have different precise isotopic masses.

Use the precise isotopic masses from the data you are given to test candidate formulae. A rounded nominal mass may fit several possibilities; the accurate molecular mass can separate them.

Calculate the precise molecular mass of each plausible formula from the precise atomic masses, then compare those values with the measured precise molecular mass. Choose the formula whose calculated value matches the measurement.

Keep enough decimal places during the comparison: the whole point is that formulae with the same nominal mass can differ slightly in precise mass. Formulae that are structural isomers of one another still have the same precise molecular mass.

With an IR spectrum, use characteristic absorption regions to identify likely bonds or functional groups. The absorption occurs when the IR frequency matches an allowed molecular vibration.

The fingerprint region contains a more complex whole-molecule pattern. Comparing it with an authentic reference spectrum can strengthen an identification when the conditions are appropriate.

If you’re connecting greenhouse gases with IR, the key is that CO₂, CH₄ and H₂O absorb infrared radiation at frequencies associated with molecular vibrations. They can then re-emit some of that energy, reducing the rate at which it escapes directly to space.

Use the AQA infrared data to match significant absorptions to bonds and functional groups, then check the whole spectrum against the proposed structure. An unexpected absorption can indicate an impurity; an expected group should normally have the relevant diagnostic absorption present.