Matching Techniques to Structural Questions

Introduction

Faced with an unknown compound, a chemist asks a series of questions. How heavy is the molecule? Which functional groups does it contain? How are the carbon and hydrogen atoms connected? Does it contain a conjugated system? No single technique answers all of them. The skill lies in choosing the right tool for each question and then combining the evidence into one consistent structure, much as a detective combines clues from different witnesses.

Core explanation

Mass spectrometry — "How heavy, and what pieces?" The molecular ion peak gives the relative molecular mass. High-resolution measurements give an exact mass that fixes the molecular formula. Isotope patterns reveal chlorine or bromine, and fragment ions hint at structural units.

Infrared spectroscopy — "Which functional groups?" Bonds vibrate at characteristic wavenumbers. A strong absorption near 1700 cm⁻¹ indicates C=O; a broad band around 3200 – 3550 cm⁻¹ indicates O–H in an alcohol. IR is quick and excellent for confirming the presence or absence of a group, but it rarely gives the full carbon skeleton.

UV-visible spectroscopy — "Is there conjugation, and how much is present?" Absorption in the UV-visible region arises from electronic transitions in chromophores such as C=C, C=O and aromatic rings. Longer conjugated systems absorb at longer wavelengths. Its greatest strength is quantitative: absorbance is proportional to concentration.

NMR spectroscopy — "How are the atoms connected?" ¹H NMR shows how many different hydrogen environments exist, how many hydrogens are in each, and how many hydrogens are on neighbouring carbons. ¹³C NMR shows the number of different carbon environments. NMR is usually the most powerful single technique for building the carbon–hydrogen framework.

Summary table:

Question Best technique Key evidence --- --- --- Relative molecular mass Mass spectrometry Molecular ion (M⁺) peak Molecular formula High-resolution MS Exact mass to four decimal places Functional groups IR Characteristic absorption wavenumbers Conjugation or colour UV-visible λmax and band intensity Concentration UV-visible Absorbance via Beer–Lambert law Number of H environments and neighbours ¹H NMR Chemical shift, integration, splitting Number of C environments ¹³C NMR Number of peaks and their shifts

Why combine? Each technique has blind spots. IR cannot easily distinguish propan-1-ol from propan-2-ol; NMR can. NMR cannot directly give the molecular mass; mass spectrometry can. Using several techniques together makes structure determination reliable.

Step-by-step reasoning

A logical order for identifying an unknown:

1. Use mass spectrometry to find the relative molecular mass and, if possible, the molecular formula. 2. Calculate the degree of unsaturation from the formula. 3. Use IR to identify functional groups. 4. Use ¹³C and ¹H NMR to build the skeleton. 5. Check that the proposed structure fits every piece of data.

Visual explanation

Draw a jigsaw with four pieces labelled MS, IR, UV-vis and NMR. The MS piece shows a mass number, the IR piece shows a C=O bond, the UV piece shows a conjugated chain, and the NMR piece shows the carbon skeleton with hydrogens. Only when all four pieces fit does the complete molecule appear.

Real-world analogy

Identifying a stranger works the same way: height and weight (mass spectrometry), clothing style (functional groups from IR), and the names of their family and neighbours (NMR connectivity). Any one clue is ambiguous; together they point to one person.

Real-world example

Pharmaceutical quality-control laboratories check each batch of a drug using several methods: IR to confirm identity against a reference spectrum, UV-visible or chromatography with UV detection to measure the amount of active ingredient, and mass spectrometry to detect impurities.

Why?

Why can IR not tell butanal from butanone reliably, while NMR can? Both contain C=O and give similar C=O stretches near 1720 cm⁻¹. The aldehyde, however, has a distinctive CHO proton at about 9.5 – 10 ppm in ¹H NMR, which the ketone lacks.

Common misconception

"NMR is the best technique, so the others are unnecessary." NMR does not directly give the molecular mass, and it is less convenient for quantifying trace amounts. Each technique is the best choice for certain questions only.

Worked example

Question: A student has two colourless liquids known to be ethanol and ethanoic acid. Which technique distinguishes them most simply, and what evidence would they look for?

Reasoning: Both contain O–H, but only ethanoic acid contains C=O. IR shows a strong C=O stretch near 1700 – 1725 cm⁻¹ for the acid, and its O–H band is much broader, spreading across 2500 – 3300 cm⁻¹.

Answer: IR spectroscopy; ethanoic acid shows a strong C=O absorption and a very broad O–H band, while ethanol shows no C=O.

Quick check

1. Which technique would you use to find the relative molecular mass of an unknown compound? Answer: Mass spectrometry, from the m/z value of the molecular ion peak.

Exam focus

Structure-determination questions award marks for linking each piece of data to a deduction — for example "peak at 1715 cm⁻¹ shows C=O present". Always name the technique and the specific evidence, and check that your final structure is consistent with all the data.

Advanced insight

Modern laboratories often couple techniques directly: gas chromatography–mass spectrometry (GC-MS) separates a mixture and records a mass spectrum for each component, and liquid chromatography with UV and mass detection does the same for less volatile compounds. Two-dimensional NMR experiments reveal which atoms are bonded to which in complex molecules.

Summary

Mass spectrometry gives relative molecular mass and formula; IR identifies functional groups; UV-visible reveals conjugation and measures concentration; NMR maps the carbon–hydrogen framework. Each has limitations, so chemists combine them, working from formula to functional groups to skeleton and checking the final structure against every spectrum.

Practice questions

1. Which technique is most useful for determining the concentration of a coloured solution? Answer: UV-visible spectroscopy (colorimetry), because absorbance is proportional to concentration. 2. Why would IR struggle to distinguish propan-1-ol from propan-2-ol? Answer: Both contain the same functional groups (O–H, C–O and C–H), so their main absorptions occur at very similar wavenumbers. 3. What does ¹³C NMR tell you about a molecule? Answer: The number of different carbon environments, and from their chemical shifts, the types of carbon present. 4. Suggest why chemists run the mass spectrum before interpreting the NMR spectrum. Answer: Knowing the molecular formula first limits the possible structures and allows the degree of unsaturation to be calculated.