Modern Analytical Techniques II (Edexcel A Level Chemistry): Flashcards

¹³C NMR spectroscopy is an analytical technique that measures the magnetic field strengths of carbon-13 atoms in organic compounds, producing a spectrum of peaks at different chemical shifts corresponding to non-equivalent carbon environments.

False.

In ¹³C NMR, peak height is not proportional to the number of carbon atoms in a given environment, unlike ¹H NMR where integration reflects the number of protons.

Two peaks, because propanone has two non-equivalent carbon environments: the carbonyl carbon (C=O) and the two equivalent methyl carbons.

Chemical shift (δ) is the position of a resonance peak on an NMR spectrum, measured in ppm relative to the reference compound tetramethylsilane (TMS) at 0 ppm.

False.

¹³C NMR spectra display sharp single signals with no complicated splitting patterns, unlike ¹H NMR spectra.

Atoms with odd mass numbers show signals in NMR. Carbon-12 has an even mass number and is NMR-inactive; carbon-13 has an odd mass number and produces signals.

In ¹³C NMR, all samples are measured relative to the reference compound tetramethylsilane (TMS) at 0 ppm.

By counting the number of ¹³C resonances: each isomer has different molecular symmetry, giving a different number of non-equivalent carbon environments and therefore a different number of peaks.

¹H NMR spectroscopy is an analytical technique that measures the magnetic resonance of protons in different chemical environments, producing a spectrum used to determine the structure of organic compounds.

In ¹H NMR, the number of sub-peaks a signal splits into = n + 1, where n is the number of protons on the adjacent carbon.

True.

The integrated area under each peak is proportional to the number of equivalent protons responsible for that absorption, giving the ratio of protons in each environment.

Chemical shift (δ) is the position of a proton's absorption on an NMR spectrum, measured in ppm relative to the reference compound tetramethylsilane (TMS) at 0 ppm.

Ethanol (CH₃CH₂OH) produces three peaks for the three environments (CH₃, CH₂, OH) in an integration ratio of 3:2:1.

True.

In a CH3CH2— group, the CH3 protons are split into a triplet by two adjacent CH2 protons, and the CH2 protons are split into a quartet by three adjacent CH3 protons.

TMS is used because it is non-toxic, does not react with the sample, has a low boiling point for easy separation, and produces a single sharp absorption peak at 0 ppm well away from most sample peaks.

Spin-spin coupling is the interaction between the spin states of protons in adjacent, non-equivalent environments, causing peaks to split into sub-peaks (a splitting pattern).

The CH signal is split into a heptet (7 sub-peaks) because there are 6 equivalent protons on the two adjacent CH3 groups (n + 1 = 6 + 1 = 7).

Thin-layer chromatography is an analytical technique used to separate and identify components in a mixture by allowing a mobile phase (solvent) to travel through a solid stationary phase (e.g. silica-coated plate), separating components by their differing interactions with each phase.

Colourless spots can be located using UV light, iodine vapour, or ninhydrin (which is harmful — handle with care). Once located, the spots are circled in pencil.

True.

The baseline must be drawn in pencil because ink would interact with the sample components and solvents, interfering with the separation.

The stationary phase is typically a metal sheet coated in alumina (Al₂O₃) or silica (SiO₂). Components adsorb onto the surface; stronger interactions with the stationary phase cause a component to travel a shorter distance up the plate.

R_f value is the ratio of the distance travelled by a component to the distance travelled by the solvent front in chromatography:

R_f = distance travelled by component ÷ distance travelled by solvent

False.

The solvent level must be below the sample spot. If the solvent covered the spot, the sample would dissolve directly into the solvent rather than separating up the plate.

In TLC, the mobile phase (solvent) flows through the stationary phase (e.g. silica), separating components by their retention.

Reference compounds allow identification of components in the sample by comparing their R_f values and spot positions with those of known substances.

High performance liquid chromatography (HPLC) is a column chromatography technique in which a liquid mobile phase is pumped through a column of very fine stationary phase particles, giving greater separation than gravity-fed column chromatography. A detector records the retention time of each component.

In HPLC, the mobile phase is pumped through rather than flowing by gravity, and the stationary phase uses much smaller particles, giving greater separation efficiency and faster, automated results.

False.

HPLC and GLC alone are insufficient for definitive identification because different compounds may share the same retention time. They are typically coupled with mass spectrometry (e.g. GC-MS) for positive identification.

In a GLC chromatogram, the area of a peak is proportional to the quantity of that compound present, and the retention time indicates its identity.

Gas-liquid chromatography (GLC) is a technique for analysing gases, volatile liquids, and solids in vapour form, in which an inert carrier gas (e.g. helium or nitrogen) acts as the mobile phase, carrying vaporised sample through a non-volatile liquid stationary phase in a coiled column.

The component with the longest retention time has the strongest interaction with the stationary phase, as it takes longest to travel through the column.

True.

An inert carrier gas (e.g. helium or nitrogen) is the mobile phase in GLC. It carries vaporised sample molecules through the non-volatile liquid stationary phase.

Retention time is the time taken for a component to travel from the point of injection to the detector in a chromatography column, used to help identify compounds by comparison with data book values.

GLC and HPLC are used in drug testing in sports (e.g. detection of anabolic steroids), forensic evidence collection, analysis of environmental pollution, and detection of explosives in baggage.