Organic Analytical Techniques (Edexcel International A Level (IAL) Chemistry): Flashcards
Exam code: YCH11
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What is high resolution mass spectrometry?
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What is high resolution mass spectrometry?
High resolution mass spectrometry is a technique that measures the molecular mass of a compound to four or more decimal places, allowing the molecular formula to be deduced or confirmed from accurate atomic mass data.
To how many decimal places does a high resolution mass spectrometer typically give data?
High resolution mass spectrometers provide data accurate to four decimal places, sometimes more. This precision allows molecular formulae to be distinguished where low-resolution instruments cannot.
State the accurate relative atomic masses of hydrogen, carbon, nitrogen and oxygen used in high resolution MS calculations.
H = 1.0078
C = 12.0000
N = 14.0031
O = 15.9949
A compound has a high-resolution Mr of 58.0417. Show that the molecular formula is C3H6O and not C4H10.
C3H6O: (3 × 12.0000) + (6 × 1.0078) + 15.9949 = 58.0417 ✔
C4H10: (4 × 12.0000) + (10 × 1.0078) = 58.0780 ✘
Only C3H6O matches the given value.
True or False?
High resolution MS can confirm the molecular formula of a compound.
True.
By comparing the measured accurate mass with calculated values using precise atomic masses, high resolution MS can confirm or deduce the molecular formula.
True or False?
High resolution mass spectrometry can determine the structural formula of a compound directly from the molecular ion peak.
False.
The molecular ion peak gives the molecular formula only. To determine structural details, fragmentation patterns from the mass spectrum must also be analysed.
In high resolution mass spectrometry, if a calculated molecular mass does not exactly match the given value, you should choose the formula with the .......... calculated mass.
In high resolution mass spectrometry, if a calculated molecular mass does not exactly match the given value, you should choose the formula with the closest calculated mass.
Calculate the accurate Mr of C2H6N2 using high resolution atomic masses.
Mr = (2 × 12.0000) + (6 × 1.0078) + (2 × 14.0031) = 24.0000 + 6.0468 + 28.0062 = 58.0530
What is 13C NMR spectroscopy?
13C NMR spectroscopy is an analytical technique that measures the magnetic field strengths of carbon-13 atoms in an organic compound, recording each distinct carbon environment as a peak at a characteristic chemical shift.
Why is carbon-13 used in 13C NMR rather than carbon-12?
Carbon-12 does not show signals on NMR because it has an even mass number. Only atoms with odd mass numbers (such as carbon-13) give NMR signals.
What reference compound is used in 13C NMR, and at what chemical shift does it appear?
Tetramethylsilane (TMS) is the reference compound. Its signal appears at 0 ppm and all other chemical shifts are measured relative to it.
The number of peaks on a 13C NMR spectrum equals the number of .......... carbon environments in the molecule.
The number of peaks on a 13C NMR spectrum equals the number of distinct (non-equivalent) carbon environments in the molecule.
How many peaks appear in the 13C NMR spectrum of propanone, and why?
Two peaks. Propanone has two distinct carbon environments: the two equivalent CH3 carbons (one environment) and the carbonyl C=O carbon (a second environment).
True or False?
In 13C NMR, peak height is proportional to the number of carbon atoms in that environment.
False.
The height of each signal in 13C NMR is not proportional to the number of carbon atoms. Unlike 1H NMR, peak areas in 13C NMR are not used for counting purposes.
How many 13C NMR peaks would 1,3-dihydroxybenzene show, and why?
Four peaks. Due to the symmetry of the molecule, there are four distinct carbon environments, so four resonances appear in the 13C NMR spectrum.
True or False?
13C NMR spectra show complicated splitting patterns similar to 1H NMR spectra.
False.
13C NMR spectra display sharp, single signals with no complicated splitting patterns. Each signal corresponds to one distinct carbon environment.
What is tetramethylsilane (TMS)?
Tetramethylsilane (TMS) is the reference compound used in NMR spectroscopy. It produces a single, sharp peak at 0 ppm. TMS is non-toxic, does not react with the sample and has a low boiling point for easy separation.
Define chemical shift (in 1H NMR)
Chemical shift is the position of a proton’s absorption peak on an NMR spectrum, measured in parts per million (ppm) relative to the TMS reference peak at 0 ppm.
What information does the area under a peak give in a low resolution 1H NMR spectrum?
The area under a peak is proportional to the number of equivalent protons in that chemical environment. Larger areas indicate more protons in that environment.
How many peaks appear in the low resolution 1H NMR spectrum of ethanol (CH3CH2OH), and what are the environments?
Three peaks, one for each environment:
–CH3 (1.2 ppm)
–CH2 (3.7 ppm)
–OH (5.4 ppm)
The ratio of peak areas for –CH3, –CH2 and –OH in ethanol is ...........
The ratio of peak areas for –CH3, –CH2 and –OH in ethanol is 3:2:1.
True or False?
1,2-dichloroethane (Cl-CH2-CH2-Cl) shows one peak in 1H NMR.
True.
All four hydrogen atoms in 1,2-dichloroethane are in the same chemical environment, so only one peak appears in the 1H NMR spectrum.
How many 1H environments are present in 2-methylpropane, and what are they?
Two environments:
The three equivalent –CH3 groups (nine equivalent H atoms)
The lone –CH hydrogen
Why is TMS used as the reference compound in NMR spectroscopy rather than other compounds?
TMS is used because it is non-toxic, does not react with the sample, has a low boiling point for easy removal and produces a single, sharp peak at 0 ppm well away from most sample peaks.
Define spin-spin splitting
Spin-spin splitting is the splitting of a 1H NMR peak into sub-peaks caused by interactions between the spin of a proton and the spins of nearby protons in different environments.
What is the n+1 rule in 1H NMR?
The n+1 rule states that a proton with n equivalent protons on adjacent carbon atoms will have its signal split into n+1 sub-peaks (a splitting pattern).
What splitting pattern is produced by a proton with two protons on an adjacent carbon atom?
A triplet (3 sub-peaks), with an intensity ratio of 1:2:1. This is because n = 2, so n+1 = 3.
What splitting pattern and intensity ratio is produced by a proton with three protons on an adjacent carbon atom?
A quartet (4 sub-peaks), with an intensity ratio of 1:3:3:1. This is because n = 3, so n+1 = 4.
A quartet and a triplet appearing together in the same 1H NMR spectrum usually indicate the presence of a .......... group.
A quartet and a triplet appearing together in the same 1H NMR spectrum usually indicate the presence of an ethyl (CH3CH2-) group.
What information does the integrated spectrum in 1H NMR provide?
The relative areas under each peak in the integrated spectrum give the ratio of the number of protons responsible for each peak, helping to identify unknown compounds.
In the 1H NMR spectrum of (CH3)2CHOH, how many peaks are present and what are the splitting patterns of the CH3 and CH signals?
Three peaks. The –CH3 signal is a doublet (split by 1 adjacent CH proton, n+1 = 2). The –CH signal is a heptet (split by 6 adjacent CH3 protons, n+1 = 7).
True or False?
Splitting patterns must always occur in pairs in a 1H NMR spectrum.
True.
Each proton splits the signal of its neighbour, so splitting patterns always occur in pairs. For example, a triplet and a quartet pair together in an ethyl group.
In the 1H NMR spectrum of propane, the CH2 signal appears as a .......... because it has six neighbouring equivalent H atoms.
In the 1H NMR spectrum of propane, the CH2 signal appears as a heptet because it has six neighbouring equivalent H atoms (n+1 = 7).
What is thin-layer chromatography (TLC)?
Thin-layer chromatography (TLC) is a technique used to separate and analyse small samples. A spot of sample is placed on a thin metal sheet coated with silica or alumina (stationary phase) and developed with a liquid solvent (mobile phase).
What is the Rf value?
Rf value is the retention factor, calculated as: Rf = distance travelled by component ÷ distance travelled by solvent front. It is a fixed value for a compound under given conditions and is used to help identify components.
What are the stationary and mobile phases in TLC?
The stationary phase is a thin metal sheet coated with alumina (Al2O3) or silica (SiO2). The mobile phase is a polar or non-polar liquid solvent that carries the sample components up the plate.
State three methods used to locate colourless spots on a TLC plate.
UV light
Ninhydrin (carcinogenic)
Iodine vapour
In TLC, the baseline must be drawn in .......... because any other medium would interact with the sample.
In TLC, the baseline must be drawn in pencil because any other medium would interact with the sample.
In TLC, what determines how far a component travels up the plate?
The distance a component travels depends on its retention by the stationary phase and its solubility in the mobile phase. Components with stronger interactions with the stationary phase travel shorter distances.
True or False?
The solvent level must cover the sample spot at the start of TLC.
False.
The solvent level must be below the sample spot (baseline). If the spot is submerged, the sample will dissolve into the solvent rather than separating up the plate.
A spot travels 3.6 cm up a TLC plate while the solvent front travels 9.0 cm. Calculate the Rf value.
Rf = 3.6 ÷ 9.0 = 0.40
Rf values are dimensionless and always between 0 and 1.
What is high performance liquid chromatography (HPLC)?
High performance liquid chromatography (HPLC) is a form of column chromatography in which the sample is pumped through a column of very fine stationary phase particles under pressure, giving greater separation. A detector records the retention time of each component.
Define retention time
Retention time is the time taken for a component to travel from the point of injection to the detector at the end of the column. It is used to identify compounds by comparison with known reference values.
What are the stationary and mobile phases in gas-liquid chromatography (GLC)?
The stationary phase is a non-volatile liquid inside a long coiled column. The mobile phase is an inert carrier gas (e.g. helium or nitrogen) that carries the sample vapour through the column.
What types of sample are suitable for analysis by GLC?
GLC is used for analysing gases, volatile liquids and solids in their vapour form. Samples must be capable of vaporisation inside the column.
On a GLC chromatogram, the relative .......... of each peak indicates how much of that compound is present in the mixture.
On a GLC chromatogram, the relative area of each peak indicates how much of that compound is present in the mixture.
State two key ways HPLC differs from simple column chromatography.
The sample is pumped through the column rather than flowing by gravity.
The stationary phase particles are much smaller, giving greater separation of compounds.
Why are HPLC and GLC often coupled with mass spectrometry?
Different compounds can share the same retention time, making positive identification unreliable from chromatography alone. Coupling with mass spectrometry (giving HPLC-MS or GC-MS) allows separated components to be identified as well as separated.
True or False?
A compound with a longer retention time has a stronger interaction with the stationary phase.
True.
The longer a component is retained in the column, the stronger its attraction to the stationary phase relative to its affinity for the mobile phase, resulting in a longer retention time.
State three applications of HPLC and GLC.
Forensic evidence analysis
Drug testing in sport
Analysis of environmental pollution (or: detecting explosives in baggage)
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