Analytical Techniques (OCR A Level Chemistry A): Flashcards
Exam code: H432
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Define stationary phase (TLC)
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Define stationary phase (TLC)
A stationary phase is the solid material on a TLC plate, commonly a metal sheet coated in alumina (Al2O3) or silica (SiO2), onto which solute molecules adsorb during separation.
What is the purpose of the baseline drawn in pencil at the start of a TLC experiment?
It marks the position where the sample and reference compound spots are placed before the solvent begins to travel up the plate.
In TLC, the solvent must not cover the .......... when the plate is placed in the beaker, and a lid is used to .......... the beaker during separation.
In TLC, the solvent must not cover the spot when the plate is placed in the beaker, and a lid is used to saturate the beaker during separation.
Why do less polar components travel further up a TLC plate?
They are more soluble in the non-polar mobile phase and interact less with the polar stationary phase, so they are carried further by the solvent.
True or False?
A higher Rf value indicates that a component has a stronger interaction with the stationary phase.
False.
A higher Rf value means the component travelled further with the mobile phase, indicating weaker interaction with the stationary phase and greater solubility in the solvent.
How can colourless spots be located on a TLC plate after separation?
Colourless spots can be located using UV light, ninhydrin spray, or iodine vapour, then drawn around in pencil.
The Rf value is calculated by dividing the distance travelled by the .......... by the distance travelled by the .......... .
The Rf value is calculated by dividing the distance travelled by the component by the distance travelled by the solvent.
Define retention time (GC)
A retention time is the time taken for a component to travel through the column in gas chromatography, which depends on its volatility and its attraction to the stationary and mobile phases.
What does the area of a peak on a gas chromatogram indicate?
The relative amount (proportion) of that compound present in the analysed mixture.
In gas-liquid chromatography, an .......... carrier gas such as helium moves the vaporised sample through a long .......... column.
In gas-liquid chromatography, an inert carrier gas such as helium moves the vaporised sample through a long coiled column.
Why do compounds with stronger interactions with the stationary phase have longer retention times in GC?
Stronger interactions with the stationary phase slow the component's progress through the column, so it takes longer to reach the detector.
True or False?
In GC, the mobile phase is a volatile liquid that carries the sample through the stationary phase.
False.
The mobile phase in GC is an inert carrier gas (such as helium or nitrogen), not a liquid.
How are unknown compounds identified using a gas chromatogram?
Their retention times are compared with data book values for known compounds under the same experimental conditions.
True or False?
Each peak on a gas chromatogram represents a separate volatile compound in the analysed sample.
True.
Each peak corresponds to a different component reaching the detector at its characteristic retention time.
Define Brady's reagent
A Brady's reagent is a pale orange solution of 2,4-dinitrophenylhydrazine (2,4-DNP) in methanol and sulfuric acid, used to test for the presence of a carbonyl group.
What is observed when bromine water is shaken with an alkene, and why does this occur?
The orange-yellow bromine water decolourises because the alkene undergoes an addition reaction across the C=C double bond.
To test for a haloalkane, it is first warmed with .......... in ethanol/water, then the released halide ion is tested with .......... solution.
To test for a haloalkane, it is first warmed with sodium hydroxide in ethanol/water, then the released halide ion is tested with silver nitrate solution.
Why must the sample be acidified with dilute nitric acid before adding silver nitrate solution in the halide ion test?
Acidification removes carbonate ions that would otherwise react with silver nitrate to produce a false positive precipitate.
True or False?
Tollens' reagent can be used to distinguish between an aldehyde and a ketone.
True.
An aldehyde reduces Ag+ ions to silver metal, producing a silver mirror, whereas a ketone cannot be oxidised and gives no visible change.
What colour change is observed when a primary or secondary alcohol is warmed with acidified potassium dichromate solution?
The solution changes from orange to green as the dichromate ion oxidises the alcohol.
Carboxylic acids react with sodium carbonate solution to produce .......... gas, which turns .......... milky when bubbled through it.
Carboxylic acids react with sodium carbonate solution to produce carbon dioxide gas, which turns limewater milky when bubbled through it.
True or False?
Bromine water decolourises and then forms a white precipitate when shaken with a compound containing a phenol group.
True.
Phenol undergoes a substitution reaction with bromine water, decolourising the solution and producing a white precipitate of 2,4,6-tribromophenol.
Define chemical shift (δ) in 13C NMR
A chemical shift (δ) is the position of a carbon-13 signal on an NMR spectrum, measured in parts per million (ppm) relative to the reference compound TMS (tetramethylsilane) at 0 ppm.
Why does propanone show two signals on its 13C NMR spectrum?
Propanone has two non-equivalent carbon environments: the carbonyl carbon (C=O) and the two equivalent methyl carbons, each appearing at a different chemical shift.
In 13C NMR, a C=O carbon in an aldehyde or ketone appears in the chemical shift range .......... ppm, while a C–C alkyl carbon appears in the range .......... ppm.
In 13C NMR, a C=O carbon in an aldehyde or ketone appears in the chemical shift range 160–220 ppm, while a C–C alkyl carbon appears in the range 0–50 ppm.
True or False?
The height of each signal in a 13C NMR spectrum is proportional to the number of carbon atoms in that environment.
False.
Unlike 1H NMR, the signal height in 13C NMR is not proportional to the number of carbon atoms in a given environment.
What chemical shift range would you expect for a C–O carbon in an alcohol or ether on a 13C NMR spectrum?
A C–O carbon in an alcohol or ether appears in the range 50–90 ppm on a 13C NMR spectrum.
Why are carbon-13 atoms rather than carbon-12 atoms detected in 13C NMR spectroscopy?
Carbon-12 has an even mass number and gives no NMR signal, whereas carbon-13 has an odd mass number and produces signals detectable by NMR.
True or False?
13C NMR spectra display complicated splitting patterns similar to those seen in 1H NMR.
False.
13C NMR spectra display sharp single signals with no splitting patterns, making them simpler to interpret than 1H NMR spectra.
What is tetramethylsilane (TMS)?
Tetramethylsilane (TMS) is the reference compound used in NMR spectroscopy, defined as having a chemical shift of 0 ppm, against which all other signals are measured.
Why does ethanol produce three peaks on a low-resolution 1H NMR spectrum?
Ethanol has three distinct proton environments (CH3, CH2, and OH), each producing a separate peak at a different chemical shift.
According to the n+1 rule, a proton with .......... adjacent protons on a neighbouring carbon will appear as a .......... in a high-resolution 1H NMR spectrum.
According to the n+1 rule, a proton with two adjacent protons on a neighbouring carbon will appear as a triplet in a high-resolution 1H NMR spectrum.
Why do OH and NH protons typically appear as broad singlets in 1H NMR spectra rather than showing splitting patterns?
Hydrogen bonding causes the peaks to broaden, and the OH/NH protons are not usually involved in spin-spin coupling with adjacent protons.
True or False?
The area under each peak in a 1H NMR spectrum is proportional to the number of protons in that environment.
True.
The relative peak areas (integrals) reflect the ratio of protons in each chemical environment, unlike in 13C NMR.
How does adding D2O to a sample help identify OH protons in a 1H NMR spectrum?
Deuterium from D2O exchanges with OH (and NH) protons, so those peaks disappear in the second spectrum, confirming their identity.
A quartet and triplet combination in the same 1H NMR spectrum usually indicates the presence of an .......... group, written as .......... .
A quartet and triplet combination in the same 1H NMR spectrum usually indicates the presence of an ethyl group, written as CH3CH2.
True or False?
Equivalent protons in the same chemical environment cause spin-spin splitting of each other's signals.
False.
Equivalent protons do not cause spin-spin splitting; only protons in different chemical environments on adjacent carbons split each other's signals.
Define molecular ion peak (mass spectrometry)
A molecular ion peak is the peak in a mass spectrum at the highest m/z value, corresponding to the unfragmented molecule and giving the relative molecular mass of the compound.
What is the correct sequence of analytical techniques used to deduce the full structure of an unknown organic compound?
Elemental analysis, then mass spectrometry, then infrared spectrometry, then NMR spectrometry, each narrowing down the possible structure.
In IR spectroscopy, a strong absorption between 1700 and 1750 cm-1 indicates the presence of a .......... group, while an absorption around 2500–3300 cm-1 indicates a .......... group.
In IR spectroscopy, a strong absorption between 1700 and 1750 cm-1 indicates the presence of a C=O group, while an absorption around 2500–3300 cm-1 indicates a O–H (carboxylic acid) group.
Why can elemental analysis sometimes be skipped when deducing an organic structure?
The combination of mass spectrometry and NMR can suggest the molecular formula without elemental analysis, making the step redundant.
True or False?
The number of peaks in a 1H NMR spectrum tells you the number of different proton environments in the molecule.
True.
Each peak corresponds to a distinct proton environment, so counting peaks reveals how many chemically non-equivalent hydrogen environments are present.
A compound has a molecular formula of C4H8O and a strong IR absorption at 1720 cm-1. What functional group does this indicate, and which two compound types are consistent with this formula?
The absorption indicates a C=O (carbonyl) group; with only one oxygen atom the compound must be either an aldehyde or a ketone.
In a mass spectrum, the .......... technique gives the molecular mass of the compound, while .......... of the molecular ion peak reveals the fragments present.
In a mass spectrum, the molecular ion peak technique gives the molecular mass of the compound, while fragmentation of the molecular ion peak reveals the fragments present.
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