Organic Analysis (AQA A Level Chemistry): Flashcards
Exam code: 7405
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What are test-tube reactions in organic chemistry?
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Cards in this collection (22)
What are test-tube reactions in organic chemistry?
Test-tube reactions are simple chemical tests carried out on small samples to identify the functional groups present in an organic compound, based on characteristic colour changes or precipitates.
If an organic compound reacts with sodium carbonate to produce bubbles of gas, what functional group is likely to be present?
A carboxylic acid group (–COOH). Carboxylic acids are acidic enough to react with carbonates, releasing CO2 gas.
A compound that is a gas at room temperature is likely to have a very .......... carbon chain length and is unlikely to have .......... .
A compound that is a gas at room temperature is likely to have a very short carbon chain length and is unlikely to have strong intermolecular forces.
True or False?
If an organic compound is soluble in water, this suggests it contains polar functional groups.
True.
Water is a polar solvent; polar groups (such as –OH, –COOH, or –NH2) allow the compound to interact with water molecules through hydrogen bonding or dipole–dipole forces, increasing solubility.
When an organic compound turns moist red litmus paper .......... and produces a pungent smell when heated with NaOH, it contains an .......... group.
When an organic compound turns moist red litmus paper blue and produces a pungent smell when heated with NaOH, it contains an ammonium group (or amine that releases NH3).
What simple physical observation might suggest that an organic compound has long C–C chains or strong intermolecular forces?
The compound is a solid at room temperature, since longer chains and stronger intermolecular forces raise the melting point above room temperature.
True or False?
A liquid organic compound at room temperature necessarily contains polar groups.
False.
A liquid state only suggests a medium carbon chain length or moderate intermolecular forces. Non-polar compounds of intermediate chain length (e.g. hexane) can also be liquids at room temperature.
What is the molecular ion (M+) in mass spectrometry?
Molecular ion (M+) is the ion formed when a molecule loses one electron during bombardment, giving the whole molecule a 1+ charge. Its m/z value equals the relative molecular mass of the compound.
What is the base peak in a mass spectrum?
The peak corresponding to the most abundant (most stable) fragment ion. It is the tallest peak in the spectrum.
In mass spectrometry, molecules are bombarded with .......... electrons, causing them to lose an electron and form a .......... charged molecular ion.
In mass spectrometry, molecules are bombarded with high-energy electrons, causing them to lose an electron and form a positively charged molecular ion.
True or False?
The [M+1] peak in a mass spectrum is due to the natural abundance of carbon-13 in the sample.
True.
About 1% of carbon atoms in nature are 13C, so a small proportion of molecules have one extra mass unit, producing the [M+1] peak. The more carbon atoms in the molecule, the taller the [M+1] peak.
In the mass spectrum of a halogenoalkane containing bromine, there are two molecular ion peaks of .......... height because bromine has two isotopes (79Br and 81Br) in a .......... ratio.
In the mass spectrum of a halogenoalkane containing bromine, there are two molecular ion peaks of equal height because bromine has two isotopes (79Br and 81Br) in a 1:1 ratio.
What is fragmentation in mass spectrometry?
Fragmentation is the process by which the molecular ion breaks apart into smaller fragment ions, each producing a peak at a characteristic m/z value that gives information about the structure of the original molecule.
In the mass spectrum of an alcohol, a peak 18 units below the molecular ion peak is commonly observed. What causes this peak?
The loss of a water molecule (H2O, Mr = 18) from the molecular ion. Alcohols readily lose water during fragmentation.
True or False?
In a time-of-flight mass spectrometer, a fragment ion with a smaller m/z value takes longer to reach the detector than a fragment ion with a larger m/z value.
False.
All ions receive the same kinetic energy when accelerated. Lighter ions (smaller m/z) move faster and arrive at the detector sooner. In TOF mass spectrometry, ions are separated by flight time, not by magnetic deflection.
What is the fingerprint region in an infrared spectrum?
Fingerprint region is the region below 1500 cm-1 that contains many overlapping peaks from complex bond vibrations. It is unique to each compound and is used for exact identification by comparison with a database.
In IR spectroscopy, a broad absorption at 2500–3000 cm-1 AND a sharp absorption at 1680–1750 cm-1 together indicate the presence of a .......... functional group.
In IR spectroscopy, a broad absorption at 2500–3000 cm-1 AND a sharp absorption at 1680–1750 cm-1 together indicate the presence of a carboxylic acid functional group.
What bond vibration causes the broad O–H absorption at 3230–3550 cm-1 in the IR spectrum of an alcohol?
Hydrogen bonding between O–H groups broadens the absorption. The O–H stretch is broad rather than sharp because of the range of hydrogen-bond strengths in the sample.
True or False?
A compound showing only a sharp C=O absorption at ~1710 cm-1 and no O–H absorption is most likely an alcohol.
False.
An alcohol shows an O–H absorption at 3230–3550 cm-1 and no C=O peak. A C=O peak without O–H indicates a carbonyl compound such as an aldehyde or ketone.
Greenhouse gases such as CO2, CH4 and H2O absorb .......... radiation emitted by the Earth through bond .......... , which traps heat in the atmosphere.
Greenhouse gases such as CO2, CH4 and H2O absorb infrared radiation emitted by the Earth through bond vibrations, which traps heat in the atmosphere.
In an IR exam answer, why must you state both the bond type AND its wavenumber range?
The AQA mark scheme requires both pieces of information. Quoting only a bond (e.g. O–H) without its wavenumber range, or a wavenumber without naming the bond, does not score the mark.
True or False?
Two isomers from the same homologous series will have identical infrared fingerprint regions.
False.
Isomers share the same functional groups (same absorptions above 1500 cm-1) but differ in their fingerprint regions below 1500 cm-1, which is unique to each compound.
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