Carbon-13 NMR Spectroscopy (Cambridge (CIE) A Level Chemistry): Flashcards
Exam code: 9701
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Define chemical shift.
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Define chemical shift.
A chemical shift (δ, in ppm) is the position of a peak on an NMR spectrum, measured relative to the TMS reference peak at 0 ppm, indicating the electronic environment of the atom.
What is the reference compound used in 13C NMR, and what solvent is typically used to dissolve the sample?
The reference compound is tetramethylsilane (TMS), set at 0 ppm. The sample is dissolved in CDCl3 (deuterochloroform), whose solvent peak at 77 ppm can be ignored.
True or False?
In 13C NMR, the height of each signal is proportional to the number of carbon atoms in that environment.
False. In 13C NMR, the height of each signal is not proportional to the number of carbon atoms. Each non-equivalent carbon produces one signal regardless of how many carbons share that environment.
A carbonyl carbon (C=O in a ketone or aldehyde) gives a 13C NMR signal in the range .......... ppm, while an sp3 alkyl carbon gives a signal in the range .......... ppm.
A carbonyl carbon (C=O in a ketone or aldehyde) gives a 13C NMR signal in the range 190-220 ppm, while an sp3 alkyl carbon gives a signal in the range 0-50 ppm.
Define carbon-13 NMR spectroscopy.
Propanone produces 2 signals: one for the carbonyl carbon (200 ppm) and one for the two equivalent methyl carbons (30 ppm). The two CH3 groups are in identical environments due to the molecule's symmetry.
carbon-13 NMR spectroscopy
Carbon-13 NMR spectroscopy is an analytical technique that detects distinct 13C environments in a molecule to determine molecular structure, producing one signal per type of non-equivalent carbon.
True or False?
13C NMR spectra show splitting patterns similar to those seen in 1H NMR spectra.
False. 13C NMR spectra show sharp single signals with no complicated splitting patterns, unlike 1H NMR where spin-spin splitting produces multiplets.
In which chemical shift range would you expect a 13C signal for a carbon bonded to an oxygen atom (–C–O)?
A carbon bonded to oxygen (sp3, next to oxygen) gives a 13C signal in the range 50-70 ppm.
In 13C NMR, only .......... carbon atoms produce signals. The more abundant .......... carbon atoms are NMR-inactive and do not appear on the spectrum.
In 13C NMR, only carbon-13 carbon atoms produce signals. The more abundant carbon-12 carbon atoms are NMR-inactive and do not appear on the spectrum.
Define equivalent carbons.
Equivalent carbons are carbon atoms in identical molecular environments due to symmetry, producing a single 13C NMR signal rather than separate signals for each carbon.
How many 13C NMR peaks does benzene produce, and why?
Benzene produces 1 peak because all six carbon atoms are equivalent due to the molecule's multiple lines of symmetry — they all occupy the same molecular environment.
True or False?
Propan-2-ol gives more 13C NMR peaks than propan-1-ol.
False. Propan-2-ol gives 2 peaks (the two equivalent CH3 groups are symmetric). Propan-1-ol gives 3 peaks (no equivalent carbons). Propan-2-ol gives fewer peaks.
To predict the number of 13C NMR peaks, first look for .......... in the molecule. Carbons that are equivalent due to symmetry produce a .......... signal.
To predict the number of 13C NMR peaks, first look for symmetry in the molecule. Carbons that are equivalent due to symmetry produce a single signal.
How many 13C NMR peaks would you predict for 1,3-dihydroxybenzene, and how are they determined?
1,3-Dihydroxybenzene gives 4 peaks. The molecule has a plane of symmetry, making some ring carbons equivalent. Counting unique environments — two types of C–OH, one C between them and one equivalent pair of unsubstituted carbons — gives 4.
Define molecular environment (13C NMR).
A molecular environment in 13C NMR is the unique chemical surroundings of a carbon atom; non-equivalent carbons in different environments appear as separate peaks on the spectrum.
True or False?
A molecule with five carbon atoms must always show five peaks in its 13C NMR spectrum.
False. A molecule with five carbons shows at most five peaks, but if some carbons are equivalent due to symmetry, fewer peaks are observed.
Why is counting the number of 13C NMR resonances a useful first step in analysing an unknown spectrum?
It immediately tells you the number of distinct carbon environments, which can distinguish between structural isomers — for example, the three isomers of dihydroxybenzene each have a different number of environments.
Pentane-1,4-diol has .......... carbon environments because it has no .......... and therefore no equivalent carbons.
Pentane-1,4-diol has 5 carbon environments because it has no symmetry and therefore no equivalent carbons.
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