What a learner can do afterwards
- Explains why a routine carbon spectrum shows singlets and why the integrals are not reliable
- Reads a DEPT experiment to classify each carbon by the number of attached protons
- Extracts a coupling constant in hertz and uses its size to argue for cis or trans geometry
- Combines carbon and proton evidence to distinguish two candidate structures
1 · Read
The carbon spectrum shows the skeleton directly, one signal per distinct carbon. Routine spectra are run with the protons decoupled: the instrument irradiates them while it records, so C-H splitting collapses and every carbon gives a singlet. Peak heights cannot be trusted for counting. Carbons relax at different rates and get different boosts from the protons (the nuclear Overhauser effect), so a carbon with no hydrogens often gives a small peak.
An editing experiment called DEPT sorts carbons by attached protons. In DEPT-135, CH and CH3 carbons point up and CH2 carbons point down. DEPT-90 shows only CH carbons. Carbons with no protons stay silent in both, which is itself a clue: compare with the full carbon spectrum to find them.
A coupling constant in hertz reports on geometry, not just neighbours. Measure the gap between two lines of a multiplet in ppm and multiply by the spectrometer frequency in MHz: a gap of 0.03 ppm at 400 MHz is 12 Hz. For two hydrogens across a C=C, trans couplings are about 12 to 18 Hz and cis couplings about 6 to 12 Hz. So given cis and trans candidates, the larger coupling fits trans.
Combine both nuclei to kill one candidate. If a carbon signal or a coupling fits only one structure, the other is out. Name the exact signal that decided it.
Carbon maps the skeleton, DEPT counts attached protons, and coupling size argues geometry.
2 · Watch
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Where it sits
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.