Interpreting and explaining the Carbon-13 13C NMR spectrum of cyclohexene

TMS is the acronym for tetramethylsilane, formula Si(CH₃)₄, whose ¹³C atoms are arbitrarily given a chemical shift of 0.0 ppm. This is the 'standard' in ¹³C NMR spectroscopy and all other ¹³C resonances, called chemical shifts, are measured with respect to the TMS, and depend on the individual (electronic) chemical environment of the ¹³C atoms in an organic molecule - cyclohexene here.

The molecular structure and naming of alkenes

Interpreting the C-13 NMR spectrum of cyclohexene

As you can see from the diagram above there are 3 different ¹³C chemical shift lines in the C-13 NMR spectrum of cyclohexene indicating 3 different chemical environments of the 10 carbon atoms of cyclohexene.

¹³C chemical shifts δ (a) to (c) on the C-13 NMR spectrum diagram for cyclohexene.

(a)  C atoms : ¹³C NMR chemical shift δ of 22.8 ppm

The ¹³C NMR resonance for the 2 x equivalent C atoms furthest from the C=C bond.

(b)  C atoms : ¹³C NMR chemical shift δ of 25.3 ppm

The ¹³C NMR resonance for the 2 x equivalent C atoms nearest to the C=C bond.

(c)  C atoms : ¹³C NMR chemical shift δ of 127.3 ppm

The  ¹³C NMR resonance for the 2 x equivalent C atoms of the actual C=C bond itself.

The presence of the pi electron cloud produces the largest downfield chemical shift for the C=C bond carbons compared to the other two pairs of carbon atoms.

The carbon-13 NMR spectra provides direct evidence of 3 different carbon atom environments for the 10 carbon atoms in the cyclohexene molecule, deduced from the presence of 3 different 13C NMR chemical shifts (ppm).

Key points about the 13C NMR spectrum of cyclohexene

Key Features of the ¹³C NMR Spectrum of Cyclohexene

• Molecular formula: C₆H₁₀ → six carbons total.
• Distinct carbon environments: Four signals due to symmetry in the ring.
• Chemical shift ranges:
  • Alkene carbons (sp²): ~125–135 ppm.
  • Allylic carbons (sp³ adjacent to C=C): ~20–30 ppm.
  • Remaining saturated carbons (sp³ further from C=C): ~25–35 ppm.
• Decoupled spectrum: In routine exam spectra, proton decoupling is used → signals appear as unsplit singlets (no multiplicity).
• No carbonyl carbons: Absence of signals >160 ppm confirms no C=O group.

Table of ¹³C Chemical Shifts and Origins for the 13C NMR spectrum of cyclohexene

Common Student Misconceptions

• Expecting six signals: Students often forget symmetry reduces the number of distinct carbons to three.
• Confusing alkene carbons with aromatic carbons: Aromatics appear further downfield (~120–150 ppm), but cyclohexene is not aromatic.
• Thinking signals should be split: In exam spectra, ¹³C is proton‑decoupled, so signals are singlets.
• Misplacing allylic carbons: Allylic carbons are slightly downfield compared to normal alkyl carbons, but not as far as vinylic carbons.

Exam Revision Tips

• Count signals first: Four signals = symmetry in cyclohexene.
• Identify vinylic carbons (~125–135 ppm): Diagnostic of alkenes.
• Check allylic carbons (~20–30 ppm): Slightly deshielded compared to normal alkyl.
• Remember decoupling: All signals are singlets in exam spectra.
• Compare with cyclohexane: Cyclohexane shows only sp³ carbons (~25 ppm), no downfield vinylic signals.
• Exam technique: When asked to assign signals, state ppm, carbon type, and reasoning (e.g., “Signal at ~130 ppm corresponds to vinylic carbons of the C=C bond”).
• Cross‑board consistency: All exam boards (AQA, Edexcel, OCR, WJEC, CCEA, CIE, IB, AP) expect recognition of signal count, approximate ranges, and diagnostic alkene carbons.

Final Comments

For A level and AP exams, focus on:

• Three signals only (due to symmetry).
• Vinylic carbons at ~125–135 ppm.
• Allylic carbons at ~20–30 ppm.
• Remaining saturated carbons at ~25–35 ppm.
• Decoupled spectrum = unsplit singlets.