Q5(e). NMR of ethene and ethyne; benzyl to tropylium cation

Introduction. (i) Chemical shift depends on the electron density around the proton and on magnetic anisotropy: the induced circulation of π electrons sets up a local field that adds to or opposes the applied field depending on where the proton sits. (ii) In the mass spectrometer the C₇H₇⁺ ion (m/z 91) from benzyl compounds is the very stable tropylium ion.

(i) Ethene (δ 5.25) and ethyne (δ 1.80)

Ethene. The π electrons circulate in a loop perpendicular to the applied field B₀ (molecular plane across the field). The induced field reinforces B₀ at the protons, which lie in the plane: they are deshielded. Together with the sp² carbon this gives δ 5.25.

Figure: Induced π circulation: ethene H lies in the deshielding region; the axial H of ethyne in the shielding cone.

Ethyne. When the molecular axis lies along B₀, the cylindrical π cloud circulates round the axis. The induced field opposes B₀ along the axis, where the protons sit: they are shielded. This outweighs the greater electronegativity of sp carbon (which alone would deshield), so ethyne appears at δ 1.80, upfield of ethene by about 3.5 ppm.

(ii) Benzyl cation to tropylium cation

Scheme: The CH₂ carbon is built into the ring: C₇H₇⁺ becomes the seven-membered aromatic ion.

1. In the benzyl cation (formed by benzylic cleavage, for example from toluene M⁺. by loss of H) the empty p orbital of CH₂⁺ is attacked by the adjacent ring C–C bond (the ortho carbon bends towards it).
2. This gives a bicyclic norcaradienyl (bicyclo[4.1.0]heptadienyl) cation, a three-membered ring fused to the six-membered ring.
3. The bond shared by the two rings breaks: the CH₂ carbon is inserted, and the tropylium ion, a planar 6π aromatic ring of seven CH units, results.

The rearrangement is supported by labelling: 13C or D placed on the CH₂ of toluene is spread over all seven carbons in the m/z 91 ion and its fragments. Tropylium loses acetylene to give C₅H₅⁺ (m/z 65).

Conclusion. (i) Ethene protons lie in the deshielding zone of the induced π current (δ 5.25); ethyne protons lie on the axis of the cylindrical π current and are shielded (δ 1.80). (ii) The benzyl cation expands through a norcaradienyl ion to the aromatic tropylium ion, C₇H₇⁺, m/z 91.

Application. The m/z 91 peak is the fingerprint of benzyl and alkylbenzene groups in forensic and drug analysis by GC–MS; anisotropy explains the deshielded aldehyde and aromatic protons used daily in NMR structure work.