Explain anisotropic effects on chemical shift

Map local fields

Benzene protons lie in the deshielding region of the induced π-electron ring current and appear near δ7.2.

The applied field creates a directional local field, so position in space matters.

π electrons respond to the applied field by circulating and producing a direction-dependent local field. A proton's shift therefore depends on where it lies relative to the π system, not simply on how close it is.

Define anisotropy

Magnetic anisotropy is direction-dependent shielding produced by circulating electrons in an applied field.

It is distinct from a simple electronegativity trend.

Magnetic anisotropy means that the induced field is not the same in every spatial direction. Shielding and deshielding regions can be mapped around alkenes, aromatic rings, carbonyl groups and alkynes.

Compare π systems

Aldehyde protons are strongly deshielded near δ9–10, while an ethyne proton can be relatively shielded near δ2–3.

The alkyne proton lies along a shielding region of the C≡C induced field.

Aromatic protons lie in the deshielding region around the outside of the ring current and commonly appear near δ 7–8. An aldehydic proton is strongly deshielded by the carbonyl anisotropy and often appears near δ 9–10.

A terminal alkyne proton lies along the C≡C axis in a relative shielding region, so it can appear around δ 2–3 despite attachment to an sp carbon. This is a useful counterexample to distance-only reasoning.

Explain benzene

The ring current reinforces B₀ outside the ring where peripheral H atoms lie.

Their larger effective field accounts for δ about 7.2.

Compare benzene, ethyne and ethanal by locating each proton in the induced-field diagram before quoting a shift region. The spatial field pattern explains why their order is not predicted by electronegativity alone.

Explain ethyne

Supplied shifts: ethyne H δ1.9 and ethene H δ5.3. In ethyne the terminal H lies on the C≡C axis; in ethene each H lies in the plane of the C=C π system.

Decide the direction of the induced π field at each proton before you use hybridisation or electronegativity.

Draw the applied field and the induced-field direction at the proton. If the induced field reinforces B₀, predict deshielding; if it opposes B₀, predict relative shielding.

Assign ethanal signals

Supplied structure: ethanal, CH₃CHO. Its two ¹H signals are δ2.2 (3H) and δ9.8 (1H).

Use the integrals to assign the groups, then explain the difference in shift using the proton positions relative to C=O and the oxygen's inductive effect.

Use anisotropy to explain a supplied shift difference, but check inductive effects as well. A defensible answer identifies which effect is being compared instead of naming ‘ring current’ for every π-containing molecule.

Common mistake: electronegativity-only reasoning

Electron density alone cannot explain why an alkyne H is relatively upfield.

Add the direction of the induced π field at the nucleus.

All π systems do not shift every nearby proton downfield. The sign and size of the effect depend on the proton's position within the anisotropic field.

Read fixed maps

After the check questions, interpret one new π-field map without notes, then return later for a different π system.

Next: hydrogen bonding.

An exam-quality explanation names the π system, locates the proton in its shielding or deshielding region and links the induced field to the observed direction of shift.