Page 39 - Practical-organic-3

❖ Chemical shift ():
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I. It is a scale measured by ppm (0-12 in H-NMR & 0-200 in C-NMR).
II. It indicates how much the proton affected by the external magnetic field.

Factors affecting
chemical shift Special notes
The presence of electronegative atom (X) as:
F > O > N > Cl > Br > C > H.
▪ Inductive effect  → the chemical shift .

1. Inductive effect ▪ Distance → the inductive effect  → milder effect on the
chemical shift.
▪ Number of electronegative substituents → Inductive effect 
→ the chemical shift.

2. Mesomeric effect The presence of double bond (SP2 carbon) (CH=CH) or triple bond

(- multiple Bond) (SP3 carbon) (-C≡C-) has a de-shielding effect.

3. Hybridization S character → de-shielding  and H appear at lower field.

Effect SP2 < SP < SP3 (exception of SP due to anisotropic effect).

In systems containing  electrons they will generate a magnetic

4. Magnetic field which may add to or oppose the applied magnetic field

anisotropy causing a de-shielding or shielding effect, respectively.

In case of (-C≡C-) it causes shielding effect

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