Page 40 - Practical-organic-3

Factors affecting
chemical shift Special notes
Polar H atoms (attached to O / N atoms) are involved in
hydrogen bonding.
▪ These polar H atoms could be exchanged easily with any

impurities present in sample and it will not appear in NMR
5. Hydrogen bonding spectra, so they are called exchangeable protons.
1
1
(in case of H-NMR ▪ As concentration of compound  H bonding  & the H
become de-shielded (appear at lower field).
only)
Dil. Sol. Conc. Sol.

OH  0.5-1  4-5

NH  4-5 Up to  12

SH  3-4  6

❖ Coupling Constant (J) = (line 1 chemical shift – line 2 chemical shift) x frequency of
NMR machine.

System Coupling Constant (J) Type of protons

Ja,b (Cis) = 6-12 Hz.
Unsaturated systems Ja,c (Trans) = 12-20 Hz.
2
(sp )
J Trans > J Cis
Ja,b (ortho) = 7 Hz

Aromatic Ja,c (meta) = 1-4 Hz
Systems Ja,d (para) = 0-2 Hz (not occur in most cases)
J ortho > J meta > J para

• Plane of symmetry results in two doublets
each integrated for 2 protons.

a
b

Para Substituted
Benzene
(e.x: p-Chloro
aniline)

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