Page 90 - Org 3 theoritical book 2024-25

Clinical Pharmacy PharmD - 2024/2025 Level 2 Pharmaceutical Organic Chemistry-3 (PC 305)

Magnetic anisotropic effect in Magnetic anisotropic effect in Alkynes
Benzene rings & Alkenes (C=C) (C≡C)

4. Hydrogen bonding (H-bonds) & Exchangeable Protons (OH, COOH, NH2, SH):

❖ This effect is related to polar H atoms (attached to O / N atoms) that 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 spectra, so they are called exchangeable protons.
❖ The hydrogen bonds are dynamic, constantly forming, breaking and forming again,
there will be a different electronic density around these polar H atoms and

consequently appear at a wide range of chemical shift.
❖ As concentration of sample  → H bonding between polar H atoms in sample  →
the chemical shift  (deshielded at higher ppm) (if appeared in spectra).
❖ So, sample must be pure and very well dried (moisture-free) in order to let

exchangeable protons appear in NMR spectra.

Diluted Sample Concentrated Sample
OH  0.5-1  4-5
NH  4-5 Up to  12
SH  3-4  6