8.4: Inductive Effects on Chemical Shift: Overview
The protons in unsubstituted alkanes are strongly shielded with chemical shifts below 1.8 ppm. Methine, methylene, and methyl protons appear at approximately 1.7, 1.2 and 0.7 ppm, while the proton signal from methane appears at 0.23 ppm. An electronegative substituent, such as chlorine, withdraws the electron density from the protons, increasing their chemical shift. Progressive substitution of the hydrogens in methane by chlorine shifts the proton signals increasingly downfield, to 3.05 ppm in CH3Cl, 5.30 ppm in CH2Cl2, and 7.27 ppm in CHCl3. The chemical shift also increases with the electronegativity of the substituent, as seen in the halomethanes. The inductive influence of an electronegative substituent is strongest at the alpha position and decreases with distance, becoming almost negligible at the gamma position.
In addition to inductive effects, other factors such as hybridization, delocalization, and hydrogen bonding also alter the electron density around protons. The greater s-character of sp2 hybridized carbons makes them more electronegative in comparison to sp3 hybridized carbons. Consequently, sp2 hybridized carbons pull the bonding electrons toward them, deshielding the attached vinylic hydrogens. As a result, the protons in ethene resonate at 5.28 ppm, compared to 0.86 ppm for ethane. In vinyl ether, the delocalization of electron density due to resonance increases the shielding at the beta-hydrogen, lowering the observed chemical shift from 5.28 ppm in ethene to 4.21 ppm.
Protons involved in hydrogen bonding are deshielded and exhibit a range of values. The extent of deshielding increases with the extent of hydrogen bonding. Therefore, factors that affect hydrogen bonding, such as concentration and temperature, also affect the chemical shifts of such protons.
