Field effect (chemistry)

[1] This field, which is substituent and conformation dependent, can influence structure and reactivity by manipulating the location of electron density in bonds and/or the overall molecule.

[3] Field effects are relatively weak, and diminish rapidly with distance, but have still been found to alter molecular properties such as acidity.

[1] The directionality of a dipole, and concentration of charge, can both define the shape of a molecule's electric field which will manipulate the localization of electron density toward or away from sites of interest, such as an acidic hydrogen.

[4] The C-X dipole is oriented away from the carboxylic acid group, and can draw electron density away because the molecule center is empty, with a low dielectric constant, so the electric field is able to propagate with minimal resistance.

[8] These effects can therefore help to tune the acidity/basicity of a molecule to protonate/deprotonate a specific compound, or enhance hydrogen bond-donor ability for molecular recognition or anion sensing applications.

[16] When the chlorines are pointed over the carboxylic acid group, the pKa is higher because loss of a proton is less favorable due to the increase in negative charge in the area.

Field effect on a carbonyl arising from the formal charge on N in a quaternary ammonium salt.
Field effect on a carbonyl arising from the dipole in a C-F bond.
A bicycloheptane acid with an electron-withdrawing substituent, X, at the 4-position experiences a field effect on the acidic proton from the C-X bond dipole. [ 4 ]
A bicyclooctance acid with an electron-witituent, X, at the 4-position experiences the same field effect on the acidic proton from the C-X bondole as the related bicylcoheptane. [ 4 ]
This octane derivative has only a single linker between the electron-withdrawing substituent and the acidic group. [ 11 ]
This octane derivative has two linkers between the electron-withdrawing substituent and the acidic group. [ 11 ]
This cubane derivative has four linkers but the acidic proton still feels the same effect from the C-X dipole because the interaction is a field effect. [ 11 ]
The dichloroethano-bridged anthroic acid isomer with the C-Cl bond dipole oriented over the carboxylic acid has pKa of 6.07. [ 16 ]
The isomer of dichloroethano-bridged anthroic acid in which the C-Cl dipole points away from the carboxylic acid has a pKa of 5.67. [ 16 ]