Encyclopedia of Chemistry

attached are excluded. Polar effect is then not synony-
mous with electronic effect.
See also FIELD EFFECT; INDUCTIVE EFFECT;
MESOMERIC EFFECT.

polarimeter A device used to measure optical activ-
ity; determines the amount of polarization of light.

polarity When applied to solvents, this rather ill-
defined term covers their overall SOLVATIONcapability
(solvation power) for solutes (i.e., in chemical equilib-
ria: reactants and products; in reaction rates: reactants
and ACTIVATED COMPLEX; in light absorptions: ions or
molecules in the GROUNDand EXCITED STATE), which in
turn depends on the action of all possible, nonspecific
and specific, intermolecular interactions between solute
ions or molecules and solvent molecules, excluding
such interactions leading to definite chemical alter-
ations of the ions or molecules of the solute. Occasion-
ally, the term solvent polarity is restricted to
nonspecific solute/solvent interactions only (i.e., to VA N
DERWAALS FORCES).
See also DIMROTH-REICHARDT ET PARAMETER;
GRUNWALD-WINSTEIN EQUATION; IONIZING POWER;
KAMLET-TAFT SOLVENT PARAMETERS; Z-VALUE.

polarizability The ease of distortion of the electron
cloud of a MOLECULAR ENTITYby an electric field (such
as that due to the proximity of a charged reagent). It is
experimentally measured as the ratio of INDUCED
DIPOLE MOMENT(μind) to the field E that induces it:

α= μind/E

The units of αare C^2 m^2 V–1. In ordinary usage, the
term refers to the “mean polarizability,” the average
over three rectilinear axes of the molecule. Polarizabili-
ties in different directions (e.g., along the bond in Cl 2 ,
called “longitudinal polarizability,” and in the direction
perpendicular to the bond, called “transverse polariz-
ability”) can be distinguished, at least in principle.
Polarizability along the bond joining a substituent to
the rest of the molecule is seen in certain modern theo-
retical approaches as a factor influencing chemical
reactivity, etc., and parameterization thereof has been
proposed.

polar molecule A molecule that has both a positive

and negative end, such as water.

polar solvent SeePOLARITY.

pollen Microscopic grains produced by plants to

facilitate reproduction. Each plant has a pollinating

period that varies, depending on the plant, climate, and

region.

polychlorinated biphenyl(PCB) Polychlorinated

biphenyls (PCBs) are a mixture of individual chemicals

that are no longer produced in the United States but

are still found in the environment. They are made up of

two benzene rings attached by a C–C bond, with vary-

ing numbers of hydrogens replaced by chlorine. Health

effects that have been associated with exposure to PCBs

include acne-like skin conditions in adults and neu-

robehavioral and immunological changes in children,

and PCBs are known to cause cancer in animals. PCBs

have been found in at least 500 of the 1,598 National

Priorities List sites identified by the Environmental Pro-

tection Agency (EPA).

Polychlorinated biphenyls are mixtures of up to

209 individual chlorinated compounds (known as CON-

GENERs). There are no known natural sources of PCBs,

which are oily liquids or solids that are colorless to

216 polarimeter

Polar molecule. A molecule that has both a positive and negative

end, such as water