Encyclopedia of Chemistry

dimerization The TRANSFORMATIONof a MOLECULAR
ENTITYA to give a molecular entity A 2. For example:

CH 3 .+ H 3 C.→CH 3 CH 3

2 CH 3 COCH 3 →(CH 3 ) 2 C(OH)CH 2 COCH 3

2 RCOOH →(RCOOH) 2

See alsoASSOCIATION.

Dimroth-Reichardt ET parameter A measure of
the IONIZING POWER(loosely POLARITY) of a solvent,
based on the maximum wave number of the longest
wavelength electronic absorption band of

in a given solvent. ET, called ET(30) by its originators,
is given by

ET= 2.859 × 10 –3ν= 2.859 × 104 λ–1

where ETis in kcal/mol, νis in cm–1, and λis in nm.
The so-called normalized ETNscale is defined as
ETN= [ET(solvent) – ET(SiMe 4 )]/[ET(water) –
ET(SiMe 4 )] = [ET(solvent) – 30.7]/32.4
See alsoGRUNWALD-WINSTEIN EQUATION; Z-VALUE.

dinuclear SeeNUCLEARITY.

dioxygenase An ENZYME that catalyzes the INSER-
TIONof two oxygen atoms into a SUBSTRATE, both oxy-
gens being derived from O 2.

dipolar aprotic solvent A solvent with a compara-
tively high relative permittivity (or DIELECTRIC CON-
STANT), greater than ca. 15, and a sizable permanent
dipole moment that cannot donate suitably labile
hydrogen atoms to form strong hydrogen bonds, e.g.,
dimethyl sulfoxide. The term (and its alternative, polar

aprotic solvent) is a misnomer and is therefore discour-

aged. Such solvents are usually not APROTIC, but PRO-

TOPHILIC (and at most weakly PROTOGENIC). In

describing a solvent, it is better to be explicit about its

essential properties, e.g., dipolar and nonprotogenic.

dipolar bond A BONDformed (actually or conceptu-

ally) by COORDINATIONof two neutral moieties, the

combination of which results in charge-separated struc-

tures, e.g.,

R 3 N: + O →R 3 N+–O–

The term is preferred to the obsolescent synonyms

coordinate link, coordinate covalence, dative bond,and

semipolar bond.

dipolar cycloaddition SeeCYCLOADDITION.

dipole The separation of charge between two cova-

lently bonded atoms; a pair of separated opposite elec-

tric charges.

dipole–dipole interaction Intermolecular or intra-

molecular interaction between molecules or groups

having a permanent electric DIPOLE MOMENT. The

strength of the interaction depends on the distance and

relative orientation of the dipoles. The term applies

also to intramolecular interactions between bonds hav-

ing permanent dipole moments.

See alsoVAN DERWAALS FORCES.

dipole-induced dipole forces SeeVAN DERWAALS

FORCES.

dipole moment The product of the positive charge

and the distance between the charges; a measure of the

POLARITYof a bond or molecule.

diprotic acid An acid that can furnish two H+per

molecule; an acid having two dissociable protons.

Examples are sulfuric acid (H 2 SO 4 ) and carbonic acid

(H 2 CO 3 ).

diprotic acid 77