Systems containing 4n π-electrons (such as
cyclobutadiene and the cyclopentadienyl cation) are
antiaromatic.
See alsoMÖBIUS AROMATICITY.
humic acid An acid substance of indeterminate com-
position found in the organic materials in soils (humic
matter) and which is insoluble in acid, methyl ethyl
ketone, and methyl alcohol, but is soluble in alkali.
Hund’s rule All orbitals of a given sublevel must be
occupied by single electrons before doubling up begins.
hybridization Linear combination of ATOMIC
ORBITALs on an atom. Hybrid orbitals are often used in
organic chemistry to describe the bonding molecules
containing tetrahedral (sp^3 ), trigonal (sp^2 ), and digonal
(sp) atoms.
hydrate A solid substance that contains a specific
percentage of bound water in it.
hydration Addition of water or the elements of
water (i.e., H and OH) to a molecular entity. The term
is also used in a more restricted sense for the process: A
(gas) →A (aqueous solution). The term is also used in
inorganic and physical chemistry to describe the state
of ions of an aqueous electrolyte solution.
See alsoAQUATION; SOLVATION.
hydration energy Energy change that accompanies
the hydration of a mole of gases or ions.
hydride A binary compound of hydrogen.hydrocarbon Compound containing only carbon
and hydrogen.hydrogenase An ENZYME, dihydrogen:acceptor OXI-
DOREDUCTASE, that catalyzes the formation or oxida-
tion of H 2. Hydrogenases are of various types. One
class ([Fe]-hydrogenases) contains only IRON-SULFUR
CLUSTERs. The other major class ([NiFe]-hydrogenases)
has a nickel-containing center and iron-sulfur clusters;
a variation of the latter type ([NiFeSe]-hydrogenases)
contains selenocysteine.hydrogenation A reaction where hydrogen is added
across a double or triple bond, usually with the assis-
tance of a catalyst.hydrogen bond A form of ASSOCIATIONbetween an
electronegative atom and a hydrogen atom attached to
a second, relatively electronegative atom. It is best con-
sidered to be an electrostatic interaction, heightened by
the small size of hydrogen, which permits the proximity
of the interacting dipoles or charges. Both electronega-
tive atoms are usually (but not necessarily) from the
first row of the periodic table (i.e., N, O, or F). Hydro-
gen bonds may be INTERMOLECULARor INTRAMOLECU-132 humic acid
Hydration. Addition of water or the elements of water (i.e., H and
OH) to a molecular entity