Encyclopedia of Chemistry

radioactive decay process, as the time required for the
activity to decrease to half its value by that process.
See alsoBIOLOGICAL HALF-LIFE; GEOLOGICAL TIME.

half-reaction Refers to one-half state of the redox
reaction, either the oxidation part (loss of electrons) or
the reduction part (uptake of electrons).

Hall process The process invented in 1886 by
Charles Martin Hall wherein he produced globules of
aluminum metal by the electrolysis of aluminum oxide
dissolved in a cryolite-aluminum fluoride mixture.
Today, alumina is dissolved in an electrolytic bath of
molten cryolite (sodium aluminum fluoride) within a
large carbon- or graphite-lined steel container (a “pot”).
An electric current is passed through the electrolyte at
low voltage but very high current, typically 150,000
amperes, and the electric current flows between a car-
bon anode (positive), made of petroleum coke and
pitch, and a cathode (negative), formed by the thick car-
bon or graphite lining of the pot. Molten aluminum is
deposited at the bottom of the pot and is siphoned off.

halocarbon One of the various compounds of car-
bon combined with any of the halogens; e.g., fluorocar-
bon has had some of the hydrogen replaced by
fluorine.

halochromism Halochromism refers to the color
change that occurs upon addition of an ACID(or BASE
or a salt) to a solution of a compound. A chemical
reaction (e.g., ion formation) transforms a colorless
compound into a colored one.

halogens Group VII elements are five nonmetallic
elements: fluorine, chlorine, bromine, iodine, and asta-
tine. Halogen means “salt-former,” and their com-
pounds are called salts. (They are labeled Group 17 in
the newer labeling convention of the PERIODIC TABLE.)

haloperoxidase A PEROXIDASE that catalyzes the
oxidative transformation of halides to XO–(X being

Cl, Br, or I) or to organic halogen compounds. Most

are HEME proteins, but some bromoperoxidases from

algae are vanadium-containing ENZYMEs.

Hammett acidity function SeeACIDITY FUNCTION.

Hammett equation(Hammett relation) The equa-

tion in the form

lg(k/ko) = ρσ

or

lg(K/Ko) = ρσ

applied to the influence of meta- or parasubstituents X

on the reactivity of the functional group Y in the ben-

zene derivative m- or p-XC 6 H 4 Y. K and k are the equi-

librium and rate constant, respectively, for the given

reaction of m- or p-XC 6 H 4 Y; koor Korefers to the reac-

tion of C 6 H 5 Y, i.e., X = H; σis the substituent constant

characteristic of m- or p-X; ρis the reaction constant

characteristic of the given reaction of Y. The equation is

often encountered in a form with lg koor lg Kowritten

as a separate term on the right-hand side, e.g.,

lg k = ρσ+ lg ko

or

lg K = ρσ+ lg Ko

It then signifies the intercept corresponding to X = H in

a regression of lg k or lg K on σ.

See also RHO (ρ) VALUE; SIGMA (σ) CONSTANT;

TAFT EQUATION; YUKAWA-TSUNO EQUATION.

Hammond principle(Hammond postulate) The

hypothesis that, when a TRANSITION STATEleading to

an unstable REACTIONintermediate (or product) has

nearly the same energy as that INTERMEDIATE, the two

are interconverted with only a small reorganization of

molecular structure. Essentially the same idea is some-

times referred to as “Leffler’s assumption,” namely,

that the transition state bears the greater resemblance

to the less stable species (reactant or reaction interme-

diate/product). Many textbooks and physical organic

chemists, however, express the idea in Leffler’s form

but attribute it to Hammond.

Hammond principle 123