1914), who discovered it indepen-
dently in the same year.
hallucinogen A drug or chemical
that causes alterations in perception
(usually visual), mood, and thought.
Common hallucinogenic drugs in-
clude *lysergic acid diethylamide
(LSD) and mescaline. There is no
common mechanism of action for
this class of compounds although
many hallucinogens are structurally
similar to neurotransmitters in the
central nervous system, such as sero-
tonin and the catecholamines.
halo A broad ring appearing in the
electron diffraction, neutron diffrac-
tion, or X-ray diffraction patterns of
materials that are not crystals. Haloes
of this type occur in gases and liquids
as well as in noncrystalline solids.
haloalkanes(alkyl halides)Organic
compounds in which one or more
hydrogen atoms of an alkane have
been substituted by halogen atoms.
Examples are chloromethane, CH 3 Cl,
dibromoethane, CH 2 BrCH 2 Br, etc.
Haloalkanes can be formed by direct
reaction between alkanes and halo-
gens using ultraviolet radiation. They
are usually made by reaction of an al-
cohol with a halogen carrier.
halocarbonsCompounds that con-
tain carbon and halogen atoms and
(sometimes) hydrogen. The simplest
are compounds such as tetra-
chloromethane (CCl 4 ), tetrabro-
momethane (CBr 4 ), etc. The
haloforms are also simple halocar-
bons. The chloroÛuorocarbons
(CFCs) contain carbon, chlorine, and
Ûuorine. Similar to these are hy-
drochloroÛuorocarbons (HCFCs),
which contain carbon, chlorine,
Ûuorine, and hydrogen, and the hy-
droÛuorocarbons (HFCs), which con-
tain carbon,Ûuorine, and hydrogen.
The *halons are a class of halocar-
bons that contain bromine.
haloform reaction A reaction for
producing *haloforms from methyl
ketones. An example is the produc-
tion of chloroform from propanone
using sodium chlorate(I) (or bleach-
ing powder):
CH 3 COCH 3 + 3NaOCl →CH 3 COCl 3
+ 3NaOH
The substituted ketone then reacts to
give chloroform (trichloromethane):
CH 3 COCCl 3 + NaOH →NaOCOCH 3
+ CHCl 3
The reaction can also be used for
making carboxylic acids, since
RCOCH 3 gives the product NaOCOR.
It is particularly useful for aromatic
acids as the starting ketone can be
made by a Friedel–Crafts acylation.
The reaction of methyl ketones
with sodium iodate(I) gives iodoform
(triiodomethane), which is a yellow
solid with a characteristic smell. This
reaction is used in the iodoform test
to identify methyl ketones. It also
gives a positive result with a sec-
ondary alcohol of the formula
RCH(OH)CH 3 (which isÜrst oxidized
to a methylketone) or with ethanol
(oxidized to ethanal, which also un-
dergoes the reaction).haloformsThe four compounds
with formula CHX 3 , where X is a
halogen atom. They are chloroform
(CHCl 3 ), and, by analogy, Ûuoroform
(CHF 3 ), bromoform (CHBr 3 ), and iod-
oform(CHI 3 ). The systematic names
are trichloromethane, triÛuoro-
methane, etc.
halogenating agentSee halo-
genation.halogenationA chemical reaction
in which a halogen atom is intro-
duced into a compound. Halogena-
tions are described as chlorination,
Ûuorination, bromination, etc., ac-
cording to the halogen involved.
Halogenation reactions may take
place by direct reaction with the261 halogenation
h