1914), who discovered it independently in the same year.
hallucinogen A drug or chemical
that causes alterations in perception
(usually visual), mood, and thought.
Common hallucinogenic drugs include *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 serotonin and the catecholamines.
halo A broad ring appearing in the
electron diffraction, neutron diffraction, 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 halogens using ultraviolet radiation. They
are usually made by reaction of an alcohol with a halogen carrier.
halocarbons Compounds that contain carbon and halogen atoms and
(sometimes) hydrogen. The simplest
are compounds such as tetrachloromethane (CCl 4 ), tetrabromomethane (CBr 4 ), etc. The
*haloforms are also simple halocarbons. The *chloroÛuorocarbons
(CFCs) contain carbon, chlorine, and
Ûuorine. Similar to these are hydrochloroÛuorocarbons (HCFCs),
which contain carbon, chlorine,
Ûuorine, and hydrogen, and the hydroÛuorocarbons (HFCs), which contain carbon, Ûuorine, and hydrogen.
The *halons are a class of halocarbons that contain bromine.
haloform reaction A reaction for
producing *haloforms from methyl
ketones. An example is the production of chloroform from propanone
using sodium chlorate(I) (or bleaching 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 secondary alcohol of the formula
RCH(OH)CH 3 (which is Ürst oxidized
to a methylketone) or with ethanol
(oxidized to ethanal, which also undergoes the reaction).
haloforms The 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 iodoform (CHI 3 ). The systematic names
are trichloromethane, triÛuoromethane, etc.
halogenating agent See halogenation.
halogenation A chemical reaction
in which a halogen atom is introduced into a compound. Halogenations are described as chlorination,
Ûuorination, bromination, etc., according to the halogen involved.
Halogenation reactions may take
place by direct reaction with the
261
halogenation
h
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hallucinogen A drug or chemical
that causes alterations in perception
(usually visual), mood, and thought.
Common hallucinogenic drugs include *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 serotonin and the catecholamines.
halo A broad ring appearing in the
electron diffraction, neutron diffraction, 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 halogens using ultraviolet radiation. They
are usually made by reaction of an alcohol with a halogen carrier.
halocarbons Compounds that contain carbon and halogen atoms and
(sometimes) hydrogen. The simplest
are compounds such as tetrachloromethane (CCl 4 ), tetrabromomethane (CBr 4 ), etc. The
*haloforms are also simple halocarbons. The *chloroÛuorocarbons
(CFCs) contain carbon, chlorine, and
Ûuorine. Similar to these are hydrochloroÛuorocarbons (HCFCs),
which contain carbon, chlorine,
Ûuorine, and hydrogen, and the hydroÛuorocarbons (HFCs), which contain carbon, Ûuorine, and hydrogen.
The *halons are a class of halocarbons that contain bromine.
haloform reaction A reaction for
producing *haloforms from methyl
ketones. An example is the production of chloroform from propanone
using sodium chlorate(I) (or bleaching 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 secondary alcohol of the formula
RCH(OH)CH 3 (which is Ürst oxidized
to a methylketone) or with ethanol
(oxidized to ethanal, which also undergoes the reaction).
haloforms The 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 iodoform (CHI 3 ). The systematic names
are trichloromethane, triÛuoromethane, etc.
halogenating agent See halogenation.
halogenation A chemical reaction
in which a halogen atom is introduced into a compound. Halogenations are described as chlorination,
Ûuorination, bromination, etc., according to the halogen involved.
Halogenation reactions may take
place by direct reaction with the
261
halogenation
h
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