by this process. The metamictization
is caused by alpha-particles and the
recoil nuclei from radioactive disintegration.
metaphosphoric acid See phosphoric(v) acid.
metaplumbate See plumbate.
metastable state A condition of a
system in which it has a precarious
stability that can easily be disturbed.
It is unlike a state of stable equilibrium in that a minor disturbance will
cause a system in a metastable state
to fall to a lower energy level. A book
lying on a table is in a state of stable
equilibrium; a thin book standing on
edge is in metastable equilibrium.
Supercooled water is also in a metastable state. It is liquid below 0°C; a
grain of dust or ice introduced into it
will cause it to freeze. An excited
state of an atom or nucleus that has
an appreciable lifetime is also
metastable.
metastannate See stannate.
metathesis A type of reaction in
which radicals are exchanged. In inorganic chemistry, it is also called
double decomposition. A simple example is
KCL + A g NO 3 → KNO 3 + A g Cl.
Metathesis of alkenes is an important
type of reaction in synthetic organic
chemistry. It involves exchange of
groups. For example
RHC=CH 2 + RHC=CH 2 → RHC=CHR
+ H 2 C=CH 2 .
Reactions of this type are catalysed
by metal alkylides (containing an
M=CR 2 grouping) and the intermediate is a four-membered ring containing the metal ion. The catalysts most
often used are the Schrock catalysts
based on molybdenum and the
Grubbs catalysts based on ruthenium.
The American chemists Richard
Schrock and Robert Grubbs shared
the Nobel prize for chemistry in 2005
for work in this field.
methacrylate A salt or ester of
methacrylic acid (2-methylpropenoic
acid).
methacrylate resins *Acrylic
resins obtained by polymerizing 2methylpropenoic acid or its esters.
methacrylic acid See 2-methylpropenoic acid.
methadone A synthetic opioid,
C 21 H 27 NO, used medically as an analgesic for chronic pains and also as a
substitute for heroin in the treatment of addiction. Methadone is itself addictive and considerable
quantities of ‘street’ methadone are
used in the UK.
351
methanal trimer
m
CH 2
CH
C
H 3
O
C
H 2
C
H 3
N
CH 3
C
H 3
Methadone
methamphetamine See amphetamines.
methanal (formaldehyde) A colourless gas, HCHO; r.d. 0.815 (at –20°C);
m.p. –92°C; b.p. –21°C. It is the simplest *aldehyde, made by the catalytic oxidation of methanol (500°C;
silver catalyst) by air. It forms two
polymers: *methanal trimer and
polymethanal. See also formalin.
methanal trimer A cyclic trimer
of methanal, C 3 O 3 H 6 , obtained by distillation of an acidic solution of
methanal. It has a six-membered ring
of alternating –O– and –CH 2 – groups.
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is caused by alpha-particles and the
recoil nuclei from radioactive disintegration.
metaphosphoric acid See phosphoric(v) acid.
metaplumbate See plumbate.
metastable state A condition of a
system in which it has a precarious
stability that can easily be disturbed.
It is unlike a state of stable equilibrium in that a minor disturbance will
cause a system in a metastable state
to fall to a lower energy level. A book
lying on a table is in a state of stable
equilibrium; a thin book standing on
edge is in metastable equilibrium.
Supercooled water is also in a metastable state. It is liquid below 0°C; a
grain of dust or ice introduced into it
will cause it to freeze. An excited
state of an atom or nucleus that has
an appreciable lifetime is also
metastable.
metastannate See stannate.
metathesis A type of reaction in
which radicals are exchanged. In inorganic chemistry, it is also called
double decomposition. A simple example is
KCL + A g NO 3 → KNO 3 + A g Cl.
Metathesis of alkenes is an important
type of reaction in synthetic organic
chemistry. It involves exchange of
groups. For example
RHC=CH 2 + RHC=CH 2 → RHC=CHR
+ H 2 C=CH 2 .
Reactions of this type are catalysed
by metal alkylides (containing an
M=CR 2 grouping) and the intermediate is a four-membered ring containing the metal ion. The catalysts most
often used are the Schrock catalysts
based on molybdenum and the
Grubbs catalysts based on ruthenium.
The American chemists Richard
Schrock and Robert Grubbs shared
the Nobel prize for chemistry in 2005
for work in this field.
methacrylate A salt or ester of
methacrylic acid (2-methylpropenoic
acid).
methacrylate resins *Acrylic
resins obtained by polymerizing 2methylpropenoic acid or its esters.
methacrylic acid See 2-methylpropenoic acid.
methadone A synthetic opioid,
C 21 H 27 NO, used medically as an analgesic for chronic pains and also as a
substitute for heroin in the treatment of addiction. Methadone is itself addictive and considerable
quantities of ‘street’ methadone are
used in the UK.
351
methanal trimer
m
CH 2
CH
C
H 3
O
C
H 2
C
H 3
N
CH 3
C
H 3
Methadone
methamphetamine See amphetamines.
methanal (formaldehyde) A colourless gas, HCHO; r.d. 0.815 (at –20°C);
m.p. –92°C; b.p. –21°C. It is the simplest *aldehyde, made by the catalytic oxidation of methanol (500°C;
silver catalyst) by air. It forms two
polymers: *methanal trimer and
polymethanal. See also formalin.
methanal trimer A cyclic trimer
of methanal, C 3 O 3 H 6 , obtained by distillation of an acidic solution of
methanal. It has a six-membered ring
of alternating –O– and –CH 2 – groups.
www.AzShimi.ir www.AzShimi.com