Dictionary of Chemistry [6th Ed.]

by this process. The metamictization
is caused by alpha-particles and the
recoil nuclei from radioactive disinte-
gration.

metaphosphoric acid See phos-
phoric(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 equilib-
rium 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 meta-
stable 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.

metathesisA type of reaction in
which radicals are exchanged. In in-
organic chemistry, it is also called
double decomposition. A simple ex-
ample is

KCL + AgNO 3 →KNO 3 + AgCl.

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 intermedi-
ate is a four-membered ring contain-
ing 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.

methacrylateA salt or ester of

methacrylic acid (2-methylpropenoic

acid).

methacrylate resins *Acrylic

resins obtained by polymerizing 2-

methylpropenoic acid or its esters.

methacrylic acidSee 2-methyl-

propenoic acid.

methadoneA synthetic opioid,

C 21 H 27 NO, used medically as an anal-

gesic for chronic pains and also as a

substitute for heroin in the treat-

ment of addiction. Methadone is it-

self addictive and considerable

quantities of ‘street’ methadone are

used in the UK.

351 methanal trimer

m

CH 2

CH

3 CH

O

C

H 2

3 CH

N CH^3

3 CH

Methadone

methamphetamineSee ampheta-

mines.

methanal(formaldehyde)A colour-

less gas, HCHO; r.d. 0.815 (at –20°C);

m.p. –92°C; b.p. –21°C. It is the sim-

plest *aldehyde, made by the cat-

alytic oxidation of methanol (500°C;

silver catalyst) by air. It forms two

polymers: *methanal trimer and

polymethanal. See also formalin.

methanal trimerA cyclic trimer

of methanal, C 3 O 3 H 6 , obtained by dis-

tillation of an acidic solution of

methanal. It has a six-membered ring

of alternating –O– and –CH 2 – groups.