frequencies. The frequencies of most
interest range from 2.5 to 16 µm;
however, in IR spectroscopy it is
common to use the reciprocal of the
wavelength, and thus this range becomes 4000–625 cm
–1 . Examples of
typical vibrations are centred on
2900 cm
–1 for C–H stretching in alkanes, 1600 cm
–1 for N–H stretching in
amino groups, and 2200 cm
–1 for
C≡C stretching in alkynes. In an IR
spectrometer there is a source of IR
light, covering the whole frequency
range of the instrument, which is
split into two beams of equal intensity. One beam is passed through the
sample and the other is used as a reference against which the Ürst is then
compared. The spectrum is usually
obtained as a chart showing absorption peaks, plotted against wavelength or frequency. The sample can
be a gas, liquid, or solid. See also
fourier-transform infrared.
Ingold, Sir Christopher Kelk
(1893–1970) British organic chemist
who worked chieÛy in London. His
main work was on reaction mechanisms and, particularly, on *electronic effects in physical organic
chemistry.
inhibition A reduction in the rate
of a catalysed reaction by substances
called inhibitors. Inhibitors may work
by poisoning catalysts for the reaction or by removing free radicals in a
chain reaction. Enzyme inhibition affects biochemical reactions, in which
the catalysts are *enzymes. Competitive inhibition occurs when the inhibitor molecules resemble the
substrate molecules and bind to the
*active site of the enzyme, so preventing normal enzymatic activity. Competitive inhibition can
be reversed by increasing the concentration of the substrate. In noncompetitive inhibition the inhibitor
binds to a part of the enzyme or *enzyme–substrate complex other than
the active site, known as an allosteric
site. This deforms the active site so
that the enzyme cannot catalyse the
reaction. Noncompetitive inhibition
cannot be reversed by increasing the
concentration of the substrate. The
toxic effects of many substances are
produced in this way. Inhibition by
reaction products (feedback inhibition) is important in the control of
enzyme activity. See also allosteric
enzyme.
inner Describing a chemical compound formed by reaction of one
part of a molecule with another part
of the same molecule. Thus, a lactam
is an inner amide; a lactone is an
inner ester.
inner-sphere mechanism See
electron-transfer reaction.
inner transition series See transition elements.
inorganic chemistry The branch
of chemistry concerned with compounds of elements other than
carbon. Certain simple carbon
compounds, such as CO, CO 2 , CS 2 ,
and carbonates and cyanides, are
usually treated in inorganic chemistry.
insecticide See pesticide.
insertion reaction A type of
chemical reaction in which an atom
or group is inserted between two
atoms in a molecule. A common example is the insertion of carbene:
R 3 C–X + CH 2 : → R 3 CCH 2 X
The opposite of an insertion reaction
is an extrusion reaction.
insulin A protein hormone, secreted by the β cells of the islets of
Langerhans in the pancreas, that promotes the uptake of glucose by body
cells, particularly in the liver and
muscles, and thereby controls its
285
insulin
i
www.AzShimi.ir www.AzShimi.com
interest range from 2.5 to 16 µm;
however, in IR spectroscopy it is
common to use the reciprocal of the
wavelength, and thus this range becomes 4000–625 cm
–1 . Examples of
typical vibrations are centred on
2900 cm
–1 for C–H stretching in alkanes, 1600 cm
–1 for N–H stretching in
amino groups, and 2200 cm
–1 for
C≡C stretching in alkynes. In an IR
spectrometer there is a source of IR
light, covering the whole frequency
range of the instrument, which is
split into two beams of equal intensity. One beam is passed through the
sample and the other is used as a reference against which the Ürst is then
compared. The spectrum is usually
obtained as a chart showing absorption peaks, plotted against wavelength or frequency. The sample can
be a gas, liquid, or solid. See also
fourier-transform infrared.
Ingold, Sir Christopher Kelk
(1893–1970) British organic chemist
who worked chieÛy in London. His
main work was on reaction mechanisms and, particularly, on *electronic effects in physical organic
chemistry.
inhibition A reduction in the rate
of a catalysed reaction by substances
called inhibitors. Inhibitors may work
by poisoning catalysts for the reaction or by removing free radicals in a
chain reaction. Enzyme inhibition affects biochemical reactions, in which
the catalysts are *enzymes. Competitive inhibition occurs when the inhibitor molecules resemble the
substrate molecules and bind to the
*active site of the enzyme, so preventing normal enzymatic activity. Competitive inhibition can
be reversed by increasing the concentration of the substrate. In noncompetitive inhibition the inhibitor
binds to a part of the enzyme or *enzyme–substrate complex other than
the active site, known as an allosteric
site. This deforms the active site so
that the enzyme cannot catalyse the
reaction. Noncompetitive inhibition
cannot be reversed by increasing the
concentration of the substrate. The
toxic effects of many substances are
produced in this way. Inhibition by
reaction products (feedback inhibition) is important in the control of
enzyme activity. See also allosteric
enzyme.
inner Describing a chemical compound formed by reaction of one
part of a molecule with another part
of the same molecule. Thus, a lactam
is an inner amide; a lactone is an
inner ester.
inner-sphere mechanism See
electron-transfer reaction.
inner transition series See transition elements.
inorganic chemistry The branch
of chemistry concerned with compounds of elements other than
carbon. Certain simple carbon
compounds, such as CO, CO 2 , CS 2 ,
and carbonates and cyanides, are
usually treated in inorganic chemistry.
insecticide See pesticide.
insertion reaction A type of
chemical reaction in which an atom
or group is inserted between two
atoms in a molecule. A common example is the insertion of carbene:
R 3 C–X + CH 2 : → R 3 CCH 2 X
The opposite of an insertion reaction
is an extrusion reaction.
insulin A protein hormone, secreted by the β cells of the islets of
Langerhans in the pancreas, that promotes the uptake of glucose by body
cells, particularly in the liver and
muscles, and thereby controls its
285
insulin
i
www.AzShimi.ir www.AzShimi.com