Dictionary of Chemistry [6th Ed.]

insecticides, and wood preservatives,
and in making dyes. They form con-
densation polymers of the bakelite
type with methanal.

chlorophyll One of a number of
pigments, including (chlorophyll a
and chlorophyll b), that are responsi-
ble for the green colour of most
plants. Chlorophyll molecules are the
principal sites of light absorption in
the light-dependent reactions of
photosynthesis. They are magne-
sium-containing porphyrins, chemi-
cally related to cytochrome and
haemoglobin.

123 chromatogram

c

CHCH 2 CH 3

C 2 H 5

CHCHCH

CH 3

CHCHCH CO

CCC

COOCHCOOCHCOOCH 333

C 20 H 39 O

CCCO

CHCHCH 222

CHCHCH 222

CH 3

H

H NNN NNN

MgMgMg

NNN NNN

CHCHCH

CH 3

CHCHCH

hydrocarbon

(phytol) chain

Chlorophyll

chloroplatinic acid A reddish crys-
talline compound, H 2 PtCl 6 , made by
dissolving platinum in aqua regia.

chloroprene See 2-chlorobuta-1,3-
diene.

chlorosulphanes See disulphur
dichloride.

chlorous acid See chloric(iii) acid.

choke dampSee blackdamp.

cholecalciferol See vitamin d.

cholesteric crystalA type of *liq-

uid crystal in which molecules lie in

sheets at angles that change by a

small amount between each sheet.

The pitch of the helix so formed is

similar to the wavelength of visible

light. Cholesteric liquid crystals

therefore diffract light and have

colours that depend on the tempera-

ture. (The word cholesteric is derived

from the Greek for bile solid.)

cholesterol A *sterol occurring

widely in animal tissues and also in

some higher plants and algae. It can

exist as a free sterol or esteriÜed with

a long-chain fatty acid. Cholesterol is

absorbed through the intestine or

manufactured in the liver. It serves

principally as a constituent of blood

plasma lipoproteins and of the

lipid–protein complexes that form

cell membranes. It is also important

as a precursor of various steroids, es-

pecially the bile acids, sex hormones,

and adrenocorticoid hormones. The

derivative 7-dehydrocholesterol is

converted to vitamin D 3 by the action

of sunlight on skin. Increased levels

of dietary and blood cholesterol have

been associated with atherosclerosis,

a condition in which lipids accumu-

late on the inner walls of arteries and

eventually obstruct bloodÛow.

cholineAn amino alcohol,

CH 2 OHCH 2 N(CH 3 ) 3 OH. It occurs

widely in living organisms as a

constituent of certain types of

phospholipids – the lecithins and

sphingomyelins – and in the neuro-

transmitter *acetylcholine. It is

sometimes classiÜed as a member

of the vitamin B complex.

chromate A salt containing the ion

CrO 4 2–.

chromatogram A record obtained

by *chromatography. The term is ap-

plied to the developed records of

*paper chromatography and *thin-

layer chromatography and also to the