dichloroethanoic acid See
chloroethanoic acids.
dichloromethane (methylene chlo-
ride)A colourless, slightly toxic liq-
uid, CH 2 Cl 2 , b.p. 41°C. It has a
characteristic odour similar to that of
trichloromethane (chloroform), from
which it is made by heating with
zinc and hydrochloric acid. It is used
as a refrigerant and solvent (for paint
stripping and degreasing).
2,4-dichlorophenoxyacetic acid
See 2,4-d.
dichroism The property of some
crystals, such as tourmaline, of selec-
tively absorbing light vibrations in
one plane while allowing light vibra-
tions at right angles to this plane to
pass through. Polaroid is a synthetic
dichroic material. See polarization.
dichromate(VI) A salt containing
the ion Cr 2 O 7 –. Solutions containing
dichromate(VI) ions are strongly oxi-
dizing.
1,2-didehydrobenzene See ben-
zyne.
dielectric constant See permittiv-
ity.
Diels, Otto Paul Hermann
(1876–1954) German organic chemist
who worked mostly at the University
of Kiel. In 1906 he discovered tricar-
bon dioxide (C 3 O 2 ). Diels also did
important work on steroids but is re-
membered for his discovery in 1928
of the *Diels–Alder reaction, which
he made with his assistant Kurt Alder
(1902–58). Diels and Alder shared the
1950 Nobel Prize for chemistry.Diels–Alder reaction A type of
chemical reaction in which a com-
pound containing two double bonds
separated by a single bond (i.e. a con-
jugated *diene) adds to a suitable
compound containing one double
bond (known as the dienophile) to
give a ring compound. In the
dienophile, the double bond must
have a carbonyl group on each side.
It is named after the German
chemists Otto *Diels and Kurt Alder.dieneAn *alkene that has two dou-
ble bonds in its molecule. If the two
bonds are separated by one single
bond, as in buta-1,3-diene
CH 2 :CHCH:CH 2 , the compound is a
conjugated diene.dienophileSee diels–alder reac-
tion.Dieterici equationAn *equation
of state for a gas of the form
P(V – b)[exp (a/VRT)] = RT,
where P is the pressure, V is the vol-
ume, T is the thermodynamic tem-
perature, R is the gas constant, and a
and b are constants characteristic of
the gas. The Dieterici equation is a
modiÜcation of van der Waals’ equa-
tion, which takes account of the pres-
sure gradient at the boundary of the
gas. At low pressures the Dieterici
equation becomes identical to van
der Waals’ equation.175 Dieterici equation
d
CH 2
CH
CH
CH 2
CH
CH
COOH
COOH
CH
CH
C
H 2
C
H
C
C H
H 2
COOH
COOH
Diels-Alder reaction