Dictionary of Chemistry [6th Ed.]

diketones are pungent-smelling yel-
low oils, whereas the aromatic com-
pounds are crystalline solids. 1,3-
Diketones (or β-diketones), R.CO.CH 2.
CO.R′, are more acidic and exist in
both keto and enol forms (see keto–
enol tautomerism); they form stable
compounds with metals. 1,4-Dike-
tones (or γ-diketones), R.CO.CH 2 .CH 2.
CO.R′, also exist and readily re-
arrange to form cyclic compounds.

dilatancySee newtonian fluid.

dilation(dilatation)An increase in
volume.

dilead(II) lead(IV) oxideA red
powder, Pb 3 O 4 ; r.d. 9.1; decomposes
at 500°C to lead(II) oxide. It is pre-
pared by heating lead(II) oxide to
400 °C and has the unusual property
of being black when hot and red-
orange when cold. The compound is
nonstoichiometric, generally contain-
ing less oxygen than implied by the
formula. It is largely covalent and has
Pb(IV)O 6 octahedral groups linked to-
gether by Pb(II) atoms, each joined to
three oxygen atoms. It is used in
glass making but its use in the paint
industry has largely been discontin-
ued because of the toxicity of lead.
Dilead(II) lead(IV) oxide is commonly
called red lead or, more accurately,
red lead oxide.

Dillie–Koppanyi testA presump-
tive test for barbituates. The Dillie–
Koppanyi test reagent has two solu-
tions: a 1% solution of colbalt acetate
in methanol, follow by a 5% solution
of isopropylamine (CH 3 CH(CH 3 )NH 2 )
in methanol. Barbiturates give a red-
dish-violet colour.

diluentA substance added to dilute
a solution or mixture (e.g. a *Üller).

diluteDescribing a solution that
has a relatively low concentration of
solute.

dilute spin species A type of nu-

cleus in which it is statistically un-

likely that there will be more than

one such nucleus in a molecule (un-

less the substance has been artiÜ-

cially enriched with isotopes having

that nucleus). An example of a dilute

spin species is^13 C since it has a nat-

ural abundance of 1.1%. This means

that for dilute spin systems it is usu-

ally not necessary to consider spin–

spin interactions between two nuclei

of the dilute spin species, e.g.^13 C–^13 C

in the same molecule. The opposite

of a dilute spin species is an abun-

dant spin species, an example being

the proton.

dilutionThe volume of solvent in

which a given amount of solute is

dissolved.

dilution lawSee ostwald’s dilu-

tion law.

dimerAn association of two identi-

cal molecules linked together. The

molecules may react to form a larger

molecule, as in the formation of dini-

trogen tetroxide (N 2 O 4 ) from nitro-

gen dioxide (NO 2 ), or the formation

of an *aluminium chloride dimer

(Al 2 Cl 6 ) in the vapour. Alternatively,

they may be held by hydrogen bonds.

For example, carboxylic acids form

dimers in organic solvents, in which

hydrogen bonds exist between the O

of the C=O group and the H of the

–O–H group.

p-dimethylaminobenzaldehyde

177 p-dimethylaminobenzaldehyde

d

O H

N

3 CH CH 3

p-dimethylaminobenzaldehyde