Dictionary of Chemistry [6th Ed.]

i.e. the rings are of carbon atoms. If

different atoms occur in the ring, as

in pyridine (C 5 H 5 N), the compound is

said to be heterocyclic.

cyclic AMPA derivative of *ATP

that is widespread in animal cells as

a second messenger in many bio-

chemical reactions induced by hor-

mones. Upon reaching their target

cells, the hormones activate adeny-

late cyclase, the enzyme that cataly-

ses cyclic AMP production. Cyclic

AMP ultimately activates the en-

zymes of the reaction induced by the

hormone concerned. Cyclic AMP is

also involved in controlling gene ex-

pression and cell division, in im-

mune responses, and in nervous

transmission.

cyclizationThe formation of a

cyclic compound from an open-chain

compound. See ring.

cyclo-PreÜx designating a cyclic

compound, e.g. a cycloalkane or a

cyclosilicate.

cycloadditionA reaction in which

two or more unsaturated compounds

form a cyclic adduct or in which a

cyclic compound is formed by addi-

tion between unsaturated parts of

the same molecule. In cycloaddition,

there is no net reduction in bond

multiplicity. The *Diels–Alder reac-

tion is an example. Cycloadditions

may be stepwise reactions or may be

*pericyclic reactions.

cycloalkanesCyclic saturated hy-

drocarbons containing a ring of car-

bon atoms joined by single bonds.

They have the general formula

CnH 2 n, for example cyclohexane,

C 6 H 12 , etc. In general they behave

like the *alkanes but are rather less

reactive.

A

• Information about IUPAC nomenclature

cyclobutadieneA short-lived

cyclic diene hydrocarbon, C 4 H 4 , with

its four carbon atoms in a square

ring. It is made by degradation of its

metal complexes, and has a short

lifetime of a few seconds. It under-

goes addition reactions with alkynes.

cyclohexadiene-1,4-dione

(benzoquinone; quinone)A yellow

solid, C 6 H 4 O 2 ; r.d. 1.3; m.p. 116°C. It

has a six-membered ring of carbon

atoms with two opposite carbon

atoms linked to oxygen atoms (C=O)

and the other two pairs of carbon

atoms linked by double bonds

(HC=CH). The compound is used in

making dyes. See also quinhydrone

electrode.

cyclohexaneA colourless liquid

*cycloalkane, C 6 H 12 ; r.d. 0.78; m.p.

6.5°C; b.p. 81°C. It occurs in petro-

leum and is made by passing ben-

zene and hydrogen under pressure

over a heated Raney nickel catalyst at

150 °C, or by the reduction of cyclo-

hexanone. It is used as a solvent and

paint remover and can be oxidized

using hot concentrated nitric acid to

hexanedioic acid (adipic acid). The cy-

clohexane ring is not planar and can

adopt boat and chair *conforma-

tions; in formulae it is represented

by a single hexagon.

cycloniteA highly explosive nitro

compound, (CH 2 N.NO 2 ) 3. It has a

cyclic structure with a six-membered

ring of alternating CH 2 groups and

nitrogen atoms, with each nitrogen

being attached to a NO 2 group. It is

made by nitrating hexamine,

cyclic AMP 158

c

2 CH

N

C

H 2

N

CH 2

N

NO 2

NO 2

O 2 N

Cyclonite