See alsoGENETIC CODE; MESSENGERRNA; OLIGONU-
CLEOTIDE; RIBONUCLEIC ACID; RIBOSOMALRNA.
nucleobase SeeNUCLEOSIDE.
nucleofuge A LEAVING GROUPthat carries away the
bonding electron pair. For example, in the HYDROLYSIS
of an alkyl chloride, Cl–is the nucleofuge. The ten-
dency of atoms or groups to depart with the bonding
electron pair is called nucleofugality.
nucleons Particles making up the nucleus of an
atom—protons and neutrons.
nucleophile(nucleophilic reagent) A nucleophile (or
nucleophilic reagent) is a reagent that forms a bond to
its reaction partner (the ELECTROPHILE) by donating
both bonding electrons. Nucleophilic reagents are
LEWIS BASES.
A nucleophilic substitution reaction is a HET-
EROLYTICreaction in which the reagent supplying the
entering group acts as a nucleophile. For exampleMeO–(nucleophile) + Et–Cl →MeOEt + Cl–
(nucleofuge)
The term nucleophilic is also used to designate the
apparent polar character of certain RADICALs, as
inferred from their higher relative reactivity with reac-
tion sites of lower electron density.196 nucleobase
Reactor hall of the Chernobyl nuclear power station, Ukraine. The floor squares in the foreground are located over the reactor’s fuel
channels. Four RBMK reactors were completed at Chernobyl during 1978–84; this is reactor 3. Each generates 925 megawatts of electric-
ity. RBMK reactor cores use graphite to promote the nuclear chain reaction and water for cooling the vertical fuel channels. The RBMK
design is flawed because its reactivity, and hence its power output, increases as coolant is lost from the fuel channels. This positive
void coefficientwas partly responsible for the major accident involving Chernobyl’s Reactor 4 in 1986.(Courtesy of Vaughan
Melzer/JVZ/Science Photo Library)