nucleophilic catalysis CATALYSISby a LEWIS BASE,
involving formation of a LEWIS ADDUCTas a REACTION
INTERMEDIATE. For example, the hydrolysis of acetic
anhydride in aqueous solution catalyzed by pyridine:
C 5 H 5 N + (CH 3 CO) 2 O →[C 5 H 5 NCOCH 3 ]++
CH 3 CO 2 –
[C 5 H 5 NCOCH 3 ]++ H 2 O →C 5 H 5 N + CH 3 CO 2 H +
H+aq
See alsoELECTROPHILIC; NUCLEOPHILICITY.nucleophilicity (1) The property of being nucle-
ophilic.
(2) The relative reactivity of a NUCLEOPHILIC
REAGENT. (It is also sometimes referred to as nucle-
ophilic power.) Qualitatively, the concept is related to
Lewis basicity. However, whereas Lewis basicity is
measured by relative equilibrium constants,
B: + A B+– A–(equilibrium constant K)nucleophilicity of a LEWIS BASEis measured by relative
RATE CONSTANTs of different nucleophilic reagents
toward a common SUBSTRATE, most commonly involv-
ing formation of a bond to carbon
B: + A–Z →B+–A–+ Z:–(rate constant k)
See also ELECTROPHILICITY; RITCHIE EQUATION;
SWAIN-SCOTT EQUATION.
nucleoside Compound in which a purine or pyrimi-
dine base is β-N-glycosidically bound to C-1 of either
2-deoxy-D-ribose or of D-ribose, but without any
phosphate groups. The common nucleosides in biologi-
cal systems are adenosine, guanosine, cytidine, and uri-
dine (which contain ribose) and deoxyadenosine,
deoxyguanosine, deoxycytidine, and thymidine (which
contain deoxyribose).
See alsoNUCLEOTIDE.nucleotide A NUCLEOSIDEwith one or more phos-
phate groups esterified mainly to the 3′- or the 5′-posi-
tion of the sugar moiety. Nucleotides found in cells are
adenylic acid, guanylic acid, uridylic acid, cytidylic
acid, deoxyadenylic acid, deoxyguanylic acid, deoxy-
cytidylic acid, and thymidylic acid. A nucleotide is a
nucleoside in which the primary hydroxy group of
either 2-deoxy-D-ribose or of D-ribose is esterified by
orthophosphoric acid.
See also ADENOSINE 5 ′-TRIPHOSPHATE; NAD+;
NADP+.nucleus The very small, dense core of the atom,
where most of its mass and all of its positive charge is
concentrated. Except for hydrogen, it consists of pro-
tons and neutrons and other subatomic particles. Also,
the nucleus of a cell, i.e., the region containing the
chromosomes.nuclide The nucleus of a particular isotope.nuclide symbol Symbol for an atom A/Z E, in which
E is the symbol of an element, Z is its atomic number,
and A is its mass number.nylon Any of a group of high-strength and resilient
synthetic polymers in which the molecules contain the
recurring amide group –CONH.→k
←nylon 197