nance (NMR) used to increase the intensities of resonance lines. The large
population differences between the
states given by the Boltzmann distribution gives rise to strong intensities
of resonance lines. In the nuclear
Overhauser effect, spin relaxation is
used to transfer population difference from one type of nucleus to another type of nucleus so that the
intensities of the resonance lines of
the second type of nucleus are increased. An example of such an enhancement, which is widely used, is
between protons and
13 C, in which
case it is possible to attain enhancement of about three times. In the
NOE proton, irradiation is used to
produce the enhancement of the
13 C
line. Another use of the NOE is to determine the distance between protons.
nucleic acid A complex organic
compound in living cells that consists of a chain of *nucleotides. There
are two types: *DNA (deoxyribonucleic acid) and *RNA (ribonucleic
acid).
A
• Information on IUPAC nomenclature
nucleon A *proton or a *neutron.
nucleon number (mass number)
Symbol A. The number of *nucleons
in an atomic nucleus of a particular
nuclide.
nucleophile An ion or molecule
that can donate electrons. Nucleophiles are often oxidizing agents
and Lewis bases. They are either negative ions (e.g. Cl
–
) or molecules that
have electron pairs (e.g. NH 3 ). In organic reactions they tend to attack
positively charged parts of a molecule. Compare electrophile.
nucleophilic addition A type of
addition reaction in which the Ürst
step is attachment of a *nucleophile
to a positive (electron-deÜcient) part
of the molecule. *Aldehydes and *ketones undergo reactions of this type
because of polarization of the carbonyl group (carbon positive).
nucleophilic substitution A type
of substitution reaction in which a
*nucleophile displaces another group
or atom from a compound. For example, in
CR 3 Cl + OH
–
→ CR 3 OH + Cl
–
the nucleophile is the OH
– ion. There
are two possible mechanisms of nucleophilic substitution. In S N 1 reactions, a positive carbonium ion is
Ürst formed:
CR 3 Cl → CR 3
+ + Cl
–
This then reacts with the nucleophile
CR 3
+ + OH
–
→ CR 3 OH
The CR 3
+ ion is planar and the OH
–
ion can attack from either side. Consequently, if the original molecule is
optically active (the three R groups
are different) then a racemic mixture
of products results.
The alternative mechanism, the
S N 2 reaction, is a concerted reaction
in which the nucleophile approaches
from the side of the R groups as the
other group (Cl in the example)
leaves. In this case the conÜguration
of the molecule is inverted. If the
original molecule is optically active,
the product has the opposite activity,
an effect known as Walden inversion.
The notations S N 1 and S N 2 refer to
the kinetics of the reactions. In the
S N 1 mechanism, the slow step is the
Ürst one, which is unimolecular (and
Ürst order in CR 3 Cl). In the S N 2 reaction, the process is bimolecular (and
second order overall).
nucleoside An organic compound
consisting of a nitrogen-containing
*purine or *pyrimidine base linked
to a sugar (ribose or deoxyribose). An
example is *adenosine. Compare nucleotide.
nucleic acid
380
n
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population differences between the
states given by the Boltzmann distribution gives rise to strong intensities
of resonance lines. In the nuclear
Overhauser effect, spin relaxation is
used to transfer population difference from one type of nucleus to another type of nucleus so that the
intensities of the resonance lines of
the second type of nucleus are increased. An example of such an enhancement, which is widely used, is
between protons and
13 C, in which
case it is possible to attain enhancement of about three times. In the
NOE proton, irradiation is used to
produce the enhancement of the
13 C
line. Another use of the NOE is to determine the distance between protons.
nucleic acid A complex organic
compound in living cells that consists of a chain of *nucleotides. There
are two types: *DNA (deoxyribonucleic acid) and *RNA (ribonucleic
acid).
A
• Information on IUPAC nomenclature
nucleon A *proton or a *neutron.
nucleon number (mass number)
Symbol A. The number of *nucleons
in an atomic nucleus of a particular
nuclide.
nucleophile An ion or molecule
that can donate electrons. Nucleophiles are often oxidizing agents
and Lewis bases. They are either negative ions (e.g. Cl
–
) or molecules that
have electron pairs (e.g. NH 3 ). In organic reactions they tend to attack
positively charged parts of a molecule. Compare electrophile.
nucleophilic addition A type of
addition reaction in which the Ürst
step is attachment of a *nucleophile
to a positive (electron-deÜcient) part
of the molecule. *Aldehydes and *ketones undergo reactions of this type
because of polarization of the carbonyl group (carbon positive).
nucleophilic substitution A type
of substitution reaction in which a
*nucleophile displaces another group
or atom from a compound. For example, in
CR 3 Cl + OH
–
→ CR 3 OH + Cl
–
the nucleophile is the OH
– ion. There
are two possible mechanisms of nucleophilic substitution. In S N 1 reactions, a positive carbonium ion is
Ürst formed:
CR 3 Cl → CR 3
+ + Cl
–
This then reacts with the nucleophile
CR 3
+ + OH
–
→ CR 3 OH
The CR 3
+ ion is planar and the OH
–
ion can attack from either side. Consequently, if the original molecule is
optically active (the three R groups
are different) then a racemic mixture
of products results.
The alternative mechanism, the
S N 2 reaction, is a concerted reaction
in which the nucleophile approaches
from the side of the R groups as the
other group (Cl in the example)
leaves. In this case the conÜguration
of the molecule is inverted. If the
original molecule is optically active,
the product has the opposite activity,
an effect known as Walden inversion.
The notations S N 1 and S N 2 refer to
the kinetics of the reactions. In the
S N 1 mechanism, the slow step is the
Ürst one, which is unimolecular (and
Ürst order in CR 3 Cl). In the S N 2 reaction, the process is bimolecular (and
second order overall).
nucleoside An organic compound
consisting of a nitrogen-containing
*purine or *pyrimidine base linked
to a sugar (ribose or deoxyribose). An
example is *adenosine. Compare nucleotide.
nucleic acid
380
n
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