Radical Substitution Reactions at
the Saturated C Atom 1
In a substitution reaction a part X of a molecule R¬X is replaced by a group Y
B
(Figure 1.1). The subject of this chapter is substitution reactions in which a part
X that is bound to an sp3-hybridized C atom is replaced by a group Y via radical
inter- mediates. Radicals are valence-unsaturated and therefore usually short-
lived atoms or molecules. They contain one or more unpaired (“lone”) electrons.
From inorganic chemistry you are familiar with at least two radicals, which by the
way are quite stable: NO and O2. NO contains one lone electron; it is therefore a
monoradical or simply a “radical.” O2 contains two lone electrons and is
therefore a biradical.
N
R sp3 X X = H, Hal, O C SMe, O C N
S S
+ Reagent,
– By-
products CO2R
Fig. 1.1. Some substrates
Y = H, Hal, OOH, CH2 CH2 , C CH and products of radical
R sp3
Y substitution reactions.
1.1 Bonding and Preferred Geometries in
C Radicals, Carbenium Ions and
Carbanions
At the so-called radical center an organic radical R. has an electron septet, which
is an electron deficiency in comparison to the electron octet of valence-saturated B
compounds. Carbon atoms are the most frequently found radical centers and most
often have three neighbors (see below). Carbon-centered radicals with their
electron septet occupy an in- termediate position between the carbenium ions,
which have one electron less (electron sextet at the valence-unsaturated C atom),
and the carbanions, which have one electron more (electron octet at the valence-
unsaturated C atom). Since there is an electron de- ficiency present both in C
radicals and in carbenium ions, the latter are more closely re- lated to each other
than C radicals are related to carbanions. Because of this, C radicals and carbenium
ions are also stabilized or destabilized by the same substituents.
Nitrogen-centered radicals 1R sp 2 2N # or oxygen-centered radicals 1Rsp 2O # are less
3 3
,stable than C-centered radicals 1Rsp 2 3C #. They are higher in energy because of
3
the higher electronegativity of these elements relative to carbon. Nitrogen- or
oxygen- centered radicals of the cited substitution pattern consequently have
only a limited chance to exist.
,2 1 Radical Substitution Reactions at the Saturated C Atom
Which geometries are preferred at the valence-unsaturated C atom of C
radicals, and how do they differ from those of carbenium ions or carbanions?
And what types of bonding are found at the valence-unsaturated C atoms of
these three species? It is simplest to clarify the preferred geometries first (Section
1.1.1). As soon as these geometries are known, molecular orbital (MO) theory
will be used to provide a de- scription of the bonding (Section 1.1.2).
We will discuss the preferred geometries and the MO descriptions of C radicals
and the corresponding carbenium ions or carbanions in two parts. In the first part
we will examine C radicals, carbenium ions, and carbanions with a trivalent
central C atom. The second part treats the analogous species with a divalent
central C atom. A third part (species with a monovalent central C atom) can be
dispensed with because the only species of this type that is important in organic
chemistry is the alkynyl anion, which, however, is of no interest here.
1.1.1 Preferred Geometries
The preferred geometries of carbenium ions and carbanions are correctly
B
predicted by the valence shell electron pair repulsion (VSEPR) theory. The
VSEPR theory, which comes from inorganic chemistry, explains the
stereostructure of covalent compounds of the nonmetals and the main group
metals. It makes no difference whether these compounds are charged or not.
The VSEPR theory analyzes the stereostructure of these compounds in the
envi- ronment of the central atom. This stereostructure depends mainly on (a)
the number n of atoms or atom groups (in inorganic chemical terminology,
referred to as ligands) linked to the central atom and (b) the number m of
nonbonding valence electron pairs localized at the central atom. If the central
atom under consideration is a C atom, n m 4. In this case, the VSEPR theory
holds in the following shorthand version, which makes it possible to determine
the preferred geometries: the compound con- sidered has the stereostructure in
which the repulsion between the n bonding partners and the m nonbonding
valence electron pairs on the C atom is as small as possible. This is the case when
the orbitals that accommodate the bonding and the nonbonding electron pairs
are as far apart from each other as possible.
For carbenium ions this means that the n substituents of the valence-
unsaturated central atom should be at the greatest possible distance from each
other:
• In alkyl cations R3C , n 3 and m 0. The substituents of the trivalent central
atom lie in the same plane as the central atom and form bond angles of 120
with each other (trigonal planar arrangement). This arrangement was
confirmed experi- mentally by means of a crystal structural analysis of the
, tert-butyl cation.
• In alkenyl cations “C ¬R, n 2 and m 0. The substituents of the divalent
central atom lie on a common axis with the central atom and form a bond
angle of 180 . Alkenyl cations have not been isolated yet because of their
low sta- bility (Section 1.2). However, calculations support the preference for
the linear structure.
the Saturated C Atom 1
In a substitution reaction a part X of a molecule R¬X is replaced by a group Y
B
(Figure 1.1). The subject of this chapter is substitution reactions in which a part
X that is bound to an sp3-hybridized C atom is replaced by a group Y via radical
inter- mediates. Radicals are valence-unsaturated and therefore usually short-
lived atoms or molecules. They contain one or more unpaired (“lone”) electrons.
From inorganic chemistry you are familiar with at least two radicals, which by the
way are quite stable: NO and O2. NO contains one lone electron; it is therefore a
monoradical or simply a “radical.” O2 contains two lone electrons and is
therefore a biradical.
N
R sp3 X X = H, Hal, O C SMe, O C N
S S
+ Reagent,
– By-
products CO2R
Fig. 1.1. Some substrates
Y = H, Hal, OOH, CH2 CH2 , C CH and products of radical
R sp3
Y substitution reactions.
1.1 Bonding and Preferred Geometries in
C Radicals, Carbenium Ions and
Carbanions
At the so-called radical center an organic radical R. has an electron septet, which
is an electron deficiency in comparison to the electron octet of valence-saturated B
compounds. Carbon atoms are the most frequently found radical centers and most
often have three neighbors (see below). Carbon-centered radicals with their
electron septet occupy an in- termediate position between the carbenium ions,
which have one electron less (electron sextet at the valence-unsaturated C atom),
and the carbanions, which have one electron more (electron octet at the valence-
unsaturated C atom). Since there is an electron de- ficiency present both in C
radicals and in carbenium ions, the latter are more closely re- lated to each other
than C radicals are related to carbanions. Because of this, C radicals and carbenium
ions are also stabilized or destabilized by the same substituents.
Nitrogen-centered radicals 1R sp 2 2N # or oxygen-centered radicals 1Rsp 2O # are less
3 3
,stable than C-centered radicals 1Rsp 2 3C #. They are higher in energy because of
3
the higher electronegativity of these elements relative to carbon. Nitrogen- or
oxygen- centered radicals of the cited substitution pattern consequently have
only a limited chance to exist.
,2 1 Radical Substitution Reactions at the Saturated C Atom
Which geometries are preferred at the valence-unsaturated C atom of C
radicals, and how do they differ from those of carbenium ions or carbanions?
And what types of bonding are found at the valence-unsaturated C atoms of
these three species? It is simplest to clarify the preferred geometries first (Section
1.1.1). As soon as these geometries are known, molecular orbital (MO) theory
will be used to provide a de- scription of the bonding (Section 1.1.2).
We will discuss the preferred geometries and the MO descriptions of C radicals
and the corresponding carbenium ions or carbanions in two parts. In the first part
we will examine C radicals, carbenium ions, and carbanions with a trivalent
central C atom. The second part treats the analogous species with a divalent
central C atom. A third part (species with a monovalent central C atom) can be
dispensed with because the only species of this type that is important in organic
chemistry is the alkynyl anion, which, however, is of no interest here.
1.1.1 Preferred Geometries
The preferred geometries of carbenium ions and carbanions are correctly
B
predicted by the valence shell electron pair repulsion (VSEPR) theory. The
VSEPR theory, which comes from inorganic chemistry, explains the
stereostructure of covalent compounds of the nonmetals and the main group
metals. It makes no difference whether these compounds are charged or not.
The VSEPR theory analyzes the stereostructure of these compounds in the
envi- ronment of the central atom. This stereostructure depends mainly on (a)
the number n of atoms or atom groups (in inorganic chemical terminology,
referred to as ligands) linked to the central atom and (b) the number m of
nonbonding valence electron pairs localized at the central atom. If the central
atom under consideration is a C atom, n m 4. In this case, the VSEPR theory
holds in the following shorthand version, which makes it possible to determine
the preferred geometries: the compound con- sidered has the stereostructure in
which the repulsion between the n bonding partners and the m nonbonding
valence electron pairs on the C atom is as small as possible. This is the case when
the orbitals that accommodate the bonding and the nonbonding electron pairs
are as far apart from each other as possible.
For carbenium ions this means that the n substituents of the valence-
unsaturated central atom should be at the greatest possible distance from each
other:
• In alkyl cations R3C , n 3 and m 0. The substituents of the trivalent central
atom lie in the same plane as the central atom and form bond angles of 120
with each other (trigonal planar arrangement). This arrangement was
confirmed experi- mentally by means of a crystal structural analysis of the
, tert-butyl cation.
• In alkenyl cations “C ¬R, n 2 and m 0. The substituents of the divalent
central atom lie on a common axis with the central atom and form a bond
angle of 180 . Alkenyl cations have not been isolated yet because of their
low sta- bility (Section 1.2). However, calculations support the preference for
the linear structure.
