Additions to the Olefinic C“C
Double Bond 3
Olefins contain a C“C double bond. The C“C double bond can be described with
two different models. According to the less frequently used model briefly B
mentioned in Section 2.4.3 (Figure 2.8), C“C double bonds consist of two bent C¬C
single bonds. With a bond energy of 73 kcal/mol, each of these bonds is 10
kcal/mol less stable than the linear C¬C bond of an aliphatic compound.
Normally, the second model is used to describe the bonding in olefins. According
to this model, a C“C double bond con- sists of a s and a p bond. The s bond with 83
kcal/mol has 20 kcal/mol more bond en- ergy than the p bond (63 kcal/mol). The
higher stability of s- in comparison to p-C¬C bonds is due to the difference in the
overlap between the AOs that form these bonds. s-C¬C bonds are produced by
the overlap of two spn atomic orbitals (n 1, 2, 3), which is quite effective
because it is frontal. p-C¬C bonds are based on the overlap of 2pz atomic orbitals,
which is not as good because it is lateral.
Regardless of which bond model is utilized, it is clear that upon exposure to a
suit- able reagent the relatively weak C“C double bond of an olefin will be
given up and a relatively stable C¬C single bond conserved instead. From the
point of view of the usual bonding model, this means that the respective
reactions of olefins are addition reactions, in which the C¬C p bond is converted
into s bonds to two new substituents, a and b. One C atom of the C“C double
bond picks up fragment a from the reagent and the other C atom picks up
fragment b.
R1 R3 R1 R3
C 2 C 2 + Reagent(s) Addition a C 3 C 3 b
sp sp
sp sp
R2 R4 R2 R4
Suitable reagents have the structure a¬b in the most simple case. However,
they may also have a different structure. The fragments a and b, which are bound
to the olefinic C“C double bond, do not necessarily constitute the entire reagent
(e.g., epox- idation with percarboxylic acids: see Figure 3.14) nor must they
originate from a sin- gle reagent (e.g., formation of bromohydrins: see Figure 3.33).
In these cases also one talks about an addition reaction.
In addition reactions to olefinic double bonds, two new sp3-hybridized C atoms
are produced. Each of them can be a stereogenic center (stereocenter) and
obviously is one if it possesses four different substituents. If stereocenters are
,produced, their configu- ration must be specified. Stated more accurately, the
first question is which absolute configuration is produced at any new
stereocenter. Then the question arises about the
,
, 86 3 Additions to the Olefinic C“C Double Bond
configuration of the new stereocenters relative (a) to each other or (b) to
additional stereocenters the molecule contained before the addition reaction
occurred.
Stereochemical aspects are therefore an important part of the chemistry of
addition reactions to the olefinic double bond. We will therefore investigate
them in detail in this chapter. In fact, the content of Chapter 3 is arranged
according to the stereo- chemical characteristics of the (addition) reactions.
3.1 The Concept of cis and trans Addition
In a number of additions of two fragments a and b to an olefinic C“C double
B
bond, a new stereocenter is produced at both attacked C atoms (Figure 3.1). For
describ- ing the configurations of these new stereocenters relative to each other
the follow- ing nomenclature has come into use: If the stereostructure (not the
preferred con- formation!) of the addition product arises because fragments a and
b have added to the C“C double bond of the substrate from the same side, a cis
addition has taken place. Conversely, if the stereostructure of the addition
product is obtained because fragments a and b were bonded to the C“C double
bond of the substrate from op- posite sides, we have a trans addition. The terms
cis and trans addition are also used when the addition products are acyclic and
thus their configuration cannot be de- scribed with the cis/trans nomenclature.
(if not identical)
R1 R3 1 a b 3
a b R R
and/o
R2 R4 r R2 R4
cis addition
R1 R3
+ reagents(s)
R2 R4
trans addition
a R3 R1 b
R1 3
fragment double bonds. b
Fig. 3.1. The cis and trans s a and b
additions of two R2 R4
to C“C
Double Bond 3
Olefins contain a C“C double bond. The C“C double bond can be described with
two different models. According to the less frequently used model briefly B
mentioned in Section 2.4.3 (Figure 2.8), C“C double bonds consist of two bent C¬C
single bonds. With a bond energy of 73 kcal/mol, each of these bonds is 10
kcal/mol less stable than the linear C¬C bond of an aliphatic compound.
Normally, the second model is used to describe the bonding in olefins. According
to this model, a C“C double bond con- sists of a s and a p bond. The s bond with 83
kcal/mol has 20 kcal/mol more bond en- ergy than the p bond (63 kcal/mol). The
higher stability of s- in comparison to p-C¬C bonds is due to the difference in the
overlap between the AOs that form these bonds. s-C¬C bonds are produced by
the overlap of two spn atomic orbitals (n 1, 2, 3), which is quite effective
because it is frontal. p-C¬C bonds are based on the overlap of 2pz atomic orbitals,
which is not as good because it is lateral.
Regardless of which bond model is utilized, it is clear that upon exposure to a
suit- able reagent the relatively weak C“C double bond of an olefin will be
given up and a relatively stable C¬C single bond conserved instead. From the
point of view of the usual bonding model, this means that the respective
reactions of olefins are addition reactions, in which the C¬C p bond is converted
into s bonds to two new substituents, a and b. One C atom of the C“C double
bond picks up fragment a from the reagent and the other C atom picks up
fragment b.
R1 R3 R1 R3
C 2 C 2 + Reagent(s) Addition a C 3 C 3 b
sp sp
sp sp
R2 R4 R2 R4
Suitable reagents have the structure a¬b in the most simple case. However,
they may also have a different structure. The fragments a and b, which are bound
to the olefinic C“C double bond, do not necessarily constitute the entire reagent
(e.g., epox- idation with percarboxylic acids: see Figure 3.14) nor must they
originate from a sin- gle reagent (e.g., formation of bromohydrins: see Figure 3.33).
In these cases also one talks about an addition reaction.
In addition reactions to olefinic double bonds, two new sp3-hybridized C atoms
are produced. Each of them can be a stereogenic center (stereocenter) and
obviously is one if it possesses four different substituents. If stereocenters are
,produced, their configu- ration must be specified. Stated more accurately, the
first question is which absolute configuration is produced at any new
stereocenter. Then the question arises about the
,
, 86 3 Additions to the Olefinic C“C Double Bond
configuration of the new stereocenters relative (a) to each other or (b) to
additional stereocenters the molecule contained before the addition reaction
occurred.
Stereochemical aspects are therefore an important part of the chemistry of
addition reactions to the olefinic double bond. We will therefore investigate
them in detail in this chapter. In fact, the content of Chapter 3 is arranged
according to the stereo- chemical characteristics of the (addition) reactions.
3.1 The Concept of cis and trans Addition
In a number of additions of two fragments a and b to an olefinic C“C double
B
bond, a new stereocenter is produced at both attacked C atoms (Figure 3.1). For
describ- ing the configurations of these new stereocenters relative to each other
the follow- ing nomenclature has come into use: If the stereostructure (not the
preferred con- formation!) of the addition product arises because fragments a and
b have added to the C“C double bond of the substrate from the same side, a cis
addition has taken place. Conversely, if the stereostructure of the addition
product is obtained because fragments a and b were bonded to the C“C double
bond of the substrate from op- posite sides, we have a trans addition. The terms
cis and trans addition are also used when the addition products are acyclic and
thus their configuration cannot be de- scribed with the cis/trans nomenclature.
(if not identical)
R1 R3 1 a b 3
a b R R
and/o
R2 R4 r R2 R4
cis addition
R1 R3
+ reagents(s)
R2 R4
trans addition
a R3 R1 b
R1 3
fragment double bonds. b
Fig. 3.1. The cis and trans s a and b
additions of two R2 R4
to C“C
