3
Stereochemistry
3.1 Hybridization and bond angles
From our discussions of bonding, we have learnt
something about the arrangement of bonds around
various atoms (see Chapter 2). These concepts are
fundamental to our appreciation of the shape of
molecules, i.e. stereochemistry. Before we delve into
these matters, let us recap a little on the disposition
of bonds around carbon.
Bonding at four-valent carbon is tetrahedral, with
four sp
3 -hybridized orbitals mutually inclined at
109.5
◦ . Remember that the tetrahedral array is demonstrated by experimental measurements, and that
hybridization is the mathematical model put forward
to explain this observation (see Section 2.6.2). We can
conveniently represent the tetrahedral arrangement in
two dimensions by using a wedge–dot convention. In
this convention, single bonds written as normal lines
are considered to be in the plane of the paper. Bonds
in front of this plane, i.e. coming out from the paper,
are then drawn as a wedge, whilst bonds behind the
plane, i.e. going into the paper, are drawn as a broken
or dotted bond (see Section 2.6.2).
C
in plane
behind plane
sp
3 hybridization
angle 109.5°
tetrahedral
in front of plane
wedge−dot
representation
As we get more familiar with this representation,
we may begin to abbreviate it by showing either the
wedge or the dotted bond, rather than both. Of course,
it is important to remember that these abbreviated
forms actually represent a tetrahedral array, and not
something with three bonds planar plus one other.
drawing stereostructures:
C
C
120°
90°
90°
120°
could lead to confusion
C
might also be drawn as
note that whilst these are OK
this
always try to keep a tetrahedral appearance
use these bond angles for nice structures
Bonding at three-valent carbon is trigonal planar
with bond angles of 120
◦ , an observation that
we account for through sp
2 hybridization plus
formation of a π bond by overlap of p orbitals (see
Section 2.6.2). Thus, an alkene double bond involves
electrons in sp
2 hybrid orbitals making σ single
bonds, and the remaining electrons in p orbitals
overlapping to produce the π-bond component of
the double bond. We can draw this as a planar
representation, all single bonds in the plane of the
Essentials of Organic Chemistry Paul M Dewick
2006 John Wiley & Sons, Ltd
Stereochemistry
3.1 Hybridization and bond angles
From our discussions of bonding, we have learnt
something about the arrangement of bonds around
various atoms (see Chapter 2). These concepts are
fundamental to our appreciation of the shape of
molecules, i.e. stereochemistry. Before we delve into
these matters, let us recap a little on the disposition
of bonds around carbon.
Bonding at four-valent carbon is tetrahedral, with
four sp
3 -hybridized orbitals mutually inclined at
109.5
◦ . Remember that the tetrahedral array is demonstrated by experimental measurements, and that
hybridization is the mathematical model put forward
to explain this observation (see Section 2.6.2). We can
conveniently represent the tetrahedral arrangement in
two dimensions by using a wedge–dot convention. In
this convention, single bonds written as normal lines
are considered to be in the plane of the paper. Bonds
in front of this plane, i.e. coming out from the paper,
are then drawn as a wedge, whilst bonds behind the
plane, i.e. going into the paper, are drawn as a broken
or dotted bond (see Section 2.6.2).
C
in plane
behind plane
sp
3 hybridization
angle 109.5°
tetrahedral
in front of plane
wedge−dot
representation
As we get more familiar with this representation,
we may begin to abbreviate it by showing either the
wedge or the dotted bond, rather than both. Of course,
it is important to remember that these abbreviated
forms actually represent a tetrahedral array, and not
something with three bonds planar plus one other.
drawing stereostructures:
C
C
120°
90°
90°
120°
could lead to confusion
C
might also be drawn as
note that whilst these are OK
this
always try to keep a tetrahedral appearance
use these bond angles for nice structures
Bonding at three-valent carbon is trigonal planar
with bond angles of 120
◦ , an observation that
we account for through sp
2 hybridization plus
formation of a π bond by overlap of p orbitals (see
Section 2.6.2). Thus, an alkene double bond involves
electrons in sp
2 hybrid orbitals making σ single
bonds, and the remaining electrons in p orbitals
overlapping to produce the π-bond component of
the double bond. We can draw this as a planar
representation, all single bonds in the plane of the
Essentials of Organic Chemistry Paul M Dewick
2006 John Wiley & Sons, Ltd