graph of potential energy against rotation angle. In the other the C–H
bonds on one group bisect the angle
between two C–H bonds on the
other. This is a staggered conformation (or bisecting conformation) and
corresponds to a minimum in the potential-energy curve. In the case of
ethane, the energy difference between the two conformations is
small (2.8 kcal/mole) and effective
free rotation occurs under normal
conditions.
In the case of butane, rotation can
occur about the bond between the
second and third carbon atoms,
MeH 2 C–CH 2 Me. The highest maximum occurs when the methyl
groups are eclipsed (they are at their
closest). Maxima of lower energies
occur when the methyl groups on
one carbon atom eclipse the hydrogen atoms on the other. The lowest
potential energy occurs when a C–Me
bond bisects the angle between the
C–H bonds (i.e. the two methyl
groups are furthest apart). This is
called the anti conformation. A lesser
minimum occurs when the methyl
group bisects the angle between a
C–H bond and a C–Me bond. This
arrangement is called the gauche
conformation.
Similar rotational arrangements
occur in other types of molecule. For
instance, in a compound such as
R 3 C–CHO, with a C=O double bond,
one conformation has the C=O bond
bisecting the angle between C–R
bonds. This is called the bisecting
conformation (note that the C–H
bond eclipses one of the C–R bonds
in this case). The other has the C=O
bond eclipsing a C–R bond. This is
the eclipsed or eclipsing conformation. (In this case the C–H bond bisects the angle between the C–R
bonds.)
See also torsion angle; ring conformations.
conformational analysis The determination or estimation of the
relative energies of possible conformations of a molecule and the effect
of these on the molecule’s properties.
conformational isomer (conformer) One of a set of stereoisomers
that differ from each other by torsion angle or angles (where only
structures that are minima of potential energy are considered).
congeners Elements that belong
to the same group in the periodic
table.
conjugate acids and bases See
acid.
conjugated Describing double or
triple bonds in a molecule that are
separated by one single bond. For example, the organic compound buta1,3-diene, H 2 C=CH–CH=CH 2 , has
conjugated double bonds. In such
molecules, there is some delocalization of electrons in the pi orbitals between the carbon atoms linked by
the single bond.
conjugate solutions Solutions
formed between two liquids that
are partially miscible with one another; such solutions are in equilibrium at a particular temperature.
Examples of conjugate solutions
are phenol in water and water in
phenol.
conjugation Delocalization of pi
electrons as occurs in *conjugated
systems. Conjugation can also involve d orbitals and lone pairs of
electrons.
connection table A way of representing a molecule as a table showing the atoms, their coordinates, and
the links between them. The MDL
*molÜle format uses a connection
table in its representation of strucconformational analysis
140
c
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bonds on one group bisect the angle
between two C–H bonds on the
other. This is a staggered conformation (or bisecting conformation) and
corresponds to a minimum in the potential-energy curve. In the case of
ethane, the energy difference between the two conformations is
small (2.8 kcal/mole) and effective
free rotation occurs under normal
conditions.
In the case of butane, rotation can
occur about the bond between the
second and third carbon atoms,
MeH 2 C–CH 2 Me. The highest maximum occurs when the methyl
groups are eclipsed (they are at their
closest). Maxima of lower energies
occur when the methyl groups on
one carbon atom eclipse the hydrogen atoms on the other. The lowest
potential energy occurs when a C–Me
bond bisects the angle between the
C–H bonds (i.e. the two methyl
groups are furthest apart). This is
called the anti conformation. A lesser
minimum occurs when the methyl
group bisects the angle between a
C–H bond and a C–Me bond. This
arrangement is called the gauche
conformation.
Similar rotational arrangements
occur in other types of molecule. For
instance, in a compound such as
R 3 C–CHO, with a C=O double bond,
one conformation has the C=O bond
bisecting the angle between C–R
bonds. This is called the bisecting
conformation (note that the C–H
bond eclipses one of the C–R bonds
in this case). The other has the C=O
bond eclipsing a C–R bond. This is
the eclipsed or eclipsing conformation. (In this case the C–H bond bisects the angle between the C–R
bonds.)
See also torsion angle; ring conformations.
conformational analysis The determination or estimation of the
relative energies of possible conformations of a molecule and the effect
of these on the molecule’s properties.
conformational isomer (conformer) One of a set of stereoisomers
that differ from each other by torsion angle or angles (where only
structures that are minima of potential energy are considered).
congeners Elements that belong
to the same group in the periodic
table.
conjugate acids and bases See
acid.
conjugated Describing double or
triple bonds in a molecule that are
separated by one single bond. For example, the organic compound buta1,3-diene, H 2 C=CH–CH=CH 2 , has
conjugated double bonds. In such
molecules, there is some delocalization of electrons in the pi orbitals between the carbon atoms linked by
the single bond.
conjugate solutions Solutions
formed between two liquids that
are partially miscible with one another; such solutions are in equilibrium at a particular temperature.
Examples of conjugate solutions
are phenol in water and water in
phenol.
conjugation Delocalization of pi
electrons as occurs in *conjugated
systems. Conjugation can also involve d orbitals and lone pairs of
electrons.
connection table A way of representing a molecule as a table showing the atoms, their coordinates, and
the links between them. The MDL
*molÜle format uses a connection
table in its representation of strucconformational analysis
140
c
www.AzShimi.ir www.AzShimi.com