graph of potential energy against ro-
tation 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 conforma-
tion (or bisectingconformation) and
corresponds to a minimum in the po-
tential-energy curve. In the case of
ethane, the energy difference be-
tween 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 maxi-
mum 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 hydro-
gen 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 anticonformation. 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 eclipsedor eclipsingconforma-
tion. (In this case the C–H bond bi-
sects the angle between the C–R
bonds.)
See also torsion angle; ring con-
formations.conformational analysisThe de-
termination or estimation of the
relative energies of possible confor-
mations of a molecule and the effect
of these on the molecule’s proper-
ties.conformational isomer (con-
former)One of a set of stereoisomers
that differ from each other by tor-
sion angle or angles (where only
structures that are minima of poten-
tial energy are considered).congenersElements that belong
to the same group in the periodic
table.conjugate acids and basesSee
acid.conjugatedDescribing double or
triple bonds in a molecule that are
separated by one single bond. For ex-
ample, the organic compound buta-
1,3-diene, H 2 C=CH–CH=CH 2 , has
conjugated double bonds. In such
molecules, there is some delocaliza-
tion of electrons in the pi orbitals be-
tween the carbon atoms linked by
the single bond.conjugate solutionsSolutions
formed between two liquids that
are partially miscible with one an-
other; such solutions are in equilib-
rium at a particular temperature.
Examples of conjugate solutions
are phenol in water and water in
phenol.conjugationDelocalization of pi
electrons as occurs in *conjugated
systems. Conjugation can also in-
volve d orbitals and lone pairs of
electrons.
connection tableA way of repre-
senting a molecule as a table show-
ing the atoms, their coordinates, and
the links between them. The MDL
*molÜle format uses a connection
table in its representation of struc-conformational analysis 140c