As discussed in Chapter 2, there are two minimum energy confirmations
for a diene such as butadiene. These are the s-cis and s-trans
conformations shown below, both of which are planar and so all the p-electrons
to be delocalized around all four carbon atoms. The s-trans conformation
will be lower in energy (ie the global minimum energy conformation) since
in the s-cis conformation there is some steric repulsion (Es)
between the hydrogen atoms at the ends of the alkenes.
Starting from the s-trans conformation, rotation around C2-C3 results in an increase in energy since the two C=C are no longer planar and so the electrons in the p-bonds can not be delocalized as effectively. The energy reaches a maximum after a 90o rotation, when the two C=C are orthogonal to one another, so no electron delocalization is possible. Continued rotation around C2-C3 brings the two C=C back into the same plane and after a 90o rotation gives the s-cis conformation. Therefore, the energy decreases during this part of the rotation. Further rotation around C2-C3 just reverses the above process, giving an energy maximum when the two C=C are orthogonal and then reforming the s-trans conformation.
A graph of the whole 360o rotation is shown below.
b)
Each sulphur atom in dimethyl disulphide has two lone pairs of electrons
and is also surrounded by two bond pairs of electrons. Thus there is a
(distorted) tetrahedral arrangement of the electron pairs around the sulphur
atoms (cf. Chapter 1). The energy / rotation diagram for rotation around
the S-S bond in dimethyl disulphide then has the same shape as the graph
for rotation around C2-C3 of butane. The structures of the conformations
corresponding to 60o rotations around the S-S bond are shown
below.
Thus, the global minimum energy conformation is the conformation in which the two methyl groups are at 180o to one another since this minimizes steric and electronic repulsions between the methyl groups. Rotation around the S-S bond then initially results in an energy increase since the methyl groups start to eclipse one of the lone pairs on the sulphur atoms. After a 60o rotation, the methyl groups and lone pairs are eclipsing and an energy maximum is reached. Although there is no steric interaction between the nuclei of the methyl groups and the lone pair of electrons, there is an electronic repulsion between the negatively charged electrons in the bond and lone pairs. Further rotation decreases the energy as the methyl group and lone pair of electrons move apart from one another until after a further 60o rotation, a second minimum energy conformation is reached since all of the lone pairs and methyl groups are again staggered to one another. This second minimum energy conformation has a higher energy than the global minimum energy conformation however, since the two methyl groups are now only 60o from one another, so there is some repulsion between them. A further 60o rotation around the S-S bond results in a large increase in energy since the two methyl groups now start to eclipse one another, and this gives a second and higher energy maximum. In this conformation, the two methyl groups eclipse one another as do the two lone pairs of electrons, so there is considerable steric and electronic repulsion between the methyl groups and lone pairs. Further rotation around the S-S bond does not give new conformations, rather the conformations discussed during the first 180o rotation are reformed, until after a total of 360o rotation, the global minimum energy conformation is regenerated.
c)
The dominant interaction will be between the two largest substituents
attached to the C2-C3 bond. These are the =CH2 group attached
to C2 and the bromine atom attached to C3. The energy / rotation diagram
for rotation around C2-C3 then has the same shape as the graph for rotation
around C2-C3 of butane. The structures of the conformations corresponding
to 60o rotations around C2-C3 are shown below.
Thus, the global minimum energy conformation is the conformation in
which the bromine and methylene groups are at 180o to one another
since this minimizes steric
and electronic repulsions between these groups. Note that in this conformation,
the bromine and hydrogen atoms are actually eclipsed. Rotation around the
C2-C3 bond then initially results in an energy increase since the methylene
group starts to eclipse one of the hydrogen atoms. After a 60o
rotation, the methylene group and hydrogen atom are eclipsing and an energy
maximum is reached. The hydrogen atom which was eclipsing the bromine atom
has now moved round to a staggered conformation relative to the bromine
atom which will be energetically favourable, however, the large methylene
and bromine atoms are now only 120o apart so there will start
to be some steric repulsion between them. Further rotation decreases the
energy as the methylene ,group and hydrogen atom move apart from one another
until, after a further 60o, rotation, a second minimum energy
conformation is reached since the methylene group is again staggered to
the bromine and hydrogen atoms. This second minimum energy conformation
has a higher energy than the global minimum energy conformation however,
since the bromine and methylene groups are now only 60o from
one another, so there is some repulsion between them. Note that the hydrogen
atom attached to C2 is also eclipsing one of the hydrogen atoms attached
to C3. A further 60o rotation around the C2-C3 bond results
in a large increase in energy since the bromine and methylene groups now
start to eclipse one another, and this gives a second and higher, energy
maximum. Further rotation around the C2-C3 bond does not give new conformations,
rather the conformations discussed during the first 180o rotation
are reformed, until after a total of 360o rotation, the global
minimum energy conformation is regenerated.
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