6. what is
conformational
isomerism?
well, first off
isomerism is a
misnomer! No
isomers here just
bond rotation...
7. rotation
of a
bond
sorry movies don’t
work in the printed
form...
8. rotation
of a
bond would have shown this bond
rotating...conformations are
different shapes of the same
molecule caused by a bond
rotating...no bonds are
broken
9. representations of
conformational
isomers
HH
H H
H H H
H H H H
H
conventional Newman
representation projection
H H
H H
H H H H
H H H H
sawhorse
projection
10. representations of
conformational
isomers
HH
H H
H H H
H H H H
H
conventional Newman
representation projection
H
three common ways of H
representing these different
H H
conformations but in fairness
only the top two are important
H H to me... H H
H H H H
sawhorse
projection
11. representations of
conformational
isomers
HH
H H
H H H
H H H H
H
conventional Newman
representation projection
H H
bold line means bond is
H
sticking upwards towards H
you and the dashed bond
H H is way from you orH
behind H
H H the page H H
sawhorse
projection
12. representations of
conformational
isomers big circle is the
bond...small black
dot is the front HH
H H atom
H H H
H H H H
H
conventional Newman
representation projection
H H
H H
H H H H
H H H H
sawhorse
projection
14. dihedral
angle
H dihedral
H angle
H
H H
H
we’ll start with
the simplest
example...ethane
15. dihedral
angle the dihedral angle (or torsional
angle) is defined, in this case, as
the angle between a C–H bond on
the near carbon and A C–H bond
on the far carbon...
H dihedral
H angle
H
H H
H
16. dihedral
angle
we are now going to look
how the energy of the
molecule changes as we
rotate the C–C bond... H dihedral
H angle
H
H H
H
17. HH HH HH HH
H H H H
H H H H H H H H
H H H H
12 kJmol–1
energy
H H H
2
H H H H H H
H H H H H H
H H H dihedral
0 60 120 180 240 300 360 angle
we get two extremes...an unstable
conformations
high energy conformation and a
stable low energy conformation
18. HH HH HH HH
H H H H
H H H H H H H H
H H H H
here the H atoms are
overlapping...I know it doesn’t look
like it (but if I had drawn it like that
12 kJmol–1
energy
then you won’t people able to see
the back atoms
H H H
2
H H H H H H
H H H H H H
H H H dihedral
0 60 120 180 240 300 360 angle
conformations
20. HH HH HH HH
H H H H
H H H H H H H H
H H H H
12 kJmol–1
in this conformation the H
energy
atoms are as far apart as
the bonds will allow
H H H
2
H H H H H H
H H H H H H
H H H dihedral
0 60 120 180 240 300 360 angle
conformations
22. torsional strain
the difference in
energy is caused by electron-
electron repulsion (like charges
torsional repel opposite attract think of a
magnet). This is called
strain torsional strain
H
H C
C
electron cloud
repulsion
23. staggered
conformation
H
H H
H H
H
H H H H
H H
H H
H H H H
24. staggered conformation
H
H H
H H
H
all these representations H H H H
show the most stable / H H
preferred conformation...the
staggered conformation... H
atoms far apart H H H
H H
25. eclipsed
conformation
HH
H
H H
H
H H H
H
H H H H
H H H H
26. eclipsed conformation
HH
H
H H
H
all these
H H H
representations show the H
least stable / disfavoured
conformation...the eclipsed H H H H
conformation...atoms as
H H H H
close as they can get
27. HH HH HH HH
as the difference is 12
H H H H
H H H HkJmol–1 and three bonds
H H H H
H H H H
are overlapping...each
bond must contribute...
12 kJmol–1
energy
H H H
2
H H H H H H
H H H H H H
H H H dihedral
0 60 120 180 240 300 360 angle
conformations
30. propane
staggered
CH3
H H H H H
CH3 H
H H
H H H
H H H H H H
H
eclipsed
if we add 1 x CH2 6 kJmol–1
and form propane H3C
we have the same H
two conformations
H
H H
H
4 kJmol–1
4 kJmol–1
31. propane
staggered
CH3
H H H H H
CH3 H
H H
H eclipsed slightly less
H H
H H H H H favoured as methyl has
H
more electrons and causes H
eclipsed
more torsional strain
6 kJmol–1
H3C
H
H
H H
H
4 kJmol–1
4 kJmol–1
32. butane with butane we can
rotate three different
C–C bonds...
H H H H
H
H
H H H H
H H 3 4 H H 4 H H
CH2CH3 CH3 H
1 2 3
2 3 1 2 4
H 1CH CH3CH2...
3
H H H H H H
33. butane
H H H H
H
H
H H H H C1–C2 & C3–C4
are dull as they are
just like propane (2
conformations of
interest)
H H 3 4 H H 4 H H
CH2CH3 CH3 H
1 2 3
2 3 1 2 4
H 1CH CH3CH2...
3
H H H H H H
34. butane
but rotation around C2–C3 far H
H H H more
interesting as we now have the relative
H
position of the two methyl groups to
worry about... H
H H H H
H H 3 4 H H 4 H H
CH2CH3 CH3 H
1 2 3
2 3 1 2 4
H 1CH CH3CH2...
3
H H H H H H
C2-C3
35. H3CCH H3C H H3C H H3CCH
3 3
H H H3C H
H H H H H H H H
H CH3 H H
now there are four
important conformations
based on staggered and
eclipsed
energy
CH3 CH3 CH3
H CH3 H H H3C H
H H H H H H
H CH3 H
dihedral
0 60 120 180 240 300 360 angle
36. H3CCH H3C H H3C H H3CCH
3 3
H H H3C H
H H H H H H H H
H CH3 H H
19 kJmol–1
16 kJmol–1
energy
4 kJmol–1
CH3 CH3 CH3
4
H CH3 H H H3C H
H H H H H H
H CH3 H
dihedral
0 60 120 180 240 300 360 angle
conformations
38. anti-periplanar
staggered
CH3
H H H H CH3 H CH3
H
H
H H CH3 H H H3C H
CH3
this is the most
important
conformation...the most
no strain
favoured / preferred...
39. anti-periplanar
staggered methyl groups (or any
other groups for that matter) are as
far apart as they can be (easiest seen
on Newman projection but must get
used to visualising on
stick diagram)
CH3
H H H H CH3 H CH3
H
H
H H CH3 H H H3C H
CH3
no strain
40. anti-clinal
eclipsed
torsional strain
H3C 4 kJmol–1
H H H CH3
CH3
H H
H3C H3C H H3C H
H H H H
H
torsional strain
6 kJmol–1
first of the
16 kJmol–1
eclipsed but not
that important...
torsional strain
41. syn-clinal (gauche)
staggered
CH3
H3C H H3C H CH3 H CH3
H3C
H
H H H H H H H
H
new kind of staggered
4 kJmol–1
conformation...no overlap
so no torsional strain...
steric strain
42. syn-clinal (gauche)
staggered
steric strain
4 kJmol–1
CH3
H3C H H3C H CH3 H CH3
H3C
H
H H H H H H H
H
but two groups are
4 kJmol–1
close...and objects don’t like
being close so they repel
each other...
steric strain
43. syn-clinal (gauche)
staggered
steric strain
4 kJmol–1
CH3
H3C H H3C H CH3 H CH3
H3C
H
H H H H H H H
H
4 kJmol–1
steric strain
...and we get steric
strain...basically you can’t
have two things occupying
the same space!
44. steric strain
...and these objects really
hate it when they eclipse /
overlap...
11 kJmol–1
45. syn-periplanar
eclipsed
steric strain
11 kJmol–1
H3C
CH3 CH3 H3C CH3
CH3
H H
H H H
H H H H H H
H
torsional strain
...so we get the least stable 4 kJmol–1
19 kJmol–1
(most disfavoured if that isn’t
too many double negatives)
torsional & steric strain
46. syn-periplanar
eclipsed
steric strain
11 kJmol–1
H3C
CH3 CH3 H3C CH3
CH3
H H
H H H
H H H H H H
H
torsional strain
...all bonds overlap (torsional 4 kJmol–1
strain) and the two methyl groups
are as close as possible (steric
strain)
19 kJmol–1
torsional & steric strain
48. two extremes most important
H CH3 H3C CH3
H
H H H
H3C H H H
learn!
CH3
H3C
H H CH3
H H H
H H
CH3 H
anti- syn-
periplanar periplanar
(staggered) (eclipsed)
51. ring strain as we can see, most
cyclic systems contain
considerable strain...
120
100
ring strain (kJmol–1)
80
60
40
20
0
3 4 5 6 7 8
ring size
52. ring strain
120
100 ...cyclopropane really
is a very unhappy
ring strain (kJmol–1)
bunny...but why?
80
60
40
20
0
3 4 5 6 7 8
ring size
53. cyclopropanes
some torsional strain
but this only amounts
to...24 kJmol–1...the
rest comes from...
torsional strain
4 kJmol–1
HH H H
H
C H H
H
HH H H
54. ring strain
109° 109°
(tetrahedral) (tetrahedral)
49° 60° 19°
90°
ring or angle 109°
strain... (tetrahedral)
1°
108°
55. ring strain
109° 109°
(tetrahedral) (tetrahedral)
remember an sp 3
carbon wants bond
angles of 109°...
49° 60° 19°
90°
109°
(tetrahedral)
1°
108°
56. ring strain
109° 109°
(tetrahedral) (tetrahedral)
49° 60° 19°
90°
...internal angle of a
triangle is 60°...so bonds are
being bent to accommodate
the difference...this causes a
109° lot of strain!
(tetrahedral)
1°
108°
57. this strain can be
harnessed in drugs...and just
for the vets, this is an anti-fungal O HO
used to treat infections of the
OH
lung (piccy of a seagull
HN N
lung)
O H O
H
H3C NH
FR–900848 O
65. H H
H H H
H
H H
H H
H H
H H
H H
H H
H H
three representations
of the same thing...
chair conformation
66. this is the most
important...if you like
chemistry (or want to
do well at it) learn to
H H draw this accurately
H H H
H
H H
H H
H H
H H
H H
H H
H H
chair conformation
67. the substituents on the
ring are given special
names depending on their
orientation
substituents
68. substituents stick out away
from the ring...they are as far
R
from anything as they possibly
can be
R R
R R
R
equatorial
position
69. R R
R
R
axial
R
these substituents are
R
vertical...above and below the
position
ring...they are still quite close
to each other...
70. ring ‘flipping’
H H H H H H
H5 H H H 5
H H 3
H H H H H H
H H H H H H
3 1
1
H H
1 H H 4 H H
H H H H H H
chair boat chair
(strain free) (strained) (strain free)
H H HH HH H H
4 1
H H H 6 2 H
6 2 4
6 2
H 1
H H H H H
H H 4
H H 1 H H
71. ring ‘flipping’
H H H H H H
H5 H H H 5
H H 3
H H H H H H
H H H H H H
3 1
1
H H
1 H H 4 H H
H H H H H H
chair boat chair
(strain free) (strained) (strain free)
H H HH HH H H
4 1
H H H 6 2 H
6 2 4
6 2
H 1
H H H H H
H H 4
H H 1 H H
simply by rotating the bonds we
can make the axial substituents
become equatorial (and vice-
versa)
72. ring ‘flipping’
H H H H H H
H5 H H H 5
H H 3
H H H H H H
H H H H H H
3 1
1
H H
1 H H 4 H H
H H H H H H
chair boat chair
(strain free) (strained) (strain free)
H H HH HH H H
4 1
H H H 6 2 H
6 2 4
6 2
H 1
H H H H H
H H 4
H H 1 H H
no bonds broken during
this...it is just a change
in conformation
73. ring ‘flipping’
H H H H H H
H5 H H H 5
H H 3
H H H H H H
H H H H H H
3 1
1
H H
1 H H 4 H H
H H H H H H
chair boat chair
(strain free) (strained) (strain free)
H H HH HH H H
4 1
H H H 6 2 H
6 2 4 process passes through
6 2
H 1
H H H
H nasty, high energy 4
a H
H H H 1 H conformation...the
H H
boat...
74. energy
29 kJmol–1
this shows the energy of the molecule
during ring flipping...note how the chair is
wonderfully stable and nothing else is...
ring ‘flipping’
75. boat conformation
disfavoured as the ‘bow’
and ‘stern’ are being
brought close together
(steric strain) and...
76. H H
H H H H
H H
H H
H H
H H HH HH
H H
torsional strain as
29
C–H bonds overlap
kJmol–1
boat conformation
77. learn to
draw the chair
conformation...it
will get you marks
drawing
in the exam!
substituents
78. howto
draw
draw a V on an angle and then
learn to draw parallel lines
(hmmm, there’s an album title
in there somewhere)
79. parallel
lines
draw this one first...same
length as before and the
bottom of the new line should
be level with the bottom of the
original two lines
80. parallel
lines
second one is parallel
and of the same length
81. parallel
line!
another parallel line
(of same length)
82. parallel
another line!
finally, close the ring
with another parallel
line
86. carbon is tetrahedral so make the
corners look like a tetrahedron
(and this is the bit none of you ever
do, it’s bl@@dy frustrating)
C
tetrahedral
87. H H
H
so the three top carbons
have a vertical line upwards
NOT down as this would
prevent the carbon looking
like a tetrahedron!
top carbons go up
97. one substituent
a single substituent will
always go for the
equatorial position... CH3
H
CH3
H
CH3
95% 5%
equatorial axial
more stable by disfavoured
8 kJmol–1
98. one substituent remember: ring flipping allows us
to change between conformations
without breaking any bonds
CH3
H
CH3
H
CH3
95% 5%
equatorial axial
more stable by disfavoured
8 kJmol–1
99. one substituent
but why equatorial??
CH3
H
CH3
H
CH3
95% 5%
equatorial axial
more stable by disfavoured
8 kJmol–1
101. axial substituent is tucked
under ring...and we get
interaction between the
three substituents on the
same face
H H
H H
CH3 H
H CH3
1,3-diaxial interactions
