conformation - organic chemistry from examville.com
TRANSCRIPT
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Unit One Part 7:conformation
just when you
thought it was safe
to look at molecules
again... came
shape!
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strain3D representations
conformations
7Unit One
Part
so very
important...but the
lecture that
routinely gets the
worst feedback
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?what shape are molecules
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?what shape are molecules
we know the shape
of each atom...but
what about the
molecules...
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what isconformational isomerism?
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what isconformational isomerism?
well, first off
isomerism is a
misnomer! No
isomers here just
bond rotation...
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rotationof abond
sorry movies don’t
work in the printed
form...
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rotationof abond would have shown this bond
rotating...conformations are
different shapes of the same
molecule caused by a bond
rotating...no bonds are
broken
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representations of
conformationalisomersH
H
Hconventional
representation
sawhorseprojection
Newmanprojection
H H
HH H
H
H
H H
H
H H
H
H H
H
H H
H
H H
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representations of
conformationalisomersH
H
Hconventional
representation
sawhorseprojection
Newmanprojection
H H
HH H
H
H
H H
H
H H
H
H H
H
H H
H
H H
three common ways of
representing these differentconformations but in fairness
only the top two are important
to me...
i f
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representations of
conformationalisomersH
H
Hconventional
representation
sawhorseprojection
Newmanprojection
H H
HH H
H
H
H H
H
H H
H
H H
H
H H
H
H H
bold line means bond issticking upwards towards
you and the dashed bond
is way from you or behind
the page
i f
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representations of
conformationalisomersH
H
Hconventional
representation
sawhorseprojection
Newmanprojection
H H
HH H
H
H
H H
H
H H
H
H H
H
H H
H
H H
big circle is the
bond...small blackdot is the front
atom
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some conformations more favourable... whilst all can
exist...some are
more important...
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dihedralangle
H
H H
H
H
H
dihedral
angle
we’ll start with
the simplest
example...ethane
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dihedralangle
H
H H
H
H
H
dihedral
angle
the dihedral angle (or torsionalangle) 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...
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dihedralangle
H
H H
H
H
H
dihedral
angle
we are now going to look
how the energy of the
molecule changes as we
rotate the C–C bond...
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H
H
H
H
H H
H
H
H
H
H H
H
H
H
H
H H
H
HH
H
H H
H
HH
H
H H
H
HH
H
H H
e n
e r g y
600 120 180 240 300 360dihedral
angle
H
H
H
H
H H
conformations
we get two extremes...an unstable
high energy conformation and a
stable low energy conformation
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H
H
H
H
H H
H
H
H
H
H H
H
H
H
H
H H
H
HH
H
H H
H
HH
H
H H
H
HH
H
H H
e n
e r g y
600 120 180 240 300 360dihedral
angle
H
H
H
H
H H
conformations
here the H atoms are
overlapping...I know it doesn’t look
like it (but if I had drawn it like that
then you won’t people able to see
the back atoms
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is this picture
clearer?
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H
H
H
H
H H
H
H
H
H
H H
H
H
H
H
H H
H
HH
H
H H
H
HH
H
H H
H
HH
H
H H
e n
e r g y
600 120 180 240 300 360dihedral
angle
H
H
H
H
H H
conformations
in this conformation the H
atoms are as far apart as
the bonds will allow
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you do not need
to learn these values!!
t i l
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H
C
C
H
electron cloudrepulsion
torsionalstrain
torsionalstrain
the difference in
energy is caused by electron-electron repulsion (like charges
repel opposite attract think of a
magnet). This is called
torsional strain
t d
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staggeredconformation
H
HH
H
H H
H
H HH
H H H
H HH
HH
t d
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staggeredconformation
H
HH
H
H H
H
H HH
H H H
H HH
HH
all these representationsshow the most stable /
preferred conformation...the
staggered conformation...
atoms far apart
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li d
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eclipsedconformation
H H
HH H
H
H
H H
H
H H
H
H
H
H
H H
all theserepresentations show the
least stable / disfavouredconformation...the eclipsed
conformation...atoms as
close as they can get
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H
H
H
H
H H
H
H
H
H
H H
H
H
H
H
H H
H
HH
H
H H
H
HH
H
H H
H
HH
H
H H
e n e r g y
600 120 180 240 300 360
dihedral
angle
H
H
H
H
H H
conformations
as the difference is 12kJmol –1 and three bonds
are overlapping...each
bond must contribute...
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what about morecomplex molecules?
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propane
H
HH
CH3
H HH
CH3
H H
H H
H H
H H H H
H Hstaggered
eclipsed
H
H
H
H3C
H H
4 kJmol –14 kJmol –1
6 kJmol –1if we add 1 x CH2
and form propanewe have the same
two conformations
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propane
H
HH
CH3
H HH
CH3
H H
H H
H H
H H H H
H Hstaggered
eclipsed
H
H
H
H3C
H H
4 kJmol –14 kJmol –1
6 kJmol –1
eclipsed slightly less
favoured as methyl hasmore electrons and causes
more torsional strain
b t
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CH3CH2...
H
H H
H H
H
CH2CH3
H H
H H
12
3 4
1 2
3
4
butane
H
H H H H
H H
H
H H
CH3
CH3
H H
H H
1
2
3
4
with butane we can
rotate three different
C–C bonds...
b t
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CH3CH2...
H
H H
H H
H
CH2CH3
H H
H H
12
3 4
1 2
3
4
butane
H
H H H H
H H
H
H H
CH3
CH3
H H
H H
1
2
3
4
C1–C2 & C3–C4
are dull as they are
just like propane (2conformations of
interest)
b t
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C2-C3
CH3CH2...
H
H H
H H
H
CH2CH3
H H
H H
12
3 4
1 2
3
4
butane
H
H H H H
H H
H
H H
CH3
CH3
H H
H H
1
2
3
4
but rotation around C2–C3 far moreinteresting as we now have the relative
position of the two methyl groups to
worry about...
H CHH C HH CH C
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CH3
H
H
H3C
H H
H
H
CH3
H3C
H H
H
H3C
H
H3C
H H
H
HH3C
CH3
H H
e n e r g
y
600 120 180 240 300 360
CH3
H
H
H3C
H H
dihedralangle
CH3
HH
CH3
H H
H
CH3H
CH3
H H
now there are four
important conformations
based on staggered and
eclipsed
H CHH C HH CH C
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CH3
H
H
H3C
H H
H
H
CH3
H3C
H H
H
H3C
H
H3C
H H
H
HH3C
CH3
H H
e n e r g
y
600 120 180 240 300 360
CH3
H
H
H3C
H H
dihedralangle
conformations
CH3
HH
CH3
H H
H
CH3H
CH3
H H
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©Graham Johnson, Graham Johnson Medical Media, Boulder, Colorado
anti periplanar
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anti-periplanar
no strain
CH3
HH CH3
H3C H
H
H
H
CH3
H HCH3
H H
CH3
H H
staggered
this is the mostimportant
conformation...the most
favoured / preferred...
anti periplanar
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anti-periplanar
no strain
CH3
HH CH3
H3C H
H
H
H
CH3
H HCH3
H H
CH3
H H
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)
anti clinal
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H
H3C
H
H3C
H H
CH3
H H
H
H3C H
CH3
HH
H
H3CH
anti-clinaleclipsed
torsional strain
16 kJmol–1
torsional strain
6 kJmol–1
torsional strain4 kJmol–1
first of the
eclipsed but not
that important...
syn clinal ( h )
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syn-clinal (gauche)staggered
steric strain
4 kJmol–1
H
HH3
C
CH3
H H
CH3
H H
H
H
H3CCH
3
H HH
H3C H
new kind of staggered
conformation...no overlap
so no torsional strain...
syn clinal ( h )
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steric strain4 kJmol–1
syn-clinal (gauche)staggered
steric strain
4 kJmol–1
H
HH3
C
CH3
H H
CH3
H H
H
H
H3CCH
3
H HH
H3C H
but two groups are
close...and objects don’t like
being close so they repel each other...
syn clinal ( h )
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steric strain4 kJmol–1
syn-clinal (gauche)staggered
steric strain
4 kJmol–1
H
HH3
C
CH3
H H
CH3
H H
H
H
H3CCH
3
H HH
H3C H
...and we get stericstrain...basically you can’t
have two things occupyingthe same space!
t i
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stericstrain
11 kJmol–1
...and these objects really
hate it when they eclipse /
overlap...
syn periplanar
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CH3
H
H
H3C
H H
CH3
H H
CH3
H H
CH3
H
H
H3C
H
H
syn-periplanareclipsed
torsional & steric strain
19 kJmol–1
torsional strain
4 kJmol–1
steric strain11 kJmol–1
...so we get the least stable
(most disfavoured if that isn’t
too many double negatives)
syn periplanar
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CH3
H
H
H3C
H H
CH3
H H
CH3
H H
CH3
H
H
H3C
H
H
syn-periplanareclipsed
torsional & steric strain
19 kJmol–1
torsional strain
4 kJmol–1
steric strain11 kJmol–1
...all bonds overlap (torsionalstrain) and the two methyl groups
are as close as possible (stericstrain)
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t i t ttwo extremes
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most importanttwo extremes
CH3
HH
CH3
H H
CH3
H3C H
H
H
H
anti-periplanar
(staggered)
CH3
H
H
H3
C
H H
CH3
H
H
H3C
H
H
syn-periplanar
(eclipsed)
learn!
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another important form of strain...
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another important form of strain...
and it has an
ace (to myjuvenile mind)
name...
ring strain
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0
20
40
60
80
100
120
3 4 5 6 7 8ring size
r i n g s t r a i n ( k J
m o l – 1 )
ring strain as we can see, most
cyclic systems contain
considerable strain...
ring strain
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0
20
40
60
80
100
120
3 4 5 6 7 8ring size
r i n g s t r a i n ( k J
m o l – 1 )
ring strain
...cyclopropane really
is a very unhappy
bunny...but why?
cyclopropanes
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cyclopropanes
H
HH
C
H
H
H
H
H
H
H
H
H
torsional strain4 kJmol–1
some torsional strain
but this only amounts
to...24 kJmol –1...the
rest comes from...
ring strain
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ring strain109°
(tetrahedral)
49° 60°
90°
19°
108°
1°
109°(tetrahedral)
109°(tetrahedral)
ring or angle
strain...
ring strain
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ring strain109°
(tetrahedral)
49° 60°
90°
19°
108°
1°
109°(tetrahedral)
109°(tetrahedral)
remember an sp3
carbon wants bond
angles of 109°...
ring strain
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ring strain109°
(tetrahedral)
49° 60°
90°
19°
108°
1°
109°(tetrahedral)
109°(tetrahedral)
...internal angle of a
triangle is 60°...so bonds are
being bent to accommodate
the difference...this causes a
lot of strain!
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H3C NH
O
O
NHN
OHO
OH
H
HO
FR–900848
this strain can be
harnessed in drugs...and just
for the vets, this is an anti-fungal
used to treat infections of the
lung (piccy of a seagull
lung)
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and the mostimportant ring...
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and the mostimportant ring...
one you’ll grow tohate by exam time...
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C
C C C
CC
H
H
HH
H H
HH
H
H HH
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N O TFLAT
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N O TFLATbenzene is flat
because it has doublebonds...
chair conformation
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chair conformation
the chair conformation(as it looks like a recliner
apparently) is the most
important and most stable...
N
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Notorsional strain
Noangular strain
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H
H
H
H
H
HHH
H
H
H
H
H
H
H
H
H
H
H
H
chair conformation
three representations
of the same thing...
this is the most
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H
H
H
H
H
HHH
H
H
H
H
H
H
H
H
H
H
H
H
chair conformation
important...if you like
chemistry (or want to
do well at it) learn to
draw this accurately
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substituents
the substituents on the
ring are given special
names depending on theirorientation
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R
RR
R
R
R
t i lposition
substituents stick out away
from the ring...they are as far
from anything as they possibly
can be
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R
R
R
R
RR
i lposition
these substituents are
vertical...above and below the
ring...they are still quite close
to each other...
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e n e r g y
this shows the energy of the moleculeduring ring flipping...note how the chair is
wonderfully stable and nothing else is...
boat conformation
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boat conformation
disfavoured as the ‘bow’
and ‘stern’ are being
brought close together
(steric strain) and...
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H
H
H
H
H H
H H
H H
HH
H
H
H
H
H
H
H
H
29 kJmol–1
torsional strain as
C–H bonds overlap
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drawingsubstituents
learn to
draw the chair conformation...it
will get you marksin the exam!
howto
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draw
howto
draw a V on an angle and then
learn to draw parallel lines
(hmmm, there’s an album title
in there somewhere)
parallellines
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parallellines
draw this one first...same
length as before and the
bottom of the new line shouldbe level with the bottom of the
original two lines
parallellines
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parallellines
second one is parallel
and of the same length
parallelline!
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parallelline!
another parallel line
(of same length)
parallelline!
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parallelline!
finally, close the ring
with another parallel
line
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?substituentswheredo
the
go
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axialaxial groups are
always vertical
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tetrahedral
carbon is tetrahedral so make the
corners look like a tetrahedron
(and this is the bit none of you everdo, it’s bl@@dy frustrating)
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H
H
H
top carbons go up
so the three top carbons
have a vertical line upwards
NOT down as this would
prevent the carbon looking
like a tetrahedron!
alternate vertical lines
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alternate vertical lines
H
H
H
H
HH ...and vice-versa for
the lower carbons
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equatorialstick outwards (and will
be parallel) and guess
what...
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tetrahedral
carbon is tetrahedral so draw it like that!
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C–Cparallel
to
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H
H
H
H
H
HH
H
parallel lines
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H
H
H
H
H
HH
H
and look...atetrahedral
carbon
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H
H
H
H
H
HHH
H
H
H
H
more
parallel
lines
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H
H
H
H
H
HHH
H
H
H
Hand even
more parallel
lines
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what happens if weadd substituents?
one substituent
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one substituentCH3
CH3
H
CH3
H
95%equatorial
more stable by
8 kJmol –1
5%axial
disfavoured
a single substituent will
always go for the
equatorial position...
one substituent
remember: ring flipping allows us
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one substituentCH3
CH3
H
CH3
H
95%equatorial
more stable by
8 kJmol –1
5%axial
disfavoured
g pp g
to change between conformations
without breaking any bonds
one substituent
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one substituentCH3
CH3
H
CH3
H
95%equatorial
more stable by
8 kJmol –1
5%axial
disfavoured
but why equatorial??
equatorial position sticks intospace...away from ring
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1 3 diaxial interactions
CH3
HH
HH
CH3H
H
i l b tit t i t k d
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1 3 diaxial interactions
CH3
HH
HH
CH3H
H
axial substituent is tucked
under ring...and we get
interaction between the
three substituents on the
same face
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one big substituent
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H
H
equatorialfavoured
axialdisfavoured
one big substituent
X
when you have a big
substituent it fixes the
ring and stop ring flip...
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H
H
H
HH
H
1 3 diaxial interactions
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H
H
H
HH
H
1 3 diaxial interactions
...because 1,3-diaxial
interaction really
disfavoured
what havee learnt?
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•conformationsof molecules
....we learnt?