lecture7: 123.101
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Unit One Part 7:conformation
just when you thought it was safe to look at molecules
again... came shape!
strain3D representationsconformations
7Unit OnePart
so very important...but the
lecture that routinely gets the worst feedback
?what shape are molecules
?what shape are molecules
we know the shape of each atom...but
what about the molecules...
what is conformational
isomerism?
what is conformational
isomerism?
well, first off isomerism is a misnomer! No
isomers here just bond rotation...
rotationof abond
sorry movies don’t work in the printed
form...
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
representations ofconformationalisomers
H
HH
conventional representation
sawhorseprojection
Newmanprojection
H H
HH H
H
H
H H
H
H H
H
H H
H
H H
H
H H
representations ofconformationalisomers
H
HH
conventional 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 different
conformations but in fairness only the top two are important
to me...
representations ofconformationalisomers
H
HH
conventional 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 is sticking upwards towards you and the dashed bond is way from you or behind
the page
representations ofconformationalisomers
H
HH
conventional 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 black
dot is the front atom
some conformations more favourable... whilst all can
exist...some are more important...
dihedralangle
H
H H
HH
H
dihedral angle
we’ll start with the simplest
example...ethane
dihedralangle
H
H H
HH
H
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...
dihedralangle
H
H H
HH
H
dihedral angle
we are now going to look how the energy of the
molecule changes as we rotate the C–C bond...
H
HH
H
H H
H
HH
H
H H
H
HH
H
H H
H
HHH
H HH
HHH
H HH
HHH
H H
ener
gy
600 120 180 240 300 360dihedral
angle
H
HH
H
H H
12 kJmol–1
2conformationswe get two extremes...an unstable high energy conformation and a stable low energy conformation
H
HH
H
H H
H
HH
H
H H
H
HH
H
H H
H
HHH
H HH
HHH
H HH
HHH
H H
ener
gy
600 120 180 240 300 360dihedral
angle
H
HH
H
H H
12 kJmol–1
2conformations
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
is this picture clearer?
H
HH
H
H H
H
HH
H
H H
H
HH
H
H H
H
HHH
H HH
HHH
H HH
HHH
H H
ener
gy
600 120 180 240 300 360dihedral
angle
H
HH
H
H H
12 kJmol–1
2conformations
in this conformation the H atoms are as far apart as
the bonds will allow
you do not needto learn these values!!
HC
CH
electron cloud repulsion
torsional strain
torsionalstrainthe difference in
energy is caused by electron-electron repulsion (like charges repel opposite attract think of a
magnet). This is called torsional strain
staggeredconformation
H
HHH
H H
H
H HH
H H H
H HH
HH
staggeredconformation
H
HHH
H H
H
H HH
H H H
H HH
HH
all these representations show the most stable /
preferred conformation...the staggered conformation...
atoms far apart
eclipsedconformation
H H
HH H
H
H
H H
H
H H
H
HH
H
H H
eclipsedconformation
H H
HH H
H
H
H H
H
H H
H
HH
H
H H
all theserepresentations show the least stable / disfavoured
conformation...the eclipsed conformation...atoms as
close as they can get
H
HH
H
H H
H
HH
H
H H
H
HH
H
H H
H
HHH
H HH
HHH
H HH
HHH
H H
ener
gy
600 120 180 240 300 360dihedral
angle
H
HH
H
H H
12 kJmol–1
2conformations
as the difference is 12 kJmol–1 and three bonds
are overlapping...each bond must contribute...
torsionalstrain
H
HH
H
H H4kJmol–1
what about more complex molecules?
propane
H
HHCH3
H HH
CH3
H H
H HH H
H H H H
H H
staggered
eclipsed
H
HH
H3C
H H
4 kJmol–14 kJmol–1
6 kJmol–1if we add 1 x CH2 and form propane we have the same two conformations
propane
H
HHCH3
H HH
CH3
H H
H HH H
H H H H
H H
staggered
eclipsed
H
HH
H3C
H H
4 kJmol–14 kJmol–1
6 kJmol–1
eclipsed slightly less favoured as methyl has
more electrons and causes more torsional strain
CH3CH2...
H
H H
H H
H
CH2CH3
H H
H H1 2
3 4
1 23
4
butaneH
H H H H
H H
H
H H
CH3
CH3
H H
H H
12
3
4
with butane we can rotate three different
C–C bonds...
CH3CH2...
H
H H
H H
H
CH2CH3
H H
H H1 2
3 4
1 23
4
butaneH
H H H H
H H
H
H H
CH3
CH3
H H
H H
12
3
4
C1–C2 & C3–C4are dull as they are just like propane (2 conformations of
interest)
C2-C3CH3CH2...
H
H H
H H
H
CH2CH3
H H
H H1 2
3 4
1 23
4
butaneH
H H H H
H H
H
H H
CH3
CH3
H H
H H
12
3
4
but rotation around C2–C3 far more interesting as we now have the relative position of the two methyl groups to
worry about...
CH3
HH
H3C
H H
H
HCH3
H3C
H H
H
H3CH
H3C
H H
H
HH3CCH3
H H
ener
gy
600 120 180 240 300 360
CH3
HH
H3C
H H
dihedral angle
CH3
HHCH3
H HH
CH3HCH3
H H
now there are four important conformations based on staggered and
eclipsed
CH3
HH
H3C
H H
H
HCH3
H3C
H H
H
H3CH
H3C
H H
H
HH3CCH3
H H
ener
gy
600 120 180 240 300 360
CH3
HH
H3C
H H
19 kJmol–116 kJmol–1
4 kJmol–1
dihedral angle4conformations
CH3
HHCH3
H HH
CH3HCH3
H H
©Graham Johnson, Graham Johnson Medical Media, Boulder, Colorado
Noyoudo not
have toremembervalues
anti-periplanar
no strain
CH3
HH CH3
H3C HH
HH
CH3
H HCH3
H HCH3
H H
staggered
this is the most important
conformation...the most favoured / preferred...
anti-periplanar
no strain
CH3
HH CH3
H3C HH
HH
CH3
H HCH3
H HCH3
H H
staggered methyl groups (or anyother groups for that matter) are as
far apart as they can be (easiest seen on Newman projection but must get
used to visualising onstick diagram)
H
H3CH
H3C
H H
CH3
H H
H
H3C H
CH3
HH
H
H3CH
anti-clinaleclipsed
torsional strain16 kJmol–1
torsional strain6 kJmol–1
torsional strain4 kJmol–1
first of the eclipsed but not that important...
syn-clinal (gauche)staggered
steric strain4 kJmol–1
H
HH3CCH3
H H
CH3
H HH
HH3C
CH3
H HH
H3C H
new kind of staggered conformation...no overlap so no torsional strain...
steric strain4 kJmol–1
syn-clinal (gauche)staggered
steric strain4 kJmol–1
H
HH3CCH3
H H
CH3
H HH
HH3C
CH3
H HH
H3C H
but two groups are close...and objects don’t like
being close so they repel each other...
steric strain4 kJmol–1
syn-clinal (gauche)staggered
steric strain4 kJmol–1
H
HH3CCH3
H H
CH3
H HH
HH3C
CH3
H HH
H3C H
...and we get steric strain...basically you can’t have two things occupying
the same space!
stericstrain
11 kJmol–1
...and these objects really hate it when they eclipse /
overlap...
CH3
HH
H3C
H H
CH3
H H
CH3
H H
CH3
HH
H3C
HH
syn-periplanareclipsed
torsional & steric strain19 kJmol–1
torsional strain4 kJmol–1
steric strain11 kJmol–1
...so we get the least stable (most disfavoured if that isn’t too many double negatives)
CH3
HH
H3C
H H
CH3
H H
CH3
H H
CH3
HH
H3C
HH
syn-periplanareclipsed
torsional & steric strain19 kJmol–1
torsional strain4 kJmol–1
steric strain11 kJmol–1
...all bonds overlap (torsional strain) and the two methyl groups
are as close as possible (steric strain)
Noyoudo not
have toremembervalues
most importanttwo extremes
CH3
HHCH3
H H
CH3
H3C HH
HH
anti-periplanar
(staggered)
CH3
HH
H3C
H H
CH3
HH
H3C
HH
syn-periplanar
(eclipsed)
learn!
another important form of strain...
another important form of strain...
and it has an ace (to my
juvenile mind) name...
0
20
40
60
80
100
120
3 4 5 6 7 8ring size
ring
stra
in (k
Jmol
–1)
ring strain as we can see, most cyclic systems contain considerable strain...
0
20
40
60
80
100
120
3 4 5 6 7 8ring size
ring
stra
in (k
Jmol
–1)
ring strain...cyclopropane really
is a very unhappy bunny...but why?
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 strain109°
(tetrahedral)
49° 60°90°
19°
108°1°
109°(tetrahedral)
109°(tetrahedral)ring or angle
strain...
ring strain109°
(tetrahedral)
49° 60°90°
19°
108°1°
109°(tetrahedral)
109°(tetrahedral)
remember an sp3 carbon wants bond angles of 109°...
ring strain109°
(tetrahedral)
49° 60°90°
19°
108°1°
109°(tetrahedral)
109°(tetrahedral)
...internal angle of atriangle is 60°...so bonds are being bent to accommodate the difference...this causes a
lot of strain!
H3C NH
O
O
NHN
O HOOH
HH
O
FR–900848
this strain can beharnessed in drugs...and just
for the vets, this is an anti-fungal used to treat infections of the
lung (piccy of a seagulllung)
and the most important ring...
and the most important ring...
one you’ll grow to hate by exam time...
cyclohexane
CC
CCC
C HH
HH
H H
HH
H
H HH
N O T FLAT
N O T FLATbenzene is flat
because it has double bonds...
chair conformation
the chair conformation(as it looks like a recliner apparently) is the most
important and most stable...
No torsional strain
No angular strain
H
HH
H
H
HHH
H
H
H
H
H
H
H
H
H
H
H
H
chair conformationthree representations of the same thing...
H
HH
H
H
HHH
H
H
H
H
H
H
H
H
H
H
H
H
chair conformation
this is the most important...if you like chemistry (or want to do well at it) learn to draw this accurately
substituents
the substituents on the ring are given special
names depending on their orientation
RRR
R
R
R
equatorialposition
substituents stick out away from the ring...they are as far from anything as they possibly
can be
R
RR
R
RR axialpositionthese substituents are
vertical...above and below the ring...they are still quite close
to each other...
ring ‘flipping’
H
H
H
HH
H
H H
H
H
H
H
HH
H
HHH
H
HH
H
H
H
HH
H
H HH
H
HH HHH
HH
H HH H
H
H
H
H
H
H H
H
H
H
H
H
H
H
H
H
H
H
1 3
5
4
261
1
1
1 1
2
2
3
4
4
5
4
6
6
chair(strain free)
chair(strain free)
boat(strained)
ring ‘flipping’
H
H
H
HH
H
H H
H
H
H
H
HH
H
HHH
H
HH
H
H
H
HH
H
H HH
H
HH HHH
HH
H HH H
H
H
H
H
H
H H
H
H
H
H
H
H
H
H
H
H
H
1 3
5
4
261
1
1
1 1
2
2
3
4
4
5
4
6
6
chair(strain free)
chair(strain free)
boat(strained)
simply by rotating the bonds we can make the axial substituents
become equatorial (and vice-versa)
ring ‘flipping’
H
H
H
HH
H
H H
H
H
H
H
HH
H
HHH
H
HH
H
H
H
HH
H
H HH
H
HH HHH
HH
H HH H
H
H
H
H
H
H H
H
H
H
H
H
H
H
H
H
H
H
1 3
5
4
261
1
1
1 1
2
2
3
4
4
5
4
6
6
chair(strain free)
chair(strain free)
boat(strained)
no bonds broken during this...it is just a change
in conformation
ring ‘flipping’
H
H
H
HH
H
H H
H
H
H
H
HH
H
HHH
H
HH
H
H
H
HH
H
H HH
H
HH HHH
HH
H HH H
H
H
H
H
H
H H
H
H
H
H
H
H
H
H
H
H
H
1 3
5
4
261
1
1
1 1
2
2
3
4
4
5
4
6
6
chair(strain free)
chair(strain free)
boat(strained)
process passes through a nasty, high energy conformation...the
boat...
ring ‘flipping’
ener
gy
29 kJmol–1
this shows the energy of the molecule during ring flipping...note how the chair is
wonderfully stable and nothing else is...
boat conformation
disfavoured as the ‘bow’ and ‘stern’ are being
brought close together (steric strain) and...
boat conformation
H
H
H
HH H
H H
H H
HH
H
H
H
H
H
H
H
H
29 kJmol–1
torsional strain as C–H bonds overlap
drawingsubstituents
learn todraw the chair
conformation...it will get you marks
in the exam!
drawhowto
draw a V on an angle and then learn to draw parallel lines
(hmmm, there’s an album title in there somewhere)
parallellines
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
parallellines
second one is parallel and of the same length
parallelline!
another parallel line (of same length)
parallelline!another
finally, close the ring with another parallel
line
carbon skeleton
?substituentswheredothego
axialaxial groups are always vertical
tetrahedralC
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)
HH
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
H
HH
H
HH ...and vice-versa for the lower carbons
equatorialstick outwards (and will be parallel) and guess
what...
tetrahedralC
carbon is tetrahedral so draw it like that!
C–Cparallel
to
parallel linesH
HH
H
H
HHH
H
H
H
H
and look...a tetrahedral
carbon
H
HH
H
H
HHH
H
H
H
H
H
HH
H
H
HHH
H
H
H
H more parallel
lines
H
HH
H
H
HHH
H
H
H
Hand even
more parallel lines
what happens if we add substituents?
one substituentCH3
CH3
H
CH3
H
95%equatorial
more stable by8 kJmol–1
5%axial
disfavoured
a single substituent will always go for the
equatorial position...
one substituentCH3
CH3
H
CH3
H
95%equatorial
more stable by8 kJmol–1
5%axial
disfavoured
remember: ring flipping allows us to change between conformations
without breaking any bonds
one substituentCH3
CH3
H
CH3
H
95%equatorial
more stable by8 kJmol–1
5%axial
disfavoured
but why equatorial??
1,3-diaxial interactionsCH3
HHH H
CH3H
H
equatorial position sticks into space...away from ring
1,3-diaxial interactionsCH3
HHH H
CH3H
H
axial substituent is tucked under ring...and we get interaction between the
three substituents on the same face
1,3-diaxial interactionsCH3
HHH H
CH3H
Hknown as 1,3-diaxial
interaction
H
H
equatorialfavoured
axialdisfavoured
one big substituent
X
when you have a big substituent it fixes the
ring and stop ring flip...
HH
H
HHH
1,3-diaxial interactions
HH
H
HHH
1,3-diaxial interactions
...because 1,3-diaxial interaction really
disfavoured
•conformations of molecules
•conformations of cyclohexane
what have....we learnt?
Picture: © Chris Ewels
©Pink Sherbet Photography@flickr
practice drawing thecyclohexane chair
©Pragmagraphr@flickr
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