conformation - organic chemistry from examville.com

8/9/2019 Conformation - Organic Chemistry from Examville.com

http://slidepdf.com/reader/full/conformation-organic-chemistry-from-examvillecom 1/106

Unit One Part 7:conformation

just when you

thought it was safe

to look at molecules

again... came

shape!



strain3D representations

conformations

7Unit One

Part

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 isconformational isomerism?



what isconformational 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 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



representations of


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



representations of


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



representations of


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



some conformations more favourable... whilst all can

exist...some are

more important...



dihedralangle

H

H H

H

H

H

dihedral

angle

we’ll start with

the simplest

example...ethane



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...



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...



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



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



is this picture

clearer?



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



you do not need

to learn these values!!

t i l



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



staggeredconformation

H

HH

H

H H

H

H HH

H H H

H HH

HH

t d



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



li d



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



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...



what about morecomplex molecules?



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



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



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



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



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



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



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



©Graham Johnson, Graham Johnson Medical Media, Boulder, Colorado

anti periplanar



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



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



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 )



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 )



steric strain4 kJmol–1


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 )





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



stericstrain

11 kJmol–1

...and these objects really

hate it when they eclipse /

overlap...

syn periplanar



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


...so we get the least stable

(most disfavoured if that isn’t

too many double negatives)

syn periplanar



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


...all bonds overlap (torsionalstrain) and the two methyl groups

are as close as possible (stericstrain)



t i t ttwo extremes



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!



another important form of strain...



another important form of strain...

and it has an

ace (to myjuvenile mind)

name...

ring strain



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



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



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



ring strain109°

(tetrahedral)

49° 60°

90°

19°

108°

1°

109°(tetrahedral)

109°(tetrahedral)

ring or angle

strain...

ring strain



ring strain109°

(tetrahedral)

49° 60°

90°

19°

108°

1°

109°(tetrahedral)

109°(tetrahedral)

remember an sp3

carbon wants bond

angles of 109°...

ring strain



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!



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)



and the mostimportant ring...



and the mostimportant ring...

one you’ll grow tohate by exam time...


http://slidepdf.com/reader/full/conformation-organic-chemistry-from-examvillecom 60/106c clohe ane

C

C C C

CC

H

H

HH

H H

HH

H

H HH



N O TFLAT



N O TFLATbenzene is flat

because it has doublebonds...

chair conformation



chair conformation

the chair conformation(as it looks like a recliner

apparently) is the most

important and most stable...

N



Notorsional strain

Noangular strain



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



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



substituents

the substituents on the

ring are given special

names depending on theirorientation



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



R

R

R

R

RR

i lposition

these substituents are

vertical...above and below the

ring...they are still quite close

to each other...


http://slidepdf.com/reader/full/conformation-organic-chemistry-from-examvillecom 74/106ring ‘flipping’

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



boat conformation

disfavoured as the ‘bow’

and ‘stern’ are being

brought close together

(steric strain) and...


http://slidepdf.com/reader/full/conformation-organic-chemistry-from-examvillecom 76/106boat conformation

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



drawingsubstituents

learn to

draw the chair conformation...it

will get you marksin the exam!

howto



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



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



parallellines

second one is parallel

and of the same length

parallelline!



parallelline!

another parallel line

(of same length)

parallelline!



parallelline!

finally, close the ring

with another parallel

line



?substituentswheredo

the

go



axialaxial groups are

always vertical



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)



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



alternate vertical lines

H

H

H

H

HH ...and vice-versa for

the lower carbons



equatorialstick outwards (and will

be parallel) and guess

what...



tetrahedral

carbon is tetrahedral so draw it like that!



C–Cparallel

to



H

H

H

H

H

HH

H

parallel lines



H

H

H

H

H

HH

H

and look...atetrahedral

carbon



H

H

H

H

H

HHH

H

H

H

H

more

parallel

lines



H

H

H

H

H

HHH

H

H

H

Hand even

more parallel

lines



what happens if weadd substituents?

one substituent



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



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



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



1 3 diaxial interactions

CH3

HH

HH

CH3H

H

i l b tit t i t k d




CH3

HH

HH

CH3H

H

axial substituent is tucked

under ring...and we get

interaction between the

three substituents on the

same face



one big substituent



H

H

equatorialfavoured

axialdisfavoured

one big substituent

X

when you have a big

substituent it fixes the

ring and stop ring flip...



H

H

H

HH

H




H

H

H

HH

H


...because 1,3-diaxial

interaction really

disfavoured

what havee learnt?



•conformationsof molecules

....we learnt?

conformation - organic chemistry from examville.com

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