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Lecture7: 123.101
 

Lecture7: 123.101

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    Lecture7: 123.101 Lecture7: 123.101 Presentation Transcript

    • Unit One Part 7:conformation just when you thought it was safe to look at molecules again... came shape!
    • Partstrain 7Unit One3D representationsconformations 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 is conformational isomerism? well, first off isomerism is a misnomer! Noisomers here just bond rotation...
    • rotation of a bond sorry movies don’t work in the printed form...
    • 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
    • representations ofconformational 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
    • representations ofconformational 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
    • representations ofconformational 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
    • representations ofconformational 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
    • some conformationsmore favourable... whilst all can exist...some are more important...
    • dihedralangle H dihedral H angle H H H H we’ll start with the simplest example...ethane
    • dihedralangle 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
    • dihedralangle 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
    • HH HH HH HHH H H HH H H H H H H H H H H H 12 kJmol–1energy H H H2 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
    • HH HH HH HHH H H HH 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–1energy then you won’t people able to see the back atoms H H H2 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
    • is this picture clearer?
    • HH HH HH HHH H H HH H H H H H H H H H H H 12 kJmol–1 in this conformation the Henergy atoms are as far apart as the bonds will allow H H H2 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
    • you do not need !to learn these values!
    • 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
    • staggered conformation H H H H H H H H H H H H H H H H H H
    • 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
    • eclipsed conformation HH H H H H H H H H H H H H H H H H
    • 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
    • HH HH HH HH as the difference is 12H H H HH H H HkJmol–1 and three bonds H H H H H H H H are overlapping...each bond must contribute... 12 kJmol–1energy H H H2 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
    • torsional strain HH H H H H 4kJmol–1
    • what about morecomplex molecules?
    • 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
    • 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
    • butane with butane we can rotate three different C–C bonds... H H H H H H H H H HH 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
    • 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
    • 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 HH 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
    • H3CCH H3C H H3C H H3CCH 3 3H H H3C HH H H H H H H H H CH3 H H now there are four important conformations based on staggered and eclipsedenergy 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
    • H3CCH H3C H H3C H H3CCH 3 3H H H3C HH H H H H H H H H CH3 H H 19 kJmol–1 16 kJmol–1energy 4 kJmol–1 CH3 CH3 CH34 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
    • No you do not have to remember values©Graham Johnson, Graham Johnson Medical Media, Boulder, Colorado
    • anti-periplanarstaggered CH3H H H H CH3 H CH3 H HH H CH3 H H H3C H CH3 this is the most important conformation...the most no strain favoured / preferred...
    • anti-periplanarstaggered 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) CH3H H H H CH3 H CH3 H HH H CH3 H H H3C H CH3 no strain
    • anti-clinal eclipsed torsional strain H3C 4 kJmol–1 H H H CH3 CH3 H HH3C 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
    • syn-clinal (gauche) staggered CH3H3C 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
    • syn-clinal (gauche) staggered steric strain 4 kJmol–1 CH3H3C 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
    • syn-clinal (gauche) staggered steric strain 4 kJmol–1 CH3H3C 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!
    • steric strain ...and these objects really hate it when they eclipse / overlap... 11 kJmol–1
    • 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
    • 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–1torsional & steric strain
    • No you do not have toremember values
    • 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)
    • another importantform of strain...
    • another importantform of strain... and it has an ace (to my juvenile mind) name...
    • 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
    • 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
    • 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
    • ring strain 109° 109° (tetrahedral) (tetrahedral) 49° 60° 19° 90° ring or angle 109° strain... (tetrahedral) 1° 108°
    • ring strain 109° 109° (tetrahedral) (tetrahedral) remember an sp 3 carbon wants bond angles of 109°... 49° 60° 19° 90° 109° (tetrahedral) 1° 108°
    • 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°
    • this strain can be harnessed in drugs...and justfor 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
    • and the mostimportant ring...
    • and the mostimportant ring... one you’ll grow to hate by exam time...
    • H H H H H C C H C H C C H C H H H Hcyclohexane
    • NOTF L AT
    • NOTF L ATbenzene 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 strainNo angular strain
    • 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
    • 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 Hchair conformation
    • the substituents on the ring are given specialnames depending on their orientation substituents
    • substituents stick out away from the ring...they are as far R from anything as they possibly can be R R R R Requatorial position
    • R R R R axial R these substituents are Rvertical...above and below the positionring...they are still quite close to each other...
    • 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 1H H H 6 2 H 6 2 4 6 2H 1 H H H H H H H 4 H H 1 H H
    • 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 1H H H 6 2 H 6 2 4 6 2H 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)
    • 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 1H H H 6 2 H 6 2 4 6 2H 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
    • 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 1H H H 6 2 H 6 2 4 process passes through 6 2H 1 H H H H nasty, high energy 4 a H H H H 1 H conformation...the H H boat...
    • energy 29 kJmol–1 this shows the energy of the moleculeduring ring flipping...note how the chair is wonderfully stable and nothing else is... ring ‘flipping’
    • boat conformation disfavoured as the ‘bow’ and ‘stern’ are being brought close together (steric strain) and...
    • 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–1boat conformation
    • learn to draw the chairconformation...itwill get you marks drawing in the exam! substituents
    • howtodraw draw a V on an angle and then learn to draw parallel lines (hmmm, there’s an album title in there somewhere)
    • 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
    • parallel lines second one is parallel and of the same length
    • parallel line! another parallel line (of same length)
    • parallelanother line! finally, close the ring with another parallel line
    • carbonskeleton
    • where do ? thesubstituents go
    • axialaxial groups arealways vertical
    • 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) Ctetrahedral
    • H H H so the three top carbonshave a vertical line upwards NOT down as this wouldprevent the carbon looking like a tetrahedron!top carbons go up
    • alternate vertical lines H H H H H H ...and vice-versa for the lower carbons
    • equatorial stick outwards (and will be parallel) and guess what...
    • carbon is tetrahedral so draw it like that! Ctetrahedral
    • parallel C–C to
    • H H H H H HH H H H parallel lines H H
    • H H and look...a H H tetrahedral carbon H HH H H H H H
    • H H H H more H H parallel linesH H H H H H
    • H H H H H HH and even H more parallel lines H H H H
    • what happens if weadd substituents?
    • one substituenta 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
    • one substituent remember: ring flipping allows us to change between conformations without breaking any bonds CH3 H CH3 H CH3 95% 5% equatorial axialmore stable by disfavoured 8 kJmol–1
    • one substituentbut why equatorial?? CH3 H CH3 H CH3 95% 5% equatorial axialmore stable by disfavoured 8 kJmol–1
    • equatorial position sticks into space...away from ring H H H H CH3 H H CH31,3-diaxial interactions
    • axial substituent is tucked under ring...and we get interaction between thethree substituents on the same face H H H H CH3 H H CH31,3-diaxial interactions
    • known as 1,3-diaxial interaction H H H H CH3 H H CH31,3-diaxial interactions
    • one big substituent when you have a big substituent it fixes thering and stop ring flip... H X H equatorial axial favoured disfavoured
    • H H H H H H1,3-diaxial interactions
    • ...because 1,3-diaxial interaction really disfavoured H H H H H H1,3-diaxial interactions
    • what have ....we learnt? • conformations of molecules • conformationsPicture: © Chris Ewels of cyclohexane
    • practice drawing thecyclohexane chair ©Pink Sherbet Photography@flickr
    • readpart 8 ©Pragmagraphr@flickr