Conformational Analysis 52t2a

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Different conformations











Different configurations







monformations of ethane Eclipsed co formatio

taggered co formatio







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Wedge-a d-das str ct res

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Saw orse projectio s





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otential energy of ethane as function of torsion angles

ástaggere conformation has otential energy minimum áecli se conformation has otential energy maximum á staggere conformation is lo er in energy than the ecli se by 2 kcal/mole (12 kJ/mole)

hy is the ecli se conformation higher in energy than the staggere conformation?

áRhe H-atoms are too small to get in each other¶s ay-steric factors make u < 10% of the rotational barrier in ethane

Rorsional strain mause by re ulsion of the bon ing electrons of one substituent ith the bon ing electrons of a nearby substituent

fille orbitals re el

m mx mx



Rhe real icture is robably a mixture of all 3 effects á Rhe rotational barrier is (12 kJ/mol) small enough to allo the conformational isomers to interconvert million of times er secon

monformations of butane





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otential energy of butane as a function of torsion angle

ÿ D D ÿ D mÿ D D ÿ D D

mÿ D D Ô

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m ! " # á malculations reveal that at room tem erature ~72% of the molecules of butane are in the ³anti´ conformation, 28% are in ³gauche´ conformation ÿ D

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monformations an monformers Butane can exit in an infinite number of conformations (6 most im ortant have been consi ere ), but has only 3 conformers ( otential energy minima)-the t o ³gauche´conformations an the ³anti´ conformations á R - -

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monformation an hy rogen bon ing

monformation of butane-2,3- iols

mycloalkanes Ring strain o of atoms in ring

3 4 6 7 8

Internal angle in lanar ring

600 00 1080 1200 128 0 130

* a measure of strain er m-atom

All internal angles 10 0m

10 0-internal angle*

4 0 1 0 1 0 -10 0 -1 0 -2 0

u 3 u u u u

áRing strain largest for 3-membere rings, then ecreases through a 4-membere ring an reaches a minimum for member ring

áre iction: lanar -membere ring shoul have the minimum level of ring strain

áRhe ring strain kee s on increasing as the rings get larger after the minimum at



á the ifference bet een any t o in series is very nearly constant at aroun ±660 kJ/mole á assuming there is no strain in the straight-chain alkanes, each extra ±mH2 grou contributes an extra 68 7 kJ/mole to the heat of combution á if cycloalkane is strain free, its heat of combustion shoul be n X 68 7 kJ/mole If there is some strain ,more energy is given out on combustion

Look at the gra h hich sho s, for each ring size: (a) angle strain er mH2 grou (b) heat of combustion er mH2 grou

á Rhe greatest strain is in the 3-membere ring

á Rhe strain ecreases ith ring size an reaches a minimum for 6-membere ring

á Rhe strain then increases, but not as quickly as the angle calculation suggeste : it reaches a maximum at an then ecreases

á Rhe strain remains constant at ~14, not increases stea ily as the angle-strain suggeste

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myclic com oun s t ist an ben to minimize the 3 ifferent kin s of strain 1 Angle strain 2 Rorsional strain 3 teric strain myclo ro ane á banana bon s oor orbital overla

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áRorsional strain

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Goo overla trong bon

oor overla eak bon

myclobutane







to re uce torsional angle myclobutane

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myclo entane

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áone carbon atom is bent u ar s áRhe molecule is flexible an shifts conformation constantly áHence each of the carbons assume the ivotal osition in ra i succession

áRhe a itional bon angle strain in this structure is more than com ensate by the re uction in ecli se hy rogens

á ith little torsional strain an angle strain, cyclo entane is as stable as cyclohexane

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myclohexane





mhair conformation



flag ole hy rogens (- ×

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um of the van er

aals ra ii = 2 4 A0

× × Boat conformation

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e man rojection of the boat conformation

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á Rhe energy ifference bet een the chair, boat, an t ist conformation of cyclohexane are lo enough to make their se aration im ossible at r t At room tem erature a rox 1 million introversions occur each other secon

á ore than % of the molecules are estimate to be in chair conformation at any given time

onosubstitute cyclohexane ±

± Rhis conformation is lo er in energy

hy? ×

±

hen X=mH3, conformer ith e in axial is higher in energy by 7 3 kJ/mol than the corres on ing equatorial conformer

Result: 20:1 ratio of equatorial:axial conformer at 200 m

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1,3- iaxial interaction



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Rhe black bon s are antieri lanar (only one air sho n)

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Rhe black bon s are synclinal (gauche) (only one air sho n)

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monc of equatorial conformer "3 monc of axial conformer

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m× 1 gauche-butane interaction 0 kcal/mol

4 gauche-butane interaction 4 x 0 kcal/mol = 3 6 kcal/mol

Difference in stability bet een the conformational isomers

3 6 - 0 = 2 7 kcal/mol

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This has 3 gauche-butane interactions

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>ery ba steric situation ~ kcal/mol (4 x 0 = 3 6 kcal/mol + ethyl-ethyl interaction) m×

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It is a resolavable molecule 2-gauche-butane interaction = 1 8 kcal/mol

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roblem: hich of the follo ing com oun s are resolvable, an hich are non resolvable? hich are truly meso? a) cis-1,2-cyclohexane iol; b) trans-1,2-cyclohexane iol; c) cis-1,3-cyclohexane iol; ) trans-1,3-cyclohexane iol; e) cis-1,4-cyclohexane iol; f) trans-1,4-cyclohexane iol

Hint: ©H ©H ©H ©H

©H ©H ©H on resolvable (easily interconvertible by fli

©H trans (resolvable) ing) ©H

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cis (meso)

trans (resolvable) ©H

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achiral (absence of chirality center)

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Rigi molecules from cyclohexane conformers

monformational equilibrium in 1- henyl-1-methyl cyclohaxane

©H

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H© m©2H Lactonization of 3-hy roxy cyclohexane carboxylic aci

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H©2m m©2H myclic anhy ri e formation from 1,3-cyclohexane icarboxylic aci

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Intramolecular H-bon ing in 1,3cyclohexane iol

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2 eliminations have anti- eri lanar transition states R 6 6 × 6 ± 6

In 2 eliminations, the ne R bon is forme by overla of the m-H x bon ith the m-X x* antibon ing orbital

2 eliminations have anti- eri lanar transition states

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roblem: ©n treatment ith the aromatic base yri ine, racemic 1,2- ibromo-1,2- i henyl ethane loses HBr to yiel trans-1-bromo-1,2- i henyl ethane; In contrast the meso ibromi e loses Br 2 to yiel trans-1,2- i henyl ethene uggets a mechanism? Hnit: Br

Br h

H

H

Br

h

h Br

H

H

H

H

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Br

Br2 loss is not favore

Br

h Racemic

H h H h

h H

hH

Br

H

Br h meso

H

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Br

H

Br

-HBr

Br

Br h h

Br Br

h h

h H

-Br2 h