conformasi cycloalkana

7/28/2019 Conformasi Cycloalkana

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Conformational Analysis and

stability of cycloalkane

Minggu ke-5


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Conformations of Ethane Stereochemistry concerned with the 3-D aspects of

molecules

bonds are cylindrically symmetrical Rotation is possible around C-C bonds in open-chain

molecules


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Conformers Conformation- Different arrangement of atoms

resulting from bond rotation

Conformations can be represented in 2 ways:


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Torsional StrainWe do not observe perfectly free rotation

There is a barrier to rotation, and some conformers are

more stable than others Staggered- most stable: all 6 C-H bonds are as far

away as possible

Eclipsed- least stable: all 6 C-H bonds are as close as

possible to each other


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Conformations of Other Alkanes

The eclipsed conformer of propane has 3 interactions:two ethane-type H-H interactions, and one H-CH3interaction


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Conformational situation is more complex for larger alkanes

Not all staggered conformations has same energy, and not all

eclipsed conformations have same energy

Conformations of Other Alkanes


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Conformations of Butane Anti conformation- methyl groups are 180 apart Gauche conformation- methyl groups are 60 apart

Which is the most energetically stable?


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Steric Strain Steric strain- repulsive interaction occurring between atoms that

are forced closer together than their atomic radii allow


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Relationship to Gauche Butane Interactions

Gauche butane is less stablethan anti butane by 3.8 kJ/molbecause of steric interference

between hydrogen atoms onthe two methyl groups

The four-carbon fragment ofaxial methylcyclohexane andgauche butane have the samesteric interaction

In general, equatorialpositions give more stableisomer


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Stability of Cycloalkanes: The Baeyer Strain

Theory Baeyer (1885): since (sp3)

carbon prefers to have bondangles of approximately109, ring sizes other than

five and six may be toostrainedto exist

Rings from 3 to 30 Cs doexist but are strained due to

bond bending distortionsand steric interactions


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The Nature of Ring Strain Rings larger than 3 atoms are not flat (planar).

Cyclic molecules can assume nonplanar conformations

to minimize angle strain and torsional strain by ring-puckering

Larger rings have many more possible conformationsthan smaller rings and are more difficult to analyze


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Types of StrainAngle strain- expansion or compression of

bond angles away from most stable

Torsional strain - eclipsing of bonds onneighboring atoms

Steric strain - repulsive interactions

between nonbonded atoms in closeproximity


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Angle Strain


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Conformations of Cycloalkanes

Cyclopropane Most strained of all the rings

Angle strain caused by 60 C-C-C bond angles

Torsional strain caused by the eclipsed C-H bonds onneighboring carbon atoms


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Bent bonds in cyclopropane: less than

maximum orbital overlap


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Cyclobutane Total strain is nearly the same as cyclopropane

Angle strain less than cyclopropane

Torsional strain more than cyclopropane because oflarger number of ring hydrogens

Not planar(puckered)

One carbon atom lies 25 above the plane of the other three


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Cyclobutane


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Cyclopentane Less strain than cyclopropane or cyclobutane

Planar cyclopentane:

Angle strain very minimal

Torsional strain large amount Twists to a nonplanar (puckered) conformation

Balance between increased angle strain and a decreased torsional strain


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Conformations of Cyclohexane

Cyclohexane

Adopts chair conformation

No angle strain

All C-C-C bonds near 109 No torsional strain

C-H bonds staggered


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Boat Cyclohexane

Cyclohexane flips through a boatconformation

Less stable than chaircyclohexane due to steric andtorsional strain

C-2, 3, 5, 6 are in a plane H on C-1 and C-4 approach each

other closely enough to produceconsiderable steric strain

Four eclipsed H-pairs on C- 2, 3,5, 6 produce torsional strain

~29 kJ/mol (7.0 kcal/mol) lessstable than chair


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Boat & Twist-boat conformations:


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Chair Conformations


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Axial and Equatorial Bonds in Cyclohexane

The chair conformation has twokinds of positions forsubstituents on the ring: axialpositions and equatorialpositions

Chair cyclohexane has six axialhydrogens perpendicular to thering (parallel to the ring axis)and six equatorial hydrogensnear the plane of the ring


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Axial and Equatorial Positions Each carbon atom in cyclohexane has one axial and one

equatorial hydrogen

Each face of the ring has three axial and three equatorialhydrogens in an alternating arrangement


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Conformational Mobility of Cyclohexane

Chair conformations readily interconvert, resulting in the exchange of

axial and equatorial positions by a ring-flip


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Bromocyclohexane When bromocyclohexane ring-

flips the bromines position goesfrom equatorial to axial and so

on At room temperature the ring-

flip is very fast and the structureis seen as the weighted average


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Bromocyclohexane


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Conformations of Monosubstituted

Cyclohexanes

The two conformers of a monosubstituted cyclohexane arenot equal in energy

The equatorial conformer of methyl cyclohexane is morestable than the axial by 7.6 kJ/mol


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1,3-Diaxial Interactions Difference between axial and equatorial conformers is due to steric

strain caused by1,3-diaxial interactions

Hydrogen atoms of the axial methyl group on C1 are too close tothe axial hydrogens three carbons away on C3 and C5, resulting in7.6 kJ/mol of steric strain


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Monosubstituted Cyclohexanes


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Conformational Analysis of Disubstituted

Cyclohexanes In disubstituted cyclohexanes the steric

effects of both substituents must be taken

into account in both conformationsThere are two isomers of 1,2-

dimethylcyclohexane. cis and trans


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Cis-1,2-dimethylcyclohexane


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Cis-1,2-dimethylcyclohexane


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Conformational Analysis of 1,2-Disubstituted

Cyclohexanes

In the cis isomer, both methyl groups same face ofthe ring, and compound can exist in two chair

conformations Consider the sum of all interactions

In cis-1,2, both conformations are equal in energy


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Trans-1,2-Dimethylcyclohexane

Methyl groups are on opposite faces of the ring

One trans conformation has both methyl groupsequatorial and only a gauche butane interactionbetween methyls (3.8 kJ/mol) and no 1,3-diaxial

interactions

The ring-flipped conformation has both methyl groupsaxial with four 1,3-diaxial interactions


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Trans-1,2-Dimethylcyclohexane


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CH3

H3C

H H

Two conformations are not equivalent; most stableconformation has both methyl groups equatorial.

H3C

HH

CH3H

CH3

H

CH3

Conformational analysis of

cis-1,3-dimethylcyclohexane


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Two equivalent conformations; each has one axialand one equatorial methyl group.

H3C H

H CH3H

H3C

H

CH3

H3C

CH3

H

H


trans-1,3-dimethylcyclohexane


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CH3H3C

H H

Two equivalent conformations; each has one axialmethyl group and one equatorial methyl group

H

CH3

H

CH3

H

H3C

H

CH3


cis-1,4-dimethylcyclohexane


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CH3

H3C

H

H

Two conformations are not equivalent; most stableconformation has both methyl groups equatorial.

H

H3C

H

CH3

H

H3C

H

CH3


trans-1,4-dimethylcyclohexane


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t-Butyl Groups


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t-Butyl Groups


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t-Butyl Groups


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Most stable conformation of

Menthol?


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Solution:CH3

OH

CH

CH3

CH

HO

H3

C CH3

H3C

H3C

more stable


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Problem: Galactose has an axial OH group at

C4. Draw the chair:


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conformasi cycloalkana

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