11.konformasi alkana dan sikloalkana

8/10/2019 11.Konformasi Alkana Dan Sikloalkana

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Conformations of Alkanes and Cycloalkanes


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I. Conformations of Alkanes

A. Ethane: torsional strain

C C

H

HH

H

HH C C

H H

H HH H

rotate 60

staggeredconformation

eclipsedconformation

in between:skewed

Chem3D

Newman projections: sight along C-C bond

H

HHH

HH H

HH

H

HH

Stereoisomers:

Isomers with thesame connectivity,

but different 3-D

orientation of their

atoms in space.


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A. Ethane: torsional strain

H

HHH

HH H

HH

H

HH

lower energy higher energy

DG~ 3 kcal/mol

K ~ 0.01

torsional strain

DG

0

3

H

HH H

HHH

HH H

HHH

HH

H

HH

Ea~ 3 kcal/mol

= barrier to free rotation

(but at room temp most

molecules have KE >Easo

rotation is essentially free)

krot~ 106s-1


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B. Butane: steric repulsions CH3CH2CH2CH3

CH3

CH3

CH3

H3C

CH3H3C

CH3

CH3 CH3CH3

CH3

CH3

I

anti

(180)

II III

gauche

(60)

IV VIV

gauche

(60)

gauche~ 0.8 kcal higher energy than anti

- van der Waals repulsions= steric strain

eclipsed: 3 kcal torsional strain

+ 0.3 kcal each CH3-H eclipse

+ ~ 3 kcal each CH3

-CH3

eclipse

Chem3D


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B. Butane: steric repulsions

I II III IV V VI

0

2

4

6

DG

3.60.8

~6

0.83.6

CH3

CH3

CH3

H3C

CH3H3C CH3

CH3 CH3CH3 CH3

CH3

I II III IV VIV


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II. Conformations of Cycloalkanes

A. Stabilities of cycloalkanesDHcomb

per CH2

Total

ring strain

166.6 kcal 31.5 kcal

162.7 26.4

157.3 7.0

156.1 0

157.0 6.3

157.3 9.6

156.2 1.2> C12

small

normal

medium

large

angle strain and

torsional strain

minimal strain

transannular

steric strain


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A. Stabilities of cycloalkanes

HH

HH

H Hpoor overlap = bond angle strain

(i.e., 109.5sp3in 60 triangle)

plus ,

H

H

H

H

H

H

all Hs eclipsed =

torsional strain

Chem3D


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A. Stabilities of cycloalkanes

H

H

H

H

H

H

H

HH

H

H

H

HH

H

H

planar, 90but all eclipsed

puckered, 88slightly more angle strain,

but less eclipsing strain

Chem3D

planar, 108

but all eclipsed

envelope

relieves eclipsing


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B. Conformations in cyclohexane

1. chair and boat conformations

H

HH

H

H

H

HH

H

HH

HH

H

H

H

H

H

HH

HH

H

H

chair conformation

- all staggered

- no eclipsing- no steric strain

no ring strain

(99.99% at room temp.)

boat conformation

- eclipsing ~ 4 kcal

- steric strain ~ 3 kcalring strain ~ 7 kcal

DG~ 7 kcal

Chem3D

skewed boat ~ 1.5 kcal

more stable than boat

(0.01% at room temp.)

flagpole interaction


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2. equatorial and axial positions

H

HH

H

H

H

HH

H

HH

H

H

HH

H

H

H

HH

H

HH

H

axialpositions equatorialpositions

3. chair-chair interconversion

H

HH

H

H

H

HH

H

HH

H

H

H H

H

H

H

H

HH

HH

H

Ea~ 10 kcal


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4. drawing cyclohexane chairs


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C. Substituted cyclohexanes

CH3

CH3

H

H

H

CH3

H

1,3-diaxial

repulsions

equatorial

(95%)

no steric strain

(anti)

axial

(5%)

steric repulsions

(gauche)

DG~ 1.8 kcal

(or 0.9 kcal

per CH3-H

repulsion)

Chem3D

Ray
http://web.uccs.edu/danderso/chem331/pp/cyclohexconf.ppthttp://web.uccs.edu/danderso/chem331/pp/cyclohexconf.ppt


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C. Substituted cyclohexanes

H

H

More pronounced effect with larger groups:

DG~ 5.5 kcal

(99.99%) (0.01%)

locked in equatorial conformation


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D. Disubstituted cyclohexanes

CH3

CH3

CH3

CH3trans- cis-1,4-dimethylcyclohexane

stereoisomers

*configurational conformational

(cannot convert from (can be converted fromone to another without to another by rotation

breaking bonds) about a bond)

*geometric isomers


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CH3

CH3

CH3CH3

H

H

CH3 H

CH3

H

CH3

CH3

DG~ 3.6 kcal

diequatorial

no repulsions

diaxial

4 1,3-diaxial repulsions

= 4 x 0.9 = 3.6 kcal


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CH3

CH3

H3C CH3

CH3

CH3

H

H

H

CH3

CH3

H

DG= 0 kcal

equatorial-axial

2 x 0.9 = 1.8 kcal

axial-equatorial

2 x 0.9 = 1.8 kcal


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CH3

CH3

CH3CH3

H

H

H CH3

CH3

H

1gaucheinteraction

= 0.9 kcal

4 1,3-diaxial repulsions

= 4 x 0.9 = 3.6 kcal

DG~ 2.7 kcal


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CH3

CH3

CH3H3C

H CH3CH3

DG~ 5.4 kcal

no repulsions 2 1,3-diaxial CH3-H = 1.8 kcal

1 1,3-diaxial CH3-CH3= 3.6 kcal


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Larger groups predominate in determining conformation:

CH3

CH3

tBu

CH3

DG~ 3.7 kcal

1.8 kcal5.5 kcal


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Question 3-1. Draw the most stable chair form of the following compounds.

Explain. Click on the arrow to check your answers.

CheckAnswer

CH3

CH3CH3CH3

CH(CH3)2


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Answer 3-1. Draw the most stable chair form of the following compounds.

Explain. Click on the arrow to check your answers.

CH3

CH3CH3

CH3CH3

CH3CH3

CH(CH3)2

All groups can be equatorial. This

chair form is more stable than the

other, where all are axial.

Isopropyl is bigger than a methyl

group, so more stable chair is where

larger group is equatorial.

CH3

CH(CH3)2


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III. Polycyclic Rings

OHdecalin borneol adamantane prismane

bicyclic tricyclic tetracyclic

Bicycloalkanes:

bicyclo[x.y.z]alkane (x y z)

numbering starts at a bridgehead,proceeds around the largest bridge first,

then around successively smaller

bridges

C

C

Cz

Cy

Cx


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III. Polycyclic Rings

bicyclo[4.0]decane

bicyclo[2.2.1.]heptane

bicyclo[4.1.0]heptane

bicyclo[3.2.1]octane


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IV. Heterocyclic Compounds

O

O

O

O

N

H

NH

ethylene oxide

oxiraneoxacyclopropane

oxetane

oxacyclobutane

tetrahydrofuran

oxacyclopentane

tetrahydropyran

oxacyclohexane

pyrrolidine

azacyclopentane

piperidine

azacyclohexane

O O

furan pyran

11.konformasi alkana dan sikloalkana

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