4.1 Conformation of Ethane - Yonsei University · 4.1 Conformation of Ethane H C HH H H H C rotate...

Ch.4 Stereochemistry of Alkanes and Cycloalkanes

stereochemistry: 3-dimensional aspects of molecules

4.1 Conformation of Ethane

H

CH H

H

HHC rotate

HC

H

HH

HHC

conformation: the different arrangements of atoms that result from rotation about a single bond

conformers: a specific conformation (conformational isomer); same connections of atoms


Sawhorse representation

H

CH H

H

HHC

Newman projection

H

H HH

HHfront carbon

back carbon


Energy barrier for rotation: not perfectly free rotation about σ-bonds

H

H HH

HH H

H

H

H

HH

staggered conformation eclipsed conformation

rotate 60o

4 kJ/mol (1.0 kcal/mol)

rotational barrier:12kJ/mol (2.9 kcal/mol)

4 kJ/mol4 kJ/mol


- the 12 kJ/mol (2.9 kcal/mol) of extra energy present in the eclipsed conformation of ethane is called torsional strain

• Torsional strain is due to repulsion between electron clouds in the C-H bonds as they pass close by each other in the eclipsed conformer

H

H

H

H

HH

eclipsed conformation


H

H HH

HH

H

H

H

H

HH0o

60o 180o 300o

360o240o120o

H

H

H

H

HH

H

H HH

HHH

H HH

HH

H

H

H

H

HH H

H

H

H

HH

Ener

gy 12 kJ/mol

A graph of potentiol energy versus bond rotation in ethane


4.2 Conformation of Propane

H

H HCH3

HH H

H

H

H3C

HH

staggered conformation eclipsed conformation

rotate 60o


4.0 kJ/mol

rotational barrier:14 kJ/mol (3.4 kcal/mol)

4.0 kJ/mol


4.3 Conformation of Butane

H3CCH3

H

H

H

HH

H CH3

CH3

HH

gaucheCH3

H HCH3

HH

anti

anti conformation: two large groups are in the opposite side

gauche conformation: two large groups are 60o apart

eclipsed conformations


H3C

H

H

H3C

HH

6.0 kJ/mol

6.0 kJ/mol 4.0 kJ/mol

total cost: 16 kJ/mol (3.8 kcal/mol)eclipsed

Eclipsed conformations


H

CH3

H

H3C

HH

least stable eclipsed


total cost: 19 kJ/mol (4.6 kcal/mol)

4.0 kJ/mol 4.0 kJ/mol

Eclipsed conformations


H

H CH3

CH3

HH

gauche

3.8 kJ/mol (0.9 kcal/mol)

Gauche conformation: 3.8 kJ/mol (0.9 kcal/mol) unstable due to stericstrain between two methyl groups

steric strain: repulsive interaction that occurs when atoms are too closer

- hydrogen atoms on methyl groups interact

H

H

HH

HH

H

HH

H


H

HH3CCH3

H HCH3

H HCH3

HH

gaucheanti

staggered

H

CH3

H

H3C

HHH3C

H

H

H3C

HH

eclipsedeclipsed

Four possible conformations of n-butane

Ch.4 Stereochemistry of Alkanes and CycloalkanesA graph of potentiol energy versus bond rotation in buthane

H

HH3CCH3

H HCH3

H HCH3

HH H

CH3

H

H3C

HH CH3

H

H

CH3

H H

gaucheanti

0o60o180o 120o

Ener

gy

16 kJ/mol

60o 180o120o

CH3

H HCH3

HH

anti

H3C

H

H

H3C

HHH

H CH3

CH3

HH

gauche

3.8 kJ/mol

19 kJ/mol


CH3

CH3

3.8 kJ/mol(0.9 kcal/mol)

Energy costs for interactions in alkane conformers

HH


HH3C


CH3H3C

11 kJ/mol(2.6 kcal/mol)

torsional strain mostly torsional strain

torsional + steric strain steric strain


• the most stable conformation of any alkanes has the C-C bonds in staggered arrangements and large substituents arranged anti to each other

H H

H H

H

H H H

H H H

H H H H H

HHHH

zig-zag conformation

• At room temperature, enough thermal energy is present to cause rotation around σ-bonds to occur rapidly so that all conformations are in equilibrium. At any given time, however, a larger percentage of molecules will be found in a more stable conformation than in a less stable one.


Practice Conformation of 1-chloropropane

ClH3C

Newman projections

Cl

H HCH3

HH H

Cl

H

H3C

HHH

H ClCH3

HH H

H

Cl

H3C

HH


4.4 Stability of Cyclohexanes: The Baeyer Strain Theory

60o90o

108o 120o

109o (tetrahedral angle)

bond angles for hypothetical planar cycloalkane structures;

Angle strain (Baeyer strain): the strain induced in a molecule when a bond angle deviates from the ideal tetrahedral value


Heat of Combustion of Cycloalkanes

(CH2)n + 3n2

O2 nCO2 + nH2O + heat

heat of combustion: the amount of heat released when a compound burns completely with oxygen; used for determination of starin energies of cycloalkanes; the more energy (strained) a compound contains, the more energy (heat) is released on combistion

ring strain: total energy of the compound- the energy of a strain-free reference compound


• the strain molecules (cyclopropane, cyclobutane) are unstable and highly reactive

• cyclopentane, cyclohexane are srtain free

• medium rings C7-C13 are srtained: 115-120o

• larger rings are unstrained ≥C14

cycloalkane strain energies, calculated by taking the difference between cycloalkane heat of combustion per CH2 and acyclic alkane heat of combustion per CH2, and multiplying by the number of CH2 units in a ring. Small and medium rings are strained, but cyclohexane ring is strain-free.


cycloalkane strain energies


4.5 The Nature of Ring Strain

H

HH

H

H

H

• rings are not flat; 3-dimensional conformations• torsional strain due to eclipsed C-H bonds in ring systems

The conformation of cyclopropane, showing the eclipsing of neighboring C-H bonds that give rise to torsional strain.

H

H

H

H

H

H

eclipsed

eclipsed


• Angle strain: the strain due to expansion or compression of bond angles

• Torsional strain: the strain due to eclipsing of bonds on neighboring atoms

• Steric strain: the strain due to repulsive interactions when atoms approach each other too closely

Cycloalkanes adopt their minimum-energy conformation for the combination of three reasons:


4.6 Cyclopropane: An Orbital View

Br Br2 Na

+ 2 Na Br

colorless gas (bp= -33 oC); first prepared by reaction of sodium with 1,3-dibromopropane

bent bond:

C

C C

109o

A typical alkane C-C bond A bent cyclopropane C-C bond

C

C C

poor overlap


4.7 Conformations of Cyclobutane and CyclopentaneCyclobutane

HH

H H

H

HH

H

H

HH

H

H

HH

H

not quiteeclipsed

not quiteeclipsed

less angle strain than cyclopropane but more torsional strain because of its larger number of ring hydrogens; total strain of cyclobutane- 110.4 kJ/mol (26.4 kcal/mol); total strain of cyclopropane- 115 kJ/mol (27.5 kcal/mol); not flat but puckered→ increase angle strain but decrease torsional strain


Cyclopentane

H

H

H

HH

H

H

HH

Henvelop

total strain of cyclopentane- 26.0 kJ/mol (6.2 kcal/mol); not planar but envelop conformation


4.8 Conformations of Cyclohexane

H H

H

H

H

H

H

H

H

H

H

H

H

H CH2H

CH2HH

HH

H

chair all staggered conformation

chair conformation: bond angle 111.5o (close to the ideal 109.5o

tetrahedral angle)

• unstrained


• drawing chair conformation: draw parallel three bonds pairs

this bond is in back

this bond is in front


4.9 Axial and Equatorial Bonds

HH

H

H

H

H

H

H

H

H

H

H

axial

equatorial

H

H

H

HH

HH

H

H

top view


4.10 Conformational Mobility of Cyclohexane

ring-flip

the energy barrier for the ring flipping is small: fast ring flipping is observed ( two conformers are in fast equilibrium)


ring flipping

Br

Brring-flip

H

H


Hax

Heq

Hax

HeqEa= 45 kJ/mol

chair

- rapid interconversion at 25 ℃

(Ea= 45 kJ/mol (10.8 kcal/mol), 20 kcal/mol available at 25 ℃)

- Hax and Heq are indistinguishable by 1H NMR at 25 ℃

- at < -70 oC, Hax and Heq are distinguishable by 1H NMR


4.11 Conformation of Monosubstituted Cyclohexanes

CH3

CH3

ring-flip

∆ E = - RT ln K

• two conformers of a monosubstituted cyclohexane are in fast equilibrium at room temperature but not equally stable


CH3

CH3

ring-flip


5 kcal/mol

at RT

109 sec-1

energy barrier

10 kcal/mol 105 sec-1

15 kcal/mol 102 sec-1

> 20 kcal/mol

energy barrier vs rate constant

A B


free energy vs % of isomer

1.0 kcal/mol 80%

free energymore stableisomer(%)

1.3 kcal/mol 90%

2.3 kcal/mol 98%

4.1 kcal/mol >99.9%

∆ G = - RT ln KA B


H

CH3

CH3

HH

H7.6 kJ/mol(1.8 kcal/mol)

1,3-diaxial interaction: steric strain, butane gauche interaction

Hax

HeqHax

HeqCH3

HeqHax

Heq

Hax

Hax

7.6 kJ/mol ~ 95% equatorial methyl preference


H

CH3

H

H CH3

CH3

HH

butane gauche


butane gauche interactions

CH3

HH

H CH3

HH

H

1,3-diaxial interaction:butane gauche interaction (x2)

3.8 x 2 = 7.6 kJ/mol


R

FClBrOHCH3CH2CH3CH(CH3)2C(CH3)3C6H5COOHCN

0.120.250.250.50.90.951.12.71.50.70.1

(kcal/mol) (kJ/mol)

0.51.01.02.13.84.04.611.46.32.90.4

Steric Strain in Monosubstituted Cyclohexanes

H

X

X

H

H


4.12 Conformational Analysis of Disubstituted Cyclohexanes

CH3

CH3

• all steric interactions in both possible chair conformations must be analyzed

CH3

CH3HH

CH3

CH3HH

1 gauche interaction (3.8 kJ/mol)2 diaxial interaction (2 x 3.8 kJ/mol)total strain = 11.4 kJ/mol (2.7 kcal/mol)


trans-1,2-dimethylcyclohexane will exist almost exclusively (>99%: 2.7 kcal/mol) in the diequatorial conformation

CH3

CH3 CH3

CH3

HH

CH3

CH3H

H

HH

1 gauche interaction (3.8 kJ/mol)

4 diaxial interaction (15.2 kJ/mol)

11.4 kJ/mol more stable


Practice Conformation of cyclohexane

CH3

CH3

CH3

CH3

CH3 CH3

CH3

CH3

CH3

CH3

CH3

CH3

more stable

equivalent


Br

Br

Practice Conformation of cyclohexanes

Br

Br

more stable

more stable

Br

Br


4.13 Boat Cyclohexane

H

H

H

H

HH

H

H

H

H

HH

boat

rel E = 29 kJ/mol (7.0 kcal/mol)

twist boat

rel E = 23 kJ/mol (5.5 kcal/mol)


Hax

Heq

Hax

Heq

Ea= 45 kJ/mol

chair

half chairtwist boat

rel E = 45 kJ/molrel E = 23 kJ/mol

rotationalbarrier


4.14 Conformations of Polycyclic Molecules

H

H

trans-Decalin

0 kcal/mol

H

H

cis-Decalin

+ 2.2 kcal/mol


HH H

H

H

H

cis-decalin can adopt two chair-chair conformations

trans-decalin can adopt only one chair-chair conformation


HO

CH3

CH3

HH

H H

Cholesterol (a steroid)

CH3CH3

HO


Norbornane(Bicyclo[2.2.1]heptane

bridgehead carbonsA 1-carbon bridge

A 2-carbon bridge

bridgehead carbon: carbon shared by two rings

O Camphor

Ch.4 Stereochemistry of Alkanes and CycloalkanesMolecular Mechanics

molecular mechanics: find the minimum energy conformations of molecules by mathematical calculations

Etotal = Ebond stretching + Eangle strain + Etorsional strain + Evan der Waals

- particularly useful in pharmaceutical research; search the molecules which complementary fit with the receptor protein; design the drug molecule and synthesis them

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4.1 Conformation of Ethane - Yonsei University · 4.1 Conformation of Ethane H C HH H H H C rotate...

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