Chapter 4 - STEREOCHEMISTRY OF ALKANES AND ...
Chapter 4 - STEREOCHEMISTRY OF ALKANES AND CYCLOALKANES Remember organic chemistry us a 3-D chemistry. C
Tetrahedral carbon
Conformation of ethane: Ethane is a two-carbon compound with all single bonds on the carbons. (Show ball and stick model of ethane). There is freedom of rotation around the single bonds. This has no consequence with the C-H bonds but does have consequence with the carbon-carbon bond. Different arrangement of the hydrogen atoms in relation to each other that are the result of the rotation around the single C-C bond. These are called CONFORMATIONS A specific conformation is called a CONFORMER. Show ethane again. There are ways of representing the 3-D aspect on paper. Newman projection Sawhorse projection H H
C H
Eclipsed conformation
H
C H
H
H C H H H
H
Staggered conformation
C H
Rotation is not completely "free". There is an energy "cost".
Staggered conformation are more stable than the eclipsed conformations. In the staggered conformer, H's are as far away from each other as possible. High energy conformer (eclipsed), H's as close as possible. The energy difference between the two extremes is 12 kJ/mol [2.9 kcal/mol]. There are three H/H eclipsed interactions in that conformer. Therefore each H/H eclipse interaction contributes 4 kJ/mol [~1 kcal/mol] to the strain of free rotation. This is known as TORSIONAL STRAIN. Best described as a barrier to rotation caused by a slight repulsion of electron clouds as they pass one another. It is not a steric strain because the electron clouds of the hydrogens are not close enough to actually be in the way of each other's "space". This barrier to rotation is represented in a potential energy diagram which plots potential energy vs. the dihedral angle in the bonds.
Dihedral angle
H H
H H H H
H H
H H
H H
H
H H
H H H
Energy
H H
H
H H H H
H
H
60˚
0˚
H 120˚
H
H
H
H
H
H
H H
240˚
180˚
300˚
360˚(0˚)
Dihedral angle
Conformations of Propane and Butane (with energy diagrams). Butane H
CH3
H3C
H
H
H
H CH3 anti
Draw energy diagram.
H
H
H H gauche
H CH3
H3C H
CH3
H H
CH3
H CH3 CH3
HH
CH3
H H
H H
H CH3
HH
H CH3
In butane not all staggered (not eclipsed) conformers are of equal energy. The anti conformation is the conformer in which the two largest groups (methyls) are as far apart as possible. This is the lowest energy isomer. As start to rotate the front carbon around, get eclipsed isomer that has one H-H eclipsed interaction, and two H-CH3 eclipsed interactions. The barrier to rotation is 16 kJ/mole (4kcal/mol). We know from ethane that the H-H energy cost is 4 kJ/mol. Thus 12kJ/mol divided by 2 = 6 kJ/mole for each H-CH3. Continuing with the rotation takes us to a staggered conformer where the dihedral angle of the two methyl groups is 60˚. This conformer is higher in energy than the anti conformer even though there are no eclipsing interactions. This is the GAUCHE conformer and there is steric strain between the two methyl groups. Steric strain is the repulsive interaction that occurs when atoms are forced closer together than their atomic radii allow. Lies 3.8 kJ/mol [0.9 kcal/mol] higher than the anti conformer. Continuing with the rotation --- this conformer has two CH3 groups eclipsed along with two H-H eclipsed interactions. This conformer has a total energy cost of 19 kJ/mol [4.5 kcal/mol] over the anti conformer. If the two H-H interactions are 8kJ/mol (4 kJ/mol each) the CH3-CH3 eclipsed interaction if 11 kJ/mol [2.6 kcal/mol]. Table 4.1 Interaction
Cause
HH eclipsed HCH3 eclipsed CH3CH3 eclipsed CH3CH3 gauche
Energy Cost kJ/mol kcal/mol Torsional strain 4.0 1.0 Mostly torsional strain 6.0 1.4 Torsional + steric strain 11 2.6 Steric strain 3.8 0.9
A point to remember: molecules are always moving. That includes molecular motion and motion within the molecules - bonds rotating, vibrating, etc. By most stable conformer we mean -
1- a molecule "spends" most of its time in the stable form. 2 - at any given instant, most of the molecules will be found in a more stable conformer than a less stable one. Conformation and Stability of Cycloalkanes Bayer Strain Theory Historically, knew that ring systems existed and many five- and six-membered rings were known but smaller and larger rings had not been prepared until well into the twentieth century. Adolph von Baeyer suggested that since carbon prefers to have a tetrahedral geometry (bond angles of ~ 109˚) ring sizes other than 5 and 6 would be too strained to be made. He assumed that three-membered rings would have 60˚ angles of an equilateral triangle, four-membered rings, 90˚ in a square and fivemembered rings, a 108˚ angle pentagon. The deviation from the “ideal” angle of the flat polygon is called angle strain. Rings larger than six would have much larger angles with a negative angle strain and thus could not be made. Actually rings are generally not planar (from cyclobutane onwards) and thus rings distort out of planarity (except for cyclopropane) to lessen this angle strain.. Actual strain is measured per CH2 group in the ring by heats of combustion - the amount of energy (heat) released on complete combustion. If we compare two isomers, more energy is released during combustion of the more strained substance because that substance had more energy to begin with. 3 -(CH2)n - n (2 O2) ---------->
n CO2 + n H 2O + Heat
Comparison by CH2 group means the size of the ring is not a factor. Then compare this to heat of combustion per CH2 in some non-strained reference and multiple by the number of carbons in the ring get Figure 4.8 in text (page 111). This comparison shows that there in mo strain in a six-membered cycloalkanes ring.
Cycloalkanes adopt their minimum energy conformation (not planar) for a combination of three reasons: 1) Angle strain (compression of bond angles) 2) Torsional strain (eclipsing of bonds on neighboring atoms). 3) Steric strain (repulsive interaction when atoms get too close together) Cyclopropane HH
H C
H HH Eclipsed hydrogens Low temperture X-ray studies have shown that there in not complete overlap of orbitals in the C-C bonds. This gives rise to bent bonds. C
C C
C C
C
Ring is very reactive and opens easily. Cyclobutane Cyclobutane has less angle strain than cyclopropane but larger torsional strain because of more ring hydrogens.
H
H
H
H
Cyclopentane Baeyer predicted that cyclopentane should have little ring strain. Combustion data indicates that the ring has a strain energy of 26 kJ/mol [6.2 kcal/mol]. There is little angle strain but much torsional strain from the neighboring hydrogens. H H H H
Conformations of Cyclohexanes Cycloalkanes are common in nature. Combustion data shows these rings are strain-free - no torsional, no angle strain. Cyclohexane assumes a puckered conformation that relieves all strain. This conformation is called the chair conformation. H
H
CH2
H H See text for drawing a cyclohexane chair.
H
H
CH2
H H
Axial and Equatorial Bonds in Cyclohexane There are two kinds of positions for hydrogens in these rings that occupy axial and equatorial positions. Each of the six carbon atoms in the ring has one of each kind of position.
+
Ring showing only axial bonds
Ring showing only equatorial bonds
Ring with all bonds
LOOK AT YOUR MODELS! Bonds on the same side of the ring are cis. Bonds on opposite sides of the ring are trans. Notice that adjacent axial bonds are trans. Likewise with adjacent equatorial bonds. Axial bonds are parallel and alternate up and down. Equatorial bonds alternate between sides around the ring. Conformation Mobility of Cyclohexane The two conformations readily interconvert. -- called ring flip. Conformational flip Use models to show that axial bonds in one chair become equatorial in the flip to the other chair and equatorial bonds become axcial. Conformations of Monosubstituted Cyclohexanes The two conformers of substitutued cyclohexanes are not equally stable.
Ring flip
CH3 axial group
equatorial group
CH3
On examination, see that the axial "methyl" substituent becomes equatorial when the ring flips. The eq. isomer is 8.0 kJ.mol [1.8 kcal/mol] more stable than the axial conformer. This means that at any instant most of the methylcyclohexane molecules will be in the conformer with the methyl equatorial. ...OR any particular molecule of methylcyclohexane will spend most of its time in the equatorial conformer. Can calculate for a particular energy difference how much of each conformer is present using the equation: K = e -(∆E/RT) For an energy difference of 7.3 kJ/mole, 95% of the molecules will be axial conformer. The energy difference is due to steric strain caused by 1,3-diaxial interactions. Steric interference
H
CH3 H
Similar to gauch interactions we saw in butane. See Table 4.2 Steric Strain in Monosubstituted Cyclohexanes For all intents and purposes, t-butylcyclohexane does not flip. The barrier to rotation is too large. Very bulky group
CH3 H3C H3C
C
Conformational Analysis of Disubstituted Cyclohexanes cis- 1,2-Dimethylcyclohexane CH3
CH3
CH3
CH3
H3C CH3 CH3
trans-1,2-Dimethylcyclohexane CH3
CH3 CH3
cis-1,3- Dimethylcyclohexane CH3
CH3
H3C
CH3 CH3
H3C CH3
CH3
trans-1,3-Dimethylcyclohexane CH3
CH3
CH3
H3C
CH3
H3C
cis-1,4-Dimethylcyclohexane CH3 CH3
CH3
CH3 CH3
H3C CH3
trans-1,4-Dimethylcyclohexane CH3
CH3
CH3
H3C CH3
Configurations of substituted cyclohexanes. A useful devise is shown below that illustrates the relationship between configurations (e.g cis/trans relatrionships) and conformations (e.g. axial/equatorial). The axial/equatorial bonds alternate around the ring as shown in the hypothetical planar cyclohexane. a
e a
e a e
e a e
a e
a
Thus group that are cis and at positions 1,3 are either diaxial or diequatorial –and which do you think they would prefer? Boat Cyclohexane As the cyclohexane ringflips, it goes through many intermediate conformations that are neither chair conformer. There is another conformer that is also free of angle strain - the boat conformer. In this conformer, hydrogens on the 1 and 4 carbon com very close to one another to produce steric strain. The other hydrogens around the ring are eclipsed. So this conformer has both seric strain and torsional strain. The conformation is about 29 kJ/mol [7.0 kcal/mol] less stable than the chair. Therre is a twist boat conformer that alleviates some of this strin but is still much less stable than the chair. "Boat" Conformer H
H H
H
H H
CH2 CH2
H H
H H
Polycyclic Molecules The conformers of polycyclic alkanes will be discussed in class. Read the section in the text covering decalin. The structure of this type of system has importance in the structure of steroids covered later in this course.
See structures below. H
H A
B H
trans- decalin - Hydrogens at the fusion points are trans to each other.
B
A
H Notice that the two bold bonds coming off the A ring to make part of the B ring are equatorial. Same is true of the dotted bonds coming off B ring to form A ring.
H
H
Axial to A ring
H A
B H
B
A
Equatroial to A ring
cis -decalin - Hydrogens at the fusion points are cis to each other.
Trans-fused decalin is rigid and cannot flip. Cis-fused decalin is not rigid and can flip to the other conformer. Bicyclic compounds CH3
CH3 CH3 O
Norbornane Bicyclo[2.2.1]heptane
Camphor 1,7,7-trimethylbicyclo[2.2.1]heptan-2-one
Tetrahedral carbon
Conformation of ethane: Ethane is a two-carbon compound with all single bonds on the carbons. (Show ball and stick model of ethane). There is freedom of rotation around the single bonds. This has no consequence with the C-H bonds but does have consequence with the carbon-carbon bond. Different arrangement of the hydrogen atoms in relation to each other that are the result of the rotation around the single C-C bond. These are called CONFORMATIONS A specific conformation is called a CONFORMER. Show ethane again. There are ways of representing the 3-D aspect on paper. Newman projection Sawhorse projection H H
C H
Eclipsed conformation
H
C H
H
H C H H H
H
Staggered conformation
C H
Rotation is not completely "free". There is an energy "cost".
Staggered conformation are more stable than the eclipsed conformations. In the staggered conformer, H's are as far away from each other as possible. High energy conformer (eclipsed), H's as close as possible. The energy difference between the two extremes is 12 kJ/mol [2.9 kcal/mol]. There are three H/H eclipsed interactions in that conformer. Therefore each H/H eclipse interaction contributes 4 kJ/mol [~1 kcal/mol] to the strain of free rotation. This is known as TORSIONAL STRAIN. Best described as a barrier to rotation caused by a slight repulsion of electron clouds as they pass one another. It is not a steric strain because the electron clouds of the hydrogens are not close enough to actually be in the way of each other's "space". This barrier to rotation is represented in a potential energy diagram which plots potential energy vs. the dihedral angle in the bonds.
Dihedral angle
H H
H H H H
H H
H H
H H
H
H H
H H H
Energy
H H
H
H H H H
H
H
60˚
0˚
H 120˚
H
H
H
H
H
H
H H
240˚
180˚
300˚
360˚(0˚)
Dihedral angle
Conformations of Propane and Butane (with energy diagrams). Butane H
CH3
H3C
H
H
H
H CH3 anti
Draw energy diagram.
H
H
H H gauche
H CH3
H3C H
CH3
H H
CH3
H CH3 CH3
HH
CH3
H H
H H
H CH3
HH
H CH3
In butane not all staggered (not eclipsed) conformers are of equal energy. The anti conformation is the conformer in which the two largest groups (methyls) are as far apart as possible. This is the lowest energy isomer. As start to rotate the front carbon around, get eclipsed isomer that has one H-H eclipsed interaction, and two H-CH3 eclipsed interactions. The barrier to rotation is 16 kJ/mole (4kcal/mol). We know from ethane that the H-H energy cost is 4 kJ/mol. Thus 12kJ/mol divided by 2 = 6 kJ/mole for each H-CH3. Continuing with the rotation takes us to a staggered conformer where the dihedral angle of the two methyl groups is 60˚. This conformer is higher in energy than the anti conformer even though there are no eclipsing interactions. This is the GAUCHE conformer and there is steric strain between the two methyl groups. Steric strain is the repulsive interaction that occurs when atoms are forced closer together than their atomic radii allow. Lies 3.8 kJ/mol [0.9 kcal/mol] higher than the anti conformer. Continuing with the rotation --- this conformer has two CH3 groups eclipsed along with two H-H eclipsed interactions. This conformer has a total energy cost of 19 kJ/mol [4.5 kcal/mol] over the anti conformer. If the two H-H interactions are 8kJ/mol (4 kJ/mol each) the CH3-CH3 eclipsed interaction if 11 kJ/mol [2.6 kcal/mol]. Table 4.1 Interaction
Cause
HH eclipsed HCH3 eclipsed CH3CH3 eclipsed CH3CH3 gauche
Energy Cost kJ/mol kcal/mol Torsional strain 4.0 1.0 Mostly torsional strain 6.0 1.4 Torsional + steric strain 11 2.6 Steric strain 3.8 0.9
A point to remember: molecules are always moving. That includes molecular motion and motion within the molecules - bonds rotating, vibrating, etc. By most stable conformer we mean -
1- a molecule "spends" most of its time in the stable form. 2 - at any given instant, most of the molecules will be found in a more stable conformer than a less stable one. Conformation and Stability of Cycloalkanes Bayer Strain Theory Historically, knew that ring systems existed and many five- and six-membered rings were known but smaller and larger rings had not been prepared until well into the twentieth century. Adolph von Baeyer suggested that since carbon prefers to have a tetrahedral geometry (bond angles of ~ 109˚) ring sizes other than 5 and 6 would be too strained to be made. He assumed that three-membered rings would have 60˚ angles of an equilateral triangle, four-membered rings, 90˚ in a square and fivemembered rings, a 108˚ angle pentagon. The deviation from the “ideal” angle of the flat polygon is called angle strain. Rings larger than six would have much larger angles with a negative angle strain and thus could not be made. Actually rings are generally not planar (from cyclobutane onwards) and thus rings distort out of planarity (except for cyclopropane) to lessen this angle strain.. Actual strain is measured per CH2 group in the ring by heats of combustion - the amount of energy (heat) released on complete combustion. If we compare two isomers, more energy is released during combustion of the more strained substance because that substance had more energy to begin with. 3 -(CH2)n - n (2 O2) ---------->
n CO2 + n H 2O + Heat
Comparison by CH2 group means the size of the ring is not a factor. Then compare this to heat of combustion per CH2 in some non-strained reference and multiple by the number of carbons in the ring get Figure 4.8 in text (page 111). This comparison shows that there in mo strain in a six-membered cycloalkanes ring.
Cycloalkanes adopt their minimum energy conformation (not planar) for a combination of three reasons: 1) Angle strain (compression of bond angles) 2) Torsional strain (eclipsing of bonds on neighboring atoms). 3) Steric strain (repulsive interaction when atoms get too close together) Cyclopropane HH
H C
H HH Eclipsed hydrogens Low temperture X-ray studies have shown that there in not complete overlap of orbitals in the C-C bonds. This gives rise to bent bonds. C
C C
C C
C
Ring is very reactive and opens easily. Cyclobutane Cyclobutane has less angle strain than cyclopropane but larger torsional strain because of more ring hydrogens.
H
H
H
H
Cyclopentane Baeyer predicted that cyclopentane should have little ring strain. Combustion data indicates that the ring has a strain energy of 26 kJ/mol [6.2 kcal/mol]. There is little angle strain but much torsional strain from the neighboring hydrogens. H H H H
Conformations of Cyclohexanes Cycloalkanes are common in nature. Combustion data shows these rings are strain-free - no torsional, no angle strain. Cyclohexane assumes a puckered conformation that relieves all strain. This conformation is called the chair conformation. H
H
CH2
H H See text for drawing a cyclohexane chair.
H
H
CH2
H H
Axial and Equatorial Bonds in Cyclohexane There are two kinds of positions for hydrogens in these rings that occupy axial and equatorial positions. Each of the six carbon atoms in the ring has one of each kind of position.
+
Ring showing only axial bonds
Ring showing only equatorial bonds
Ring with all bonds
LOOK AT YOUR MODELS! Bonds on the same side of the ring are cis. Bonds on opposite sides of the ring are trans. Notice that adjacent axial bonds are trans. Likewise with adjacent equatorial bonds. Axial bonds are parallel and alternate up and down. Equatorial bonds alternate between sides around the ring. Conformation Mobility of Cyclohexane The two conformations readily interconvert. -- called ring flip. Conformational flip Use models to show that axial bonds in one chair become equatorial in the flip to the other chair and equatorial bonds become axcial. Conformations of Monosubstituted Cyclohexanes The two conformers of substitutued cyclohexanes are not equally stable.
Ring flip
CH3 axial group
equatorial group
CH3
On examination, see that the axial "methyl" substituent becomes equatorial when the ring flips. The eq. isomer is 8.0 kJ.mol [1.8 kcal/mol] more stable than the axial conformer. This means that at any instant most of the methylcyclohexane molecules will be in the conformer with the methyl equatorial. ...OR any particular molecule of methylcyclohexane will spend most of its time in the equatorial conformer. Can calculate for a particular energy difference how much of each conformer is present using the equation: K = e -(∆E/RT) For an energy difference of 7.3 kJ/mole, 95% of the molecules will be axial conformer. The energy difference is due to steric strain caused by 1,3-diaxial interactions. Steric interference
H
CH3 H
Similar to gauch interactions we saw in butane. See Table 4.2 Steric Strain in Monosubstituted Cyclohexanes For all intents and purposes, t-butylcyclohexane does not flip. The barrier to rotation is too large. Very bulky group
CH3 H3C H3C
C
Conformational Analysis of Disubstituted Cyclohexanes cis- 1,2-Dimethylcyclohexane CH3
CH3
CH3
CH3
H3C CH3 CH3
trans-1,2-Dimethylcyclohexane CH3
CH3 CH3
cis-1,3- Dimethylcyclohexane CH3
CH3
H3C
CH3 CH3
H3C CH3
CH3
trans-1,3-Dimethylcyclohexane CH3
CH3
CH3
H3C
CH3
H3C
cis-1,4-Dimethylcyclohexane CH3 CH3
CH3
CH3 CH3
H3C CH3
trans-1,4-Dimethylcyclohexane CH3
CH3
CH3
H3C CH3
Configurations of substituted cyclohexanes. A useful devise is shown below that illustrates the relationship between configurations (e.g cis/trans relatrionships) and conformations (e.g. axial/equatorial). The axial/equatorial bonds alternate around the ring as shown in the hypothetical planar cyclohexane. a
e a
e a e
e a e
a e
a
Thus group that are cis and at positions 1,3 are either diaxial or diequatorial –and which do you think they would prefer? Boat Cyclohexane As the cyclohexane ringflips, it goes through many intermediate conformations that are neither chair conformer. There is another conformer that is also free of angle strain - the boat conformer. In this conformer, hydrogens on the 1 and 4 carbon com very close to one another to produce steric strain. The other hydrogens around the ring are eclipsed. So this conformer has both seric strain and torsional strain. The conformation is about 29 kJ/mol [7.0 kcal/mol] less stable than the chair. Therre is a twist boat conformer that alleviates some of this strin but is still much less stable than the chair. "Boat" Conformer H
H H
H
H H
CH2 CH2
H H
H H
Polycyclic Molecules The conformers of polycyclic alkanes will be discussed in class. Read the section in the text covering decalin. The structure of this type of system has importance in the structure of steroids covered later in this course.
See structures below. H
H A
B H
trans- decalin - Hydrogens at the fusion points are trans to each other.
B
A
H Notice that the two bold bonds coming off the A ring to make part of the B ring are equatorial. Same is true of the dotted bonds coming off B ring to form A ring.
H
H
Axial to A ring
H A
B H
B
A
Equatroial to A ring
cis -decalin - Hydrogens at the fusion points are cis to each other.
Trans-fused decalin is rigid and cannot flip. Cis-fused decalin is not rigid and can flip to the other conformer. Bicyclic compounds CH3
CH3 CH3 O
Norbornane Bicyclo[2.2.1]heptane
Camphor 1,7,7-trimethylbicyclo[2.2.1]heptan-2-one
