Chemistry 1301A/B Lecture 26: 5.3 Alkanes, Cycloalkanes and Alkenes
27 Nov 2018
School
Department
Course
Professor
Alkanes
Cycloalkanes
Alkenes
Alkynes
Bonds
C-C
C-C Ring
C=C
C≡C
General Formula
CnH2n+2
CnH2n
CnH2n
CnH2n-2
Alkanes
Straight or branched-chained compounds
-
Contain only single C-C bonds
-
Methane + CH2chain
-
Increased size of alkanes = higher boiling point
-
Constitutional (Structural) Isomers
Compounds with the same molecular formula but different bonding
sequences
-
Number of possible constitutional isomers greatly increases with molecular
size
-
Example of Isomers of Alkanes
Formula
Isomers
CH4
1
C4H10
2
C6H14
5
C8H18
18
C10H22
75
C20H42
366, 319
C40H82
62, 481, 801, 147, 341
Constitutional isomers have different chemical properties
ex. C5H12
○
-
When drawing constitutional isomers, start with the longest possible chains
and progress to the shortest
As pictured above
○
-
Drawing 3D Structures
Dot-Line-Wedge Formulas
Dotted lines represents bonds behind the plane
-
Wedged lines represents bonds in front of the plane
-
Sawhorse Projections
Structure is drawn in the shape of a sawhorse using only plain lines
-
Only useful for simple molecules
-
Newman Projections
Viewed along the C-C axis
-
Bonds drawn in front of and along the edge of a circle
-
Used for showing conformations
-
Conformations
Transient (non-permanent) spatial arrangements of atoms in a molecule
produced by rotations about single bonds
-
Two key conformations of alkanes
Staggered
○
Eclipsed
○
-
In an eclipsed conformation, C-H bonds on the rear C are seen directly
behind the front C-H bonds
In a staggered conformation, they are between the front C-H bonds
○
Staggered conformation of ethane is formed by a 60° rotation of the
CH3group around the C-C bond
○
-
Staggered Conformation
Steric repulsion between the front and rear H atoms is minimized
Therefore have a lower energy
○
More stable than eclipsed conformers
○
-
Energy difference is so small that the conformations interconvert very
quickly (106/s) and cannot be isolated in pure form
-
Relative Energies of Conformations
In this example (Figure 4), look at position of CH3groups
Largest group thus the first conformation is least stable
○
-
Angle between methyl groups is known as the dihedral angle
-
Staggered conformer with lowest energy is the anti conformer
-
Staggered conformers with higher energy than anti- are gauche
conformers
-
Eclipsed conformers will be higher in energy than any staggered conformer
-
Cycloalkanes
Hydrocarbons containing a C-ring connected by single C-C bonds
-
Cycloalkanes with rings containing 4 or 5 C's have all C's nearly in the same
plane
Substituents above and below the plane
○
4 or 5 atom rings bend (pucker) slightly out of plane to reduce
eclipsed interactions
○
-
Cyclohexane Conformations
6-membered rings are the most common of all cycloalkanes
-
Two distinct cyclohexane conformations; all carbons have a normal
tetrahedral geometry associated with sp3hybridization
Boat and chair conformations
○
-
Boat Conformation
Substituents on carbons 2 and 3, and carbons 5 and 6, are eclipsed
-
Substituents on carbons 1 and 4 interact with each other
Flagpole interaction
○
Not favourable, thus not very common
○
-
Chair Conformation
All C-H bonds on adjacent C's are staggered
-
No flagpole interactions
-
Six substituents directed straight up or down are called axial substituents
-
Six substituents directed laterally or outward are called equatorial
substituents
-
Equatorial substituents are less crowded since they are more distant from
other substituents in the ring
Large groups prefer to be in an equatorial position
○
-
A chair conformation with large substituents in an equatorial position is
preferred and more stable
Due to steric repulsion in an axial position
○
-
More stable chair conformation will be more present in eqm mixture
Since they rapidly interconvert, they are not isomers
○
-
Constitutional Isomers in Cycloalkanes
ex. C8H16
-
Stereoisomerism in Cycloalkanes
Same molecular formula and same bonding sequence, but different non-
interconverting 3D structures
-
Cis-Trans Isomers
When a cycloalkane ring has two or more non-H substituents on different
ring carbons, they can occupy two different positions relative to the ring
plane
-
Cis (same) isomers have the substituents on the same side of the ring plane
-
Trans (opposite) isomers have the substituents on opposite sides of the ring
plane
-
Can affect the molecule's properties
-
Line-wedge formulas do not represent any particular conformations of the
C ring
-
Cis-trans isomers do not interconvert at room temperature
However they can exist in their usual variety of conformations
○
Be sure to recognize cis-trans relationships of chair conformations of
cyclohexane compounds
○
-
Alkenes and Alkynes
Alkenes: Hydrocarbons with one or more C=C double bond
Alkynes: Hydrocarbons with one or more C≡C triple bond
Both are said to be unsaturated
Have less than the maximum number of H atoms
○
Saturated: Maximum number of H atoms in the hydrocarbon
○
-
Removal of two H atoms from an alkane represents one unit of unsaturation
Each unit of unsaturation could be a pi-bond or a ring
○
-
Structure of Alkenes and Alkynes
In an alkene, each C in C=C bond is bonded to three atoms and is sp2
(planar) with bond angles of 120°
Three sp2 orbitals on each C atom forms three sigma bonds
○
Remaining p orbital overlaps sideways with p from other C and forms a
pi-bond
○
Planar structure
○
-
In an alkyne, each C in C≡C bond is bonded to two atoms and is sp (linear)
with bond angles of 180°
Two sp orbitals of each C atom form two sigma bonds
○
Remaining p orbitals overlap sideways to form two pi-bonds
○
-
Stereoisomerism of Alkenes
Multiple C-C bond prevents free rotation about the CC axis
Would require breaking the pi bond
○
Not possible under natural conditions
○
Thus cis-trans isomers exist in alkenes
○
-
In rings with seven members or less, a double bond can only have cis
geometry
Trans is too strained
○
-
C=C bond with two different substituents on each C can give rise to two
isomers, cis or trans
n such bonds = 2nisomers
○
-
Example Vitamin A
All trans stereoisomer
○
Enzyme is required to break a pi bond, rotate molecule about a C-C
bond and restore pi bond
Since cis-trans isomers do not interconvert on their own
§
○
Trans-retinal -> cis-retinal
○
-
When light strikes the cis-retinal-opsin complex, cis-retinal is returned to the
all-trans state via a cis-to-trans isomerization reaction
Causes opsin to undergo a conformational change
○
Results in concomitant transmission of a signal to the brain
○
All-trans retinal is detached from opsin, completing the cycle
○
-
E,Z Nomenclature for Alkene Stereoisomers
Employs a set of rules to assign priorities to each of the two substituents on
each C in the C=C bond
-
High Priority Side Pneumonic
Z Same Zame Zide
EOpposite
Assigning Priorities to Substituents
Directly bonded to C: Higher atomic number receives higher priority
If this leads to a tie, proceed to rule 2
a.
1.
Make two lists of atoms bonded to each of the centres involved in the tie
and arrange each list in order of decreasing atomic number
Compare lists atom by atom
a.
At the earliest difference, the entire set containing the atom with a
higher atomic number receives higher priority
b.
Continue down the list until priority is assigned
c.
2.
If double or triple bonds are present, they are given single-bond
equivalencies
C=O-C is O,O,C
a.
C≡N is N,N,N
b.
3.
When assigning E,Z nomenclature to larger molecules with more than one
alkene, assign labels to each double bond
-
Pentane (b.p. = 36°C) Isopentane (b.p. = 28°C) Neopentane (b.p. = 9°C)
Increasing
Boiling Point
Figure 1
Figure 2
Figure 3
Figure 4
Highest Energy
(Least Stable)
Lowest Energy
(Most Stable)
Figure 5
Figure 6
Figure 7
Figure 8
Figure 9
Figure 10
Figure 11
Figure 12
5.3 Alkanes, Cycloalkanes and Alkenes
Tuesday, November 13, 2018
5:50 PM
Alkanes Cycloalkanes Alkenes Alkynes
Bonds C-C C-C Ring C=C C≡C
General Formula CnH2n+2 CnH2n CnH2n CnH2n-2
Alkanes
Straight or branched-chained compounds
-
Contain only single C-C bonds
-
Methane + CH2chain
-
Increased size of alkanes = higher boiling point
-
Constitutional (Structural) Isomers
Compounds with the same molecular formula but different bonding
sequences
-
Number of possible constitutional isomers greatly increases with molecular
size
-
Example of Isomers of Alkanes
Formula Isomers
CH41
C4H10 2
C6H14 5
C8H18 18
C10H22 75
C20H42 366, 319
C40H82 62, 481, 801, 147, 341
Constitutional isomers have different chemical properties
ex. C5H12
○
-
When drawing constitutional isomers, start with the longest possible chains
and progress to the shortest
As pictured above
○
-
Drawing 3D Structures
Dot-Line-Wedge Formulas
Dotted lines represents bonds behind the plane
-
Wedged lines represents bonds in front of the plane
-
Sawhorse Projections
Structure is drawn in the shape of a sawhorse using only plain lines
-
Only useful for simple molecules
-
Newman Projections
Viewed along the C-C axis
-
Bonds drawn in front of and along the edge of a circle
-
Used for showing conformations
-
Conformations
Transient (non-permanent) spatial arrangements of atoms in a molecule
produced by rotations about single bonds
-
Two key conformations of alkanes
Staggered
○
Eclipsed
○
-
In an eclipsed conformation, C-H bonds on the rear C are seen directly
behind the front C-H bonds
In a staggered conformation, they are between the front C-H bonds
○
Staggered conformation of ethane is formed by a 60° rotation of the
CH3group around the C-C bond
○
-
Staggered Conformation
Steric repulsion between the front and rear H atoms is minimized
Therefore have a lower energy
○
More stable than eclipsed conformers
○
-
Energy difference is so small that the conformations interconvert very
quickly (106/s) and cannot be isolated in pure form
-
Relative Energies of Conformations
In this example (Figure 4), look at position of CH3groups
Largest group thus the first conformation is least stable
○
-
Angle between methyl groups is known as the dihedral angle
-
Staggered conformer with lowest energy is the anti conformer
-
Staggered conformers with higher energy than anti- are gauche
conformers
-
Eclipsed conformers will be higher in energy than any staggered conformer
-
Cycloalkanes
Hydrocarbons containing a C-ring connected by single C-C bonds
-
Cycloalkanes with rings containing 4 or 5 C's have all C's nearly in the same
plane
Substituents above and below the plane
○
4 or 5 atom rings bend (pucker) slightly out of plane to reduce
eclipsed interactions
○
-
Cyclohexane Conformations
6-membered rings are the most common of all cycloalkanes
-
Two distinct cyclohexane conformations; all carbons have a normal
tetrahedral geometry associated with sp3hybridization
Boat and chair conformations
○
-
Boat Conformation
Substituents on carbons 2 and 3, and carbons 5 and 6, are eclipsed
-
Substituents on carbons 1 and 4 interact with each other
Flagpole interaction
○
Not favourable, thus not very common
○
-
Chair Conformation
All C-H bonds on adjacent C's are staggered
-
No flagpole interactions
-
Six substituents directed straight up or down are called axial substituents
-
Six substituents directed laterally or outward are called equatorial
substituents
-
Equatorial substituents are less crowded since they are more distant from
other substituents in the ring
Large groups prefer to be in an equatorial position
○
-
A chair conformation with large substituents in an equatorial position is
preferred and more stable
Due to steric repulsion in an axial position
○
-
More stable chair conformation will be more present in eqm mixture
Since they rapidly interconvert, they are not isomers
○
-
Constitutional Isomers in Cycloalkanes
ex. C8H16
-
Stereoisomerism in Cycloalkanes
Same molecular formula and same bonding sequence, but different non-
interconverting 3D structures
-
Cis-Trans Isomers
When a cycloalkane ring has two or more non-H substituents on different
ring carbons, they can occupy two different positions relative to the ring
plane
-
Cis (same) isomers have the substituents on the same side of the ring plane
-
Trans (opposite) isomers have the substituents on opposite sides of the ring
plane
-
Can affect the molecule's properties
-
Line-wedge formulas do not represent any particular conformations of the
C ring
-
Cis-trans isomers do not interconvert at room temperature
However they can exist in their usual variety of conformations
○
Be sure to recognize cis-trans relationships of chair conformations of
cyclohexane compounds
○
-
Alkenes and Alkynes
Alkenes: Hydrocarbons with one or more C=C double bond
Alkynes: Hydrocarbons with one or more C≡C triple bond
Both are said to be unsaturated
Have less than the maximum number of H atoms
○
Saturated: Maximum number of H atoms in the hydrocarbon
○
-
Removal of two H atoms from an alkane represents one unit of unsaturation
Each unit of unsaturation could be a pi-bond or a ring
○
-
Structure of Alkenes and Alkynes
In an alkene, each C in C=C bond is bonded to three atoms and is sp2
(planar) with bond angles of 120°
Three sp2 orbitals on each C atom forms three sigma bonds
○
Remaining p orbital overlaps sideways with p from other C and forms a
pi-bond
○
Planar structure
○
-
In an alkyne, each C in C≡C bond is bonded to two atoms and is sp (linear)
with bond angles of 180°
Two sp orbitals of each C atom form two sigma bonds
○
Remaining p orbitals overlap sideways to form two pi-bonds
○
-
Stereoisomerism of Alkenes
Multiple C-C bond prevents free rotation about the CC axis
Would require breaking the pi bond
○
Not possible under natural conditions
○
Thus cis-trans isomers exist in alkenes
○
-
In rings with seven members or less, a double bond can only have cis
geometry
Trans is too strained
○
-
C=C bond with two different substituents on each C can give rise to two
isomers, cis or trans
n such bonds = 2nisomers
○
-
Example Vitamin A
All trans stereoisomer
○
Enzyme is required to break a pi bond, rotate molecule about a C-C
bond and restore pi bond
Since cis-trans isomers do not interconvert on their own
§
○
Trans-retinal -> cis-retinal
○
-
When light strikes the cis-retinal-opsin complex, cis-retinal is returned to the
all-trans state via a cis-to-trans isomerization reaction
Causes opsin to undergo a conformational change
○
Results in concomitant transmission of a signal to the brain
○
All-trans retinal is detached from opsin, completing the cycle
○
-
E,Z Nomenclature for Alkene Stereoisomers
Employs a set of rules to assign priorities to each of the two substituents on
each C in the C=C bond
-
High Priority Side Pneumonic
Z Same Zame Zide
EOpposite
Assigning Priorities to Substituents
Directly bonded to C: Higher atomic number receives higher priority
If this leads to a tie, proceed to rule 2
a.
1.
Make two lists of atoms bonded to each of the centres involved in the tie
and arrange each list in order of decreasing atomic number
Compare lists atom by atom
a.
At the earliest difference, the entire set containing the atom with a
higher atomic number receives higher priority
b.
Continue down the list until priority is assigned
c.
2.
If double or triple bonds are present, they are given single-bond
equivalencies
C=O-C is O,O,C
a.
C≡N is N,N,N
b.
3.
When assigning E,Z nomenclature to larger molecules with more than one
alkene, assign labels to each double bond
-
Pentane (b.p. = 36°C) Isopentane (b.p. = 28°C) Neopentane (b.p. = 9°C)
Increasing
Boiling Point
Figure 1
Figure 2
Figure 3
Figure 4
Highest Energy
(Least Stable)
Lowest Energy
(Most Stable)
Figure 5
Figure 6
Figure 7
Figure 8
Figure 9
Figure 10
Figure 11
Figure 12
5.3 Alkanes, Cycloalkanes and Alkenes
Tuesday, November 13, 2018
5:50 PM
Alkanes Cycloalkanes Alkenes Alkynes
Bonds C-C C-C Ring C=C C≡C
General Formula CnH2n+2 CnH2n CnH2n CnH2n-2
Alkanes
Straight or branched-chained compounds
-
Contain only single C-C bonds
-
Methane + CH2chain
-
Increased size of alkanes = higher boiling point
-
Constitutional (Structural) Isomers
Compounds with the same molecular formula but different bonding
sequences
-
Number of possible constitutional isomers greatly increases with molecular
size
-
Example of Isomers of Alkanes
Formula Isomers
CH41
C4H10 2
C6H14 5
C8H18 18
C10H22 75
C20H42 366, 319
C40H82 62, 481, 801, 147, 341
Constitutional isomers have different chemical properties
ex. C5H12
○
-
When drawing constitutional isomers, start with the longest possible chains
and progress to the shortest
As pictured above
○
-
Drawing 3D Structures
Dot-Line-Wedge Formulas
Dotted lines represents bonds behind the plane
-
Wedged lines represents bonds in front of the plane
-
Sawhorse Projections
Structure is drawn in the shape of a sawhorse using only plain lines
-
Only useful for simple molecules
-
Newman Projections
Viewed along the C-C axis
-
Bonds drawn in front of and along the edge of a circle
-
Used for showing conformations
-
Conformations
Transient (non-permanent) spatial arrangements of atoms in a molecule
produced by rotations about single bonds
-
Two key conformations of alkanes
Staggered
○
Eclipsed
○
-
In an eclipsed conformation, C-H bonds on the rear C are seen directly
behind the front C-H bonds
In a staggered conformation, they are between the front C-H bonds
○
Staggered conformation of ethane is formed by a 60° rotation of the
CH3group around the C-C bond
○
-
Staggered Conformation
Steric repulsion between the front and rear H atoms is minimized
Therefore have a lower energy
○
More stable than eclipsed conformers
○
-
Energy difference is so small that the conformations interconvert very
quickly (106/s) and cannot be isolated in pure form
-
Relative Energies of Conformations
In this example (Figure 4), look at position of CH3groups
Largest group thus the first conformation is least stable
○
-
Angle between methyl groups is known as the dihedral angle
-
Staggered conformer with lowest energy is the anti conformer
-
Staggered conformers with higher energy than anti- are gauche
conformers
-
Eclipsed conformers will be higher in energy than any staggered conformer
-
Cycloalkanes
Hydrocarbons containing a C-ring connected by single C-C bonds
-
Cycloalkanes with rings containing 4 or 5 C's have all C's nearly in the same
plane
Substituents above and below the plane
○
4 or 5 atom rings bend (pucker) slightly out of plane to reduce
eclipsed interactions
○
-
Cyclohexane Conformations
6-membered rings are the most common of all cycloalkanes
-
Two distinct cyclohexane conformations; all carbons have a normal
tetrahedral geometry associated with sp3hybridization
Boat and chair conformations
○
-
Boat Conformation
Substituents on carbons 2 and 3, and carbons 5 and 6, are eclipsed
-
Substituents on carbons 1 and 4 interact with each other
Flagpole interaction
○
Not favourable, thus not very common
○
-
Chair Conformation
All C-H bonds on adjacent C's are staggered
-
No flagpole interactions
-
Six substituents directed straight up or down are called axial substituents
-
Six substituents directed laterally or outward are called equatorial
substituents
-
Equatorial substituents are less crowded since they are more distant from
other substituents in the ring
Large groups prefer to be in an equatorial position
○
-
A chair conformation with large substituents in an equatorial position is
preferred and more stable
Due to steric repulsion in an axial position
○
-
More stable chair conformation will be more present in eqm mixture
Since they rapidly interconvert, they are not isomers
○
-
Constitutional Isomers in Cycloalkanes
ex. C8H16
-
Stereoisomerism in Cycloalkanes
Same molecular formula and same bonding sequence, but different non-
interconverting 3D structures
-
Cis-Trans Isomers
When a cycloalkane ring has two or more non-H substituents on different
ring carbons, they can occupy two different positions relative to the ring
plane
-
Cis (same) isomers have the substituents on the same side of the ring plane
-
Trans (opposite) isomers have the substituents on opposite sides of the ring
plane
-
Can affect the molecule's properties
-
Line-wedge formulas do not represent any particular conformations of the
C ring
-
Cis-trans isomers do not interconvert at room temperature
However they can exist in their usual variety of conformations
○
Be sure to recognize cis-trans relationships of chair conformations of
cyclohexane compounds
○
-
Alkenes and Alkynes
Alkenes: Hydrocarbons with one or more C=C double bond
Alkynes: Hydrocarbons with one or more C≡C triple bond
Both are said to be unsaturated
Have less than the maximum number of H atoms
○
Saturated: Maximum number of H atoms in the hydrocarbon
○
-
Removal of two H atoms from an alkane represents one unit of unsaturation
Each unit of unsaturation could be a pi-bond or a ring
○
-
Structure of Alkenes and Alkynes
In an alkene, each C in C=C bond is bonded to three atoms and is sp2
(planar) with bond angles of 120°
Three sp2 orbitals on each C atom forms three sigma bonds
○
Remaining p orbital overlaps sideways with p from other C and forms a
pi-bond
○
Planar structure
○
-
In an alkyne, each C in C≡C bond is bonded to two atoms and is sp (linear)
with bond angles of 180°
Two sp orbitals of each C atom form two sigma bonds
○
Remaining p orbitals overlap sideways to form two pi-bonds
○
-
Stereoisomerism of Alkenes
Multiple C-C bond prevents free rotation about the CC axis
Would require breaking the pi bond
○
Not possible under natural conditions
○
Thus cis-trans isomers exist in alkenes
○
-
In rings with seven members or less, a double bond can only have cis
geometry
Trans is too strained
○
-
C=C bond with two different substituents on each C can give rise to two
isomers, cis or trans
n such bonds = 2nisomers
○
-
Example Vitamin A
All trans stereoisomer
○
Enzyme is required to break a pi bond, rotate molecule about a C-C
bond and restore pi bond
Since cis-trans isomers do not interconvert on their own
§
○
Trans-retinal -> cis-retinal
○
-
When light strikes the cis-retinal-opsin complex, cis-retinal is returned to the
all-trans state via a cis-to-trans isomerization reaction
Causes opsin to undergo a conformational change
○
Results in concomitant transmission of a signal to the brain
○
All-trans retinal is detached from opsin, completing the cycle
○
-
E,Z Nomenclature for Alkene Stereoisomers
Employs a set of rules to assign priorities to each of the two substituents on
each C in the C=C bond
-
High Priority Side Pneumonic
Z Same Zame Zide
EOpposite
Assigning Priorities to Substituents
Directly bonded to C: Higher atomic number receives higher priority
If this leads to a tie, proceed to rule 2
a.
1.
Make two lists of atoms bonded to each of the centres involved in the tie
and arrange each list in order of decreasing atomic number
Compare lists atom by atom
a.
At the earliest difference, the entire set containing the atom with a
higher atomic number receives higher priority
b.
Continue down the list until priority is assigned
c.
2.
If double or triple bonds are present, they are given single-bond
equivalencies
C=O-C is O,O,C
a.
C≡N is N,N,N
b.
3.
When assigning E,Z nomenclature to larger molecules with more than one
alkene, assign labels to each double bond
-
Pentane (b.p. = 36°C)
Isopentane (b.p. = 28°C)
Neopentane (b.p. = 9°C)
Increasing
Boiling Point
Figure 1
Figure 2
Figure 3
Figure 4
Highest Energy
(Least Stable)
Lowest Energy
(Most Stable)
Figure 5
Figure 6
Figure 7
Figure 8
Figure 9
Figure 10
Figure 11
Figure 12
5.3 Alkanes, Cycloalkanes and Alkenes
Tuesday, November 13, 2018 5:50 PM
Document Summary
Increased size of alkanes = higher boiling point. Compounds with the same molecular formula but different bonding sequences. Number of possible constitutional isomers greatly increases with molecular size. When drawing constitutional isomers, start with the longest possible chains and progress to the shortest. Wedged lines represents bonds in front of the plane. Structure is drawn in the shape of a sawhorse using only plain lines. Bonds drawn in front of and along the edge of a circle. Transient (non-permanent) spatial arrangements of atoms in a molecule produced by rotations about single bonds. Figure 1 produced by rotations about single bonds. In an eclipsed conformation, c-h bonds on the rear c are seen directly behind the front c-h bonds. In a staggered conformation, they are between the front c-h bonds. Staggered conformation of ethane is formed by a 60 rotation of the. Steric repulsion between the front and rear h atoms is minimized.
