Conformations

1
Conformations

• Conformations are different arrangements that
  occur around the carbon-carbon (C-C) single bond
• Important point In alkanes the C-C single bond
  is a freely rotating bond and a molecule can take
  on an infinite number of conformations. These
  specific conformations are called conformers

2
Conformers (cont.)

• Conformers can be represented in several ways.
  The two primary types for alkanes are
• Sawhorse representation Shows all C-C bonds,
  and represents the atoms at a particular angle
• Newman projection Two carbon atoms are
  represented by a circle, and H atoms are shown

3
Ethane Molecule (C2H6)

• Bonding orbitals of carbon (sp3 hybridized) are
  directed toward the corners of a tetrahedron (
  ie., exhibits tetrahedron electronic geometry).
• Bond angles between C-H and C-C bond 109.5o
• Bond length 1.10 A

4
Conformations of Ethane

• Although ethane can theoretically can exhibit an
  infinite number of conformations, we will
  concentrate only those conformations which are
  produced for each 60o turn (ie, six
  conformations, since there are 360o in a circle)

5
Conformations (cont.)

• Three primary conformations exists for ethane
• Eclipsed
• Staggered
• Skew

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Eclipsed Conformation

• C-H bonds are as close to each other as possible
  (ie., H atoms attached to carbon are as close to
  each other as possible)
• Conformation exhibits high energy, therefore, low
  stability(see potential energy vs. bond rotation
  graph)

7
Staggered Conformation

• C-H bonds are as far away from each other as
  possible (ie., H atoms in the ethane molecule are
  at their maximum distance from each other)
• Conformations are of lower energy, or higher
  stability than that of the eclipsed conformation

8
Skew Conformation

• These are the intermediate conformations that lie
  between the staggered and the gauche
  conformations
• The molecule can exhibit an infinite amount of
  skew conformations

9
Torsional Strain

• There is a 12 kJ/mol energy barrier between the
  eclipsed and the staggered conformations
• The 12 kJ/mol energy barrier present in the
  eclipsed conformation is due to torsional strain,
  which is strain due to the energy necessary to
  rotate the molecule about the C-C single bond

10
Torsional Strain (cont.)

• The torsional strain is also contributed to the
  increasing closeness of the H atoms in the ethane
  molecule in the eclipsed conformation
• This torsional strain decreases as the molecule
  is rotated from the eclipsed conformation,
  bringing the H atoms further apart

11
Conformations in Propane (C3H8)

• Very similar to ethane in terms of conformations,
  except in this case, there is an extra CH3 group
  being rotated
• The extra methyl group contributes to a higher
  energy barrier than in ethane (14 kJ/mol vs. 12
  kJ/ mol in ethane)

12
Conformations of Propane (cont.)

• In propane, two eclipsing interactions costs 4
  kJ/mol whereas one other eclipsing interaction
  (in which the CH3 group eclipses the H atom)
  costs 6 kJ/mol, giving a total of 14 kJ/mol

13
Conformations of Butane (C4H10)

• In this case, two methyl groups (CH3) are
  involved in the rotation
• In n-butane, not all staggered conformations have
  the same energy
• n-butane consists of primarily one anti and two
  gauche conformations (in addition to the infinite
  number of skew conformations)

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Anti and Gauche Conformations

• Anti conformation The two methyl groups are as
  far apart from each other as possible (ie.,
  180o). This tends to be the most stable of all
  the conformations of n-butane
• gauche conformation The two methyl groups as 60o
  from each other
• Both the anti and gauche conformations are
  analogous to the staggered conformations in
  ethane propane

15
Anti and Gauche Conformations (cont.)

• The gauche conformation is higher in energy the
  staggered conformation, although there are no
  eclipsing interactions within the molecule itself
  (why?)

16
Steric strain

• Steric strain is the repulsive interaction that
  occurs when atoms ( or groups of atoms) are
  forced closer together than their atomic radii
  allow (ie., strain that is due to increasing
  crowding)
• The gauche conformations in n-butane has a large
  amount of energy than anti conformation due to
  both torsional and steric strain

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Steric strain (cont.)

• Which one of the n-butane conformations in the
  plot has the highest energy or lowest stability?
  Why?

18
Steric Strain (cont.)

• As the dihedral angle between the two methyl
  groups reach 0o, an energy maximum is reached.
  This results in a very high energy eclipsed
  conformation
• Energy Cost 11 kJ/mol for the two methyl groups
  eclipsing each other, plus 4 kJ/mol for each pair
  of H eclipsing. Therfore total energy for this
  conformation is 19 kJ/mol

19
Conformations of Cycloalkanes

• Conformations were originally based on Baeyers
  strain theory
• Baeyers strain theory makes the following
  assumptions
• Very small rings (3 to 4 carbons) and very large
  rings (seven or more carbons) are too strained to
  exist
• Cyclopentane (5 carbons) was assumed to be
  strain free

20
Conformations of Cycloalkanes (cont.)

• Drawbacks of Baeyer strain theory
• Did not take into consideration of measuring
  strain energies by utilizing heats of combustion
• The theory assumed that all cyclic rings were
  flat, when in reality, they are not. Most
  cycloalkanes possess what are called puckered
  rings. Only cyclopropane is flat

21
Conformations of Cycloalkanes (cont.)

• In reality, based on heats of combustion studies,
  small and medium rings tend to be highly
  strained, whereas cyclohexane tends to be
  essentially strain-free

22
Cyclopropane

• The three carbons in the ring makes cyclopropane
  highly strained due to
• A relative large difference between the C-C bond
  angle in cyclopropane (60o) and the tetrahedral
  bond angle (109o)
• A large amount of torsional strain exist in
  cyclopropane, because the hydrogen atoms are in
  the eclipsed position

23
Cyclobutane

• Cyclobutane has considerable less angle strain,
  but more torsional strain than cyclopropane, due
  to the larger number of ring hydrogens. The
  total strain between cyclopropane and cyclobutane
  are nearly the same

24
Cyclopentane (C5H10)

• Cyclopentane exhibits very little angle strain,
  but has a large amount of torsional strain
• Four of the cyclopentane atoms are in the same
  plane, but one of them is out of plane

25
Cyclohexane (C6H12)

• Chair Conformation Virtually strain-free under
  all aspects (angle, torsional, or steric)
• Boat Conformation Free of angle strain but has
  higher torsional strain than the chair
  conformation

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Chair conformation of cyclohexane

• Most stable of cyclohexane conformation,
  practically strain-free
• All hydrogen atoms on chair conformation is in
  the staggered position
• C-C bond angles are approximately 110o, which is
  very close to the tetrahedral bond angle of
  109.5o

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Axial and equatorial positions

• Two types of bond positions exist in cyclohexane,
  axial position and equatorial position
• axial position Bonds lie perpendicular to the
  chair, above or below the plane (axis)
• equatorial position Bonds lie with the plane of
  the chair (equator)
• Chair conformation contains six axial and six
  equatorial positions

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Conformations of Cyclohexane (cont.)

• Each carbon atom has one axial and one equatorial
  hydrogen attached to it
• Different chair conformations interconvert,
  resulting in a ring flip

29
Conformations of Monosubstituted cyclohexanes

• Two possible conformers are possible in
  monosubstituted cyclohexane one which the
  substituent is located in the axial position and
  one in which the substituent is located in the
  equatorial position
• The equatorial position tends to be more stable,
  due to steric strain caused by the diaxial
  interactions(1,3-diaxial positions)

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Polycyclic Molecules

• These are molecules in which two or more rings
  are fused together, e.g., decalin
• In decalin, the two carbon atoms that join the
  ring are called bridgehead carbons
• Two molecules of decalin exist cis and trans
• Steroids ,such as cholesterol are very common
  examples of polycyclic molecules

31
Polycyclic Compounds (cont.)

• Steroids have three six-membered and one five
  membered rings
• Compounds which consists of two fused ring
  systems are called bicyclic compounds (bi means
  two)
• Homework Question What is the IUPAC name for
  camphor