CH264
1
CH264/2
Organic Chemistry II
Conformational Analysis
Dr Andrew Marsh C515
a.marsh@warwick.ac.uk
Dr David J Fox B51...
CH264
2
Today’s Lecture
1. Stabilisation of conformation in simple alkanes
2. Strain in acyclic and cyclic molecules
3. Cy...
CH264
3
Ethane
Ethane is able to exist in many different CONFORMATIONS because it has free
rotation around its central C-C...
CH264
4
Dihedral Angle
Ha
HH
H
Hb
H
view
Staggered
H
HbH
H H
Ha
φ
If we chose a pair of hydrogen atoms in a Newman project...
CH264
5
Dihedral Angle φ
H°
Hb
HH
Ha
H
H
Hb
HH
H
H
Ha
Hb
HH
H
Ha
H
Hb
HH
Ha
H
H
H
HbH
H H
Ha
Hb
HH
H H
Ha
H
HHb
H H
Ha
Pot...
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6
Stabilisation of conformation
Filled orbital has a stabilising interaction with empty antibonding orbital if
antip...
CH264
7
Butane
CGW p.365 – 366
There are now several distinct conformations with different energies which are
revealed by ...
CH264
8
D ih e d r a l A n g le φ
H °
P o t e n t ia l E n e r g y - D ih e d r a l A n g le
C H 3
HH
H 3 C
H
H
H
C H 3H
H...
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9
Klyne-Prelog Nomenclature
CH3
HH
H3C
H
H
H
CH3H
H3C
H
H
H
CH3H
H H
CH3
CH3
HH
H H
CH3
CH3 groups: synperiplanar – ...
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Energetic Costs for Steric Interactions
Type of interaction Energy (kJ mol-1
)
H - H steric strain (gauche / sync...
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11
1,3-Strain in acyclic molecules
Syn-pentane (g+ g- pentane) interactions. So-called due to presence of two
consec...
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Cyclic Molecules - Angle Strain
Note that the bond angles in a planar ring are always further from 109° than if t...
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You should be able to:
(i) Draw Newman projections for ethane and butane.
(ii) Predict the lowest energy conforme...
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Year 2 Organic Chemistry - Mechanism and Stereochemistry Lecture 2

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CH264 Year 2 Organic Chemistry - Mechanism and stereochemistry Lecture 2

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Year 2 Organic Chemistry - Mechanism and Stereochemistry Lecture 2

  1. 1. CH264 1 CH264/2 Organic Chemistry II Conformational Analysis Dr Andrew Marsh C515 a.marsh@warwick.ac.uk Dr David J Fox B510 d.j.fox@warwick.ac.uk
  2. 2. CH264 2 Today’s Lecture 1. Stabilisation of conformation in simple alkanes 2. Strain in acyclic and cyclic molecules 3. Cyclohexane
  3. 3. CH264 3 Ethane Ethane is able to exist in many different CONFORMATIONS because it has free rotation around its central C-C bond. There are however two extreme conformers, the eclipsed (left) and staggered (right). H H H H H H H HH H H H view view H HH H H H Eclipsed Staggered H HH H H H Revision: CGW pp. 363 – 364
  4. 4. CH264 4 Dihedral Angle Ha HH H Hb H view Staggered H HbH H H Ha φ If we chose a pair of hydrogen atoms in a Newman projection of ethane, then the angle made between these two atoms, φ is called the DIHEDRAL ANGLE.
  5. 5. CH264 5 Dihedral Angle φ H° Hb HH Ha H H Hb HH H H Ha Hb HH H Ha H Hb HH Ha H H H HbH H H Ha Hb HH H H Ha H HHb H H Ha Potential Energy - Dihedral Angle 0o 60o 120o 180o 240o 300o 360o 2.8 kcal mol-1 Calculate rate constant at 298 K for a 3 kcal/mol barrier, using k = Ae(-Ea/RT)
  6. 6. CH264 6 Stabilisation of conformation Filled orbital has a stabilising interaction with empty antibonding orbital if antiperiplanar H HH H H H H HH H H H antiperiplanar H H H H H H Newman projection HH H H H H filled orbital (HOMO) - empty (LUMO) overlap eclipsed C-HLUMO C-HHOMO C-HLUMO C-HHOMO CGW p.364
  7. 7. CH264 7 Butane CGW p.365 – 366 There are now several distinct conformations with different energies which are revealed by Newman projections.
  8. 8. CH264 8 D ih e d r a l A n g le φ H ° P o t e n t ia l E n e r g y - D ih e d r a l A n g le C H 3 HH H 3 C H H H C H 3H H 3 C H H H HH 3 C H 3 C H H C H 3 HH H 3 C H H H C H 3H H H C H 3 C H 3 HH H H C H 3 H HH 3 C H H C H 3 0 o 6 0 o 1 2 0 o 1 8 0 o 2 4 0 o 3 0 0 o 3 6 0 o 0 .9 k c a l/ m o l 6 k c a l/ m o l 3 k c a l/ m o l 0
  9. 9. CH264 9 Klyne-Prelog Nomenclature CH3 HH H3C H H H CH3H H3C H H H CH3H H H CH3 CH3 HH H H CH3 CH3 groups: synperiplanar – synclinal – anticlinal antiperiplanar gauche anti H CH3H H H CH3 SYN ANTI H H3C CH3 H CH3H H H CH3 –+ H CH3H H H CH3 PERIPLANAR PERIPLANAR CLINALCLINAL
  10. 10. CH264 10 Energetic Costs for Steric Interactions Type of interaction Energy (kJ mol-1 ) H - H steric strain (gauche / synclinal) 0 Methyl - methyl steric strain (gauche / synclinal) 3.6 H - H torsional strain (eclipsed) 3.6 Methyl - H steric and torsional strain (eclipsed) 5.3 Methyl - methyl steric and torsional strain (eclipsed) 13 – 21 Note sometimes you will see values in kcalmol-1 : 1 kcalmol-1 = 4.184 kJmol-1
  11. 11. CH264 11 1,3-Strain in acyclic molecules Syn-pentane (g+ g- pentane) interactions. So-called due to presence of two consecutive differently arranged gauche interactions Certain conformations of linear chains are higher energy than others: linear lowest energy. Two gauche interactions in certain directions are much higher. H3C CH3 H3C linear pentane all anti- CH3 HH H H CH3 H3C H H H H g + H3C H3C H3C H H H H g+ g- CH3 H H HH H2 C close H ... H contact H syn-pentane H3C H H H H H H envelope cyclopentane
  12. 12. CH264 12 Cyclic Molecules - Angle Strain Note that the bond angles in a planar ring are always further from 109° than if the ring is puckered. 24.5° 9.5° 0.5° -5° "angle strain" (divided between two ring bonds) 9.17 6.58 1.24 0.02 38.4 27.5 5.19 0.09 strain per CH2 group (kcal mol-1) strain per CH2 group (kJ mol-1) "angle strain" is (109° - ring angle)/2
  13. 13. CH264 13 You should be able to: (i) Draw Newman projections for ethane and butane. (ii) Predict the lowest energy conformer for simple alkanes. (iii) Discuss the role of orbital stabilisation of conformation (iv) Discuss the role of 1,3 strain in conformation of acyclics Outputs

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