chapter 6-2003

The dihedral angle of H1-C(1)-C(2)-H2 (6): 180o
6.5 Cyclohexane
8 7 tr ans-1,2-dimethylcyclohexane
Relationship of 5 and 7: Enantiomers General statement: For a substitutent X, trans-1,2diX-cyclohexanes have either diaxial or diequatorial and exist as a pair of enantiomers.
6.2 Cyclopropane
3 carbons: lie in a plane
All C-H bonds: eclipsed
The angle of CCC: 60o
6.2 Cyclopropane
Practice:
1. Which of the following compound(s) is (are) chiral? Meso?
6.5 Cyclohexane
Some common AValues (kcal/mol)
6.5 Cyclohexane
OH Me HO Me
Locked chair conformation
Very bulky substituent group: Overwhelming preference for equatorial position.
6.3 Cyclobutane
The crystal structure, determined by X-ray diffraction, contains chains of hydrogen-bonded molecules. The cyclobutane ring is planar with C-C single bond lengths of 1.567 ± 0.006 and 1.552 ± 0.006 Å (longer than normal C-C, 1.537 Å ).
6.5 Cyclohexane II. Disubstituted cyclohexanes
1. Both substitutents can be axial
1) 1,2-disubstituted cyclohexanes
The dihedral angle of C-C(1)-C(2)-C (5): 180o
Three Types of Cyclic Strain
1. Angle strain (Baeyer Strain) 2. Torsional strain (Pitzer Strain)
3. Steric strain (Prelog Strain)
6.1 Types of Strain
1. Angle strain (Baeyer Strain theory): Proposed by Adolf von Baeyer (Nobel Prize in 1905), an explanation of the relative stabilities of cyclic molecules.
6.1 Types of Strain
2. Torsional strain (Pitzer Strain): Butane
6.1 Types of Strain
3. Steric strain (Prelog Strain): Steric strain (van der Waals strain) is the unfavorable interactions of ring substituents on nonadjacent carbons, arises from a lack of space in the interior of the ring (distance in between less than the sum of their van der Waals radii), which forces substituents into conflict with one another.
6.1 Types of Strain
2. Torsional strain (Pitzer Strain):
Is the repulsion caused by the electrons between X and Y groups in a system of X-C-C-Y (1,2-interaction) when they pass by each other during the rotation around the C-C sigma bond.
6.5 Cyclohexane
1,3-diaxial interaction
6.5 Cyclohexane
Practice: Draw both chair conformers of cis-1-chloro3-methyl-cyclohexane. Is this compound chiral? Consider whether a plane of symmetry exists in any energetically accessible conformation.
2.
Draw cis-2-chlorocyclopropane-1-carboxylic acid and its mirror image. What is the stereochemical relationship between them and compound 4?
6.3 Cyclobutane
6.5 Cyclohexane
2) 1,3-disubstituted cyclohexanes
The dihedral angle of C-C(1)…C(3)-C (9): 0o The dihedral angle of H1-C(1)…C(3)-H3 (10): 0o
General statement: cis-1,3-diX-cyclohexanes have either diaxial or diequatorial and are meso (two X are identical, symmetry plane exists).
trans-cyclobutane-1,3-dicarboxylic acid
JACS, 1967,89:2, 223-226
6.3 Cyclobutane
When
the number of carbon atoms in the ring, n, is even, and the substituents are located at carbon 1 and 1+(n/2), their diastereomers are achiral.
Indigo dye
6.1 Types of Strain
1. Angle strain (Baeyer Strain theory): Occurs when C-C-C single bond angles deviate significantly from the ideal value of 109.5o
6.1 Types of Strain
Ring Strain vs. Cycloalkane Size
kcal/mol
10 9 8 7 6 5 4 3 2 1 0 3 4 5 6
Cyclohexane the most stable
Ring Strain
7
8
9
10
11
12
Cycloalkane Ring Size
Not flat, one carbon is ca.
25o above the plane defined by the other three. 88o.
The internal CCC angle is Certain
substituted cyclobutanes are planar (trans-cyclobutane-1,3dicarboxylic acid).
6.4 Cyclopentane
Puckered
conformation (envelope-type conformation) around the ring (pseudorotation).
The puckering moves rapidly
puckering
6.4 Cyclopentane
6 Stereochemistry of Cyclic Compounds
6.1 Types of Strain
6.2 Cyclopropane
6.3 Cyclobutane
6.4 Cyclopentane
6.5 Cyclohexane
6.6 Summary of Key Points
6.1 Types of Strain
Pseudorotation of cyclopentane (VALISA)
Int. J. Mol. Sci., 2003, 4, 107-118
6.5 Cyclohexane
C-C-C angle: 109o29’, close to the
tetrahedral value.
Little strain A C3 axis Two principal conformers (chair and boat) Hydrogens at axial and equatorial positions
6.5 Cyclohexane
Chair Conformer
6.5 Cyclohexane