Stereochemistry of Alkanes and Cycloalkane CHM136: Intro to Organic Chemistry
Stereochemistry: the 3D arrangement of atoms in a molecule of interest
Models are very useful for visualizing this material! Alkane Conformations (Use Models!!) • bonds are cylindrically symmetrical
• Rotation is possible around C-C bonds in open-chain molecules, e.g. ethane… 2 Representing Conformations Conformation: any three-dimensional arrangement of atoms in a molecule that results from rotation about a single bond Newman projection: a way to view a molecule by looking along a carbon-carbon bond rotate rotate 180o dihedral (torsion) angle 60o Conformations Staggered conformation: a conformation where atoms on one carbon are as far apart as possible from the atoms on the adjacent carbon Eclipsed conformation: a conformation where atoms on one carbon are as close as possible to the atoms on the adjacent carbon Energy of Conformations Torsional strain: a destabilization that arises during rotation as bonds eclipse each other. Torsional strain drives molecules away from eclipsed conformations. +12 kJ/mol eclipsed staggered Conformations of Ethane dihedral angle (degrees) torsional strain Steric Strain Steric strain: the strain that arises when atoms not bonded to each other are forced abnormally close to one another gauche conformation: substituents 60o from one another. Consider butane: anti conformation: substituents 180o from one another. no torsional strain steric strain is approximately 3.8 kJ/mol no steric strain no torsional strain Strain Conformations of Butane 10 Butane 11 Zig-zag Drawings • Why do we represent alkanes with a zig-zag line?
12 Cycloalkane Stereoisomerism • Cycloalkanes less flexible than open-chain alkanes
• Much less “conformational freedom”
Stereoisomerism Stereoisomers: compounds having the same atom connectivity but different 3D atomic arrangements in space
13 Stereoisomerism These two isomers (diastereomers) cannot interconvert through bond rotation
Isomerism common in multi-substituted cycloalkanes
14 Problem 15 Draw a skeletal structure to represent trans-1-methyl-2-(2-methylpropyl)cyclobutane 16 Cycloalkane Stability • Rings larger than three atoms are not flat
• Cyclic molecules adopt non-planar conformations (the ring “puckers”) to minimize angle strain and torsional strain
• Larger rings are harder to analyze: have many possible conformations 17 Cycloalkane Stability
Rings from 3 to 30 C’s exist but many are strained due to steric interactions 18 Strain Summary • Angle strain: expansion or (often) compression of bond angles away from most stable
• Torsional strain: eclipsing or alignment of bonds on neighbouring atoms
• Steric strain: repulsive interactions between non- bonded atoms in close proximity Cycloalkane Conformations: Cyclopropane • 3-membered (3-sided) ring must be planar!
• Symmetrical with C–C–C bond angles of 60°
• Requires that sp3-sp3 bonds are bent (weakened)
• All C-H bonds eclipsed
19 20 Cycloalkane Conformations: Cyclobutane • Less angle strain than cyclopropane but more torsional strain because of more ring H’s
• One C atom about 25° above plane of other three
• Ring bend increases angle strain BUT decreases torsional strain: paper airplane conformation
Cycloalkane Conformations: Cyclopentane • Planar cyclopentane would have NO angle strain BUT very high torsional strain
• Non-planarity reduces torsional strain
• Four carbon atoms are in a plane, fifth is above the plane: envelope conformation 21 22 Cyclohexane Conformations • Substituted cyclohexane rings occur widely in nature (cholesterol has two): free of angle strain and torsional strain
• Tetrahedral angles between all carbons: chair conformation Drawing Cyclohexane 23 24 Drawing Cyclohexane 25 Axial & Equatorial Bonds in Cyclohexane • Chair conformation has two kinds of positions for the 12 H atoms: axial and equatorial
• Chair cyclohexane has six axial C-H bonds perpendicular to the ring and six equatorial C-H bonds near the ring plane 26 Drawing Axial & Equatorial Bonds 27 Cyclohexane Conformational Mobility • Chair conformations readily interconvert, resulting in exchange of axial and equatorial positions by a ring- flip (build/use a model!) 28 Don’t draw H’s like this!! Either show the H or don’t show the bond. 29 H H H H ring flip Note:
“Up” hydrogens still up after flip. “Down” hydrogens still down after flip. Axial and equatorial swap. “Back rest” and “foot rest” swap. Need one “up” and one “down” H on each C. 30 Conformations of Monosubstituted Cyclohexanes 30 • Cyclohexane ring rapidly flips between chair conformations at room temperature
• Two conformations of any monosubstituted cyclohexane aren’t equally stable
equatorial conformation of methylcyclohexane more stable than axial by 7.6 kJ/mol
CH3 H H CH3 1,3-Diaxial Interactions • Difference between axial and equatorial conformations due to steric strain caused by 1,3-diaxial interactions
• H atoms of axial CH3 group on C1 too close to axial hydrogens on C3 and C5, resulting in 7.6 kJ/mol of steric strain 31 1,3-Diaxial Interactions
32 Strain of one H-Y 1,3-diaxial interaction: - for Y = CH3, ∆G o = +3.8 kJ/mol Size and shape of the substituent affects the magnitude of steric strain –Cl 1.0 –Br 1.0 –F 0.5 Y kJ/mol –CN 0.4 –CH(CH3)2
4.6
–OH
2.1
–CH3
3.8
–CH2CH3
4.0
–C(CH3)3
11.4
–C6H5
6.3
–COOH
2.9
Bulkier alkyl groups even more likely to be equatorial (e.g. t-butyl, -C(CH3)3) Conformational Analysis of Disubstituted Cyclohexanes
Two isomers of 1,2-dimethylcyclohexane exist
33 Disubstituted cycloalkanes can exist as cis-trans stereoisomers Stereoisomerism exists in cycloalkanes with two or more substituents cis trans Conformational Analysis of cis-1,2-dimethylcyclohexane Conformational Analysis of trans-1,2-dimethylcyclohexane Problem 36 Determine whether each of the following is a cis isomer or trans isomer. Cl Br Br Cl Br cis trans cis trans cis In-Class Problem 37 Draw the least stable and the most stable chair conformations of trans-1-tert-butyl-3-isopropylcyclohexane.
