CHEM 2211 1st Edition Lecture 6 Outline of Last Lecture I II III IV Alkyl Halides Ethers Amines Conformational Isomers Outline of Current Lecture I II III Conformational Analysis Cycloalkane Angle Strain Conformers of Cyclohexane Vocabulary Staggered conformer Gauche interaction Equatorial bond Eclipsed conformer VSEPR theory Ring flip Anti conformer Angle strain Boat conformer Gauche conformer Chair Conformer Half chair conformer Steric strain Axial bond Current Lecture I Conformational Analysis A Molecules constantly switch between staggered and eclipsed conformers millions of times per minute B Staggered conformers are more stable than eclipsed conformers because their bonding orbitals and Antibonding orbitals are parallel Staggered Conformer of Butane Eclipsed conformer of Butane C Anti conformers have two groups 180 away from each other in a Newman projection Axial Conformer of Butane D Gauche conformers have two groups 60 away from each other in a Newman projection Gauche Conformer of Butane II E Both types of conformation leads to Steric Strain i Steric strain happens when the electron clouds of atoms or molecules are close enough to repel each other ii Gauche conformers have greater strain because they have closer substituents This is called a gauche interaction iii As atomic or molecular size increases so does steric strain Cycloalkane Angle Strain A VSEPR theory tells us that a sp3 carbon orbital has a bond angle of 109 5 B However Adolf von Baeyer pointed out that planar cycloalkanes have different bond angles than the ideal bond angle Good bond angle with 109 5 angle C He believed that the difference between the ideal bond angle and the actual bond angle caused angle strain and decreases stability i He predicted that cyclopentane would have the most bond stability and that any higher number of carbons would increasingly destabilize the ring structure D Good sp3 overlap happens when two orbitals directly bond to each other Poor overlap due to decreased or increased bond angle causes angle strain as well Poor bond angle causes angle strain III E Contrary to Baeyer cyclohexane is the most stable of the ring structures because it like all cycloalkanes after cyclopropane is not a planar molecule It puckers out of the plane to increase bond stability and decrease the number of eclipsed hydrogens that it has Conformers of Cyclohexane A Cyclohexane is the most common cycloalkane in nature B It forms a chair conformer with 111 angles and completely staggered bonds This is cyclohexane s most stable conformer C Because the hydrogen bonds are staggered we must differentiate between axial bonds and equatorial bonds i Axial bonds are vertical bonds Axial bonds of cyclohexane ii Equatorial bonds are horizontal bonds Equatorial bonds of cyclohexane iii If an axial bond is pointing upward then an equatorial bond is pointing downward at a slant If an axial bond is pointing downward then an equatorial bond is pointing upward at a slant Both axial and equatorial bonds of cyclohexane D Cyclohexane moves between several conformers This is called ring flip i It exists as a boat conformer which has no angle strain as well but has steric strain due to the closeness of the hydrogens at the bow and the stern The boat conformer is less stable than the chair conformer because of the eclipsed hydrogens ii It also exists as a half chair conformer
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