CHEM 2211 1st Edition Lecture 6 Outline of Last LectureI. Alkyl HalidesII. EthersIII. AminesIV. Conformational IsomersOutline of Current LectureI. Conformational AnalysisII. Cycloalkane Angle StrainIII. Conformers of CyclohexaneVocabularyStaggered conformerEclipsed conformerAnti-conformerGauche conformerSteric strainGauche interactionVSEPR theoryAngle strainChair ConformerAxial bondEquatorial bondRing flipBoat conformerHalf-chair conformerCurrent LectureI. Conformational AnalysisA. Molecules constantly switch between staggered and eclipsed conformers millionsof times per minuteB. Staggered conformers are more stable than eclipsed conformers because their bonding orbitals and Antibonding orbitals are parallelStaggered Conformer of ButaneEclipsed conformer of ButaneC. Anti-conformers have two groups 180° away from each other in a Newman projectionAxial Conformer of ButaneD. Gauche conformers have two groups 60° away from each other in a Newman projectionGauche Conformer of ButaneE. Both types of conformation leads to Steric Straini. Steric strain happens when the electron clouds of atoms or molecules are close enough to repel each otherii. 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. II. Cycloalkane Angle StrainA. 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 angleGood bond angle with 109.5° angleC. He believed that the difference between the ideal bond angle and the actual bond angle caused angle strain and decreases stabilityi. He predicted that cyclopentane would have the most bond stability and that any higher number of carbons would increasingly destabilize the ring structureD. 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 strainE. 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 outof the plane to increase bond stability and decrease the number of eclipsed hydrogens that it has.III. Conformers of CyclohexaneA. Cyclohexane is the most common cycloalkane in natureB. 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 bondsi. Axial bonds are vertical bondsAxial bonds of cyclohexaneii. Equatorial bonds are horizontal bondsEquatorial bonds of cyclohexaneiii. 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 cyclohexaneD. 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
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