Lecture 9Outline of Last Lecture:I. Viewing/Drawing Conformations of Ethane (Newman Projections)II. CycloalkanesIII. Naming CycloalkanesIV. Cycloalkane Conformational FreedomV. Cis-Trans Geometrical Isomers CycloalkanesOutline of Current Lecture:I. Stereoisomers vs. Constitutional IsomersII. Ring Strain in CycloalkanesIII. Characteristics of Cyclic MoleculesStereoisomers vs. Constitutional Isomers:Isomers: same molecular compositionStereoisomers: Same connections, but different 3D geometryConstitutional Isomers: Different connections between atoms, but same molecular formulaRing Strain in Cycloalkanes:-When the 3D geometry deviates (either under or over) 109 degree formationEx. Cyclopropane ( 60 degrees)-Is highly reactive, and explosive when in contact with oxygen-Boiling Point = -33 degrees CelsiusEx. Cyclobutane (90 degrees), Boiling Point = 12 degrees Celsius **more ring strain more reactive** ring strain (or “angle strain”) is directly related to deviation from 109 degree conformation inANY directionRing Strain reality: Cyclopropane (planar) and cyclobutane (not as planar, slightly more flexible) = least “happy” conformations**rings that are greater than 3 atoms are NOT flat Chem 261 1st Edition-Cyclic molecules assume non-planar conformations to minimize strain (angle strain and torsion strain)Characteristics of Cyclic Molecules:Cyclopropane:-torsion strain and ring strain = very unhappy molecule-3 membered ring, 60 degree bond angles, all symmetrical, sp3 bent shape, all C-H bonds are eclipsedCyclobutane:-less angle strain than cyclopropane, but more torsion strainCyclopentane:**if planar**, less angle strain but high torsion strain (not the case, there is some angle strain)-Envelope shapeCyclohexane:-Most flexible, not flat- “chair” conformation**know how to draw and interpret chair diagramsAxial and Equatorial Bonds in Cyclohexane (Chair) : Chem 261 1st Edition-6 axial hydrogens, 6 equatorial hydrogens. Each carbon is bound to one of eachAxial: Carbon up = up bond, Carbon down = down bondEquatorial: Bonds are in parallel to where the opposite bond is going Chem 261 1st
View Full Document
Unlocking...