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Lecture

chapter4.pdf

14 Pages
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Department
Chemistry
Course Code
CHEM 281
Professor
John Carran

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Description
Physical Properties of Alkanes & Cycloalkanes • boiling points & melting points Physical Constants of Cycloalkanes Sigma Bonds & Bond Rotation • two groups bonded by a single bond can undergo rotation about that bond w/rt each other ◦ conformations – temporary molecular shapes result from a rotation about a single bond ◦ conformer – each possible structure of conformation ◦ conformational analysis – analysis of energy changes occur as a molecule undergoes rotations about single bonds Newman Projections How to Do a Conformational Analysis lowest energy conformer Conformational Analysis of Butane The Relative Stabilities of Cycloalkanes: Ring Strain • cycloalkanes do not have the same relative stability due to ring strain • ring strain comprises: ◦ angle strain – result of deviation from ideal bond angles caused by inherent structural constraints ◦ torsional strain – result of dispersion forces that cannot be relieved due to restricted conformational mobility Cyclopropane • internal bond angle (θ) ~60 (~49.5 deviated from the ideal tetrahedral angle) Cyclobutane o o o • internal bond angle (θ) ~88 (~21 deviated from the normal 109.5 tetrahedral angle) • cyclobutane ring is not planar but is slightly folded • if cyclobutane ring were planar, the angle strain would be somewhat less (the internal angles o o would be 90 instead of 88 ), but torsional strain would be considerably larger because all eight C–H bonds would be eclipse Cyclopentane • if cyclopentane were planar, θ ~108 , very close to the normal tetrahedral angle of 109.5 o • however, planarity would introduce considerable torsional strain (i.e. 10 C–H bonds eclipsed) • therefore cyclopentane has a slightly bent conformation Conformations of Cyclohexane: The Chair & the Boat • The boat conformer of cyclohexane is less stable (higher energy) than the chair form due to ◦ eclipsed conformation ◦ 1,4-flagpole interactions • the twist boat conformation has a lower energy than the pure boat conformation, but is not as stable as the chair conformation Energy diagram - more complicated when start substitutingfor H - most stable:whenbiggest groups inequatorialposition inchair Substituted Cyclohexanes: Axial & Equatorial Hydrogen Atoms • equatorial hydrogen atoms in chair form • axial hydrogen atoms in chair form Substituted cyclohexane: two different chair forms *always eqm! • the chair conformation with axial G is less stable due to 1,3-diaxial interaction • the larger the G group, the more severe the 1,3-diaxial interaction and shifting the equilibrium from the axial-G chair form to the equatorial-G chair form 1,3-diaialinteraction → about same size as H, F does't stable equatorialpositionthat much isopropyl tert-but Disubstituted Cycloalkanes: Cis-Trans Isomerism Cis-Trans Isomerism & Conformation Structures of Cyclohexanes Trans-1,4-Disubstituted Cyclohexanes (slightly) down up Trans-1,4- dimethyle upper-lower bonds means the groups cyclohexane are trans Cis-1,4-Disubstituted Cycl
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