CHEM 261 1st Edition Lecture 8Outline of Last Lecture:I. Snow Day- No lecture material covered Outline of Current Lecture:I. Viewing/Drawing Conformations of Ethane (Newman Projections)II. CycloalkanesIII. Naming CycloalkanesIV. Cycloalkane Conformational FreedomV. Cis-Trans Geometrical Isomers CycloalkanesViewing/Drawing Conformations of Ethane (Newman Projections):-Twisting = TorsionEx. Ethane, Newman Projection (For ethane, one carbon is in the “front” and one carbon is in the “back”. Displayed as a circle for the back carbon, and lines for the front carbon)-Image on the left = eclipsed conformation (HIGH energy…not preferred. Less stable)-Image on the right = staggered conformation (LOW energy…preferred! More stable)-When given energy, conformation can twist! (eclipse conformation is usually a transition state)**barrier free rotation does not exist**LOW ENERGY IS MORE FAVORED…molecule is less “happy” in eclipsed conformationEx. Propane (bigger molecule than ethane)Ex. Butane Most preferred conformation= staggered with methyl groups opposite of each other (image farthest on the right)… an anti-conformation. Most energy efficientSomewhere in between= gauche (staggered with methyl groups next to each other. Second image from the left)Least favorable is eclipsed with methyl groups “on top of each other”. Staggered with a methyl group “on” a hydrogen is more favorable, but still unfavorable overall. Terms:Torsional strain- from twistingSteric strain: repulsive interaction between atomsCycloalkanes: -Lots of compounds/molecules have ring structures-Cycloalkanes: saturated, are hydrocarbons, but in a ring structure“CnH2n”**very important = cyclohexane**Naming Cycloalkanes:1. Find parent # of carbons (in the ring)How do you know what direction to number the parent carbons? Number them so that the substituents have the lowest possible numbers)Cycloalkane Conformational Freedom:-Cycloalkanes are far less flexible than alkanes, which causes for 2 faces of the same moleculeCis = same (same face/same side of molecule”Trans= oppositeCis-Trans Geometrical Isomers in Alkanes:Ex. 2 CH3 on the same side/face of cyclopropane-“geometrical isomers”-Cannot convert w/o breaking bonds, then they are referred to as
View Full Document
Unlocking...