Lecture 8Outline of Last Lecture I. Constitutional IsomersII. Nomenclature of alkanesIII. Classification of carbonIV. Nomenclature of cycloalkanes CHEM 333 1st EditionOutline of Current LectureI. 3D Shapes of Alkanesa. Conformationb. Dihedral anglec. Eclipsed vs. staggeredd. StrainsII. CycloalkanesCurrent LectureI. 3D Shapes of Alkanes: help us understand bond angles, distance between atoms, and strains, such as torsional, angle, and steric.a. Conformation: 3D arrangement resulting from rotating around a single bondi. Example:ii. Example (Newman Projection): The “Y” shape denotes front C and substituentsThe circle denotes back C with 3 lines off of it for substituentsb. Dihedral angle: Angle in Newman Projection between C-H bond on front C andC-H bond on back Cc. Eclipsed vs. staggeredi. Eclipsed: conformation where front H eclipses back H1. Example:ii. Staggered: conformation furthest apart1. Example:d. Strains:i. Torsional strain: from eclipsing interactionsii. Steric strain: when nonbonded atoms separated by four or more bonds are forced closer than atomic radius will allowiii. Angle strain: bond angle is expanded or compressed from its optimal value1. Example:119 degrees versus optimal 109.5 degrees. Has angle strain to relieve steric strain.iv. Gauche: two substituents (one on front C, one on back C) with a 60 degree anglev. Anti: two substituents are 180 degrees apartII. Cycloalkanesc. Cyclopropane: 60 degree angles, but wants 109.5 degree angles, so there is lots of angle strain. 6 pairs of eclipsing hydrogens, so there is lots of torsional strain.d. Cyclobutane: 90 degree angles, but wants 109.5 degree angles, so there is lotsof angle strain. Has a little flexibility, so it makes a “puckered” form. Torsional strain is released when this happens because H’s are no longer eclipsing, but this means that the angle strain increases.e. Cyclopentane: 108 degree angles when the molecule is planar, but 10 H’s are also eclipsing when it is planar, therefore, there is a lot of torsional strain. The ring bends to relieve this torsional strain and it twists into an “envelope” shape. Then the angle is 105 degrees, where the angle strain is now increased to releieve the torsional
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