Sect 4.3: Conformations of Cycloalkanes

Small rings have two problems: ANGLE STRAIN A carbon with four bonds requires a tetrahedral hybrid with 109 o angles. If that carbon is inside a small ring, the desired 109 o angle cannot be achieved. TORSIONAL STRAIN Issues in small rings Issues in small rings If the rings are planar, there are many eclipsed hydrogen atoms, representing torsional strain. BOTH FACTORS RAISE THE ENERGY OF THE SYSTEM C H H H H H H

Cyclopropane Cyclopropane has angle strain and torsional strain

Planar cyclobutane Planar cyclobutane (actual molecule is not planar) notice that all hydrogens are = TORSIONAL STRAIN eclipsed = TORSIONAL STRAIN internal angle = 90 o = ANGLE STRAIN

CYCLOBUTANE - PUCKERED RING relieves eclipsing which relieves TORSIONAL STRAIN Actual molecule is puckered as shown - it folds in the middle about 25 o from planar “PUCKERING” Still has angle strain H H HH HH H H

CYCLOPENTANE 108 o essentially no ANGLE STRAIN puckering (envelope) relieves eclipsing TORSIONAL STRAIN and, therefore, TORSIONAL STRAIN planar pentagon

ENERGYENERGYENERGYENERGY ~3.0 kcal/mol 0 kcal/mol planar cyclopentane would have all hydrogens = TORSIONAL STRAIN eclipsed = TORSIONAL STRAIN PUCKERING RELIEVES TORSIONAL STRAIN IN CYCLOPENTANE IN CYCLOPENTANE envelope conformation The pucker moves around the ring. planar

equatorial methyl axial methyl METHYLCYCLOPENTANE

ENERGYENERGYENERGYENERGY equatorial axial EQUATORIAL METHYLCYCLOPENTANE HAS LOWER ENERGY THAN AXIAL 0.9 kcal/mol 0 kcal/mol 3.4 kcal/mol PLANAR CH 3 H H H H H H H H H

Molecules viewed with Chime Click on START, Click on PROGRAMS Click on Netscape Communicator (4.7), then launch Netscape Navigator Using Google, type in the address for the Dept. of Chemistry, WWU: Select, course materials, select “WWU virtual molecular model set” You may need the free program, Chime, to run this program. Note: Internet Explorer and Netscape 7.1 won’t work!

Sect 4.4: Evaluation of Angle Strain

Sect 4.5: Cyclohexane

If cyclohexane were planar all of the hydrogens would be eclipsed, resulting in torsional strain. PLANAR CYCLOHEXANE Cyclohexane is not planar... (doesn’t exist) There would also be angle strain - a hexagon has 120 o internal angles.

Cyclohexane is puckered (chair conformation) The perfect molecule! No angle strain and no torsional strain.

axial equatorial COMPLETE STAGGERING ABOUT ALL BONDS COMPLETE STAGGERING ABOUT ALL BONDS

Cyclohexane (boat conformation) bowsprit-flagpole interaction Torsional strain along side of boat and steric strain at top, but there is no angle strain.

ENERGYENERGYENERGYENERGY boat chair THE CHAIR CONFORMATION HAS LOWER ENERGY THAN THE BOAT 29 kJ/mol 0 kJ/mol

Cyclohexane (axial bonds)

Cyclohexane (equatorial bonds)

Cyclohexane (showing both axial and equatorial bonds

CHAIR CONFORMATION axial equatorial

NITROGEN AND OXYGEN HETEROATOMS CAN REPLACE CARBON All of these atoms have sp 3 hybridization.

RING INVERSION axial and equatorial hydrogens are interchanged when the ring is inverted axial red axial blue equatorial blue equatorial red inverted ring starting ring

THE CHAIR IS A RIGID CONFORMATION IT DOES NOT FLEX THE BOAT IS A FLEXIBLE CONFORMATION IT CAN FLEX TO RELIEVE TORSIONAL AND STERIC STRAIN

THE BOAT IS A FLEXIBLE CONFORMATION IT WILL TWIST OR FLEX TWIST BOAT SKEW BOAT twisting relieves the eclipsing at the bottom of the ring... … and the bowsprit-flagpole hydrogens move apart H H

ENERGYENERGYENERGYENERGY THE TWIST BOAT HAS LOWER ENERGY THAN THE BOAT THAN THE BOAT boat twist boat 6 kJ/mol 0 kJ/mol

VIRTUAL MOLECULAR MODEL SET Click on START at the bottom left of the computer on University computers. Click on PROGRAMS Click on Netscape Communicator (4.7) to launch Netscape Navigator. Using Google, type in the address for the Dept. of Chemistry, WWU: Select course materials, select “WWU virtual molecular model set” You may need the free program, Chime, to run this program. Note: Internet Explorer and Netscape 7.1 won’t work! DON’T FORGET