ORGANIC CHEMISTRY 3540

CONFORMATIONS

NEWMAN PROJECTIONS

CONFORMATIONS OF ALKANES

THE SHAPES OF MOLECULES ARE IMPORTANT!

THERE ARE COUNTLESS NUMBER OF THESE STRUCTURES

DIFFERENT ARRRANGEMENT OF ATOMS THAT RESULT FROM THE ROTATION ABOUT A SINGLE BOND.

Call these CONFORMATIONS

COULD YOU EASILY SEE HOW EACH ONE OF THOSE CONFORMATIONS DIFFERED IN THE "SAWHORSE" REPRESENTATION?

NEED AN EASY WAY TO VIEW VARIOUS CONFORMATIONS.

NEWMAN PROJECTION

A QUICKTIME MOVIE IS AVAILABLE WHICH SHOWS THE RELATIONSHIP BETWEEN A PERSPECTIVE VIEW OF A MOLECULE AND ITS NEWMAN STRUCTURE.

HOW MANY CONFORMATIONS OF ETHANE ARE THERE?

WE ARE CONCERNED WITH "STAGGERED" AND "ECLIPSED" CONFORMATIONS SINCE THEY ARE ENERGY MINIMA AND MAXIMA!!!

TORSIONAL STRAIN = 1 Kcal/H-H eclipse

PLOT OF ENERGY vs. DIHEDRAL ANGLE

DIHEDRAL ANGLE ANIMATION
(Requires Shockwave Plugin)

A QUICKTIME MOVIE IS AVAILABLE WHICH SHOWS THE RELATIONSHIP BETWEEN THE ENERGY OF A CONFORMATION IN ETHANE AND THE ANGLE OF ROTATION.

HOW ABOUT PROPANE?

TORSIONAL STRAIN = 1.4 Kcal/CH₃-H eclipse

HOW ABOUT BUTANE?

A QUICKTIME MOVIE IS AVAILABLE WHICH SHOWS THE RELATIONSHIP BETWEEN THE ENERGY OF A CONFORMATION OF BUTANE AND THE ANGLE OF ROTATION.

STERIC STRAIN {{FORCE TWO ATOMS TO OCCUPY THE SAME SPACE}}

CH₃-CH₃ eclipse = 2.6 Kcal

CH₃-CH₃ gauche = 0.9 Kcal

CONCLUSION: MOLECULES GENERALLY FAVOR STAGGERED CONFORMATIONS AND IN PARTICULAR, THE ANTI CONFORMATION. Note: this does not mean that these are the only conformations, just the ones with the highest populations!