Conformations and Stereoisomerism: Rings and Double Bonds Chapter 4 Conformations and Stereoisomerism: Rings and Double Bonds

Chapter 4 Problems In-Text Problems 1 - 5 8 - 22 24 1 - 5 8 - 22 24 End-of-Chapter Problems 25a, b 26a, b 27a, b 28a, b 31 - 35 37, 39, 40, 42, 43, 47, 51 52a, c, d, f, h 54a, c, d Read Sect. 4.4 and 4.10 (no lecturing) Read steroid (pp 321-324) and terpenes (pp 349-351) for interest.

Homework Assignment Molecular Modeling Assignment: Spartan Download this exercise from my website. Note that there are two pieces of material: Molecular Modeling Instructions Molecular Modeling Report Sheet. This exercise will be omitted this quarter

How to find Spartan on campus computers 1)” My Network Places” should be on the desktop on computers in all university computer labs. 2) Select Novell and look for Mendeleev. If you find it then work your way through the menus, as follows: mendeleev\data1\programs\PC Spartan Pro 3) If Mendeleev doesn’t appear as one of the choices, search for it from Novell. 4) Once you have found Mendeleev, now find PC Spartan Pro from data1, programs, PC Spartan Pro 5) You want PC Spartan Pro and not PC Spartan! 6) Click on WFPro.exe to launch the program

Another way to find Spartan Pro on campus computers Click on RUN Type in the following: \\mendeleev\DATA1\PROGRAMS\PCSpartan Pro\WFPro.exe 4) Click OK Click on START

Sect 4.1: Conformations Different arrangements of atoms within a molecule that can be converted into one another merely by rotation about single bonds.

Conformations Different internal arrangements of the atoms in a molecule that differ by rotation(s) about one or more of the single bonds. CONFORMATIONS ARE NOT ISOMERS Conformations are different arrangements of the SAME MOLECULE. The connection pattern of the atoms does not change. .

Ethane: staggered conformation End View Side View Sawhorse projection

Ethane: eclipsed conformation End View Side View eclipsed projection

Two conformations of ethane: Newman Projections ECLIPSED STAGGERED rear bonds stop at edge bonds on front carbon meet only the front carbon shows H H H H H H H H H H H H Looking down the carbon-carbon bond.

CONFORMATIONS DIFFER IN ENERGY numerous other conformations in between EXTREMES 12.1 kJ/mol eclipsed E N E R G Y 3 Kcal/mol staggered 0 kJ/mol 0 Kcal/mol The staggered conformation has a lower energy.

Torsional Strain Extra potential energy that a molecule possess due to the presence of eclipsed bonds. Eclipsed ethane has torsional strain because of the electron pair repulsions between adjacent bonds. Staggered ethane has no torsional strain.

Torsional strain versus angle of rotation about the carbon-carbon bond in ethane etc ………. eclipsed 3.0 Kcal 3.0 Kcal Potential Energy staggered ETHANE Dihedral Angle

. PROPANE CH3-CH2-CH3 eclipsed E N E R G Y staggered C1 - C2 CH3 H 3.4 Kcal/mol (14.2 KJ/mol) H CH3 staggered 0 Kcal/mol As with ethane, the staggered conformation has a lower energy. Potential Energy Curve is similar to Ethane (sinusoidal)

. BUTANE We will look down the C2 - C3 bond. CH3 C2 - C3 view H 1 2 3 4 We will look down the C2 - C3 bond.

Butane (anti conformation) end view side view no torsional strain or any other strain

Butane (gauche conformation) end view side view no torsional strain, but has steric strain

Steric Strain Extra potential energy that a molecule possesses because groups are crowded together or are forced to approach each other to within the sum of their effective radii

Butane (syn conformation) end view side view has torsional and steric strain

Conformations of butane: Newman projections CH3 H ANTI (180o) CH3 H SYN (0o) CH3 H GAUCHE (60o) CH3 H ECLIPSED (240o) CH3 H GAUCHE (300o) CH3 H ECLIPSED (120o)

( ) Eclipsed always higher energy than staggered! CH3 H CH3 CH3 CH3 H syn E C L I P S E D HIGH ~6 CH3 H CH3 H E N E R G Y eclipsed 3.4 CH3 H CH3 H S T A G G E R E D gauche 1.0 ( ) Kcal mol CH3 H anti LOW

Torsional strain (Kj) versus angle of rotation BUTANE

C C X X Y Y Guideline however, 1,2-dihaloalkanes are an exception ….. X and Y large groups X X Y When large groups are attached to adjacent carbon atoms in a chain the anti conformation is preferred. Y Gauche is preferred second. however, 1,2-dihaloalkanes are an exception …..

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: http://www.chem.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 4.2: Conformations of halides: molecules with polar bonds IS ANTI ALWAYS BEST ?

1,2-DICHLOROETHANE Cl-CH2-CH2-Cl WHY….. ? Prefers the anti conformation in the gaseous state or in non-polar solvents. Cl H non-polar solvent anti But, prefers the gauche conformation in polar solvents. Cl H polar solvent gauche WHY….. ?

Cl Cl Cl 1,2-DICHLOROETHANE d- d- d+ d+ d- d+ d+ d- DIPOLE MOMENT bond dipoles add to zero d+ d+ Cl d- d+ NO NET DIPOLE MOMENT bond dipole vectors add to a positive value d+ d- PREFERRED CONFORMATION DEPENDS ON SOLVENT DIPOLE MOMENT = 0 DIPOLE MOMENT > 0 Preferred conformation in a non-polar solvent = anti Preferred conformation in a polar solvent = gauche