Lecture 9Outline of Last Lecture I. 3D Shapes of Alkanesa. Conformationb. Dihedral anglec. Eclipsed vs. staggeredd. StrainsII. CycloalkanesOutline of Current LectureI. Cyclohexanesa. Chair conformationb. How to drawCHEM 333 1st Editionc. Other conformations of cyclohexaned. Chairs with ring flipe. Chairs with a substituenti. Diaxial interactionsf. Multiple substituents on ringsCurrent LectureI. Cyclohexanes: has numerous different conformationsa. Chair conformation gives a 109.5 degree angle. This does not have angle strainor torsional strain.i. H’s have two different orientation, axial and equitorial1. Example:b. How to draw:1) Two parallel lines2) One carbon down and one carbon up3) Draw in axial H’s4) Draw in equitorial H’s at slight angle parallel to C-C bonds found in chairFinal structure is steps 3 and 4 combined into one structurec. Other conformations of cyclohexane:i. Boat has torsional and steric strainii. 99.9% of cyclohexane is found in char conformation at room temperatured. Chairs with ring flip: chairs interconvert and so do their H’sThese are equal in energy.e. Chairs with a substituenti. Example: Chair with a methylOne chair has a methyl in axial position and the other has methyl in equitorial position. These are not equal in energy. 95% is found with methyl group in equitorial position.ii. Diaxial interactions: form of steric strain between an axial substituent and an axial H or another group on the the same side of cyclohexane ringiii. The larger the substituent, the greater the energy difference between equitorial and axial1. Example:Right structure has larger difffernece in between axial vs. equitorial2. Exception to this rule: Halidesa. Example:Size: F < Cl < Br < IPrefer equitorial: F < I < Cl < BrWhy? Because C-I bond is so long that iodied doesn’t interact much with other axial H’s.g. Multiple substituents on ringsi. cis: substituents on same side of ringii. trans: substituents on opposite side of ringiii. Example:Cis on left, trans on
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