lec 12 - Newman Projec-ons Chairs Lecture#12...

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Unformatted text preview: Newman Projec-ons Chairs Lecture #12 Thursday, July 28 Sigma Bond Rota-on   In general, there is free rotation around sigma bonds.   Conformers – different 3-D orientations a molecule assumed by single bond rotation.   Generally, conformers cannot be isolated because they interchange too rapidly.   Different conformers typically have different energies. Wedges and Dashes   When drawing 3 ­D structures in 2 ­D, we have to define different bond orienta-ons: In the plane of the paper/screen. Projects out of the paper/screen toward the viewer (wedge). Projects into the paper/screen away from the viewer (dash). Newman Projec-ons   Consider this two carbon molecule:   If you looked down the carbon ­carbon bond like this, you would see the front carbon but not the back carbon. Front carbon Newman Projec-ons   From this perspec-ve, the hydrogen isotopes would be in the back.   From this perspec-ve, the halogens would be in the front. Front carbon Newman Projec-ons   Newman projections represent the conformation of a σ bond.   Dihedral angle, θ – the angle between two groups, one on each C. conformers Ethane Rota-on   The energy required for ethane to rotate around its C-C bond is only 2.9 kcal/mol. That is so low there is free rotation around ethane’s C-C bond.   The methyls rotate ~1011 times per second at RT!   Ethane has ∞ number of conformers, but 2 are important: ≡ staggered ≡ eclipsed Eclipsed Conforma-on   It is easier to see with a space filling model that the eclipsed conforma-on is less stable. Ethane Rota-on Butane   The anti conformation of butane has eclipsing bonds (torsional strain) and steric hindrance from the methyl groups approaching too closely. θ = 0° Conforma-ons of Butane   Rank these conformers from most to least stable: Butane Strain   Strain – sources of instability within a molecule.   Torsional strain – eclipsing bonds   Angle strain – non-idealized bond angles   Steric hindrance or interactions – atoms/groups approaching too closely   Ring strain – angle + torsional strains in cyclic molecules Ring Strain   Cyclopropane and cyclobutane have very high ring strains.   Cyclohexane has no ring strain.   As rings get very large, ring strain drops.   Unbranched alkanes have no rings, ∴ no ring strain. higher ring strain = less stable molecule Cyclopropane   The only flat cycloalkane.   60° bond angles (severe angle strain).   All C-H bonds are eclipsed (torsional strain). Cyclobutane   88° bond angles (angle strain).   C-H bonds are partially eclipsed (torsional strain).   Puckered (≠ flat) to minimize eclipsing C-H bonds. Cyclobutane   UV light damages DNA via a cyclobutane intermediate.   >1 M new cases of skin cancer per year in the U.S. + thymidine UV Cyclohexane Chair   Cyclohexane adopts a “chair” conformation. The upper half is further from the viewer. ≡ The lower half is closer to the viewer. flat cyclohexane (doesn’t exist, easy to draw) chair cyclohexane (exists 99.5% of the time) Cyclohexane Chair   Cyclohexane adopts a “chair” conformation.   109.5° bond angles (no angle strain).   All bonds staggered (no torsional strain). Cyclohexane Chair   Cyclohexane adopts a “chair” conformation.   109.5° bond angles (no angle strain).   All bonds staggered (no torsional strain). How to Draw a Chair   Chairs have 3 sets of parallel lines. Start by drawing two parallel lines at an angle slightly offset from each other.   Convert the top bond to a symmetrical “V” and the bottom to an upside-down “V”.   Connect the final two bonds to form a ring. Cyclohexane Chair   Cyclohexane chairs are important in nature. Saccharides (sugars, cotton, paper, cellulose, wood, starch, carbohydrates) Steroids (cholesterol, bile acids, estrogen, hydrocortisone, birth control medication, anabolic steroids, testosterone) Diamond infinite pattern of chairs Cyclohexane Boat   A high-energy transition state.   Unfavorable “flagpole” interactions (steric hindrance) .   8 eclipsing bonds (torsional strain). There are 2 chair conformers, the boat is a transition state between them. Axial and Equatorial   A chair has 2 types of bonds, axial and equatorial.   Axial – alternately point up or down   Equatorial – around the perimeter of the ring Equatorial Bonds   Equatorial bonds run parallel to a chair’s carbon-carbon bonds. Ring Flips   When 2 chairs interconvert, axial bonds become equatorial and vice versa. ⇄ Subs-tuted Cyclohexanes   Methylcyclohexane has its –CH3 equatorial 95% of the time. tert ­Butyl Anchors   The tert-butyl group is so large, it is equatorial 99.99% of the time. 1,3 ­Diaxial interac-ons   Any atom or group besides H prefers to be equatorial.   1,3-Diaxial interactions - steric hindrance (a gauche interaction) between an axial group at C1 and axial Hs at C3 and C5. tert ­Butyl Anchors   The size of a group dictates how often it is found in the equatorial position. group Cl equatorial-axial ratio 70 : 30 OH C H3 83 : 17 95 : 5 CH2CH3 96 : 4 CH(CH3)2 97 : 3 C(CH3)3 9999 : 1 cis ­trans Isomers   It is easy to envision cis and trans cyclohexane isomers when the molecule is flat: ≡ cis-1,2- dichlorocyclohexane cis ­trans Isomers   It is easy to envision cis and trans cyclohexane isomers when the molecule is flat: ≡ cis-1,2- dichlorocyclohexane cis ­trans Isomers   Which chair would be preferred? ⇄ Chair Flips   When a chair flips,   Axial bonds become equatorial bonds   Equatorial bonds become axial bonds.   Up bonds remain up bonds.   Down bonds remain down bonds. Chair Flips 1   What is the most stable conformation of: 2 3 number the carbons clockwise, it doesn’t matter where you start ...
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This note was uploaded on 08/06/2011 for the course CHEM 221x taught by Professor Broyer during the Summer '11 term at USC.

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